Collins 30S-1 Amplifier

Contact me! Email Gord (VA7GP)

In October 2018, I bought a used Collins 30S-1 HF amplifier off Craigslist! The seller and I carefully loaded the amp into my trailer, where I wrapped and secured it. The trip from the small oceanside community of Tsawwassen, BC to my home required about 30 minutes' highway driving. The Fall day was absolutely perfect!

A Bit of History

My childhood radio-mentor had a Collins S-Line transmitter and receiver; in addition he had the 30L-1 compact desktop-amplifier to go with these. It was all the best gear of the 1970s, but far out of the reach of a 15-year-old paper-delivery-boy's meager earnings. Eventually I *did* manage to save up enough to buy an amplifier, but it was a well-used homebrew amp with 6KD6 TV sweep-tubes. Even then, I wasn't really happy with this amp...

Thus, my desire for an amplifier took root, and high regard for Collins gear was firmly instilled.

Along the path of life, I de-constructed my 6KD6 amp, removing the good parts for re-use, and embarked on constructing a kind of clone of the 30L-1 / Clipperton-L amp with 4x 572B tubes. Unfortunately, this stalled due to life's busy-ness, new priorities, and increasing difficulty of finding builder-parts for radios.

Now, over 40 years later, I was able to purchase an amp: A legendary Collins 30S-1 - heavier-duty than anything I'd had before, and a true radio legend!

Here, the amplifier arrives home:

Preparation

While the amplifier was still outside, I fired up my air-compressor and blew a cat's-worth of dust out of everything. I cleaned all the relays and switch-contacts, did a basic check of all the fuses, and carefully struggled to get this 160-pound amp into my ham-room.

I changed the plug on the amplifier's power-cord to match my existing wall-socket (6-20R). I removed the 4CX1000A power-amplifier tube, before applying any power.

First Power-Up

First power-up brought a lot of smoke :-( It seems the Bias Transformer T203 was very, very unhappy. I had hoped to turn the amp on, and quickly be using it; now, clearly I was actually going to have to look deep inside. My blissful ignorance slipped away...

Repairs

• Bias Transformer T203 was utterly toasted... burned and bubbled. Fortunately Peter Dahl / Hammond offer a top-quality replacement - Whew! Nearby components and the wiring-harness appeared miraculously undamaged!
• Bias rectifiers had been changed to silicon diodes, mounted on a terminal-strip. However, the terminal-strip was bent over, shorting at least one (perhaps two) leads to the chassis.
• In a similar vein on the underside of the Relay Shelf, Bias divider resistor R218 was bent to touch the chassis.
• Next to this bent/touching resistor, I then saw the cracked-and-broken 10-ohm R232 (limits screen-supply inrush current).
• And, the RF output connector had been changed from the N-flanged original, to a loose-and-sloppy SO-239 barrel (featuring mangled threads which prevented tightening).
• Time-delay thermal relay K202 was only 35-seconds, not 3-minutes as Collins designed (to ensure V101's cathode is properly heated before use).
• Rectifier-filament transformer T202 had cracked-and-broken secondary leads. Because this amplifier had replacment solid-state HV rectifiers, I decided to simply remove the no-longer-needed T202.

Here is a photo of the taped-up schematic, with my annotations and some flags showing the components I decided to replace:

Beyond these quickly-visible electrical problems, I noted that there were some other changes and physical areas needing my attention too:

• The 3B28 rectifiers have been changed to solid-state modules. These modules were mounted to a thin piece of plywood, and look electrically OK... but it is not a thing of beauty
• the 12AL5 dual-diode ALC rectifier-tube has also been changed to a plug-in solid-state diode. This looked tidy. It functionally implements Collins' Service Bulletin SB-3, Nov 1/69 (space-charge standoff voltage from 12AL5 tube)
• the "TUNING" dial had some rubbed/dissolved printing 'way down at the low-end of the dial. The dial functioned, but I knew this would truly annoy me.
• the "LOADING" dial had cracked and been repaired, with a visible glue-line. The repair was very neatly done, but I would find it a constant aggravation simply knowing it was there.
• The 12V filter-capacitors C211A and C211B (a two-section can) looked sketchy: A previous repair fished up into the internals of the can to effect a connection to one of the caps (!), and a scary bare wire snaked through (grounded/conductive) obstacles from the cap to the circuit. It's possible all this did work, but I'd lose sleep if I ignored this area...

