Univox 4100 top of waveform collapses under load

[DrGonz78]

https://youtu.be/ylSlwSb_g1U

So I have a Univox 4100 that under a load collapses the top part of the waveform. Power amp of the U130 is almost identical but uses a few different transistors. This U4100 schematic differs from the amp and the U130 matches closer to what is in the amp. B+ voltage is at 83v and midpoint adjustment only gets to 37.5v. All electrolytic capacitors have been changed. Bias pot is at coldest setting since adjusting it doesn’t make a change. I have installed mj15003’s on the outputs replacing the RCA 2N4348’s. Before that the original driver transistors were producing the same issue and they eventually cracked off at the legs so I had to replace them too. I had to install NTE equivalents and I don’t suspect that they are the issue.

Under no load it produces a fairly decent sine wave with a bit of clipping on the top of the waveform as volume is increased. Plug in a load and it just collapses. I removed the bias and midpoint pots to make sure they were good which upon reinstalling them helped restore the waveform with the load not attached. Before doing this it wouldn’t even display a healthy sine wave whether on load or not. Oh I soldered all the joints and have done tons continuity checks to rule out bad connections. So any advice on what to do next would be greatly appreciated.
I will post u130 schematic too but having a problem finding my copy on my phone right at the minute.

[The Dude]

What are the voltages on the output transistors. Also measure B-E voltages.

[Helmholtz]

Is the D5 zener voltage good (22V)?

[g1]

I don't think the unloaded waveform is good. I suspect the issue is earlier than the outputs, and maybe before the drivers.

[The Dude]

This is of course speculation at this point, but I suspect there is a problem with the top side output transistor circuit and the driver transistor is trying to drive the load. Voltage measurements should tell the story.

[DrGonz78]

Is the D5 zener voltage good (22V)?

No Zeners on board the U4100 that's where it differs from the U130. Some values of components on this particular U4100 are more in line with U130 schematic. Probably later over lapping production.

What are the voltages on the output transistors. Also measure B-E voltages.

The B-E votages are 12 volts apart. Just can't figure this out right now, head scratching time. lol

I don't think the unloaded waveform is good. I suspect the issue is earlier than the outputs, and maybe before the drivers.

[DrGonz78]

I am adding another earlier voltage reading picture. I may have mixed up B to E readings on Q8 which is now the NTE331 transistor. Although I was very thorough but I will double check when I get up. I had the same problem with the amp with the original driver transistors when they were installed. I would not have replaced them but I think the legs were ready to crack. Only took a bump or two and just cracked off. We had a thread about that phenomenon as I recall or I saw a YouTube video about it. Probably both. But here are voltage readings from when I first installed the NTE331 and the 332.

[g1]

If Q8 is for sure NPN, it's base can not be more than 0.7V above the emitter. So it is either blown, or reverse biased like numbers in post #6.

[Helmholtz]

There are essential voltages (more exactly voltage or potential differences) that should always be measured directly, like B-E voltage or voltage drop across the emitter resistor.
As always with DC: Polarity matters.

[DrGonz78]

I am pretty sure I am my own worst enemy. It’s probably reversed biased because I installed it backwards. I know exactly what I did now. The collector and base legs on the rca45192 (ECB) cracked off right flush to the body of the transistor. I macgyvered a solid core wire to reconnect those legs to the transistor. My main mistake was upon installing that now frankenstein transistor that I installed it backwards. Then when installing the new nte331 I never noticed that mistake thus putting the orientation of my new transistor backwards as well. Here's a funny picture that finally made realize the orientation was not correct. I took other pictures before removing it but I just didn't notice. The Emitter and Base were then reversed on the 45192 transistor and in the pic below it's facing the wrong way!! Thanks so much for your last post G1 since it made me go back and look at all the pictures I took.

I had a 7mm stone sitting in my ureter and like 4 other stones in my kidney when I made this mistake. I had to wait a year to get back on insurance to get two lithotripsy surgeries to get those damn things out. But during that time period I was just not doing very well and this goes to show. Finally got the stones out this past March & April and then a month ago I had tight chested feeling prompting more probing from the medical world. But as the fog in my head starts to clear I am finally starting to get back to projects that I was just too sick to work on. One stupid mistake is all it was the whole time. Well I am gonna go flip that transistor around and hope it wasn't damaged.

[DrGonz78]

Yup just went in the shop and flipped that transistor around. It’s alive!! Thanks to all you guys for all being here to help get my brain tuned up again. G1, The Dude and Helmholtz you all are such knowledgeable techs. But thank you most for always sharing your knowledge on MEF.

[g1]

Glad to hear you got that cleared up.
And the amp.