Vox AC30VR - oddities

[jondoe]

Hi All,

I have an odd problem with this Vox, it came to me with a blown fuse and power amplifier chip which are quite hard to get hold of, but I managed to find some NOS to try. When I removed the old one it was shorted from V+ to Output. I checked the power rails and they looked good at the time, so popped another one in. To my surprise this bit the dust instantly, the dim bulb didn't save it, it had 20+ DC on the output, so out it came. I checked the V+/ V- rails again and they were sagging slowly to almost nothing, then, after a short while, back to normal.

I went around all the hot looking solder joints to look for cracks and touched many of them just in case, but didn't find a smoking gun. I tested the power rails at various points, all good now and holding steady. I then tested the preamp section and it's good all the way to the power amp chip. I then put another chip in thinking the first one was a dud, but this had the same effect.

The only odd thing I've discovered is that at Pin 3 and Pin 8 of the LM3875 I have DC voltage on them with no chip install. It also varies, at times can be between +2V and -1V, both pins are identical in readings. At the moment it's floating around +0.2v

The attached schematic isn't quite the same as the issue 3 board I have, but very close, there is an addition of a 7805 for the 5v rail on this board. I don't suppose anyone has a newer version of it incase I'm missing something?

Thanks.

[g1]

Are you 100% sure the new chips are actual NOS? It wouldn't be the first time for fakes being sold as NOS.
Can you clarify what exactly happened with the 3rd chip? It is blown also, or just DC on output?
I don't see any path for DC to pins3 and 8 with chip removed aside from a defective circuit board. Unless you are not getting a proper ground reference, which might also explain the collapsing supply voltages you saw earlier.

[jondoe]

You make a very valid point, I can't be 100% sure they are NOS. I bought them from somewhere that has sold me NOS in the past that have worked fine, so it's as good as I can get under the circumstances.

I took a hammer to the original chip and one of the dead ones, splitting them down the middle to compare construction/internals, they were identical in construction and die size, of course that doesn't mean a great deal, but they are not obviously different. Annoyingly the third chip lost a leg while being removed from the board so it's a write off, but there were no dead shorts on the remaining legs.

I took the voltage readings with the PCB in the chassis, with two screws completing the grounds, so I think I'm good in that respect. There are no signs of overheating on the board, no signs of cracks, flexing the board doesn't change the readings. I left it on over night and this morning those pins are floating around 0.1 to 0.5v.

I have 1 "new" amplifier chip left, at this point I don't really want to fit it

[x-pro]

I checked the power rails and they looked good at the time, so popped another one in.

How good? What is the voltage on C13 and C22 individually?
Unsolder and check C13, C22.

[jondoe]

How good? What is the voltage on C13 and C22 individually?
Unsolder and check C13, C22.

Excuse the delay I reply. While on a dim bulb (which isn't bright) I have;

+32v
-32v

+15v
-15v

+5v

I've tested C13 and C22 on a basic ESR tester and they check out fine.

[The Dude]

......I've tested C13 and C22 on a basic ESR tester and they check out fine.

FWIW: An ESR test will not show a leaky cap. You'll need to do a leakage test with a cap tester, or measure it's resistance on a high ohms scale to detect leakage. Even then, some caps do not show leakage until higher voltage is applied. If you short your ESR tester leads together, you'll see what I mean. A dead shorted cap will show fantastic ESR.

[x-pro]

Without the output chip, what is the DC voltage across the collector of Q6?
If there is no DC voltage there and the power supply is OK, put in a new chip. But do not connect the load and check the DC voltage at pin 3, relative to the Common.

[Mars Amp Repair]

Without the output chip, what is the DC voltage across the collector of Q6?
If there is no DC voltage there and the power supply is OK, put in a new chip. But do not connect the load and check the DC voltage at pin 3, relative to the Common.

Yes I agree, I NEVER connect the load when powering up a SS amp that I have rebuilt in the case of a discrete amp or even with an IC power amp Chip. Monitor the speaker out before the protection relay (if it has one) to be certain there is no offset.
You may see some offset esp in older discrete amps until you pass abt 40VAC on the variac, as the bias circuit on older amps has not kicked in yet at the lower AC in voltage. But there shouldn't be much offset DC at the power IC output.
I guess if the Coupling cap C71 were leaky, that would cause offset. Be certain R47 .022ohm is not open.
Let us know how it turns out.
Glen - 12th Fret Music Service Dept

[jondoe]

Without the output chip, what is the DC voltage across the collector of Q6?
If there is no DC voltage there and the power supply is OK, put in a new chip. But do not connect the load and check the DC voltage at pin 3, relative to the Common.

Q6 reads as follows;

E 0
C 0
B +0.6v

I've sent back the chip(s) that I purchased so I can't put one in to test. I'm currently on the hunt for some alternatives, I may buy a few from different sellers and see how I fair with the dice roll