Here's a visual summary:

Dealing with the Overheated and Cracked R232

R232 broken into two pieces. Somewhere along the line, a previous repair has replaced the Collins 10-ohm 14W resistor with this small (2W???) resistor; it's overheated then cracked as a result. The purpose of this resistor is to limit screen-supply (inrush) current, in the same manner as R222 and R223 do for the HV plate-supply.

Awesome new 10-ohm 20W R232! This Ohmite replacement brings higher-dissipation, and solid mounting. I wanted to mount R232 very solidly, so I opted for a tubular-ceramic resistor with mount-clips. I also moved to higher-dissipation capacity (20W) than Collins specified (14W), and I chose a new placement with improved air circulation and reduced heat-soak of nearby components - along the edge of the Relay Shelf. Mounting hardware is all 18-8 stainless: #6-32 x 3/8 Phillips pan-head machine-screws, internal-tooth lockwashers and hex-nuts. On the outer (visible) edge of the Relay Shelf, I aligned the Phillips-head crosses :-) In this location, and for other repairs, my wire of choice is either 22ga or 18ga (19 or 34 strands of 0.007 for maximum flexibility), with 3kV silicone insulation.

Dealing with the RF Output

I unsoldered, and unscrewed the double-female SO-239 barrel-connector which arrived with my 30S-1 amplifier. The only good part was the use of wide copper-strapping for the rf-ouput. But the loose double-female connector flopped around and rotated whenever an antenna-connection was made, which in turn twisted and fatigued this wide copper strapping. Furthermore, the strapping was routed very close to the relay-frame and the low-voltage wiring. I wasn't comfortable at all with the chassis-ground-connection provided by the wobbly and sloppy barrel connection - even if it "mostly worked", this could lead to one of those intermittent and puzzling annoyances, like fluctuating received-signals or flakey jumping of transmit plate-current.

I harvested some RG-8x-mini coax for it's braid to use as the RF output connection. Nominal operating conditions for this braid connection are expected to be in the neighbourhood of 300V and 5A - I have ample margin on both parameters. I twisted and pushed the braid all the way into the bottom of the SO-239 and soldered it prior to installing the connector to the chassis; I slightly opened the other end of the braid to allow a slip-fit all the way over the relay-terminal. The braided-conductor was then dressed neatly (angle of photo doesn't clearly show the 1/2-inch clearance to the chassis side-wall). I connected my TS-850 transceiver to the RF Input, and my antenna to the RF Output. I received signals; pressing the top of the Antenna Changeover Relay K101 removed the signal.

The flanged SO-239 features Teflon dielectric, silver-plating, and a gold-plated center-conductor. Mounting hardware is all 18-8 stainless: #6-32 x 3/8" Phillips pan-head machine-screws, internal-tooth lockwashers and hex-nuts. On the rear of the amplifier, I aligned the Phillips-head crosses :-) The final touch was the addition of a right-angle PL-259-to-SO-239 adapter out the back; this allows my RF cable to tuck neatly down the back of the amplifier.

Dealing with the Shorted BIAS Diodes

The original selenium rectifiers were long-gone, having been replaced with a terminal-strip and newer silicon diodes. Rather than replace or simply fix what was already there, I wanted to prevent another bent-and-shorted kind of tragedy (if it can happen once...). I borrowed Collins' own idea from their nearby 12V rectifiers: I bought a triple-fuse mounting block (designed for AGC tubular glass fuses), and mounted my replacement 1N4005 diodes to this fuse-holder. The diode-leads were lightly clinched, then soldered to the fuse-clips, while the circuit connections were made to the holder's solder-tabs. These diodes aren't going anywhere! And the printed component-labelling may help me (or someone else) in the future.

