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Fisher 500-C stereo beacon circuit

RS Steve

RS Steve

Tube Junkie
Subscriber
I passed on one of my 500-C receivers to a fellow AK member that I restored, and upon arrival to him the stereo beacon stays on at all times. Not sure what the problem is or if in the shipping something got jarred enough to cause the problem. It's even possible that I never noticed the issue, but unlikely. Having eight of these at one time and trying to find new homes for some make it a blur. I'm hoping it's something simple and can be fixed fairly easy. He did say that the beacon will go out when he pushes on the relay, but it must not be functioning on its own. The unit has had all the usual capacitors replaced, and tubes checked. Hopefully someone that's an expert with this circuit can chime in and help diagnose this. Obviously, I'm not in possession of the receiver, so I will try and get the person to chime in.
 
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Hi all,

I was the member who got this beautiful example of 500C from Steve.
Everything works great with the exception of the above mentioned hiccup.

The problem is with the stereo beacon and the associated FM stereo / mono switching.
It remains switched on from the moment power is applied. Doesn’t make a difference which one of the selector settings is applied.
When it’s in the FM auto or stereo settings, I can hear the mpx adapter doing its thing and when I depress the relay plate the lamp turns off and it reverts to FM mono. Doesn’t matter whether the 6gk5 is in or out of socket.
It even remains on when it is switched to the AUX input.

Lou
 
It could be the relay itself. Not sure if the relay is NO or NC. If the coil of the relay is bad or not getting power it will not engage. Relay coil disconnected should have more than 0 ohms (continuity), if not it is bad. Is it possible the relay is wired wrong? Does the unit come in and out of Stereo mode? (you should be able to hear it). Trying to determine if the problem is just the control of the lamp, or if MPX is not going in and out.

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Looks like the relay operates from current flow through v7. If I1 is the lamp, it would need V7 to conduct and pull the relay. It can't do that immediately on power up, so if the light is on as soon as you turn it on, I'd guess the relay is mechanically jammed in the stereo position

It also cant do that with V7 out

Other possibility, if the wiring to the plate of V7 is shorted to ground the relay will pull as soon as the power is switched on. If you can see tge relay move, check that.
 
It’s pretty much going to be the tube or the relay. Of course, a goofy problem with the tube socket contacts or an open R60 would do it as well.

Dave
 
Thanks all!
I’ll have at it tonight and see what’s up.
(1)Check the tube
(2) Check the continuity of the relay
(3) clean the tube socket
(4) check the resistor value

Fingers crossed….
 
Sounds similar to what mine is doing but from a cold start it is fine, but when it warms up the problem starts. I'll be taking it to the tech that did and excellent restoration on it-but it does have this hic-up. I changed the 6gk5 which did not help.
 
JHS2RT said:
Sounds similar to what mine is doing but from a cold start it is fine, but when it warms up the problem starts. I'll be taking it to the tech that did and excellent restoration on it-but it does have this hic-up. I changed the 6gk5 which did not help.
Click to expand...
Miss Sasha looks like one of our felines, miss Toes the polydactyl.

This one’s problem appears to be limited to the relay itself.
 
The coil will normally measure around 10K Ohms when good. If the coil is in fact open, there will be no plate voltage on the 6GK5 tube, either. Best to get one from a junker. They have shown themself to be quite durable over time, so an open coil is quite rare. Even if the 6GK5 developed a direct plate short (near impossible), it still wouldn’t take out the relay coil because of the series connected 10K dropping resistor limiting current flow. The original is of a class called a “Plate Relay”, which worked in conjunction with vacuum tubes. When tubes were phased out back in the 60s, so were plate relays since they were no longer needed.

Glad you found it!

Dave
 
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