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Univox U65G DC on speaker

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

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galaxiex

I have this U65G amp that has DC approx 4V on the speaker leads when the speaker is DIS-connected and .007 VDC with the speaker hooked up.
There is some low level hum with the vol at 0. Somewhat more when operating, and a bit of hiss.

The amp sounds fine otherwise, and running it at full volume with a lamp limiter it barely makes the 25W lamp filament glow.

Here's the schematic.... as far as I can tell... the DC seems to be coming from the emitter of Q2. Is this normal for this amp?

I have replaced the 330uf at the speaker output, the 33uf at Q2, and the .001 at Q9, even tho they tested ok.

I should mention that if I power the amp with ONLY the voltmeter on the speaker leads, the meter reads approx 12V at power on, and then rapidly falls to ~4V.

With the amp running and Vol on 10, speaker hooked up, I get 3.9V at the emitter of Q2.
What should I check next?

Thanks for any and all help.  :)
If it ain't broke I'll fix it until it is.

Enzo

Note your outputs are totally isolated from earlier circuits by the drive transformer.  And there is a speaker cap isolating them from the speaker.  So you are probably right, the 4v is coming from the emitter of Q2.  That doesn't mean it is bad.  it does disappear when loaded.

See the 12k and a couple others next to the speaker?  Let's just call all that a 12k.  When there is no load, no current flows through that 12k.   The speaker was its path to ground.  So essentially the voltage there will be whatever the voltage is at Q2 emitter.   I'll believe 4v, after all that emitter bypass cap is only rated 6.3v.

Once you load the circuit, then that 12k is more or less from emitter to ground - the 4 or 8 ohms of speaker hardly changes the 12k resistance.  And 4v across 12k is less than a milliamp of current.  So no harm is happening.

So as far as I can tell, there is nothing wrong here to fix.

What to check next?  The refrigerator.  Have a beer.

Loudthud

With no speaker connected, the 4V or so on the output is probably normal for this circuit. It comes from Q2 through the feedback resistors.

Enzo beat me to it ha ha.

galaxiex

Awesome!
Thanks guys!

I think I'll go have a beer now....  :)
If it ain't broke I'll fix it until it is.

J M Fahey

 8|   I hope you understand that it being a Brazilian made amp, you MUST have one of these or the amp will instantly lose all Mojo

galaxiex

Quote from: J M Fahey on November 23, 2015, 06:13:36 AM
8|   I hope you understand that it being a Brazilian made amp, you MUST have one of these or the amp will instantly lose all Mojo

Hmmmm, I better check my local liquor store for those.... Is it ok if I drink some Brazilian Rum instead?
If it ain't broke I'll fix it until it is.

DrGonz78

Is that one beer for every other song in the setlist?  :lmao:
"A person who never made a mistake never tried anything new." -Albert Einstein

gfr

That's a headache for every song in the set list :)

Try one (or two :)) of these:

galaxiex

So I have done some tinkering with this amp....  :o

I did not like the Tone control filter response and it operated "backwards" to what I am used to.
("0" = Treble and "10" = Bass)

Reverb was quite over the top on "10" and really... you could barely crack the Reverb control and still have too much.  :loco

The last issue was Q2 seemed to cause distortion with aggressive pick attack.

This was the most concerning to me. I'll try to describe it...

With volume maxed, when picking softly all seemed ok, loud but ok.
With only slightly harder pick attack the volume took a HUGE jump and came with very harsh distortion.
Not musical at all.

Hooked scope to Q2 collector and during soft pick attack showed normal waveform.
Slightly harder picking and the waveform "jumped" from normal to off the screen.
It would do this at lower "normal" vol settings too.

It's almost like "gating" where a threshold has been exceeded.
No gradual transition from the soft picking to slightly harder picking where you would expect the volume to proportionally increase.

Any thoughts/ideas on what would cause this?

Attached is a new cleaned up schematic.

Note that the distortion/volume jump was there even before I made the below changes.

Changes made from the original circuit.

R8, 9, 10, C5, VR1, VR2,

R27 (tames the reverb)

R10 provides some attenuation and almost fixes the loud jump/distortion issue.
If I increase R10 it gets rid of the distortion but the amp gets very quiet.

Scope on any point before Q2 collector shows normal signal waveform.

The collector of Q2 is the first point where I could observe the distortion/volume jump.
Obviously, any point father down the circuit shows the same...

Original Q2 tests good both with a Meter and my component checker.
Tried replacing Q2 anyway with a 2N5210. No change.

Could there be something about the Q2 bias or other R values around it?

What's with the 56 ohm R15 and all Q2 etc sitting on top of R15?

Sorry for the long post. :-[

Thanks for any help.  :)
If it ain't broke I'll fix it until it is.

galaxiex

Been doing some research on the issue with this amp and it seems like what "might" be happening is blocking distortion on Q2.

Gonna play with some values to see if it can be reduced or even eliminated.
If it ain't broke I'll fix it until it is.

phatt

Maybe a wacky Feedback circuit,,, that's just a guess :-X
Phil.

galaxiex

Thanks Phil,
That (feedback issues) came up in the research on blocking distortion.

I considered disconnecting the FB network, will probably try that at some point.

So far, some small success with reducing the value of C6.
Went down to a 220nf and it got better.
Lost a minor amount of bass response.
If it ain't broke I'll fix it until it is.

galaxiex

#12
Ok, some tinkering has yielded some small success.  :)

Rearranged the Reverb control somewhat, and that required a bit of trace cutting and some jumpers.
New 100K Reverb pot.
The schematic shows the circuit as it stands now.
Still have some undesirable distortion on peak input/playing with aggressive pick attack, but it can be tamed by increasing the value of R10.
Doing that loses some volume, makes the amp quieter that I "think" it should be.

Have not yet looked at the FB network, that's next.

Edit; noticed something else, at full vol it sounds a bit compressed.
Not a problem, just an observation.
If it ain't broke I'll fix it until it is.

phatt

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.

Enzo

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.