Replacement 30S-1 BIAS Transformer T203, and Associated Circuitry

The replacement Peter Dahl transfomer! It took 2-1/2 months from initiating the order, until delivery. I made the actual purchase through Digi-Key, who provided the customer-facing fulfilment for Hammond. The transfomer also includes a 12.6VAC (at 800ma continuous commercial service) winding, which is used for three distinct purposes within the 30S-1: the AC voltage from this winding powers the incandescent dial and meter bulbs rectified and filtered DC runs through the HV-interlock circuit rectified and filtered DC is used to operate the Antenna Relays (when an external short-to-ground occurs - often an Amp T/R relay inside the exciter)

T203 installed, laced and soldered into it's new home! When I look at the previous T203, especially the type of insulation and the under-chassis un-laced wiring, it makes me wonder if my burned T203 was already a replacement...? Hmmm... I will thoroughly check each and every circuit associated with this transformer, because it seems possible that this BIAS transformer, in this amp, has been an on-going issue. I intend to firmly quash all the problems now!

C209 and C210 upgraded, mechanically secured, and labelled A pair of new capacitors - same 50uF, but uprated from 150VDC to 250VDC, working temperature range -20C to +85C. The capacitors are labelled, as is the T203 Bias-Transformer centre-tap connection (I may be the next person helped by this clear labelling!). An additional layer of clear heat-shrink was added to ensure HV sturdiness, and the nylon mounting clamps are spaced a further 8mm off the chassis.

I broke this brittle terminal-strip While working underneath the Relay Shelf, I broke a terminal-strip. Unable to find an exact replacement, I opted for two smaller terminal-strips, turned 90-degrees to the original. And, this allowed me to neatly separate the two functions on the original: the Step-Start circuit, and the BIAS connections.

Replacement C203 step-start capacitor, securely mounted I replaced C203, making sure it's well-secured, and well-insulated. There should never be a problem of fat-fingers causing future short-circuits! The new C203 is rated at 200VDC, which improves on the original 150VDC rating, especially comforting considering how the incoming AC line-voltage has increased over the decades: designed with 115VAC/230VAC in mind, my current voltage is ~6% higher at 122/244 (just measured it).

Underside of Relay Shelf, now tidied Whew! All the parts I indicated on the schematic with yellow arrows have now been replaced - YAY! Now, on to some of the upgrades...

Screen Supply Overhaul

Much (much!) earlier, when I removed the Relay Shelf, I caught a glimpse of a dark smudge in the shadows near the front-fuses. But I didn't dwell on that - I was focussed on the BIAS area... Now, I decided to look deeper. I was suspicious when I looked closely; I was aghast when I removed the screen rectifier board - there were obvious signs of combustion, and weird melted-looking goop on the rectifier board. The rectifier diodes had been replaced at some point in the past, and the equalizing-capacitors C251-C258 were entirely gone. I rolled up my sleeves...

I discarded Collins' rectifier board, then cleaned the chassis. I salvaged the 0.01uF / 1.6kV C221 ceramic cap, and re-used it with... a modern rectifier module. This module is rated at 1.2kV 35A which ought to do the job nicely! What's more, it's mechanically robust, and compact. If there is a screen-fault, I believe the 1A fuse F205 will NOW do it's job, and save all the downstream components (rather than the components blowing up to save the fuse :-O ).

While I was working on the screen-supply, I looked at the filter-capacitor C204. The original was rated at 40uF / 350V, so a modern 47uF / 400V @85degC seemed like a gentle improvement. When I looked closely at C204, my decision to replace it was confirmed: The original can was dimpled out-of-round by the mounting clip; time and heat had also taken it's toll and the mounting-clip had dug through the insulating sleeve.

The replacement capacitor got an additional jacket of insulating heat-shrink, then I mounted this new C204 using a nylon clamp. High-voltage fiberglass acrylic insulating sleeve (2.5kV, MIL- and NEMA-rated) on both leads will again prevent careless-finger problems

Later, I increased the Screen Supply, and felt this replacement C204 was marginal. So, I mod'd my mod :-) by installing a higher-rated 500V/105C capacitor, with slightly nicer lead-arrangement.

