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SUNN Stage Lead power issue

Started by markorock37, December 20, 2011, 11:19:34 AM

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markorock37

A bit more testing I found about 10V at pins 11 and 14 from driver transformer to Q2. Pins 12 and 13 from driver transformer to Q1 is less than a volt. Sounds like the driver transformer is bad to me. Its a model 28-4300

Loudthud

Quote from: markorock37 on December 27, 2011, 06:45:21 PM
A bit more testing I found about 10V at pins 11 and 14 from driver transformer to Q2. Pins 12 and 13 from driver transformer to Q1 is less than a volt. Sounds like the driver transformer is bad to me. Its a model 28-4300

10V is a little low. Pins 11 and 14 should be at half the supply voltage (+47). They should go to that voltage when Q1 and Q2 are removed and should be about the same when a good pair of transistors are installed. You can see that R128, R137, R136 and R139 form a voltage divider. Some current goes through R132 but not much. C1 could be bad, but it wouldn't affect the voltage when no speaker is connected.

These transformers develop shorts between windings. Disconnect 11, 12, 13 and 14 and then check that those wires measure infinite ohms to 15 and 16 and to ground. Mercury Magnetic$ makes the only replacement transformer available. If you find a short, there are two more windings on the transformer that were not used in this amp. They probably just cut the wires off. Any chance you can see them and maybe solder a wire to them? One pair is Red/Orange, the other is Yellow/Green.

Do you know how to check transistors out of circuit with an ohm meter? Basically you should see a diode between base and collector and another diode between base and emitter. Collector to emitter should show a high resistance or open.


J M Fahey

Oh well.
It´s discouraging when it happens, but we must go on.
Hope it´s not the driver transformer.
*If* it is, considering what MM usually asks for their iron, I´d ditch the Power amp and put a small chipamp there, fed by that PSU and driven from that Preamp.
Sound would be very good.

Anyway, let´s try it once more:
I have 2 suspects for what happened:
1) fake 2N3055, specially if from a small supplier (the 2 or 3 "big" ones can still be trusted, sort of).
The situation is so bad that I had to stop using them after 35 years of faithful service, over 8000 amps built around them.
2) Overbiasing and thermal destruction.
I suggested earlier you would find "500/550mV" across BE junctions (across R137/139) , you found around 650mV.
Too high in my book.
Yet, "since Sunn made it that way" I said nothing.
Please solder a 100 ohm resistor (1/4W will do) in parallel with each 22 ohm one.
Measuring without the output transistors you should have around 500mV across each of them, and 1/2 47V (1/2 +V rail) on point 5.
Please confirm.
You might also have R142 open (so all current must pass through R137 = "red hot"); check R140 too.
Do not replace transistors (which should be checked "outside" the amp) until posting found voltages.
Good luck.

EDIT:just checked MM´s ordering page:
QuoteSUNN-CONL-INT     Concert Lead -- Drop-in Upgrade!    170.00
ARRRGHHH !!!!!!!!!!!!!!!!!!!!!!! 170 bucks !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

markorock37

Well, looks like the driver trans is good, it measured infinite on all secondary leads. I did remove R140 and R142 and both measure around 2.5 ohms = should be .33 ohms. For whatever reason, pins 11 and 14 are at 24V now with the transistors removed. One of the new transistors is bad, and one of the originals was bad too. I have 3 good ones left. I'll get a couple new .33 resistors installed and then take a reading with those 100 ohm resistors installed in parallel with the 22 ohm ones.

Loudthud

Quote from: markorock37 on December 30, 2011, 01:56:41 PM
For whatever reason, pins 11 and 14 are at 24V now with the transistors removed.
That's what you should expect.
Quote from: markorock37 on December 30, 2011, 01:56:41 PMOne of the new transistors is bad, and one of the originals was bad too.

There is the possibility that something in the circuit killed one of the new ones when you installed it. Did you find a short when you checked the transistors with an ohm meter? Check for shorts from the case of those output transistors to the chassis before you power up the amp. This is a good time to have one of those light bulb limiters, it will usually save the transistors if there is still a problem.

