Pulling in stereo radio on 500-C - very distorted

[thornev]

I'm having problems with pulling in stereo radio stations. I've read Dave's post about adding a 47K resistor to the MPX circuit in order to lower the stereo threshold detection circuitry. I will try that. I am also going to replace the 6GK5 tube.

The problem I have is that when I set the receiver in FM STEREO mode, ALL the stereo stations are incredibly distorted as if they're being over-driven. If I play with the little blue adjustment knob next to tube V102, that helps to reduce the distortion, but it also reduces the amount of stereo signal. (what is that adjustment for anyway? Oh.. the service manual says it's the MPX Separation Control, R215)

Has anyone else had this problem and how did you fix it? I'm new to electronics so please don't get too technical on me. Thanks, Thorne

 

[RS Steve]

I'm having problems with pulling in stereo radio stations. I've read Dave's post about adding a 47K resistor to the MPX circuit in order to lower the stereo threshold detection circuitry. I will try that. I am also going to replace the 6GK5 tube.

The problem I have is that when I set the receiver in FM STEREO mode, ALL the stereo stations are incredibly distorted as if they're being over-driven. If I play with the little blue adjustment knob next to tube V102, that helps to reduce the distortion, but it also reduces the amount of stereo signal. (what is that adjustment for anyway? Oh.. the service manual says it's the MPX Separation Control, R215)

Has anyone else had this problem and how did you fix it? I'm new to electronics so please don't get too technical on me. Thanks, Thorne

Start with making sure the tubes in that section are healthy, most problems I encountered like that turned out to be a poor functioning tube.

 

[gadget73]

There is also an electrolytic cap under the MPX unit that can go bad. The output gets distorted if that happens. Worth changing if you haven't.

 

[larryderouin]

Concur with Gadget. Replace the 1uf 350v electrolytic cap on the side of the MPX unit with a 1uf 400v film cap. Here's a pic of where it is. This is a very old pic showing an electrolytic on my 1st 400. The mpx unit in the 500c is identical. Replace the cap, then RESET the separation control. Rotate thru it's range. Determine which side of the pot the pointer on the knob points when the knob is at the end of travel and you have a distorted signal in one channel. That channel will also appear to get louder. Now back off the knob to 5:30 or 6:30, depending on which side the pointer is. This is close enough without a full alignment for separation.

 

[Dave451]

For stereo reception you need a well-functioning and aligned FM receiver (with all good tubes, as Steve notes) and the multiplex (MPX) decoder (which deciphers stereo from the broadcast 'multiplex' signal) must be functioning properly or stereo signals will be distorted. Certain failed components (like the ratio detector cap gadget describes) can also cause this problem.

The blue control you are twisting is, as you note, the stereo (MPX) separation control that adjusts the MPX decoder for maximum stereo separation between left and right channels. It's about the last thing that gets adjusted in the alignment process. Put it in the middle for now--it's not the problem.

So, as the guys say above 1) check tubes in the tuner section and multiplex decoder and replace bad, 2) if that doesn't do it, replace the ratio detector electrolytic cap (C85 8 uF), and if that doesn't do it 3) get the tuner aligned properly. If you have multiple bad tubes, an alignment may be called for anyway, particularly since you moved the separation control. Changing the stereo threshold resistor you mentioned likely won't do anything for this problem, but it can help the automatic stereo control turn on at lower signal strengths (I changed it on my 1800 and the Stereo Beacon now activates in automatic mode on more stereo signals).

Good luck with the 500C--outstanding receivers

 

[thornev]

Thank you, All. I've ordered everything that's been recommended.

Dave - When you say "get the tuner aligned", do you mean ensuring the FM station pointer is pointing correctly? Or is this some kind of bench procedure that only a professional can do? I am getting a limited amount of tools just for replacing parts (solder, desolder, etc.). I purchased 2 restoration kits so I plan to follow those instructions. Thorne

 

[larryderouin]

A Tuner alignment is analogous to a 4 Wheel alignment on a car. It takes a special procedure, with special gear designed just for that purpose and a tech that has been trained with that purpose in mind. Turning coils, and trimmers indiscriminately can throw it so far out of alignment that it can be a nightmare for the tech to get back in alignment. Part of the procedure is to align the tuning dial, and a small part of it. The rest is aligning the RF and IF sections to very specific frequencies so that it will actually capture, amplify and split the FM signal into a defined left and right signal that can be amplified by the audio section of the radio.

 

[Dave451]

What he said (well said, Larry!). Thorne, yes it takes bench equipment to do and careful work. Hopefully, looking at the tubes and replacing the ratio cap and adjusting the separation control as above will get you there. If not, you may need to have someone troubleshoot it further and align it for you.
Let us know how it works out.

 

[Pio1980]

I gave up on vintage tuners and receivers because I cannot align the FM sections. I have a 500T that is basically a door stop due mainly to FM problems.