While I was in the neighbourhood (the back panel), I felt it would be a good idea to check the Screen bleeder-resistor R203, the High-Volage bleeders (R205, R206 and R207), along with the meter's HV divider-resistors (R229, R210-R214. All check "OK", and there appears no HV or Screen shorts.

12V Safety and Control

The Bias Transformer includes 12VAC for panel-lights, HV safety interlock, and the RF T/R relays. These areas also require investigating, to make sure they weren't part of the short-circuit smoke-show I had. And I already knew, in this area, that C211A/C211B were flakey. Digging deeper...

C211 was beyond redemption. One of the wires had an internal-break and provided no connection. Both wires were spliced behind the front-panel fuse area, and both showed signs of the same combustion I noticed with the screen rectifiers. I made another shelf-edge bracket, and replaced the two halves of C211 with a pair of individual electolytics. I cut back, then extended the capacitor-wires, removing the crispy parts. Given the general provenance I've found so far, I also replaced both the diodes CR206 and CR209 with some 1N4005.

DC meter-testing showed no faults. I included as much of each circuit as I could - for the T/R circuit, I metered right out to the rear-panel RCA jack.

BIAS Testing

Test-time had arrived. I carefully metered the entire Bias circuit, and didn't like the way the "Bias Adjust" potentiometer contacts were nearly touching the front-panel. A slight bend, and two layers of 600V electrical tape eased my concern. All connections seemed sound, and there were no detectable shorts.

I made up an AC power-cord with alligator-clips, and hooked onto the (not-yet-wired-in) Bias Transformer primary, with the 12V secondary still un-connected and capped-off. This would test the Bias supply only.

Mode switch in the CW position, internal meter set to "Bias Voltage", my Fluke meter connected to the V101-socket grid-connection and the common + of C209 and C210. YAY! -90V showing on both meters. And especially ... NO SMOKE Continuing, I placed the mode-switch in the SSB postion, and found I could easily adjust the SSB negative bias from about -22V to the CW level of 90V. Having read that I would get the right plate-idle-current with around -55V to -60V bias, I set it to -60V for now. Again, it was nice to see agreement between the Collins internal meter and my Fluke.

This first power-application test was really gratifying, after the long idle and rebuilding periods. And because I believe this was the troublesome circuit that caused my smoke-out, I'm actually elated to see this first section working!

12V Safety and Control Testing

"How many previous owners does it take to change a light-bulb?" Hmmm...

My 12V test-strategy involved *NO* connection yet of the T203 12V winding, but selectively using a current-limited bench-supply:

• Use the bench-supply to inject 12V upwind of CR206, to verify wiring K203 through K101 Antenna Change-Over Relay, to the rear-panel T/R RCA-jack.
• Inject 12V downwind of CR209, and selectively bypass S205 Lower Door Safety Switch, and the 3-min timer K202, and see if pressing S203 "ON" switch will engage relay K203.
• Inject 12V where the transformer will connect, bringing up all lights, allowing HV Relay to be enaged with front-panel "ON", and allowing the rear T/R jack to switch the antenna once the HV Relay has switched. The whole 12V enchilada :-)

Good! Injecting 12V onto CR206 anode allows K101 Antenna Change Over Relay to be controlled by the rear panel. Maximum current-draw is 0.31A with K101 and K205 engaged.

More Good! Injecting 12V onto C211b let me try the "ON" button, and watch K203 engage! Maximum current-draw with K203 engaged was 0.21A.

Then Badness :-( Applying 12V to the terminal-strip at the back of the Relay Shelf, where T203 12V winding will eventually connect, caused over-current (3 amps!) trip. Clearly, something was unhappy. At this point, I just introduced about 24" of wire, and the panel-lights. I hadn't planned to pull the front-panel yet, but now I had to. I found panel-light connections shorting - solidly. Behind the front panel, I found shorted socket-connections, and (behind the dials) contact between the clip-socket-contacts and the chassis! I must say, I never expected a deadly-serious problem in such a simple area as a light-bulb :-O

I re-soldered all light-connections, adding heatshrink sleeves over every terminal. I added large tubing over the entire socket for the two dial-bulbs. I sleeved the interconnecting wires to prevent wire-chafing. I added judicious bits of electrical-tape to prevent bracket-contact. APPARENTLY my 30S-1 is missing some black sleeving on the dial-sockets, which led to the full-shorting of the 12V transformer. Looking also at the broken/repaired LOAD dial, I wonder if a previous-owner tried to sneak the bulb out from behind the dial, breaking it in the process and also losing the essential sleeve insulator :-O Did abundant grief follow a simple bulb-change?!?!?!

