Need help Fisher x100c buzzing

mp53

Active Member
Hello, Got through with rebuilding and modifying my Fisher except for setting up my feedback circuit in the amp section. I used 16 ohm resistors across my speaker 16 ohm jacks for testing . Had my connection open between my volume pot and input of my amp with the input grounded. So all my voltages were close to specifications when done and nothing getting too hot or smelling hot. Hooked up speakers and I get buzzing. If I connect up my volume pot it gets worse and starts running away. Also before it starts running away if I touch the chassis the buzz changes so I new I must have a hot chassis. With the amp off there is no short on my meter between any of the primary legs of my transformer and chassis. But with the amp on I get ac voltage on my chassis. The chassis voltage is different depending on what primary leg you test on. So if I run my primary up to about 60 volts ac , one leg will read like 45 volts and the other 15 volts to ground. Of course Im using a variac to do this. My thoughts on these readings means I must have a very slight short to ground on one side of the power transformer, probably inside the transformer touching the bell housing. Any thoughts is appreciated.
 
Initially my thinking was you had a leaky "death cap" but the 100C doesn't have one. You could install a .01uf 300vAC X2 Rated Safety cap between the AC lines before the switch. Try that for shits and giggles and see if the voltage drops down or goes away completely.

Check R-69, R43, R44, R33,R34, Plus a few others that are direct to ground if 10% off replace.

If you had an internal short to the case of the transformer the fuse would blow and it would never start up, buzz or Hum. More likely is an out of spec resistor or a leaky cap.

What exactly did you "modify" on the amp.
 
All resistors have been replaced and double checked before installing. Most capacitors have been replaced also except for the coupling capacitors in the preamp sections which are euro II types. I was very careful when building this amp and double checking myself.
I did have a capacitor set up like you suggested on the ac side like what the Fisher 400 has.
My modifications are 10k output transformers out of a Fisher 400, tube based screen grid regulator, 7199 driver tubes in the amp section using the RCA design. I also took out the original floating ground scheme for the speaker jack circuitry and deleted any headphone circuitry. My feedback circuit for the amp section in not installed with the proper resistor and capacitor. I have a scope and using my phone generator app for that.
Back to the main problem. I did some more measuring last night . This time with the power switch off and I'm inclined that the problem is a short inside the steel insulator tube the runs the ac current to the on/off switch. Will get ro it later today and report back.
 
It will be hard to help without a schematic of the modified amplifier or pics of its installation. I'm not sure what you mean when you say "before it starts running away" (oscillating maybe?), but it would seem to me that you're potentially describing either a grounding issue, shielding issue, or at least significant instability. Can you expand on the symptoms?

Dave
 
Talking about grounding then and before going to anything else, let me ask this question: Does the output transformers need ro be grounded to the chassis?
 
The core is grounded by virtue of being mounted. If you're referring to the secondary, it has to be grounded at some point if the NFB system is to work. Powered center channels grounded the 4Ω tap. Otherwise, the Com lead should be grounded.

Dave
 
That means I made a mistake because I insulated the output transformers with rubber grumets. So I will make a fix on that and report back.
 
Got the output transformers grounded and that fixed the running away symptom . But still have a buzzing. Now it is coming out the left channel only. I see another potential for the buzzing and that is the wire that runs from the preamp to the volume pot is uninsulated. I will fix that and see what happens.
 
Take and use a shielded cable. A RCA interconnect cable will do the job (Cut the jacks off the ends). Just don't ground the shield at both ends or you'll get a ground loop and a hum that will have you running in circles for a week(UNLESS THE ORIGINAL cable is grounded at both ends. If the other channel is fine duplicate the cable and mounting of the cable.).if it's grounded on one end only, on the ungrounded end leave no part of the shield exposed (heat shrink tubing) and expose the center lead no more than 1/2 inch.

Is the buzzing in ALL SELECTOR POSITIONS?? or Just one?
 
Ok , thats what I ended doing as you instructed because thats what Fisher had done for the right side channel. But still have the buzzing in the left channel after the fix.So I swapped the 7199 tubes. And buzz is now on the right side channel. With a small amount now coming from the left. So seams like I have a tube problem. But I have a hard time believing that because these don't have much time on them. I bought them NOS from Antique Audio in Tempe Arizona. Used them in another amp with no problem years ago.
 
Time doesn't matter with most of the tri-pent tubes. The darn things are just prone to heater cathode hum in the pentode section. No doubt, that is why Fisher tried them in two of their products (TA-600 and TA-800) and quite abruptly stopped using them. That, and in direct coupled circuits, any off tolerance in the screen grid in the pentode section can really upset the operating point of the triode section as well.......

Dave
 
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