# YBA-1 issues



## J-75 (Jul 29, 2010)

I have a 60's YBA-1 (choke, silicon diodes) that has a couple of problems.
When I turn up the volume, it flattens out at about '3' and doesn't increase loudness much after that. This happens on both channels.
There appears to be a parasitic oscillation on both plates of V2, causing background noise.
I've switched V1 with V2 - no change.
The circuit doesn't conform to a schematic I have of a '66 model, particularly in the power supply/filtering parts.
All components are original at this point.
Guesses?


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## nonreverb (Sep 19, 2006)

If it is oscillation, it could be lead dress somewhere in the preamp is the culprit.....the fun is discovering where. Best to start by consulting the schematic. 
Is the C11 47pf suppressor cap present at the phase inverter output? What's the ripple like on the filter caps?


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## jb welder (Sep 14, 2010)

Is it 7027A output tubes or 6CA7/EL34 ?
I have a schem for a 6CA7 version with choke from around '67 but can't attach the pdf is too big.


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## nonreverb (Sep 19, 2006)

Better yet, here you go...
http://www.lynx.bc.ca/~jc/traynorSchmArchive.html


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## jb welder (Sep 14, 2010)

Thanks nonreverb. The schematic I spoke of is on page 6 of this pdf: http://www.lynx.bc.ca/~jc/YBA1Schems.pdf


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## J-75 (Jul 29, 2010)

jb welder said:


> Thanks nonreverb. The schematic I spoke of is on page 6 of this pdf: http://www.lynx.bc.ca/~jc/YBA1Schems.pdf


Yes, I think that's it. I'll have a better look-see this weekend. Thanks


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## J-75 (Jul 29, 2010)

nonreverb said:


> If it is oscillation, it could be lead dress somewhere in the preamp is the culprit.....the fun is discovering where. Best to start by consulting the schematic.
> Is the C11 47pf suppressor cap present at the phase inverter output? What's the ripple like on the filter caps?


Yep, the 47 pf is present. The ripple on the paralleled 40/40 mfd Mallory is 7 V p/p (which is less than 2% of a nominal 430 VDC) and the other Mallory is clean on both sections.
The 8 mfd connected to the diode on the bias line is 2V p/p, the second 8 mfd is clean.

All DC levels seem to be reasonable throughout the amp, as far as I've checked.

I can presumably clean up the noise by replacing the electrolytics, but I'm not getting why the volume control on either channel flattens out. The pots are all original.

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jb welder said:


> Thanks nonreverb. The schematic I spoke of is on page 6 of this pdf: http://www.lynx.bc.ca/~jc/YBA1Schems.pdf


It's the one that has number "3" encircled in the top right corner, dated June 1967.

Thanks jb!


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## nonreverb (Sep 19, 2006)

Any access to a scope?


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## J-75 (Jul 29, 2010)

nonreverb said:


> Any access to a scope?


Yep - got a Tek 454


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## Tyler Savage (Nov 16, 2009)

here's a wild guess - check the pots, are they stock? Are they log-taper? 

Also, it doesn't increase much in loudness, but I'm guessing it distorts a hell of a lot more? I have a tube-rectified 60s traynor and she doesn't get much louder after about 3-4, just kind of nature of the beast. 

as for the oscillation, don't rule out problems with the tube sockets - I've had socket problems with old traynors as well


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