I'm really in need of some help here, here is the story:

I changed some components on my Penta II (only one of my pair for now..) with the idea of giving them many more years of health.

Last intervention on my working Penta (which was already modified/restored: capacitors of the main power supply, opamp, and signal path capacitors) was yesterday, changes (only worked on the amp card) listed here:

 -changed the opamp IC2 to opa 2134 after having tried a linear chip I didn't like so much

 -changed some old capacitors for new ones (C27, C29, C31)

 -changed some more capacitors (the ones around the +-15v regulators, C18,19,20,21 and 22)

Then I got tried to measure and adjust the idle current as I usualy do before reassembling everything and.... no idle current at all !!!

All the logic and control worls at it should though, red without signal, green as soon as signal is applied, standby timer, all ok.

Of course I checked everything again few times, polarity of the new capacitors, solder, loose cables, etc..

Couldn't find anything, so I spent hours (I'm definitely functioning beyond my technical knowledge, so I'm slow and puzzled..) measuring with voltmeter and oscillo, the opamp part is 100% ok, so are both power supply, so my guess is that the problem lies in the part of circuitry which is full of discrete transistors (and which I didn't touch appart from 2 capacitors, of course I may have unvolontarily fried something?)... alas this is not easy for me to troubleshoot, I do understand the basics of a transistor, but no chance here.

As much as I can tell it seems the transistors do not activate. (except maybe T8? I measured 1.9v Base-Emitter)

Let me insert this aera of the circuit here (from service manual..), I added on it 2 things I found, for one I can't probe with voltmeter or oscillo along the line underlined in pink as it sends the amp in overload mode (blocked with orange light).

Second I noticed the signal gets amplified and looks clean till a certain point (C25, or T3, proof that the opamp part is ok), beyond that lots of noise, higher frequencies, are added. (the 68v supply is clean originaly)

Edit: I commented wrongly the schematics, the point indicated (just before C25 or T3) is where the high freq noise already appears. before R27 things look clean.

I thought T3 might be responsible somehow, so I removed it completely and that didn't change anything...

I hope someone here will have the knowledge to help me troubleshot this or give me some hint about the right methodology to use in order to find the origin of the trouble.

Thanks in advance!

I have had a problem with BeoLab Penta which did not switch on. When I started measuring on the front PCB things even got more weird, all the voltages I measured were wrong. After some investigation it turned out that the solderings of the connector from the main PCB the front PCB were bad. After resoldering them the problem was fixed!

I'm so happy.... problem solved !

what a little, old and annoying amp..

On a previous intervention few months ago, I had replaced the R73 which was quite fried.

Doing so (new resistor had larger leads) I damaged slightly the tracks of the circuit, but things worked, contact was ok.... till my last intervention. For some reason the precarious contact there decided to break off, and as I didn't change anything around that resistor I didn't check well enough it's area.

Thanks a thousand times to you mister Blackrix, I had an other go at inspecting and resoldering all I could see (though I knew my problem was, unlike yours, unrelated to the front panel and its connectors, as mentioned earlier the control, logic and switching all were working perfect).

Thanks god I saw the hair line crack on the track, overwise I would have had again a sleepless night...

A bit of wire to shunt altogether the pesky tracks, now everything seems to work, I will reassemble everything tomorrow morning.

-Now out of curiosity, I measured again the area around C25, lots of noise on my oscillo, if I inject a sinus say around 100hz, well one can still guess it's a sinus, but it looks plain ugly. However if I measure on the output transistors.. tadaaaa, a beautiful and perfect sinus is back!

Would someone care to explain this miracle to me? I thought the signal path is Opamp-C25-TR8-etc.., how could the signal be denatured in the middle and come out ok? Either I do not understand the path (wouldn't be surprising ), or I do not measure correctly for this case?

-Last thing, I measured the +15v and -15v supply (both at the principal decoupling caps C19 and C21, and at the Opamp pins, same results), they are not perfectly clean:

I have a 1.5mVp on the +15V, shape is bump 1/2 period, flat 1/2 period more or less.

I have a 2.5 to 3.0mVp on the -15V, looks like just noise.

1 quick question: -should I worry about that residual garbage, or is it ok in this magnitude?

Anyway so glad I can now just reassemble the damn thing and, provided I like the OPA 2134, I won't open it again anytime soon.

(Well of course I would then need to make the other channel which I, for now, left in original state for comparaison purposes, and once the amp part is solved, I will finally change the aging parts of both crossovers, so not quite finished yet...)