Genz Benz GBE 600 DC Fault Protection

[Jazz P Bass]

Here it is.

[Kenrod]

I have a GBE600 that let the smoke out of something. I found IC3 with the input lead burnt off the board.
2 questions.
1) should i start a new thread?
2) as a starting point, what exactly does this part of the circuit do? I see a zener in there, so I know we're deriving a specific voltage, but I'm not sure what else is being accomplished. These components are on the power supply board, but aren't show there on this schematic.

[Jazz P Bass]

IC3 is the +15 volt regulator.
LM7815.
The zener diode is limiting the +24Vdc going into the regulator through Q31 (pass transistor circuit)

It is shown at the top/ right of the output section.

[Kenrod]

IC3 is the +15 volt regulator.
LM7815.

It is shown at the top/ right of the output section.

yeah, that's what i meant, it's not shown in the power supply PCB section, but elsewhere.

IC3 is the +15 volt regulator.
LM7815.
The xener diode is limiting the +24Vdc going into the regulator through Q31 (pass transistor circuit)

Should is suspect R65 of failing to limit current into the input of IC3? or a failure of Q31, or the Zener?

I haven't fooled with electronics in quite some time, so I'll probably ask some seemingly dumb questions. I really would like to get very good at this as it interests me as a hobbyist.

[Jazz P Bass]

Should I suspect R65 of failing to limit current into the input of IC3? or a failure of Q31, or the Zener?.

When faced with a failure, everything is suspect.
Start by:
Checking that all of the resistors in the +15Vdc circuit are good.
Diode check the zener diode & the transistor.

If something downstream pulled too much current through IC3, then it would fail.(although the IC should shut itself down before that happens).
In that case look for a failed R74.

If the 'pass transistor' 24 volt circuit feeding IC3 failed, it may have overloaded the input to the IC.

[Kenrod]

Zener is open
Q31 shorted across C-E

Funny thing is I didn't find that until I had replaced the regulator and it powered up. I suppose i was sending full VCC to the regulator, but didn't blow it.
I don't know how I missed the failures in the zener and Q31. I was sure I had checked everything and determined that perhaps i simply had a regulator failure.

BTW, how much is VCC?

As I think i now understand the regulator circuit better, VCC is likely higher than the 7815 limit of 30V, due to the use of the zener to drop it to 24V, to then be regulated at 15V by the 7815. Is my thinking correct?

[Jazz P Bass]

"BTW, how much is VCC?"

VCC voltage is not stated on the schematic.

You can measure it on the left side of R75.(where it meets the output transistor collector)

The power supply shows 80Vdc capacitors.
So it is at least 70Vdc.

[Kenrod]

"BTW, how much is VCC?"

VCC voltage is not stated on the schematic.

You can measure it on the left side of R75.(where it meets the output transistor collector)

The power supply shows 80Vdc capacitors.
So it is at least 70Vdc.

Can't find R75, R74, D23, D24 or R76, R77, D25, D26 on the neg side. Are they required? could they be omitted safely from my revisions of the amp?
I've followed the trace from the collector of Q31 back to the power amp board and unless the components are on the underside, i can't find them (R75, D23 & 24). followed the output of the 7815 and can't locate R74

"VCC voltage is not stated on the schematic" is why i was wondering if anyone knew otherwise.
I figured to to be well above 50V, closer to 75 maybe, based on the rated power of the amp.
I have a 400W bass amp and seems like its about 76V

Since i'm still learning as i go, was my assumption on the use of the 24V Zener correct?

[Jazz P Bass]

That's what the schematic calls for: a 24 volt zener diode.
Wattage is not called out though.

[mozwell]

24V zener fed through 15K from approx 75V (max), zener current is 3.4mA, zener wattage is 0.08W, use 500mW zener or higher

With no speaker load connected, make sure all power supplies +/-15V and the main +/- VCC are ok before we go further.

[Kenrod]

VCC is 72V , but only 9.45V out of the 7815. I have a spare that I'll swap when I swap the Zener and Q31

[Jazz P Bass]

Hmm, I thought that you did change out the 7815.
If so, then something downstream is loading it down.

[Kenrod]

I did change it out, but wonder if i overheated it it during install, partially damaging it, or maybe it was mislabeled?
Since the zener is open, I've got 72V on the input of the 7815. I'm sure it doesn't like that, but i've only powered it up twice since i discovered that the zener had opened, which i can guess was the original failure, maybe have shorting the C-E junction of Q31 and blew the input leg of the original 7815 completely off the board.
I have 24V Zeners ordered, can get a TIP31 at RS locally, have a spare 7815.

BTW, the neg side of the regulator circuit looks good, with 23V on the input and 14.95V on the output of the 7915.

[mozwell]

the 7815 may have been stressed, replace the zener & fit a new 7815
If the 15V supply is still low, then something is loading it down, we need to find out where that is.
Try replacing IC1 TL072 and the two clamp diodes D6 & D7

[Kenrod]

Replaced the 7815 again, Q31 and the Zener. I powered it up and fed it some music from my ipod and it sounds like its old self. I haven't check any voltages yet, just giving it the music test.

Something I failed to mention in earlier posts, IIRC, was that after I replaced only the regulator it had a squeal and lower output than I thought it should have.

Thanks for the help.