Thunderbolt 305. New to reading schematics, could use some help!

[crappykraco]

I want to understand what is going on in this circuit. I can identify some of the components, but want to understand the design of the amplifier circuit.

I may be a lost cause but even if you can point me in the direction of a book to read, or video series to watch, that would be helpful.


I can see certain things on the schematic and then identify the parts in the amplifier. L1 and L2 are the HV transformers. I see rectifier diodes then the bank of 3 80uf 450v capacitors. I see a pot labeled "power" I assume it must vary the voltage to the driver tube? I don't know.

Seems like K1 and K2 are relays and K1A and K1B etc are the relay contacts? The whole bottom right corner of the schematic with the 2905a transistors is a mystery to me. Preamp circuit and?
The variable capacitors labeled 39 39 and 313? Those are the tuning capacitors.

What is the section with the 330k pot and what is the "m" symbol?




What are components you would consider checking and replacing? I do not see any leaking capacitors, but they appear to be original so they have some age. Any help appreciated, and take it easy on me, I obviously have a lot of learning to do.

 

[brandon7861]

This is what I think everything does, ignoring a little blocking and filtering anyhow.

The troubleshooting advice will have to come from one of the experienced tube guys.

 

[crappykraco]

Thank you for taking the time, Brandon. Incredibly helpful. I think I understood a little more than I gave myself credit for, and some of it I had just plain wrong.

The amplifier was working with nice clear sounding audio reports on SSB. I took it upon myself to clean it, and now get reports of garbled or distorted audio. I'm fairly certain I inadvertently bumped the bias pot out of adjustment. Whats the procedure for proper adjustment?

 

[brandon7861]

There are people here far more qualified to answer this as I have never worked on a tube amp, not once in my life, but I will take a crack at it while we wait for the professionals.

It would seem (to me anyhow) that those pentodes are being run in a tetrode class-c configuration (both screen and suppressor grids grounded with a negative voltage at the control grid). When the RF waveform at the filament drops below the control grid voltage, the tube conducts. If your audio suddenly got garbled and you think you may have bumped that bias pot, I would assume it got turned in a way that raised the voltage on the grid making it conduct for more of the RF waveform. I think you need to adjust that pot so that the grid voltage goes more negative so that the tube conducts for less of the cycle. With the voltage on the control grid too close to ground, more of the negative half of the RF waveform can turn on the tube and that probably has an effect similar to saturating a transistor (flat topping and distortion). My recommendation (if a better one doesn't appear) would be to make that control voltage as negative as you can (with respect to ground) and then increase it until it sounds good with decent output power, backing it back down if distortion occurs. This seems like a job for a scope honestly.

 

[nomadradio]

If you're hip to what an electrolytic capacitor is, you'll need to start by replacing every one of them. The chemistry inside them breaks down after 3 or more decades as a rule. Age alone is the problem, even if the amplifier's mileage is low.

73

 

[crappykraco]

If you're hip to what an electrolytic capacitor is, you'll need to start by replacing every one of them. The chemistry inside them breaks down after 3 or more decades as a rule. Age alone is the problem, even if the amplifier's mileage is low.

73

Thanks, I will start there. I have some parts on the way.

 

[crappykraco]

First picture is before i did anything. Dust bunnies and mystery brown schmoo.

I managed to get both boards free enough to work on them. Waiting on more parts.

Any reason to check any of the other parts on the HV board while I have it out of there?

 

[nomadradio]

I always replace the bleeder resistors with more-modern carbon-film or metal-film resistors. The carbon-composition resistors in the picture were the favorite choice 50 years ago, but they have a bad habit of drifting, changing resistance value. This could cause premature failure of a filter cap. When three of them are in series, you really want the DC voltage to be divided equally across all three. If one of them gets a "share" that's over 450 Volts bad things can happen. New bleeder resistors are cheap insurance.

