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Post subject: Crackly hum after Showman SF AB763 restoration
Posted: Tue May 03, 2016 12:07 pm
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Guys, thanks for all the advice so far.
Last Sunday I was able to finally power up the amp on a dummy load and current limiter. Everything is in working order and sounds amazing! This was a pretty big restoration project so I'm very relieved that it went as well as it did.

All new filter and bypass caps, power tube sockets, bias board, bias pot and new tubes all around.
The amp sounds great overall but when I take it out of standby there's a buzz/hum that I cannot seem to isolate. I cant tell if its a 60-cycle hum or buzz from somewhere else in the circuit. Its on both channels and gets louder as the volume increases.
In the video you can hear what's going on.



Any thoughts on where I might start probing?


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Tue May 03, 2016 1:33 pm
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Based on the symptoms there are three areas that should be investigated -- the power supply, the phase-inverter stage, and the power amp. With the latter, check that the blue and brown wires supplying plate voltage to the output tubes were not reversed when the chassis was re-wired. The blue wire should be powering V5 and V6 while the brown wire should be connected to V7 and V8.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 1:35 am
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oKors wrote:
. I cant tell if its a 60-cycle hum or buzz from somewhere else in the circuit. Its on both channels and gets louder as the volume increases.
In the video you can hear what's going on.



Any thoughts on where I might start probing?


Not big noise.
Problem is on the circuits before volume.
I'd check lead dress
I'd replace all plate resistors


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 8:31 am
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Okay,
So last night I pulled the chassis and put it in the rack again. Powered it on with the speaker cab and probed around a bit. I noticed the buzz is independent of either volume pots and if I turn one up and the other down, the buzz would increase the closer my hand got to the chassis; specifically directly over the knob. Turn the one down and the other one up and same thing.. like a Theremin. Obviously touching the chassis directly over the pot had the most dramatic effect.
Nothing obvious in the circuit, or wiring, including the power supply portion.

Here's another video for clarity:


So now the weird part. Powered everything down, drank a beer, came back about 20 mins later. Powered everything back on again and nothing... practically dead quiet (mind you, I had done multiple power cycles throughout the evening). At full volume there's a bit of a hiss on the Normal channel but that's it. Vibrato is near silent. No buzz or hum or any of the noise from earlier.

I'm not a fan of problems that go away on their own so I turned it on again this morning and its still quiet.

Rewind a bit; while probing around I did find an old cold/cracked solder joint at the end of the red wire tying to the two 68K resistors on the Normal input jack. When I rubbed the wire insulation with my chop stick I could hear it through the speaker. Some flux and a reflow with good solder cleaned that right up. This is had no affect on the buzz since the buzz was still present after the fix.

Not sure what's going but hopefully it sorted itself out.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 10:23 am
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oKors, check the soldering of the ground points on the brass grounding strip, under the pots. The grounding wires should not be touching the chassis, but go directly to the brass strip.

Also, check any grounding wiring or resistors tied to the back of any pots. Pots should be snugged up against the chassis. Tighten the nuts, if need be. Tighten to only 10-12 foot pounds.

Good luck!


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 11:50 am
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I see your amp on video , lead dress is poor. Many wires may act like antenna and make noise.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 12:38 pm
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stratele52 wrote:
I see your amp on video , lead dress is poor. Many wires may act like antenna and make noise.

Can you be specific?
I understood the importance of lead dress in these amps going into this project, which is why I've literally taken hundreds of photos of every detail at every angle prior to even plugging in the soldering iron. With excruciating detail I put every bend, twist and lay back into each wire I pulled up or replaced. If the wire from pin 7 went under pin 1 and over pin 8's wire, that's how I routed it back together. I even measured any old/charred wire before replacing it with the new.
With the exception of the Mercury Magnetics power transformer, if the lead dress is poor its because it came that way from Fender.

If you can provide any specific lead dress issues you spotted on the video I'd be happy to take all that into consideration. But to say its "poor" and leave no details is unhelpful to me.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Wed May 04, 2016 1:38 pm
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oKors wrote:
stratele52 wrote:
I see your amp on video , lead dress is poor. Many wires may act like antenna and make noise.

Can you be specific?


Sorry I can't explain here, how which lead must be run to have a quiet amp. I see very well your amp need some lead / wire dress.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Thu May 05, 2016 8:44 am
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Powered up the amp again last night and still no ground hum.
I decided to reflow the filter circuit ground wire on the brass strip just because it looked a little cold. No change in hum either way.

For now I will just leave it. The Normal channel has a slight hiss at max volume. The Vibrato channel is noticeably quieter but also sounds better than the Normal channel so I'm likely to play out of that side anyway.

But speaking of Vibrato, when I have it enabled and the Intensity turned up is it normal to hear an audible pulse sound, "wub-wub-wub-wub-wub..."?
There's no doubt its working, I just don't know if its normal to hear the oscillation like that or if it should be dead quiet and ONLY affect the instrument signal.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Thu May 05, 2016 9:36 am
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For the ghost buzz, clean the Input jack switches
Tighten the Input jack nuts Speaker jack nuts too.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Thu May 05, 2016 2:32 pm
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TimsAudio wrote:
For the ghost buzz, clean the Input jack switches
Tighten the Input jack nuts Speaker jack nuts too.


Sometimes the contact stay open, not easy to fix and most of the time I put a new input jack


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Fri May 06, 2016 4:55 am
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"For the ghost buzz, clean the Input jack switches
Tighten the Input jack nuts Speaker jack nuts too."

I've been wanting to address this issue to amp jacks as well as pedal jacks.
Short of disassembling units to clean jacks.....is there something that can be coated onto plugs that keep jack contacts conducting properly?


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Fri May 06, 2016 5:01 am
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ckmckool wrote:
..is there something that can be coated onto plugs that keep jack contacts conducting properly?


Not in my opinion, coating on a new jack last long time. Add coating , I neverd heard about product.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Fri May 06, 2016 5:38 am
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stratele52 wrote:
ckmckool wrote:
..is there something that can be coated onto plugs that keep jack contacts conducting properly?


Not in my opinion, coating on a new jack last long time. Add coating , I neverd heard about product.

I was thinking in terms of a male plug (cleaning plug) you could insert to clean jacks.
Probably no such thing but never hurts to ask.


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Post subject: Re: Crackly hum after Showman SF AB763 restoration
Posted: Fri May 06, 2016 5:50 am
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ckmckool wrote:
stratele52 wrote:
ckmckool wrote:
..is there something that can be coated onto plugs that keep jack contacts conducting properly?


Not in my opinion, coating on a new jack last long time. Add coating , I neverd heard about product.

I was thinking in terms of a male plug (cleaning plug) you could insert to clean jacks.
Probably no such thing but never hurts to ask.


These contacts can't be clean to last. it was a fine protective metal coat gone with years and use. They could stay "clean " if you use them very often, in and out. No warranty.

The switch on input jack, the shorting contact to ground use to keep amp quiet may need to be clean with contact burnishing tools or paper money.
I see sometimes those contact stay open and unable to fix them.

Best ( and often the only way ) is to replace the complete jack with new one , Switchcraft brand


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