Weird problem with Fisher 500 C radio

[robbarrow]

You're right, I didn't believe it either. I did switch between the various settings on the selector switch. I started on aux then FM mono then FM stereo auto and FM stereo mpx and around to the phono settings and noted a small drop when a radio setting was selected. I switched clips from the micro clips to the larger ones and checked the voltage of an old battery which came in at 1.48 volts dc. I think I have to believe my measurements at this juncture. Pin 4 readings were steady at .003 volts dc and 7.18 volts ac so the filament is hot. I cleaned all of the selector contacts with deoxit and a lint free applicator or three. Initially when I switch it on, the readings are all over the place from above 300 volts dc and then jumping between 1.0 something on aux and phono choices and then between .8 and .9 on radio settings. I just repeated these reading without significant differences. I also measured the resistance in situ of R37 and got 3.43K. I have added an IBAM to the system since those first measurements an the current across each of the power tube grids was 34ma at zero volume. The B source voltage is at 26volts at attachment to the negative side of the capacitors. This is with my variac at 120. The unit sounds great when I run my phone through the aux.

 

[dcgillespie]

When the unit is set up for FM operation, I would start at R37 and work backwards towards the power supply to find out where the voltage first starts to appear. Then you'll know where to concentrate on to find the problem.

Dave

 

[robbarrow]

I now believe this is the area where the problem lies. The indexed photo does not reveal the locations of the MPX connections. I believe this small board contains those connections. I assume that the small diode floating out in front of the board is CR100. I would really like to identify R205 because I believe a voltage measurement here might answer the voltage issue.
I did go through this area replacing capacitors and might have connected something wrong.
I had to put the jumpers on the diode because I clipped one of the leads removing a capacitor. Probably should replace the diode. Help?

 

[thornev]

I believe the 4 diodes are on the board side that faces the MPX. Only R89 is on that side. The other side is all resistors IIRC. I just buttoned up my 500-C so I can't look.

C100 should be at or connected via wire to pin 3 of tube V3.

R205 is in the MPX. To find it, find IF transformer Z101 (it's the shorter of the 2) and follow 1 of the 3 leads to R205. R205 is RED-RED-YEL, should be easy to find.

If you took a diode out, yeah, you should definitely replace it.

EDIT: See if my latest PHOTOFACT helps with location:
https://www.audiokarma.org/forums/i...ate-model-project.912239/page-4#post-13851090

Keep in mind that I did my best. Check my ID's against the parts in your 500-C to verify correctness and please let me know about any mistakes.

 

[dcgillespie]

Rob -- The board you are showing in your pic is not part of the MPX unit, but is the diode matrix board with CR1-CR4 located on the back side of it relative to the picture. This circuit supports the FM Automatic feature that allows for automatic switching between FM Stereo mode, and FM Mono mode. The MPX unit is the sub-chassis located behind the matrix board in your picture. R 205 is in the MPX sub-chassis and is a 220K resistor that connects to pin 6 of V100. The other side of this resistor is fed from the same switched power source that provides power to the RF front end on the other side of the chassis.

In this case however, you really only need to look at the T-Strip where R36 and R37 attach to. Both of these resistors are fed from the same power source, with R37 supplying power to V1, and R36 supplying power to both V2 and V18. Here is the bottom line of the bottom line: If the point where R36 and R37 connect together at doesn't have any power, then believe me, your FM section would be dead as a door nail, no ifs, ands, or buts. It means that the RF Amplifier, Oscillator, and Mixer tubes would all be dead, which represents ALL tubes in the RF sub-chassis. The only power those tubes would be receiving is heater power, and heater power alone won't allow the RF section to operate. But one of the basic premise we've been operating on is that the FM section in fact works well enough when V1 is removed, so if that is the case, then there must be B+ voltage at the point where these two resistors connect together (at least 160 vdc as indicated on the schematic when an FM function is selected). If that is true, and yet there is truly no power at pin 5 of V1, then it means that either R37 is open, L4 is open, or there is a bad connection associated with these components. C105 is a feed-through cap that R37 connects to on the outside of the RF Can, and L4 connects to on the inside of the Can. I suppose it's possible that the feed-through terminal has gone open in C105, but that is only a very remote possibility.

You're quickly getting into that realm I describe as "Something you think or know to be true, is not, or visa versa". When you get to that realm, it can really foul up troubleshooting efforts as then you are operating on faulty information. If that's where you are, then I suggest you go back and reprove (or not) everything (and I do mean everything) that you think is true in terms of facts that you're operating on. It's the only way to untangle such messes. I've been there enough to know!