While here, I also performed a cool mod: I re-wired the dial-lights so they will illuminate only after the warm-up delay has passed and K202 closes. Not only does this indicate visually that the amp has completed the 3min warm-up and ready to hit the "ON" push-button, but it also confirms 12V is available to energize K203 - maybe this will help troubleshooting one day. (Technical note: this "delay completed" lighting moves the dial bulbs from 12VAC to ~15VDC operation). Final touch for the dial-light mod: hand-lacing!

Working around the panel switches led me to examine CR216. The old-school top-hat rectifier probably had 6-decades of mileage under it's belt, and carrier-recombination likely softened it. Replace! I still had 1N4005's on-hand... Once removed, CR216 exhibited 3.2-ohms in both directions - it was completely useless as a diode, and vindicated my decision to update it! Collins added this diode CR216 and some associated components with Service Bulletin SB-1, Jun 21/61.

The new CR216. I also checked the associated resistors. These follow Collins' Service Bulletin SB-1 (1961). I wonder if previous owners may have seen crazy grid-metering?

SUCCESS! Everything 12V-related now works with the bench-supply. The Safety switches are all doing their thing (un-bypassing opens K203 nicely), the red "Amp Ready" light works, the front-panel "ON" and "OFF" push-buttons do their thing, the rear T/R works. Even the new lighting mod is nice: just the outer meter-lights turn on until the Safety interlock + timer is completed, then the dial-lamps turn on to let you know you're good-to-go! I was interested to see that the TOTAL 12V current-draw, with all lights and relays energized, is 0.75A. My new Peter Dahl transformer is rated 0.800A Continuous Commercial Service - a nice fit. It is entirely possible that the light-bulb short I discovered was the cause of my T203 smoke-show.... I'll never know, for sure, because there seems to be several plausible causes :-O

At this point, I ditched the bench-supply, soldered-in my new T203 12V-windings, then powered the primary - YAY! T203 is working... I have both my Bias voltage and my 12V Safety and Control! I can finally close the main chapter that started this journey, but there are a few other chapters left...

A Quick Detour

I am not happy with the charred and crispy wires in the corner near the fuses. I am also not very happy about cutting-back the nicely-marked factory wires. But balancing these two un-happinesses, I'll cut them back ~6" and re-make safe wiring.

Ah - much better! Laced, placed, soldered and sleeved. The sleeve was harvested from the removed T202 (above), then split down the side; the idea is to prevent possible wire-chafe on the sharp edge of the fuse panel. I traced the wires before connecting, to ensure that the hot/supply circuit runs to the rear of the fuseholder, for safety when changing.

This one is interesting... I did look at the bakelite cover on the AC-input when I first brought it home. At any rate, the jumpers were hidden from view, behind the bakelite cover, and I ASSumed it was good-to-go for 240VAC. Seeing the actual jumpers now, and viewing the schematic while working on the fuses/wiring suggested an error. Deeper digging: the 1964 manul I am using differs from the later 1976 manual, for this connection. The Bakelite printing and my schematic do not jumper 2 to 3&4. But 1976 does. Connecting 2 to 3&4 would have forced the neutral (mid-point of my 240VAC) onto the center-connection of the two series-connected HV transformer primary windings. T201 never powered-up, but if it had, then this center-connection would have naturally been at the mid-point anyway. Later, I pulled the fuses, and found one open-circuit. With the Terminal 2 jumpered as-pictured, the 1/2-primary with the open fuse would now act as a third secondary winding, and be live! The "real" secondaries would still be powered, allowing plate/screen to have some supply despite a blown primary fuse :-O I am not quite sure what to think on this one...

