Buzzing Traynor YRM SC1

knuckleberryfinn

I'm stumped.  Did my first gig with the Traynor I got off here about a year ago (it's done rehearsals, but never been pushed) and now it's buzzing like a mad thing.  But there's something weird going on...

With nothing plugged in, there's no buzzing.  With a lead plugged and no guitar, there's the usual buzzy hum.  With a guitar on the end of the lead, volume down: nothing.  But with the volume up, there's a very loud saw-toothed buzzing.  This lessens when touching the strings, and if I move the guitar.  It happens with all my guitars, and irrespective of humbucker or single coils it's the same volume.

I've pulled the valves one by one.  With V1 out, the buzzing is reduced dramatically, but it is still there in the background.  With the PI pulled, it disappears completely.  These results don't change if the reverb and tremolo valves are pulled too.  (I've tried changing the valves for known good ones too).

The voltages appear to be fine.  The PI is balanced.  I haven't been able to change the power valves as I have no spares.  The filter and bias caps have been changed recently, and it's a professional job.

I am a little bit worried this is an OT problem, though I'm not sure why I think it is!  There is quite a lot of acoustic transformer noise with the amp in standby, so I suppose that's why I think it might be the OT.  But surely it's not if the guitar being plugged in is what is causing the buzzing!

So, as I said, I'm stumped.  Can anybody help?  Here's a schematic in case it's useful:

https://drive.google.com/file/d/0BwSlpEBYJ9ved2lOSEt2YTlhUGc/view?usp=sharing

knuckleberryfinn

Also, it's definitely not the lead. I've tried others, and other amps are fine. Also it doesn't change if the amp is moved or plugged into a different mains supply. Any tips? I really don't want to get the old oscilloscope out the cupboard and attempt to learn how to use it again.

DJH83004

I'm afraid, its going to be down to some good old fashioned fault finding on this one. What I can say is that on all the Traynors I've worked on over the years I've never had a transformer issue, they are over sized and rock solid and a great manufacturer (Hammond generally). Unfortunately removing the PI doesn't prove a lot and can easily take you down a wrong path. So sig gen on the input, nice clean 1kHz sine wave and check with the stage by stage where it starts to go horribly wrong - but don't crank up the gain, keep it sinusoidal. Leaky stage coupling / decoupling caps are an obvious one, but also don't discount something as simple as a grounding issue. Just basic stuff really, but not easy to diagnose remotely. 
       

knuckleberryfinn

Ah okay, well I suppose that's better than a known expensive fault.  Thanks for the reassurance on the OT @DJH83004 .

It does sound and act like a grounding issue, but I just don't know how that could occur simply by it being played through (and not tampered with).  A cap having gone bad sounds likely - so I'll drag the old oscilloscope out of retirement.  Will report back...

ICBM

If moving the guitar changes the noise and turning the guitar volume down to zero stops it entirely then the noise is coming from the guitar not the amp, and if the guitar wiring is OK then it's external noise being picked up.

If there is any fault with the amp it can only be that it isn't earthed properly which is allowing the noise to appear. Check the continuity from the earth pin on the mains plug to the chassis and to the ground contact on the input jack.

ecc83

ICBM said:

If moving the guitar changes the noise and turning the guitar volume down to zero stops it entirely then the noise is coming from the guitar not the amp, and if the guitar wiring is OK then it's external noise being picked up.

If there is any fault with the amp it can only be that it isn't earthed properly which is allowing the noise to appear. Check the continuity from the earth pin on the mains plug to the chassis and to the ground contact on the input jack.

+1 Some older designs used a 'handy' transformer mounting screw for the chassis earth and I had a case on another forum* where this had worked loose. I believe this is now illegal? Earth conns' HAVE to go to a dedicated fixing?

*This was complicated by the fact that a lot of ancillary gear was involved and an earth loop was suspected. Turned out that the fault worked 'the other way' and it was the LACK of an earth that was the problem not too many!

Dave.

shaunm

I assumed from the title thread you were from Manchester and really loved your amp.

knuckleberryfinn

Ha! I do love it, and I used to live in Manchester. So I suppose you're right.

To my shame I discovered that the mains earth had slipped out of the pin. This possibility did occur to me in a lucid moment a few days ago but I promptly forgot it and failed to check. Bloody dangerous! But there's a lesson there: check the simplest stuff first.

Legba

Old(ish) thread I know... but having worked on several Traynors; it pays to check the tightness of all earth points, check that the input sockets and pots are all tight and earthed on the front and back panels,  Another favorite is loose valve socket pins; use a fisherman’s hook pin behind the valve socket inserts and tension them to the middle so that they 'grab' the valve pins firmly when inserted.  These things relate to all vintage valve amps and worth doing as preventitive mainternance. 

Oh, make sure that the amp is switched off, plug removed from mains and that the HT voltage is dissapated.  Valve amp voltages can kill and can still be active even when the amp is switched off!