Gallien Krueger 800RB Dead from the head end back!

[Sowhat]

Got a Gallien Krueger 800RB amp in with the complaint that it just stopped working… how very descriptive!

I opened it up and after checking the fuse I checked over the power amplifier section and everything looked fine statically. I turned my attention towards the power supply board and the +-15 volt regulator was in shambles. R3 and R4 were crispy critters, Q3 was blown open revealing it’s die. I didn’t check any further with this regulator, I just replaced all the regulator silicon plus those resistors which fixed things right up for the power supply. I used TIP41C’s for U1 and U4… basically the same thing as TIP31B, just handles more power and most importantly, I had some laying around in the shop.

The power amplifier board has an op amp on it requiring +-15 volts so I disconnected the preamp and checked the +-15 volt rails… they were just fine and the power amplifier seems to check out good so far, but as for the preamp, that’s another situation all together. I’t rails are a dead short so… we got’s to go on a short hunt in the preamp. The first thing I notice on the preamp is that the two 220uf 25v filter caps were bulged and leaking dialectric so, these came right out. I just removed all 6 of the op amps and inspected them individually… oh yeah, once the op amps and those caps were no longer in the circuit the shorted rails disappeared. I guess theres some parts to be replace here but my findings on the op amps just makes me scratch my head… what could have caused this failure?

I checked the power pins on the op amps and this is what I found.

U1 open
U2 good (about a megohm)
U3 short
U4 open
U6 short
U7 short

U5 is a SS Switch and looks to be OK.

Almost every op amp in the entire preamp was either shorted or open, I have never seen this before, what do you think could have caused this?

[Enzo]

Who knows?

A power supply failure can run excess voltage or worse, verse voltage, down the rail and fry ICs on it.

OR...

A failed IC can kill the power supply, which can then do further damage.

OR...

A large static discharge into the jacks can damage ICs etc.

OR...

Their story isn't the truth, and someone tried to fix it, and caused further blow ups.

OR...

something else entirely.

[Sowhat]

Who knows?

A power supply failure can run excess voltage or worse, verse voltage, down the rail and fry ICs on it.

OR...

A failed IC can kill the power supply, which can then do further damage.

OR...

A large static discharge into the jacks can damage ICs etc.

OR...

Their story isn't the truth, and someone tried to fix it, and caused further blow ups.

OR...

something else entirely.

I'm leaning towards your first assumption. The +-15 volt regulator is using +-60 volts as a source so it's working pretty hard already choking down that 60 volts. If it let loose, there would be +-60 volts on the preamp which would be a bad day in small signalville, that is until R3 and R4 vaporized. It don't look like anyone has been in there mucking around before... yeah, maybe just an unfortunate random regulator component failure started this sequence of destruction.

[J M Fahey]

^^^^^^ what Enzo says

Now :

* replace whatever´s needed to have +/-15V back

* replace bulging/suspect filter caps

* replace all Op Amps with fresh ones, don´t even try to recycle the old ones, use TL072 which will work fine thgere and hiss a little less.
Most important: abbundant and inexpensive, less than $1 each.
After replacing each one, turn amp on and recheck you still have +/- 15V there.

* last but not least replace TL604 and *pray* it works fine, because it´s somewhat unavailable (to put it mildly).

* your preamp should work.

[Sowhat]

* last but not least replace TL604 and *pray* it works fine, because it´s somewhat unavailable (to put it mildly).

I know the schematic says TL604 but it's actually a DG419DJ which is not very rare at all, at least not here in Southern California... they are a couple bucks, and that's getting replaced as well... semiconductors that have been stressed can never be trusted again, out with the old and in with the new.

[g1]

DG419 can not be used as a direct sub for a TL604, different pin out.
Either that is not the correct schematic, or someone has put the wrong part in there.
I would guess in later models they re-worked the board. See if you can find the correct schematic or you may run in to other problems using an incorrect diagram.

Edit: see page 12 of 800rb svc manual attached, it shows the DG419 version.

[Sowhat]

DG419 can not be used as a direct sub for a TL604, different pin out.
Either that is not the correct schematic, or someone has put the wrong part in there.
I would guess in later models they re-worked the board. See if you can find the correct schematic or you may run in to other problems using an incorrect diagram.

Edit: see page 12 of 800rb svc manual attached, it shows the DG419 version.

Yeah... my schematics are a mismosh of different versions, I see that the pass transistors in the regulator went from TIP29c's to TIP31b's and other little things here and there but they did a nice job silk screening the boards and there's a nice 419 in the rectangle for U5 so I feel confident that it's the right part.

[Enzo]

REvision level is probably printed on the board somewhere.

[J M Fahey]

Well, if it´s factory silkscreened on the PCB, then that settles it.

Just being anally obsessive compulsive (no, not you, but *me* ) please check +/-15V reach proper pins.
meaning if it´s actually DG419 you should find them on pins 4 and 7 respectively, while if meant for TL604 they should be on pins 8 and 5 ...... I know I would

[g1]

This is a bit off topic, but seeing their changes as shown in the service manual makes me think DG419 would work fine dead-bugged into other TL604 applications, like Peavey etc.
(the Peavey retrofit kit is over $10 and I think it probably uses the DG419 anyway)

[g1]

attached is the GK revision, schematic was dated 1995.

[Sowhat]

Things have been a little distracting at the shop these last few days but I have the parts and all I need now is to get a few minutes alone with it and solder things up. I hope the emergency rush jobs that have been flying in settle down a bit here... How does it go? "poor planning on your part does not necessitate and emergency on my part"... I wish that was really true.

[Sowhat]

Finally got some time with it and everything is good, it works fine now. In the final analysis I replaced everything on the +-15V rails just because I no longer trust devices made for that rail that have been exposed to +-60 volts. My best guess is that the +-15 volt regulator, especially Q3 (the one that exploded) was just time to die... maybe some microscopic die crack or inclusion that passed inspection but not the test of time. No customer fault here... they're an innocent victim, it was more like an act of god.

[nevetslab]

Ah, the weak link in the GK 800RB, as well as the successors. I had one GK 800RB that kept coming back into the shop with distortion problems. I had gone thru and increased the coupling cap sizes in all the stages from the front end thru and past this circuit (which was what GK had done on the 1001RB, not knowing that at the time). But, even with C93 and C95 increased so the clipped LF signal was square and not sloped, this stage clips much sooner than the op amp stages. Meaning, as a bass player, you dial in enough gain on the front end, dial in your tone, press the contour switch in, turn up the Woofer level, then crank up this Boost control, the amp sounds awful. This stage runs out of headroom. Just the limitation of the single JFET stage J113. Signal looks just fine coming thru the analog switch ahead of it.