This .PDF memo from Collins explains that the change in AC primary connections will reduce cabinet-hum, for some 30s-1 amplifiers. Mine doesn't appear to suffer from transformer-induced hum, so I'll leave terminal 2 disconnected... I feel this is more-sound, electrically:

Another Quick Detour

Overcurrent capacitor C260 was original-looking. I decided I wanted problem-free operation, so I updated C260 too. This capacitor is connected in-parallel with overload-plate-current-sensing relay K204 - it's purpose is to bypass audio-current-peaks so they don't trip the overcurrent relay. Earlier schematics show C260 as 1000uF; later schematics and my 30S-1 have 2000uF. Collins added this capacitor with Service Bulletin SB-2, Nov 1/69.

Bonus!

I decided, in order for this bad boy to get out and play, I would change the blower-motor to the popular Dayton 4M093E 3000rpm motor, dramatically increasing the airflow. Some correspondance and digging through Collins' maillist archives led me to choose a 50-ohm series-resistor, primarily to keep the motor from overheat-tripping. Another detour :-)

The line-up of prime-suspects for the blower-motor change :-)

Motor changed, resistor mounted to housing, blower re-assembled. Home at long last :-) The lower mounting bolt took me 4hrs just to get it installed. I later read Mr. Carn's article in "The Signal" which says it should be a stud - they say the best advice comes just after the job is completed :-O The only one good thing about my (re-)using a bolt: it was easy to adjust the front mount, so that the weight of the motor is hanging equally from the top-mounts - just look at the "angle of the dangle" for this bolt, in the power-supply compartment. It's neutral and balanced when the bottom bolt hangs perfectly vertical. The 50-ohm power resistor is screw-mounted on the lip of the blower intake, where it will receive some cooling, but not noticeably obstruct airflow.

My cut-off tool got a workout: the Dayton motor has front and rear mounting-bolts; I didn't like the knuckle-slicing appearance of the rear ones so I cut them off and added some protective heatshrink. I trimmed the front bolts down, to allow more lattitude in adjusting the squirrel-cage. And my 1/4" nut-driver was too long to fit into the blower compartment, so I cut that too :-)

For this 50-ohm resistor, I measured:

• 0.980 amps without resistor nor any nozzle-restriction (121VAC)
• 0.76 amps with resistor in steady-state (10-12s after start)
• 93VAC across motor, with resistor, in steady-state

Pretty much, this shifts the original motor-dissipation from 120W without a resistor, down to 70W in the motor and 32W in the resistor. Subjectively, the airflow seems the same, but the motor-temperature will be 'way down.

Test driving the new blower made my beach-towel wave in the wind, from 3 feet away :-) Too much is just about right :-)

An idea of how this sytem will work, when the forces of Hot and Cold do battle! Looks like the forces of Cold will win! This graph was generated with 1500W dissipated in the 4CX1500B plus 20% margin... I won't operate at this point, so I'll have even more margin when I operate at more-modest levels. To push the tube up against the cooling-limits would require something like 3kW DC input!!! Even bleary-eyed, at the end of a long contest, starved for food and water, I am incapable of damaging my 30S-1 :-)

Dials

Yay! The front panel is back together again. Also shown: my favourite lube for things like shaft-bushings. I had to apply partial-drops to the end of a piece of wire, then guide the small amount of lubricant into several tight areas. Capacitor-, switch- and pulley-shafts were all treated to this stuff (which, BTW, worked miracles on the screeching helical-gears in my TS-850's antenna-tuner). Some quick notes: dials pretty much go all the way back, very nearly touching the lamps. Otherwise they will contact the back of the escutcheon lightly snug the two Bristol set-screws, test-fit the panel, and if the red line isn't where you want, you can gently use your fingers on the outer edge of the dial to rotate it until you are satisfied with the line-up. Lots of panel-on / panel-off... Snug the Bristols. the SSB/CW switch-wires will contact a pulley-bracket. The bracket is insulated, but it bears close inspection - I used a shaving-mirror to look up from the Power Supply compartment - but really, all I could tell was that it was scrunched :-(

Power Supply & Lower Compartment

I had to balance my wish to dismantle and scrub everything, against the known-fact that T202 had broken leads - I wanted to do "enough", but not so much that further damage occurs. In the end, I disassembled stuff along the right-hand side, leaving the items down the left-side and back un-touched, for gentle wipe-in-place cleaning. This choice arose from difficulty/ease of disassembly, and the location of obvious griminess and sludge.

