Hey folks, new here. Hope I'm posting in the right spot.

I have a Peavey Special 212 combo I'm working on, 90s Transtube series. Very similar to the Peavey Supreme transtube head I believe. I'll try to post the schematic in here. For reference, I do have experience working on both tube and solid state amps already.

I got this amp in working condition, though it needed some repairs (slight buzzing, sounded like filter cap leak, and scratchy pots along with a couple damaged jacks). Also, not sure if it was my doing when I was testing the amp after replacing the filter caps, but one or 2 of the output transistors shorted.

The power section is setting off my light bulb limiter. (Red wire from the transformer). The preamp section does not set it off with the power section unplugged, BUT channel switching isn't working at the moment (maybe the power section needs to be plugged in?)


So, I've replaced all of the big electrolytics (triple checked orientations), replaced all pots, replaced output transistors with onsemi equivalents (15003g and 15004g), replaced driver transistors that sit before those, and all power section transistors tested good. I've also replaced the diodes from CR53 down to CR71 (check schematic). Nothing is fried resistor wise. I checked voltages at key points and they're all there, but when I plug the transformer into the power section, the LEDs fade out and everything seems to get dragged down. Running out of ideas, and I'm asking for some help or suggestions to get this thing going again.

Link to the schematic: SPECIAL 212 SCHEMATICS

#1  Do not connect any speaker or load until everything is working properly, all voltage correct.
If you had a speaker or load connected, recheck.

Secondly, it is not clear what you mean about disconnecting red wire of transformer.  Please be more specific.

Lastly, this is a bit of an unconventional design, often called 'flying rails' where the normal spot we usually see the output is instead grounded. Where the center point of the pwr.amp supply is normally grounded, it instead goes to the speaker (+) terminal.
If you are not familiar with this type of design, it can be confusing.

Quote from: g1 on June 19, 2023, 08:27:52 PM#1  Do not connect any speaker or load until everything is working properly, all voltage correct.
If you had a speaker or load connected, recheck.

Secondly, it is not clear what you mean about disconnecting red wire of transformer.  Please be more specific.

Lastly, this is a bit of an unconventional design, often called 'flying rails' where the normal spot we usually see the output is instead grounded. Where the center point of the pwr.amp supply is normally grounded, it instead goes to the speaker (+) terminal.
If you are not familiar with this type of design, it can be confusing.

Thanks for the reply. I won't connect a load until it passes the light bulb limiter test.

Regarding the transformer, if you look at the schematic there's 2 points where the plugs for the transformer plug in. Orange (labeled O / O/W and another O) at the rectifier section and Red for the power amp section ( labeled R, etc.). So, when i plug in J20/21/22, the light bulb illuminates. Otherwise it's off.

And yes, I did read about flying rail designs a bit. I just can't figure out why this is drawing so much current. I'm using a 150w bulb, by the way.

Could it be the TO-3 insulators, maybe you have a short from the power transistors to the heatsink.

Quote from: Tassieviking on June 20, 2023, 07:56:01 AMCould it be the TO-3 insulators, maybe you have a short from the power transistors to the heatsink.

I actually did think about that, was getting around to checking that next. I couldn't tell with a multi since the emitter resistors are only .33ohms. Thanks for the second opinion.

My advice with Peavey designs that failed is to check every single little thing that resembles a diode. Those transistors count hugely but don't forget the 1N4148 diodes on the bias string for example. I am pretty compulsive with such a repair but I had issue with a peavey just like that. Thought I had checked everything but I missed one.

If you used thermal paste on the transistor insulators, make sure that you are using non-conducting paste.
It is very common to use conductive thermal paste in computers and other places where there is no need for any insulation between the component and the heatsink.
Thermal heatsink paste is a big no-no in amps, I refuse to have any at home just in case I make a mistake one day.
Good quality paste and just a very thin film of it is all you need, if you have the cheaper stuff it will dry up and become useless faster the the good stuff.