Nobody gave you an answer before, but the MJ15003 is a good substitute for 2N3055.

markorock37

Light bulb limiter has been on it since I lost a transistor. Good to know MJI5003 will work..thANKS. Did check them with an ohmeter. I didn't get any time to really get at it today, but should get somewhere this weekend.

markorock37

I put in new R140 and R142, also soldered in the 100 ohm resistors in parallel with the 22 ohm ones. Reading a more stable 501mv at Q1 and 489mv at Q2. I haven't tried it yet but will later tonight. Problems only occur once the speaker is plugged in so hopefully its a go this time which makes me wonder about that cap in C1. The local parts guy didn't have that in stick so may have to order one if its the issue.

J M Fahey

Yes, C1 might be bad.
Easy to test: connect a 100 ohm resistor from its negative to ground (as if it were a speaker) and read DC voltage across said resistor. You should have nothing.
If the resistor goes up in smoke .... there it is.
Another suspect: replace both C123 and R141, or simply measure that R141 still is 22 ohms, easy, and replace the capacitor; you can use 2 x .1uF caps in parallel if .22 not available.
This is the "Zobel" stability network, and if open, amp looks good but starts oscillating as soon as you plug a real world speaker there.
Might be what´s happening to you.
We will leave no stone unturned.
*When* you plugthose speakers back, still leave the lamp limiter connected, the amp should still be able to supply 10/15W , enough to test it.
Only when everything fine can te limiter be bypassed.
Good luck.

markorock37

Replaced c123 and r141 was a little out of spec so replaced that too. C1 tested fine. Fired it up on the limiter and had rough sound so I Shut it down and cleaned all the jacks And pots with Deoxit. Sound is still intermittent.

markorock37

Got it figured out, its making great sound now on the limiter, the light bulb gets brighter the louder the amp volume is turned or how hard you play. Haven't plugged it in straight to the outlet yet.

J M Fahey

Looks fine to me.
If in doubt, put a bigger lamp in the limiter socket, I have a 150W and a 200W ones for that purpose, you can reach rehearsal levels with them.
If all looks fine, go ahead.
You might start by plugging the amp straight into the power line, no sound, no speaker attached, turn it on and measure DC voltages, such as +B (those nominal 47V), having 1/2 that at the centerline , proper around 500mV base to emitter.
If fine, connect the speaker, still no signal, cheak thing still stay the same, that transistors do not overheat, etc.
If fine, inject some music and play it at low power for, say, 5 minutes.
If fine, play at will.
A *VERY* cautious, step by step approach.
The *only* unknown might be fake transistors, which might pass all low power rtests, but overheat and burn at high power.
But then you have already shown your proper troubleshooting skills, simply you need to find a trusty supplier (which is not easy).
So far I´ve found all Toshibas fakes, and most the ON/Motorolas , both because of perceived prestige; while ST have been good, being relatively unglamorous.
Most "strange" unknown brands have been a fiasco.
Good luck.

markorock37

Sounds good I will give it a try tonight. ST is what I put in there , I have a local electronics shop that is pretty well stocked. Thanks for all of your help, I will let you know how it goes.

markorock37

Took it cautious like you said and now I'm rockin! It started to sound better the more I played it probably the caps getting fully charged. It has really good sound I have to admit I am surprised. Sounded even better with an overdrive pedal. I gave it the run through for about an hour, put the knobs back on and the amp is back in the cabinet. Thanks for all the help in locating the issues Mr Fahey and Loudthud, I learned a lot on this one and gained a ton of knowledge in the process. I'll get a mp3 up soon so you can hear how it sounds!

joecool85

Quote from: markorock37 on January 07, 2012, 02:53:17 AM
I'll get a mp3 up soon so you can hear how it sounds!

I look forward to it.  My brother rocks out an old Sunn bass amp and loves it.

**edit**
He is a bass player though, not playing guitar through a bass amp.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

markorock37

Here it is finally had time to record an mp3 of the Sunn Stage Lead. It was early morning so my chops weren't up to speed yet..lol.

http://www.isound.com/mark_hawkins