The messiest component was HV cap C206 - it had sticky grunge all over it, which looked to me like spewed electrolyte, from above - there certainly were a few Relay Shelf electrolytics which were changed before me, and could have caused this collateral damage! I cleaned it off, polished all vitreous components and wiped them medically-clean with alcohol - I was rewarded with no measurable leakage (to 50Mohm anyway). The cabinet floor at the right front and down along the right side also had more of this tarry gunk - Simple Green and elbow-grease took care of it. A troubling amount of hardware was also missing, and HV nuts were loose or only finger-tight - again, signs that I must again be extra-attentive in this lower compartment too.

I swear, the previous owner must have had the Fire Department on speed-dial!! Here's a beauty - an awesome high-voltage blow-through coming straight off the HV transformer :-O Un-repaired too... but wait, there's more! The two insulators are different, meaning a previous repair in this area. And the crowning jewel: fabric-insulation off the top of the resistor, with crummy soldering... pure gold! I sure hope my repairs will be solid.

The laced pair of white+whatever cut wires are the 120VAC primary supply wires, for the now-removed T202. I covered the ends in heatshrink, then labelled the wires in case a future use appears. And now, C206 looks resplendent after cleaning :-) The date-code on C206 indicates it was made around mid-June 1963. That might place my amplifier's year of manufacture at 1964. I wonder if this amplifier began life on a crisp wintry morning, a proud new owner eager to add some DX with his Christmas gift?!?! :-)

Those long 1N4872 diodes have a date-code of 1976... I think they've survived several battles after 4-1/2 decades. I can't really test them (forward-drop is 23V), but I find they are not shorted... I'll keep'em. Basic DC tests show no shorts-to-ground, just the bleeder-values. All hardware is snug. Gentle wire-tugs show nothing coming apart :-O

Testing - 120V Initial

• I installed the original 4CX1000A tube which came in my amplifier. Only the 3.2A fuse was installed, and I applied power!
• The meters illuminated; after the time-delay relay closed, the dials also illuminated. The "ON / OFF" switch functioned.
• Opening either the top-lid or the front-panel removed the dial-illumination, and prevented operation of the "ON / OFF" switch.
• I tested the Thermal Overload switch K102 by moving it out of, and back into the airstream - it works, and disables the "ON / OFF" switch when there is no airflow over the sensor.

The Safety and Control seem to work. The Bias and Filament adjustments work.

I clipped some test-leads right onto the tube-socket, in order to measure and adjust filament-voltage accurately. Eimac specifies 6.0 +/-5%, so that's what I gave it. The panel-meter actually reads the incoming 115VAC supply, using that as a proxy for the actual filament voltage - you can see this does a pretty good job.

With an eye on the amp, I ran it this way for 8 hours, to getter the tube. And to verify that the blower doesn't go into thermal-shutdown. And to get the various smells flushed out (especially that new Dayton motor - it runs quite hot).

Testing - 240V

After warm-up, I pressed "ON", and got a nice "clunk" from relay K203. But no High Voltage :-( I metered and monitored the primary AC windings of Transformer T201 - nothing! That pretty much points to K203 as the culprit. Out came the Relay Shelf, for a focussed look. Removal notes:

• Remove the bakelite AC power cover, and disconnect all the power leads
• Remove the 5 front / 2 rear screws, and shift the Shelf over - to allow removal of the HV shorting wire (hidden behind edge of compartment)

Servicing Note:

• use a paint-stir-stick, behind the fuse-panel, to wedge and hold open the HV short, and keep closed the 12VDC interlock switch. Eyes Open!

With the Relay Shelf flopped onto it's side, I determined that the rearmost contact of K203 was failing to close. I burnished the contacts, then adjusted both such that they close when the relay armature has moved about 2/3 of it's travel.