Quote from: Amestech on June 20, 2023, 09:35:50 PM

Quote from: Tassieviking on June 20, 2023, 07:56:01 AMCould it be the TO-3 insulators, maybe you have a short from the power transistors to the heatsink.

... I couldn't tell with a multi since the emitter resistors are only .33ohms.

You want to measure from the transistor collectors (case) to the metal of the heatsink.
If the transistor is not shorted from emitter to collector, a low resistance reading will indicate the collector to heatsink insulator is not good.

Quote from: g1 on June 21, 2023, 02:09:28 PM

Quote from: Amestech on June 20, 2023, 09:35:50 PM

Quote from: Tassieviking on June 20, 2023, 07:56:01 AMCould it be the TO-3 insulators, maybe you have a short from the power transistors to the heatsink.

... I couldn't tell with a multi since the emitter resistors are only .33ohms.

You want to measure from the transistor collectors (case) to the metal of the heatsink.
If the transistor is not shorted from emitter to collector, a low resistance reading will indicate the collector to heatsink insulator is not good.

Oops, I had it backwards. Thanks for the tip! Gonna do that.

Quote from: DrGonz78 on June 21, 2023, 01:57:25 AMMy advice with Peavey designs that failed is to check every single little thing that resembles a diode. Those transistors count hugely but don't forget the 1N4148 diodes on the bias string for example. I am pretty compulsive with such a repair but I had issue with a peavey just like that. Thought I had checked everything but I missed one.

Yes! I saw a post through my google searches that those diodes would get fried. I've replaced or tested pretty much every diode, including the zeners. Only thing I haven't been able to confirm is the reverse bias voltage for those zeners, forward bias was about .8v drop which seems good. I replaced the 1n5393s and the mr502s seem good.

Quote from: Tassieviking on June 21, 2023, 10:57:22 AMIf you used thermal paste on the transistor insulators, make sure that you are using non-conducting paste.
It is very common to use conductive thermal paste in computers and other places where there is no need for any insulation between the component and the heatsink.
Thermal heatsink paste is a big no-no in amps, I refuse to have any at home just in case I make a mistake one day.
Good quality paste and just a very thin film of it is all you need, if you have the cheaper stuff it will dry up and become useless faster the the good stuff.

I used arctic silver 5. Google says it's non conductive, but I think it does carry some capacitance. I may go through and see if I need to clean some up. These tips are great, thank you guys!

I wouldn't use arctic silver for anything other than CPU's on a computer build. It says it has trace amounts of silver in it.

Quote from: DrGonz78 on June 22, 2023, 09:58:48 AMI wouldn't use arctic silver for anything other than CPU's on a computer build. It says it has trace amounts of silver in it.

I 100% agree.  I only recommend plain thermal paste, no additives.  Or you can use silicone pads.

Well, the good news is the amp isn't pulling so much current now. I cleaned up the paste and reseated the transistors. I think one of the bolts was touching the heat sink when I tightened it down or something.

The bad news is smoke is coming from somewhere now, lol. It seems to be coming from the PNP side, maybe the driver transistor? Q37.

Channel switching is back, some wires were loose. But channel switching does not work when the power section is plugged (see my earlier post to see what I mean). So, I'm guessing the voltage is still getting pulled down somewhere to where the relay won't switch.

Update on this amp.

There was a small solder bridge on an output transistor. Fixed that, replaced Q37 and a resistor, and now the power section works beautifully. The bulb limiter doesn't even light up now on startup.

However, the preamp isn't working.

Voltages at the bias points for 3.5, 2.5, and 1.5v are like 12.9v, 13.4, etc. The 15v coming in reads about 15.7v. In the schematic, this section is just above the rectifier bridge. B+ is about 40v. Scratching my head about what's wrong there, maybe voltage is leaking through somewhere.

Also, if I need to replace any of the 2sc1740 transistors, would BC547 be a good replacement (provided the pins are checked).