 

[thornev]

Step 1: I replaced the three 12AX7 tubes with new Tung-Sol's. No good. As a matter of fact it made the situation worse - I got no stereo stations in FM AUTOMATIC mode. I read somewhere to leave those tubes alone if they worked and I guess that's right. On to the next step...

 

[notdigital]

My experience with FM reception and the Fisher receivers requires that you not only change the 1uf electrolytic but also the mylar caps buried within the module itself. If I recall there are 3. The mica caps and ceramics you can leave alone. After that, the MPX module should be aligned. It can be removed from the chassis and separately aligned from the rest of the tuner circuit. That should be done with good tubes installed. Once that is done you're 85 to 95% there. I've been satisfied with that level of performance except in my stand alone tuner which deserved the extra yard of attention. Obviously, if you want the entire tuner circuit taken care of, that would require finding someone with the proper equipment and expertise and that's a bit of an expensive proposition since you'd have to either lug or ship the entire receiver. Doing the MPX alone will be considerably cheaper IF you can handle the removal and re-installation yourself.

I'm in NYC and enjoy FM immensely so it was always worth addressing whatever deficiencies came with the several FM units I've been lucky to obtain.

Oh yeah: While these things were designed to use a simple dipole and such is perfectly adequate, nothing beats an external antenna.

 

[thornev]

Thanks, notdigital. (and THANKS everyone else who replied) I was raised in NYC... Allison Steele ! Scott Muni ! Johnathan Schwartz ! WNEW 102.7 ! I miss the days of "real" FM radio.

I do intend to replace as much as possible in the MPX circuit. I'd be happy with just pulling in some stereo FM radio. I don't need the perfection that alignment would give me and hopefully it won't be required.

Thorne

 

[gadget73]

I've got 3 of those MPX units, the only thing I did to any of them was change the electrolytic. The original mylar caps seem to be working sufficiently well that I didn't want to deal with it. One is in a Fisher Philharmonic chassis, one is built as a stand-alone external, the third is currently living on my bench until I build it into an external decoder.

 

[thornev]

gadget... Which electrolytic? All the capacitors in the MPX are electrolytic, aren't they? Oh, sorry, no they're not (my schematic is weird). Do you mean C210? Thorne

 

[larryderouin]

C210 is the 1uf 350v lytic sitting on the side of the MPX. It's fairly easy to get to.. Replace it with a film cap of your choice (1uf 400v). And replace the ratio detector lytic (8uf 50v) C85. A 10uf 50v is recommended.

 

[thornev]

Well I installed the 47K ohm resistor across resistor R204 and it indeed improved the receiver's ability to go into stereo mode. However, now I'm getting that distortion on any stereo station. So the situation is worse now in that I cannot listen to any stations in stereo. On to the capacitor replacement next.

 

[thornev]

I replaced lytic C210 with a 1uf 400V lytic and still I'm getting the stereo distortion. Sigh. I'll keep going with capacitor replacement in the MPX.

In case anyone is wondering what kind of distortion I'm getting... If you know what a Ring Modulator does to sound, that's very close to what this distortion sounds like.

 

[thornev]

Holy moly ! I was reading somewhere that switching the three 12AX7 tubes on the MPX can cause issues with FM so I decided to move them around and guess what ? That's what the distortion was ! I must have put them back the way they were because the ring modulator distortion is gone !! I am SO happy ! This is testing with headphones - it's 3:45am and I don't want to wake up the wife. But I assume if the problem is gone in headphones, it's gone in speakers. Woopy !

UPDATE - I need to qualify. The distortion is gone when the receiver is set to FM AUTOMATIC mode. When I set it to FM STEREO mode, just about every station is distorted.

 

[dcgillespie]

Glad you found it! -- But I must tell you that when operating correctly, the MPX sub-chassis is normally quite INsensitive to tube placement or replacement, as their individual characteristics just don't play into the operation of the circuit that much, versus how much some of the tubes can in the circuits of an FM tuner proper.

Now, this comment only refers to those instances when quality vintage American or foreign tubes are being used, as it's becoming more and more widely reported that some of the modern reissue Russian tubes are having trouble in some positions of the MPX sub-chassis (usually V100 or V101). So if you are using these types of tubes in the MPX section of your receiver, then it may in fact be quite sensitive to the placement of the tubes. If you're using vintage tubes however, then I would suggest getting them tested in a tube tester of known caliber, as if that is the case, you may simply be dealing with some well worn out tubes.

If the problem is gone in your headphones, then it will in fact be gone in your speakers as well, but in the mean time, get some sleep!

Dave

 

[thornev]

Well I guess I spoke too soon. Some stations still exhibit the distortion when the receiver is in FM STEREO mode although it's not as bad as it was. Hmmm. When in FM AUTOMATIC mode, all the stations that trigger stereo sound really good. Man this receiver is amazing sounding !