Quick test: I couldn't get any plate-idle-current. Recall, I had pre-set the BIAS Voltage to -60V, to be on the conservative side and start out with a reduced idle-current. But now, I had no idle-current. A further quick test showed infinite input-SWR for the exciter. Two problems to look into... So, next step: let's thoroughly check screen supply circuitry. I got my 12VDC bench-supply, and verified the switching of K205, and also that K101 would also trigger K205. Both were good. Basic ohmmeter checking showed connectivity from the screen supply to the cathode-chock L103. Hmmm... let's thoroughly check the AC and DC values related to the Screen supply.

I got three meters, and carefully measured: AC input voltage on primary of T201, after fuses, switches, relays and wiring: 240VAC AC output voltage from Screen secondary of T201: 260VAC DC output of the Screen supply, across C204: 235.2VDC

We have connectivity, we have all voltages. Hmmm... maybe I just have to crank harder on the BIAS Adjustment. So, I did. I managed to achieve the prescribed 200mA idle-current in SSB mode, but only when the BIAS was down to -36V! I wonder if my tube is nearly-shot?

Now... RF in! But wait - I have infinite input SWR?!?! :-(

I metered every cable and connection along the RF input path, and found that the culprit was K205 at the back of the power compartment. It appears that my burning T203 deposited a tarry film on the open contacts. After cleaning, I tested K205 only - now good! Along the path of debugging, I removed and cleaned the input band-switch (cut strips of bond paper, spray contact-cleaner on the strips, use tweezers to sneak the paper between the contacts and drag it through).

RF in now brings RF out, but it isn't very impressive with this particular tube. Roughly, I drove it with 50W CW input, 900W DC to the final, and 400W out. As I increased the drive, I began to note slight movement toward -0.2mA grid-current... this tube simply wasn't going to give anything more. Time to get Contestant #2 - a 1979 used 4CX1000A which looked a lot less darkened and still has Eimac paint on it.

A day later...

Tube #2 is much better - I obtained a gain of about 7.3: 75W RF input drive, 900W DC plate input, 550W RF output to the dummy-load. I'll set this aside as a tested spare.

The Main Event

Screen Boost

1. When the user alters the AC input-jumper, this boost will follow and operate from either 240VAC (as now), or 120VAC, just like T201.
2. This connection should give 100V screen-supply boost in SSB, and a lower 65V boost in CW
3. The boost will only come alive once the front-panel ON HV button is pressed. If I wire this transformer to the 120V which powers the blower, BIAS, etc then the boost would come on right-away, before the main Screen or Plate voltages

Repeating the same (no-load) measurements as above:

Again, three meters show the boost for SSB, and CW. I'm a bit puzzled why the CW Boost is lower-than-expected, though.

The connections to the barrier-strip are clearly labelled; the wires are neatly laced - pretty much the way Collins might have done it :-) These screen-boost values should be pretty good, and I really like how they change with the class of operation. More Tube info: 2020 Eimac Summary and 1986 Technical Data.

Installing the 4CX1500B

Date-code of (mid-December) 1984. 12 hours of filament-only to getter the tube, after such long storage.

This photo shows 1kW into my dummy-load, making it sweat a bit :-) But my calculation of the DC input power, now compared with the RF output-power, leads me to question my Plate Current meter accuracy. Another detour...

Plate Current Meter Detour

My 30S-1 Plate Current Meter is almost 20% optimistic

Inelegant. Practical. Joy-less. I removed the meter and used a bench-supply to collect data and determine how wrong my meter was. While it was out, I undid the three red-Glyptol screws around the perimeter of the meter-body, removed the inner guts, and tried to adjust the spring-tension. Unfortunately, the upper pointer-adjuster quick ran up against the meter-stop without worthwhile improvement. I disassembled, cleaned (with Scotch tape) the movement-motor itself, and re-assembled carefully. I may investigate a meter-rebuild (does this even exist?) or guts-replacement to maintain the Collins bezel. For now, operation will involve the minor mental gymnastic of multiplication.