Fisher 400 PT hum

jvstratman

Well-Known Member
One of my 400's has a slight hum upon power up. It's immediate upon switching on. It doesn't get louder or go away. It's annoying between tracks.

What is the best way to troubleshoot/isolate components? This receiver doesn't employ the metal cover on the rear of the on/off switch. Could that have any effect?

I was contemplating lifting all secondaries to see if it dissapears but, don't know if that is safe for the xfmr.

Any thoughts/guidance is appreciated.

Btw, I've installed the surge protection component per sticky thread on both units. One hums and the other is dead quiet. Also, no rust at all on xfmrs.
 
My on/off switch doesn't have the cover too. I don't have the hum so it's something else. I always think "grounding problem". I'm not sure how to debug that. Look for loose connections among components for starters. BUT BE CAREFUL - electrolytic capacitors may retain a strong charge that can hurt you. Discharge them first or have a pro take care of it.
 
JV -- Assuming the hum is coming from the speakers (and not the transformer itself), does the hum appear immediately, as in instantly when the power switch is activated? Or does it take about 15-20" to appear after the tubes have warmed up? Hum that appears literally instantly can really only happen from magnetic coupling between the power transformer and output transformers, but when this happens (usually when the OPTs are located physically very close to the power transformer), it will be louder in one channel, in any event very soft (nearly impossible to hear from any normal listening position), and should in fact go away once the tubes warm up.

If the hum happens after the tubes warm up, then two questions:

1. Is the hum in both channels? Equal, louder in one, etc.?

2. Is the hum affected by the setting of the volume control at all? From your comments, I would think not, but just wanted to confirm.

Let us know!

Dave
 
The hum is instantaneous. It's actually coming from the PT. Screwdriver handle to ear confirms. Can I isolate/lift secondaries (one at a time) to see if it stops?

Volume control @ zero.
 
Other POI: Both utilize original 2 prong power cord which are in good shape with shiny prongs. I'm using the same outlet for both. I've rotated the plug in the outlet. No change in hum.
 
No amount of lead disconnection will solve this problem. The problem is either that the through bolts securing the lam stack are lose, the transformer is not securely mounted to the chassis, or the coil form has become lose in the center of the lam stack. If your transformer is riveted together, you might temporarily use some C-clamps to help compress the lam stack. If this helps, at least it proves the problem is due to the rivets loosening, rather than the coil form becoming lose. If the coil form is lose, then the end bells will need to be removed to gain access to where the wedge is that's inserted between the coil forms and the lams to tightly hold the coil form against the lam stack passing through it. If it is lose, taping the wedge inward slightly will act to tighten the coil form against the lams to minimize the mechanical noise.

Dave
 
I agree. I think it's internal. Found a stethoscope in the closet and am able to discern that it's more a mechanical buzz/rattle than electrical. I loosened the mounting bolts then I tightened the lam bolts and re-tightened the mounting bolts with no dicernable improvment. So, this wedge is located under one or both of the bells? Can I access the wedge without removing the xfmr from the chassis?

Yep, I know I'm using a stethoscope on a tube amp. I might have a slight OCD.
 
Hey -- old school on old school! I like it! Even if you can access the wedge without removing the transformer, I think you'll wish you had removed it, as it will likely be more work to do it that way, than if you had simply removed it.

Dave
 
One more question. Is there any harm in applying power to black primaries to test for fix while on the bench? All secondaries would be wire nutted/taped.
 
It would be best to apply and remove the power through a variac in that scenario, bringing up the power and dropping the power over a 5-7 second period for and at each event. Dropping the power instantly to a transformer that has no load attached to it can make for extremely high kick voltages in the secondary as the magnetic field collapses. In extreme cases, it can cause internal arcing within the transformer. Dropping the voltage gradually over the period suggested will prevent that from happening.

Dave
 
20180317_134345.jpg Well, I found and tighted the wooden shim. It moved easily. There was a considerable gap on the opposite side with no shim. I made and inserted one there too. It's a little quieter but, still noticable if you listen for it. Before, it was audible immediately, now you have to hunt for it so, it's a definite improvement.

Here's the patient. I made walnut sides for my 400's. They incorporate (removable) acrylic lids. I like to look at them as well as listen to them. I would really like to find some miniature struts so I could open them from the rear during operation. Any ideas?

Thanks Dave!
 
Here's a side view. I drilled/tapped the chassis and used 10-32 machine screws. I also dressed-up the SS monster I purchased new in the 70's. It'll peel paint...
 

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Phonograph top hinges mounted on the front. Add a catch on the back. Clear acrylic looks nice but still traps heat like wood or metal covers. I'd drill a pattern of ventilation holes in the top to release trapped heat when it's closed. From 3/8" in from the back edge to approx 1/3 to 1/2 the depth and side to side starting at 3/8" to 1/2" from edge. 3/16" holes 3/8" apart edge to edge. This will also keep little fingers out of it and intact. Kids with burned fingers are NOT conducive to marital harmony! And if it's grandkids it's worse as Daughter in laws 's can be as bad or worse than Mother in law's.
 
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Hi Larry, I read your thread regarding your health and have to commend you for staying strong. Your a tough man and it rubs off on others afflicted with illness/disease. It's difficult to maintain a positive attitude when uncertainty strikes.

Regarding the hinges you mentioned, much appreciated and looking into.

The ones attached look interesting too.
 

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View attachment 1139994 Well, I found and tighted the wooden shim. It moved easily. There was a considerable gap on the opposite side with no shim. I made and inserted one there too. It's a little quieter but, still noticable if you listen for it. Before, it was audible immediately, now you have to hunt for it so, it's a definite improvement.

Here's the patient. I made walnut sides for my 400's. They incorporate (removable) acrylic lids. I like to look at them as well as listen to them. I would really like to find some miniature struts so I could open them from the rear during operation. Any ideas?

Thanks Dave!


Could you tell me more about how you made the shim and out of what material? My PT buzzes on and off and will stop if I hold the metal end cap tight to the body.
 
The original appeared to be a dark wood but, maybe it got varnish dip and appears dark. I fashioned a wedge from a door shim I had lying about. Pine, I think. Good luck!
 
To clarify regarding the shim I used. It was from a pack of shims used for door jamb hanging from Lowes. Couple bucks for a pack.
 
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