Sonzera 50 buzz

[Mars Amp Repair]

Hey folks,
Working on this amp. I can't find a schemo anywhere for It (Sonzera 50).
It had one EL34 filament to grid short (according to the Hickock 600A).
Replaced those...checked for any fried resistors and then set bias.
All works well but get intermittent buzz in the sound. Tapping, flexing has no effect but sometimes you turn it on and the buzz is gone.
It sounds like a ground loop or when the filament balance resistors burn up. The amp has those resistors and they are fine.
If i remove V1, V2, and V3 the buzz actually gets better but not completely gone (you can see it in the scope somewhere in the signal path (without schemo very difficult to determine exactly where I'm probing). Without schemo will have to spend more time drawing it out.
Turning the master volume down eliminates the buzz. Power supplies all clean.
If i plug in V1 even without V2 or V3 installed the buzz immediately gets considerably less. I suspect that's when the filament is connecting. Yank the phase inverter and buzz goes away...no too surprising.
It appears V1 and V2 filaments are run off Dc that seems to be the same tap as the bias but really cannot tell. I started drawing the path to the DC filament source but
unfortunately it is a double sided pcb and very tough to trace.
Anyway, has anyone heard of this issue. Almost seems like a design flaw of Somme short. I see on a PRS forum where PRS had some design issues with hiss in another model so to me a design issue is not out of the question...I'll call customer to see if it always did this.
So, does anyone have a source for the schemo....that would greatly save time.
Thanx, glen - Mars Amp Repair

[Jazz P Bass]

There were some issues with the Sonzero 20 with the FX loop causing hiss.
Not sure if that is what you are describing.
https://www.diyaudio.com/forums/inst...-20-noise.html

[Mars Amp Repair]

Thanx jazzbass... yes, i saw that post, too. Unfortunately, it's not hiss but buzz. I'll have to get back on it and just draw out the DC filament portion of the circuit.
Sure be nice if a schemo were available.
Thanx again...glen

[Mars Amp Repair]

Solution found: Bzz was very intermittent. At one point I thought it was as easy as a filament short in one of the preamp tubes when it got hot, but alas the hum came back. It was still so intermittent made it difficult to trace. You could see the hum on the DC filament line.
I drew out the DC filament circuit for V1 & V2 preamp 12AX7's and identified the four diodes involved in the rectification of the 6.3Vac filament source. It was actually resolved jumping the 2 10Kuf filter caps but still wasn't right and it seemed very unlikely as new as this amp is that the filters would be bad - esp not intermittent filters.
I was able to identify with heat & cooling that one or possibly 2 of those diodes was intermittent. In retrospect that makes sense as they took a jolt from one of the EL34's that shorted Filament to grid. I don't usually test power tubes when the fuse is blown & it's obvious it was a power tube, but I have a newly obtained Hickcock 600A and just wanted to test it's accuracy on shorts.
Anyway, resolved for all to hopefully benefit from...Thanx for all the responses.

[Mars Amp Repair]

Intermittent buzz as described in the posts figured out. Sorry I forgot to post the solution to the issue I was having in first post.
I found D10 & D8 in the filament rectification section were intermittently shorted. I of course replaced all four diodes in that circuit D7-D10.
Also intermittent sound & tone is caused by the crimp connections on all the Alpha pots. You can just press on the front pcb with your chopstick to bring the issue to the forefront.
I merely squeezed the rivets with my small side cutters on the two outer rivets of the pots where they connect to the carbon traces. You have to be careful or you'll break the phenoalic wafer. Of course you could just replace all the pots but I believe it's the rather stiff ribbon connectors to the front pcb that put an inordinate amount of pressure on the pcb that is the cause.
There are also 2 screws on the main pcb that also ground certain points to the chassis. If loose will cause noise issues. I'd suggest adding star washers or small spit washers to them.

[Jazz P Bass]

Thanks for the update.
Especially on the poyentiometer rivets!

[SoulFetish]

Thanks for the update.
Especially on the poyentiometer rivets!

for a second I was like "what the F*@# is a poyentiometer rivet?" I get it now.

Anyway, just had one in with the same issue. the diodes in the DC filament supply were the issue. Important to mention that these tested okay out of circuit. It wasn't until some heat was applied that there was an audible difference in the buzz/hum. Replacing all 4 and the problem was solved.

[Jazz P Bass]

"poyentiometer rivet". (potentiometer rivet)
Sorry about that Chief.
It is a tad confusing.
I should have used the 'spell checker'.

[SoulFetish]

"poyentiometer rivet". (potentiometer rivet)
Sorry about that Chief.
It is a tad confusing.
I should have used the 'spell checker'.

no worries, boss.