73

 

[crappykraco]

I always replace the bleeder resistors with more-modern carbon-film or metal-film resistors. The carbon-composition resistors in the picture were the favorite choice 50 years ago, but they have a bad habit of drifting, changing resistance value. This could cause premature failure of a filter cap. When three of them are in series, you really want the DC voltage to be divided equally across all three. If one of them gets a "share" that's over 450 Volts bad things can happen. New bleeder resistors are cheap insurance.

73

I removed these and they have indeed drifted (upward). I am not able to read all of the values on the schematic, so i'm just going by the actual part I see.

Some of the values I can read on the schematic don't match what I find in the box. Do I follow what it says or what I see? For example the schematic says " 21-23 3t #16 on 33ohm 2 watt res.", but what I find in reality are 27ohm. Maybe I'm worrying about nothing.

 

[crappykraco]

The burnt looking resistor.. is that a red band or orange band? I'm pretty sure I've circled the correct resistor on the schematic..the green wire pictured goes to the potentiometer marked "power". looks more like 15k to me than 1.5k. I wish I could read the damn numbers.

d

 

[nomadradio]

Neither resistor looks burned to me. The upper one is a 15k half-Watt made on a ceramic body. That's its natural background color. The lower one is a 2.7k half-Watt carbon-comp resistor. The plastic outer layer on that one is brown.

73

 

[brandon7861]

I think that amp is missing one very important part. An RFC to ground at the ANT jack. If the DC blocking cap between the plate supply and the filter network fails, you will have high voltage on the antenna jack. With a suitable RFC to ground, the supply fuse will blow if that blocking cap ever shorts. As is, there is only one capacitor between you and the grave. Adding one is cheap insurance. Neither the standby switch nor the TX relay turns off the HV, you don't need to be on the key for HV to get there if that cap fails.

 

[crappykraco]

I think that amp is missing one very important part. An RFC to ground at the ANT jack. If the DC blocking cap between the plate supply and the filter network fails, you will have high voltage on the antenna jack. With a suitable RFC to ground, the supply fuse will blow if that blocking cap ever shorts. As is, there is only one capacitor between you and the grave. Adding one is cheap insurance. Neither the standby switch nor the TX relay turns off the HV, you don't need to be on the key for HV to get there if that cap fails.

Thank you. I don't know how to do it, but at least I'm aware I should damn sure figure it out.

 

[nomadradio]

missing one very important part. An RFC to ground at the ANT jack

Wow, didn't catch that. Textbook name I learned for that part is "safety choke". Serves as a dead short to ground if that blocking capacitor shorts. Meant to trip the fuse and protect anyone who could come into contact with the antenna. If it's connected from the hot side of the Load control to ground, it serves one other purpose. Reduces voltage stress on gap between the plates of the Load control. That blocking cap is in series with the plates of the Load control. If there is no DC circuit to ground in parallel with the Load control, that DC voltage gets divided between the two capacitors. DC voltage across two capacitors in series gets divided inverse to the capacitance of each one. Higher capacitance gets less voltage. Lower capacitance gets a larger share of the DC voltage. This puts a high DC potential across the plates of the Load control. Probably won't cause an arc by itself, but when you key the mike and add the RF voltage to that DC voltage you'll get an arc on your modulation peaks. This is not kind to the aluminum plates of the control. D&A was very sloppy about this and we learned to put a safety choke across the Load control of every Maverick or Phantom we serviced. Hearing that "crunch" sound when turning the Load knob of a Phantom became a cliche. The hazard also depended on your antenna. If it has a DC short through a matching coil at the base, this takes care of it. And if it's a series-fed antenna you really need that safety choke.

72

 

[brandon7861]

@nomadradio For 27MHz, what would an ideal choke value be? I assume a couple kohms reactive, but I lack the experience to recommend a value here. The internet is telling me 50-60uH is sufficient, but that borders ferrite land. Mind sharing what you use for this job?

Maybe, since it is on the 50ohm side, even less would work, perhaps 1 or 2 kohm. I just read about people putting them in parallel with dummy loads (as a test) to ensure the coil isn't changing the SWR. I assume a change in SWR is a result of approaching the coil's self resonance frequency.