Dave

 

[robbarrow]

Rob -- The board you are showing in your pic is not part of the MPX unit, but is the diode matrix board with CR1-CR4 located on the back side of it relative to the picture. This circuit supports the FM Automatic feature that allows for automatic switching between FM Stereo mode, and FM Mono mode. The MPX unit is the sub-chassis located behind the matrix board in your picture. R 205 is in the MPX sub-chassis and is a 220K resistor that connects to pin 6 of V100. The other side of this resistor is fed from the same switched power source that provides power to the RF front end on the other side of the chassis.

In this case however, you really only need to look at the T-Strip where R36 and R37 attach to. Both of these resistors are fed from the same power source, with R37 supplying power to V1, and R36 supplying power to both V2 and V18. Here is the bottom line of the bottom line: If the point where R36 and R37 connect together at doesn't have any power, then believe me, your FM section would be dead as a door nail, no ifs, ands, or buts. It means that the RF Amplifier, Oscillator, and Mixer tubes would all be dead, which represents ALL tubes in the RF sub-chassis. The only power those tubes would be receiving is heater power, and heater power alone won't allow the RF section to operate. But one of the basic premise we've been operating on is that the FM section in fact works well enough when V1 is removed, so if that is the case, then there must be B+ voltage at the point where these two resistors connect together (at least 160 vdc as indicated on the schematic when an FM function is selected). If that is true, and yet there is truly no power at pin 5 of V1, then it means that either R37 is open, L4 is open, or there is a bad connection associated with these components. C105 is a feed-through cap that R37 connects to on the outside of the RF Can, and L4 connects to on the inside of the Can. I suppose it's possible that the feed-through terminal has gone open in C105, but that is only a very remote possibility.

You're quickly getting into that realm I describe as "Something you think or know to be true, is not, or visa versa". When you get to that realm, it can really foul up troubleshooting efforts as then you are operating on faulty information. If that's where you are, then I suggest you go back and reprove (or not) everything (and I do mean everything) that you think is true in terms of facts that you're operating on. It's the only way to untangle such messes. I've been there enough to know!

Dave

Thanks so much Dave. Part of the problem is now solved. I found a cold solder joint at C91 where the voltage for this comes from. I found plenty of voltage at the capacitor but less than a volt (like I was finding at R37 and at pin 5) one wire later. An ugly solder joint later the radio works. The stereo aspect of it is still non functional. I didn't actually remove that diode but I had to use jumpers as seen in the photo to reinstall it. There is a lot of interference but I think that is likely from the flourescent lighting in the shop. Any clues about the stereo thing?

 

[dcgillespie]

Rob -- The component you have extended the leads on is not a diode, but is C68, which is part of the mono (only) de-emphasis network -- meaning it only functions when the unit has been triggered into FM Mono mode.

OK. Am I understanding you to say that when the unit is operating in FM Stereo mode (either triggered, or forced), there is no sound at all???

And to finish up the previous issue, is the FM tuner now working properly in FM Mono mode with V1 installed?

Dave

 

[robbarrow]

The radio is working in FM mono and FM auto modes. It does not tune in stereo stations although that is pretty much all there are in this area - at least not in stereo. There is no sound in Stereo mode. V1 is installed and appears to be working as it did not before so I deduce this is a step in the correct direction.

 

[dcgillespie]

OK. So FM Mono works properly, whether it is forced into that mode, or it automatically switches to that mode, but there is zero sound in Stereo mode -- whether in forced stereo mode, or automatically triggered. The problem will either be in the diode matrix switching circuit (possible, but doubtful), or in the MPX unit. If the unit is automatically switching in and out of stereo mode as the receiver is tuned across the dial, then V100 and V101 are likely operating OK, so concentrate your efforts on V102. If the unit is not switching in and out of stereo mode as you tune across the dial, then concentrate on V100 first, and then V101.

Dave

 

[robbarrow]

Of course since my post yesterday, I have discovered further issues. After I got the radio working, I put the cover back on the coil area and replaced the bottom plate and turned it right side up. Now the signal (with antenna still attached) is unintelligible if arguably still present. I'll have to turn it back over and remove the steel plates to see if my "repair" is still functional! With all the steel plates removed it is still mostly non functional until I attach the multimeter lead to pin 5 of V1...