SUNN Stage Lead power issue

[markorock37]

I planned on replacing the output transistor at the same time, I am aware sopmething else is wrong down the line, why else would resistors blow, I figured that may be the cause as it usually is. I have bands of orange, orange, black, silver on the resistor, that is a 33 ohm and it looks a lot bigger than a 1 watt.

[J M Fahey]

What is it connected to?
Try to find it in the schematic.
Even better, "correct" the schematic by going over it part by part and labelling them with the actual values used in the board.
Voltages too.
I was referring to the emitter resistors, don´t know what on the schematic is that "33 ohm" one.

[markorock37]

If I use the Studio Bass schem its R140 or R142 33ohm 10W. Glad I found that 10W figure. I'll get my parts today but may be after xmas before I get any more time in it.
Thanks for the info JM.

[Loudthud]

That amp (Stage Lead) is the same as the Studio Lead. The Studio Lead and Studio Bass use the same PCB, they just leave off the parts for the reverb. R140 and R142 should be 0.33 ohm 10W. If one or both are blown, you have a shorted output transistor, Q1 and/or Q2. Remove Q1 and Q2, power up the amp and check that the collector of Q1 (terminal 5 on the PCB) goes to about 25V. This will verify that the biasing resistors are ok and there is not a short in the driver transformer.

[J M Fahey]

Loudthud´s suggestion is great, because the driver transformer separates the power transistors from the rest of the circuit, (speaking of DC), so you can safely test for the voltage he indicated (which is about 1/2 the Rail voltage, what you have on the top transistor´s collector)
I add that you should check the Base to Emitter voltage on each of the output transistors (without transistors, just at the PCB connection points)
They should be both the same and around 500/550mV.
Post results before re-mounting said transistors.

[markorock37]

Sounds like a great idea, I will try that and post what I find.

[Loudthud]

This should help. The schematic and layout from the Studio Lead which should be the same as what you have. What are the numbers on the power transformer? The Studio Lead used 28-1650.

[markorock37]

Thanks loudthud! This is the real schematic, mine uses PT 28-1650 as well. According to the new schem, its R137 that burnt, 22 ohm, 2W resistor. Apparently what I thought was orange bands on that are actually just really faded red ones! Again thanks for the schematic, I'll get the parts today and do testing Q1 and Q2 out of the circuit. I have some extras laying around is there a way to test these?

[markorock37]

Pulled Motorola 22-3055 transistors out. I have a bunch of MJ15003 and MJ15004 - are they interchangeable with the 3055's?

[markorock37]

I picked up some 3055's at the local electronic supply. Replaced the burnt resistor with same spec, fired it up w/o the transistors in place and looks like its running now. Q1 on pin 5 was 24.5v and Q2 pni 8 52.1v, Q1 base to emitter 654mv, Q2 base to emitter 632 mv. Fire away if anything sounds bad or good before I put the rest back together.

[J M Fahey]

Sounds reasonably good.
Put the transistors and turn the amp on; still with the lamp limiter in series and without speaker connected.
Re measure and post voltages.
Should read fine.

[markorock37]

Plugged it in with the transisitors in with the lamp limiter,  bulb barely glows. I was going to plug it in to a speaker but the cab wasn't wired up when I got it, anyone have any ideas how the speakers were wired and what ohms is the output?

[J M Fahey]

Given that you have 4 x 16 ohm speakers and the low (47V) power rail voltage, I think they probably wired all in parallel, as to give a total 4 ohm load and an approximate 40/50W RMS which sounds reasonable.
I very much doubt they built a 4x10" amp to get just 12W into 16 ohms, which would be the other option.
You can wire the 3 alive speakers in parallel, pull the bad one for future reconing, and cover its hole with a plywood square bolted there.
You should end up with a fine practice/Club clean guitar amp.
Even overdriving it won´t sound bad at all, because of the transformer driven stage.
Try it.
Any MP3 will be welcome

[markorock37]

Sounds good. I found the speaker plug with 2 pairs of leads I figured it must have been wired series parallel. Couldn't find a speaker on ebay at the moment so sounds like I'll be sending it out to get reconed. I'll put a mp3 up soon.

[markorock37]

Got everything hooked up and had some sound for about 15 seconds before R137 started smoking again. Pulled transistors back out, one tested bad. Put 2 new ones back in that test good (I didn't test the new ones the first time) with an ohmeter. Replaced R137 although it tested ok, it did get red hot. With it back together I get sound but its really weak like its not the full signal or power. I tried it with another cabinet too, since I only tested the original speakers with a 9 volt, same thing.
Checked R136, 138, 139 all are in spec. I'm not wanting to trust Q106 if thats playing a part in this.