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Univox U65G DC on speaker

Started by galaxiex, November 22, 2015, 05:41:35 PM

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galaxiex

Quote from: phatt on June 25, 2016, 10:04:15 PM
Sounds like Q2 or Q7 is hitting the rail to fast,,, maybe the Quiescent point (offset) is too close to the rail.

C4 ,, C6 could be leaking DC upsetting the Q balance of Q2 and Q7 which are DC coupled.

Also try removing the reverb return line from the circuit it's possible that maybe causing issues, Unlikely but you never know.
Phil.

Thanks Phil,
C4 thru C6 have all been replaced, no change.
Same with removing the reverb line.
If it ain't broke I'll fix it until it is.

galaxiex

Quote from: Enzo on June 25, 2016, 11:07:28 PM
Well of course.  Look at the schematic.  R10 and VR1 form a voltage divider.  Well, R8,9,10, and VR1 really.  So increasing R10 is the same as turning down VR1, as far as Q2 is concerned. So if turning down the volume stops the clipping, then so would increasing R10.

If by FB you mean the feedback from the output to Q7, I suggest doing that first.  That is the power amps way of correcting itself, and may be contributing to your problem.

Thanks Enzo,

Yes I see the voltage divider, thanks for pointing that out, (NOT meant as sarcasm) sometimes it's easy to overlook the obvious.

Disconnecting the Feedback network results in a very loud and extremely noisy amp. (hum, buzz, crackle)
I basically cut the trace at the emitter of Q7.

I confess to being quite lost re; this FB network.
I can figure most things out, but I'm at a loss as to what to do, if anything.

I suppose I could play with some of the values in the FB network.

Any ideas where to start? What to try?

Thanks!
If it ain't broke I'll fix it until it is.

galaxiex

Arrrrgghh! I feel like such a doofus!  :-[

I made a mistake when copying the original schematic.

The feedback line actually goes to the emitter of Q2, NOT Q7 as in all previous schematics.

Here is the corrected schematic with a couple small modifications.

Added R45 to tame the Reverb. Not sure if C29 was really necessary but threw it in to ensure DC isolation.

Here I'll include the original schematic for reference.

If it ain't broke I'll fix it until it is.

galaxiex

OK! Fixed! Yay!  :)

It was C10, the 47uf cap that bypasses the emitter resistor for Q7.

I normally don't go in and wholesale replace all the electros on these old amps.
This may be a case where I should have.....  :P

There didn't actually seem to be anything wrong with C10...
No bulging/swelling/leaking.
According to my component tester, it does measure on the high side at 98uf.  :o
If it ain't broke I'll fix it until it is.

galaxiex

#19
Here is the final schematic!

Notice the absence of the attenuating resistors compared to the previous schem.

Also installed a 10uf for C10 instead of a 47uf. seems to work well.
Played the amp for an hour and no issues.

At max volume it now has "natural" transistor breakup.
Not particularly nice distortion compared to a tube amp, but passable for a solid state.  ;)

It's actually quite a loud little beast! Could gig with it in a small venue.

The previous spiky harsh distortion from the bad cap is all gone!  8)
If it ain't broke I'll fix it until it is.

phatt

You have done well :dbtu:
It makes you feel good when you finally nail it ay?  8)
Phil.

galaxiex

Quote from: phatt on July 03, 2016, 08:15:17 AM
You have done well :dbtu:
It makes you feel good when you finally nail it ay?  8)
Phil.

Thanks Phil!  :)

Indeed, it does feel good to finally sort it out.  <3)
If it ain't broke I'll fix it until it is.