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Gallien Krueger 700RB distorted sound

Started by pontiacpete, June 23, 2023, 09:45:24 AM

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pontiacpete

When this amp came to me I plugged it in and it made sound but soon started to smoke and then an electrolytic cap blew, C21 10uf/50v.You cannot view this attachment.

Also the LM3886 chip was smoking and had burn marks under the legs. I replaced both the cap and the LM3886. The amp is making sound but it is distorted. So far I check all the transistors under the heat sink for any shorts in circuit. All of them seem to be ok. I pulled Q6 which was reading a dead short between B-E but out of circuit it measures to be good. Q6 is a MPSA06 but on the schematic I have from GK says it's an 2SC3478. Confusing to me. THe trim pot R9 is not changing the distorted sound. Any ideas where I should go from here? Thanks!

g1

2SC3478 is ECB pinout, MPSA06 is EBC.
Find out which pinout is correct by following the traces to D4, R21, and R85.
Then tell us where the short is located on the board.

pontiacpete

The pin out of the MPSA06 is EBC, it's also written on the board but the schematic clearly says Q6 is the 2SC3478, confusing.

pontiacpete

Finally got the correct schematic.

g1

With Q6 removed, do you still measure the short at the traces?

Tassieviking

If that is the correct schematic then Q6 is in a totally different part of the circuit, and J1 shorts the B & E legs on Q6.
Is Q6 in the main power amp section or in the circuit with the LM555 IC (U1)?
There are no stupid questions.
There are only stupid mistakes.

pontiacpete

Q6 is the circuit near the LM555 and it is shorted by J1. Is this the fault circuit? What is J1? I can't seem to find out where that is. ANyway,The cap that blew up c21 is on pin 8 of the LM3886 which was smoking and then replaced. This could be difficult to repair. There are parts on this schematic that I can't find on the board and there are parts added that aren't part of the PCB, that is, with a designated location. I suppose because this is an early model and it never made it to the service manual.

g1

J1 should be a thermal breaker somewhere, it opens if the amp overheats and triggers the shut-down circuit.  The other schems show it as 85 degree C.

You have distortion, is it in the low amp, the high amp, or both?
R9 is a critical adjustment.  If you did not put it back exactly where it was, the amp may overheat.

pontiacpete

yes, it's distorting in both hi and low amp. I have tried to turn R9 but change in sound.

Tassieviking

I noticed that one variant of the 700RB-II power amplifier uses a 10uF 50V NP capacitor where your C21 is, perhaps your model should have a NP (NON-POLARITY) capacitor in there.
If the cap is polarized and gets fed a reverse voltage it would most likely smoke and pop like yours did.

Can you trace where the track goes to from pin 8 on the LM3886 ?
It most likely goes through a resistor to the negative supply voltage somewhere. (Mute 2) ??

Do you have the preamp schematic for the early 700RB ?

There are no stupid questions.
There are only stupid mistakes.

Tassieviking

#10
700RB-11 Manual page 10 shows the 10Uf 50V NP capYou cannot view this attachment.You cannot view this attachment.You cannot view this attachment.
There are no stupid questions.
There are only stupid mistakes.

g1

Why is that 204-0104-c pdf schematic labelled as such (posts #1 and #10)?
It is clearly marked 'Rev.D'.  All copies should be corrected and posts should be edited.
Pontiac Pete, if you have the pdf version that those jpg's came from (real Rev.C), please post it.
Otherwise, I am attaching Rev.F which includes the preamp.  It should be the same circuit, although component names may be different like in the power amp.

I think the Cap at pin8 of 3886 IC only gets reverse voltage when the chip blows.  So they probably went to non-polar cap to deal with that.
The polarized ones were probably getting damaged but overlooked when IC blew, so going with non-polar eliminates that possibility.  Normally, 'Mute 2' line would only have negative voltage.

Try using 'FX Return' jack as input, and make sure 'boost' is turned all the way down.  Still distorted in both amps? 

Tassieviking

Thanks g1 for pointing out that error in the file naming, I did not realise that that C was for the revision, I think I have fixed it on post 10 ok now.(and in my computer as well).
I would love to see revision E if you have it g1, it would be great if you could post it.
There are no stupid questions.
There are only stupid mistakes.

pontiacpete

#13
sorry I dropped out for a while, had to get away and the of course a bunch of amps came in.
Also sorry about the confusion with the schematics. The first one I posted, I thought was the C version of the board. When I realized this wasn't that at all GK help location the correct on. Obviously it's a photo of the original.
There is no DC on the output and I check connections on the LM3886. Thought I might have messed up something there but it looks good. There is the possibility that the chip is not go, I suppose.
So when I plug into the effects return with the boost down it is the same level of distortion as in the input. The cap that blew up was definitely not a Non-polarized cap. I still have the casing which shows the polarity.

g1

Quote from: pontiacpete on July 21, 2023, 12:30:33 PMSo when I plug into the effects return with the boost down it is the same level of distortion as in the input.
In both the high and low amps, or just one of them?