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Fisher 500C miswire?

K

Kelly Moore

New Member
Hello AK’ers,

I purchased a 500C in decent condition. It is my first Fisher and I’m super excited to restore it.
I initially brought it up on my variac and got absolutely nothing. The current was reasonable.
I bought a restoration kit from Hifi Audio. It’s been convenient to have all the parts and nice step-by-step directions.

When I got to replacing the bridge and 1000uf caps, I traced the destination and found something confusing.
This is the circuit:

IMG_1252.jpeg

I started tracing down the V15 and V14 line and found the wire at V14 pin 4 grounded.

You_Doodle_2026-10-19T01_26_42Z.jpeg

This would ground the heater voltage on V15. I don’t think the tube would pass any signal without the heater.
It looks like it hasn’t been touched and like this came from the factory. I’m not sure how this could’ve been possible.
V15 is the L-R high filter amp.

If I clip the wire short, it fixes the problem but swaps position of V14 and V15 on the schematic. But this is weird.

I looked out online for some hi-rez photos of the area to see if this was normal. One I saw kinda looked the same as mine and another looked like the white wire on V15 pin 5 was moved to pin 4 which would deliver voltage to V15.

What do you all think? Do any if you have pictures of this area on your 500C’s?

Thanks for your input,

Kelly
 
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All looks normal to me. The link you are referring to in your pic is not actually grounding pin 5 of V15, but merely connecting the center post of the socket to pin 5. Since the heater system represents a very low impedance, the link then effectively "grounds" the post relative to any hum and noise that no connection to the post might otherwise create. The center post then actually represents a potential of about -12 vdc due to its connection to pin 5 of V15. Over at V14, the same link connects the center post of that socket to pin 5 as well, but at that socket, pin 5 is also in fact connected to ground by way of the chassis ground lug located near by (somewhat hidden in your pic). Because both sockets include this link, both sockets "appear" then to have pin 5 grounded but in fact, only pin 5 of V14 actually is.

I hope this helps!

Dave
 
Hey Dave!

Wow, thanks for this. I guess I was assuming that the center post was grounded maybe because I saw it grounded on other sockets. I thought I checked it but I will definitely ohm this out and confirm for myself what you are describing.

I'm looking forward to bringing this up. I have finished installing all the electricals and just have bulb replacements remaining. I have checked all the tubes and all but 1 are strong. I believe that initial powerup should be all the tubes, except for the 7591's, check all power levels. If good, power down, install 7591's, connect speakers, and bring it back up on the variac.

Here we go! :banana:

Kelly
 
Hey Dave,

I saw another thread on AK about the installation of 100 Ohm Screen Stability resistors at each output tube socket. Is that something I should do before trying to bring it up on the variac?

Like this?

IMG_1254.jpeg

Kelly
 
If the original power supply caps are still in place (and of course still good), you can go ahead and bring the unit up on a Variac with little concern for the tubes. But if new power supply caps are in place, I would highly recommend installing Screen Stability resistors before powering the unit up.

Dave
 
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