Sunn Coliseum Lead railitis

[Euthymia]

Jason rises from the lake

If R218 burnt, then Q5 is immediately suspect.

Q5 tests good.

OK, base of Q206 is 70v pos from neg rail, whuch at 45v rail means it sits at about +25. That is fairly consistent with the +29 or whatever on Q207. So the whole bias string is slammed up towards +. So either Q203 is goofy or there is trouble down in R215,216 or C205 is shorted. What is at the base of Q203? IS C205 OK? what voltage is at each end of R215, 216?

(all readings ref. ground with amp running through lightbulb)

Base of Q203: +35V
R215/R216 jct.: -31V
R215/CR206 jct.:+30V
R216 (-rail): -36V

[Enzo]

Wait. Your - rail is 36v, not 50? Check your HV rails, are they both 50v? If one is low by 15v, I'd be looking at it on a scope for huge ripple, which would indicate a loss of filter cap. No scope? Set your meter to AC volts and measure the -rail. That will ignore the DC and just measure the ripple there. better not be more than about a volt. Or if the +50 is OK, look at how much ripple it has as a reference.

If the neg rail is really low, then the average between pos and neg will not be zero, and it will skew everything.

WHatever the voltage, the two rails should track.What is the voltage across R212 up by the top? Then we can calculate the current through the whole bias string.

[Euthymia]

Wait. Your - rail is 36v, not 50?

'member, there is a 100W lightbulb between the amp and the wall....

[Euthymia]

Wait. Your - rail is 36v, not 50?

All voltages are measured with the amp powered up on a lightbulb limiter, no load on the output.

The thing kills components if I power it up connected directly to the wall.

WHatever the voltage, the two rails should track.What is the voltage across R212 up by the top? Then we can calculate the current through the whole bias string.

The positive rail is 36V; the negative rail is -36V.

R212 is 470 ohms and has 1.3V across it. 2.7mA, by my calculation.

[Euthymia]

Any ideas? is it dumpster-bound?

[Enzo]

Without it in front of me, my insight at this point is stil that the trouble is in that transistor circuit at the top of the bias string. SOmething up there is funny. Or the left half of the diffy pair is not controlling it properly.

[wizard333]

I feel for you.

Solid state drives me nuts to sometimes. Enzo has helped me get a lot more comfortable with it.

Reading through this reminds me of a Polytone mini brute 1 from the 70s someone brought me where the power amp had caught fire. I rebuilt most of the board and replaced darn near everything as most of it was charred, except the xformers and the power filter caps. Got it all together and it came on but nearly no output. Spent silly time trying to figure it out, the readings didnt make sense, turned out to be those .3ohm 10 watt buggers mentioned early in this thread had gone sky high, several hundred ohms. Someone had surgically cut the 3 prong connection to, which made some voltage references look funny on a meter.

When you find the issue, it will probably be something that will make you smack you're head and think you should have seen it before. Its ok most of us have been through that more than once.

[Euthymia]

Without it in front of me, my insight at this point is stil that the trouble is in that transistor circuit at the top of the bias string. SOmething up there is funny. Or the left half of the diffy pair is not controlling it properly.

I will poke around again in the neighborhood of that transistor.

Where is the differential pair of which you speak? Is it that pair of transistors at the far left of the schematic?

[Euthymia]

I feel for you.

Solid state drives me nuts to sometimes. Enzo has helped me get a lot more comfortable with it.

...Its ok most of us have been through that more than once.

Thanks for the kind words.

Looking back to the date on my first post in this thread, I've been at it off and on for over a month. And I didn't post until I had exhausted my own troubleshooting abilities, which was not immediately.

During that time, I have repaired many other amps.

[Enzo]

The differential pair of transistors is indeed the two at the left. The input feeds the left one, and feedback from the output feeds the right one. They share a common emitter circuit. The left one controls the signal into Q203.

If Q201 is on too hard or leaky, it will draw current through 203 and that will pull the voltage at the botttom of 203 up. And since they work together, if Q202 is funny, it can affect what 201 does.

And note R224. the pot below them - that is the offset adjust, so make sure it is OK.

[Euthymia]

The differential pair of transistors is indeed the two at the left. The input feeds the left one, and feedback from the output feeds the right one. They share a common emitter circuit. The left one controls the signal into Q203.

If Q201 is on too hard or leaky, it will draw current through 203 and that will pull the voltage at the botttom of 203 up. And since they work together, if Q202 is funny, it can affect what 201 does.

And note R224. the pot below them - that is the offset adjust, so make sure it is OK.

Here are the voltages I measured today, with the amp running through a lightbulb limiter:

Q201
e -.78VDC
b -.16VDC
c 34.3VDC

Q202
e 29.9VDC
b 30.3VDC
c 35.5VDC

The offset pot tests good.

Any insights?

[Enzo]

Well, yeah. The emitters of Q201,202 are wired together in the schematic. But on your amp you measure them as 30v apart. FInd out what that is all about.

[Euthymia]

Well, yeah. The emitters of Q201,202 are wired together in the schematic. But on your amp you measure them as 30v apart. FInd out what that is all about.

Oh for crying out loud....

What that was all about was all the transistors on that driver board are socketed, even the small-signal ones.

Q202's socket has an extra, unconnected hole right next to the hole for its emitter.

Plug the damn transistor in correctly (that is, stick the emitter lead into the emitter hole and not the one .005" to its side) and it all works. No more DC on the output.

I just played guitar through this beast that's been weighing down my shelf since mid-August.

No doubt about it, Enzo, You Are The Man. A thousand thanks.

One last (I hope) question: I know your method of setting the bias by monitoring overall current draw and turning the pot until you see that the output transistors are barely conducting, but what am I looking for at that "offset" pot?

Good lord, what an achilles heel to have in an amp: if one transistor lead fails to make contact (due to my having pulled it to test it and then replacing it improperly), every downstream active device and its load resistor smokes.

[Enzo]

You mean the differential balance control? You still hopping between schematic versions?

In either case, offset means DC offset from zero. Remove the load, set a voltmeter to the speaker output terminals, and adjust the offset pot for zero volts on the speaker jack.

[Liam]

Good lord, what an achilles heel to have in an amp: if one transistor lead fails to make contact (due to my having pulled it to test it and then replacing it improperly), every downstream active device and its load resistor smokes.

Enzo, you know I was questioning the fact that so much in these transistor amps is DC coupled...could they not make things a little safer with a few caps..or at least limit the damage when things go wrong?

Must say, people must have cursed the transistor when they first started repairing them. Valve amps are a lot more straight forward and intuitive IMO.

That was at least 2 transistor amps that burst back into life yesterday. How does the Sunn sound? The Fender Stage 160 has one of the least pleasant "scooped-mid" distortion sounds I've ever heard.

Liam