Have you re-soldered the 2 "big" transistors in the middle of the board ? They are bend downwards with a heat sink on. These can be pushed down by the wires just above them when the amp is put together, and may cause them stress over time. I have seen many amps where these have bad connections. It is very easy to see if you look at the bottom of the PCB while carefully wiggling them. But be careful, the PCB traces might also be loosing their grip to the PCB.

Try re-soldering them.

Also, a re-solder of the connections might not be enough. Many times the traces on the PCB are actually broken, so you need to test with a multi-meter if there are connection between each pin and to the first(or any) component it is connected to.

/Jacob

Great suggestion Jacob, thank you! I have not yet re-soldered or tested those transistors, I'll give that a go shortly and report back with my results.

I'll also go over the traces this weekend, I have noticed while replacing components on both boards that they have been losing their grip as you say.

Matt

I checked those transistors over the weekend and their solder was definitely stressed. I renewed the solder on them and tested them out of circuit, everything checked out ok. 

I also went over pretty much every trace on the board and couldn’t find a fault there either. Plugged in to retest and the same fault persisted. I recorded a snippet this evening of the issue to give a clearer picture. I used the start of Hotel California (Hell Freezes Over) as a reference track.

Sounds to me like a grounding problem. Check ground continuity.
Check also for cracked solder joints (reflow them all) where the ribbon cable from the board inside the frontpanel is soldered to the main amplifier board. 

Martin

Thank you Martin, we have good continuity between the negative/ground input terminals and the chassis of the amp, as well as on the negative output. Would you recommend I work through the schematic and check every ground or would that be sufficient? I reflowed those joints a couple of weeks ago based on a recommendation of yours in an older thread, and I've also tried exchanging the front panels between the two amps just to *probably* rule those out.

I've attached a picture of my amp for reference, just in case there is a glaring failure that I have overlooked 😕 

link

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Did you change the pre-amp integrated circuits and the signal router i.c.(4066)

I often used to put these Penta amp's back together only to find intermittent noises and nasties afterwards,so for what they cost,I alway's replace those chips too.

Does Martins kit include all the electrolytic caps?The standby supply decouplers are often out of spec,and can cause humming etc.

Nick

The kit includes all electrolytics except the two large main supply cans in the transformer holder.

I never experienced a bad 4066 IC in a Penta.
The 4066, being a CMOS chip, is very sensitive to static electricity, so if work was done in a non-ESD environment any and all CMOS could
be suspected, and indeed the 4066 could cause noise.

Martin

@ Solderon29, 

what do you mean by The standby supply decouplers, i'm trying to repair 6 beolab 150 amps and need all

the info i can get :-)

Recap with Nichicon UFW and UFG and BIAS tune is what i'm doing right now.

I've 3 amps with standby issue and 2 getting really hot.

Greetz,

Rob

Thanks for the tip Nick! My local electronics store has stock of these and for AUD$1 each it's worth a try 😎
Who knows what/who may have touched these amps in their life before me! 

Would failing standby decouplers produce a hum in the amp itself or in the speakers?

Matt