Getting the Most From Fisher FM Stereo MPX Tuners and Receivers

[Sam Cogley]

As a point of order, I believe the 800B receiver did use the WX decoder. I'll check mine on Sunday.

It would be interesting to see these tests on a later generation MPX-100 external decoder.

 

[fred soop]

When restoring these vintage receivers and tuners, there is a new consideration that I have not yet seen mentioned. The multiplex adapters have filters for 19 kHz, 38 kHz, and 67 kHz (SCA). BUT, today, there are also subcarriers at 57 kHz and 92 kHz. So, for the best sound, the 67 kHz trap should really be replaced with a steep 55 kHz low pass filter.

 

[Sam Cogley]

Verified: my later 800B (output tubes behind the OPTs) has the WX decoder with 12AT7s. The parts 800B that I had used the same decoder.

 

[streetwise]

Dave what resistor is the 2.7k 2watt you mentioned changing. It was R123 on your 800c I believe. What is it on the late model 400? is it worth changing ontop of your other mods?

 

[dcgillespie]

Depending on which version of the 400 you have, the equivalent resistor in the 400 is either R104 for the early version, or R40 in the later version. As to whether you need to change it or not, all you can do is check the B+ voltage going to the phase inverter circuits versus the schematic for that point. If the measured voltage is significantly higher than that shown on the schematic, then changing out the original 1.2K for a 2.7K 2W piece would likely correct this. But much later versions used a power transformer producing lower overall B+ levels, so in those versions the change may not be required.

I hope this helps!

Dave

 

[streetwise]

Depending on which version of the 400 you have, the equivalent resistor in the 400 is either R104 for the early version, or R40 in the later version. As to whether you need to change it or not, all you can do is check the B+ voltage going to the phase inverter circuits versus the schematic for that point. If the measured voltage is significantly higher than that shown on the schematic, then changing out the original 1.2K for a 2.7K 2W piece would likely correct this. But much later versions used a power transformer producing lower overall B+ levels, so in those versions the change may not be required.

I hope this helps!

Dave

Dave I am low where the schematic says
435v i am at 395v
390v is 349
355 is 323
285 is 310
and 210 is right on 210?
This is with the EFB and IBBA
What do these reading mean?

 

[larryderouin]

All of the readings are consistantly just under 10% Low. Fisher allowed a 10-15% variance on voltages due to differences in individual components. These voltages correspond to an input voltage of117vac at the fuse/cord. As long as all voltages are consistantly off you should be ok. 99% of my gear is between 10-15% off at maximum and does not affect operation. With the voltages being low, a line voltage increase would increase the voltages slightly but they would still be in spec. As far as the IBBA and EFB is concerned, Dave would have to address that, but I don't believe it would be a negative development.

 

[streetwise]

Thanks Larry everything is performing fine it just seemed consistently low

 

[dcgillespie]

Street -- The 435 volt B+ shown on my schematic is simply what my unit develops when operating straight from the line voltage here at my location, which is typically around 121 vac most of the time. If your unit is only producing 395 vdc at that point, then your line voltage may be lower, or you may have a current limiter installed, which my unit did not at the time. Also, it seems that some of the later units had power transformers that intentionally produced a slightly lower B+ level, as discussions between Larry and myself have noted.

Regarding the numbers you posted:

1. I'm not sure what the 390 volt point is that you posted, as it doesn't appear on any of my schematics, or my modified schematic of the power amplifier circuit.

2. The 355 volt point reading only 323 volts is consistent with the lower overall B+ from the power supply.

3. May I assume that you inverted the numbers for the 310 volt and 285 volt data? That you're intention was to state that the 310 volt point is indicating 285 volts? If so, that too would be entirely consistent with the lower overall B+ from the power supply.

Question: What is the voltage that the EFB board is supplying to the screen grid terminals of the output tubes (pins 1&7)?

Dave

 

[streetwise]

Street -- The 435 volt B+ shown on my schematic is simply what my unit develops when operating straight from the line voltage here at my location, which is typically around 121 vac most of the time. If your unit is only producing 395 vdc at that point, then your line voltage may be lower, or you may have a current limiter installed, which my unit did not at the time. Also, it seems that some of the later units had power transformers that intentionally produced a slightly lower B+ level, as discussions between Larry and myself have noted.

Regarding the numbers you posted:

1. I'm not sure what the 390 volt point is that you posted, as it doesn't appear on any of my schematics, or my modified schematic of the power amplifier circuit.

2. The 355 volt point reading only 323 volts is consistent with the lower overall B+ from the power supply.

3. May I assume that you inverted the numbers for the 310 volt and 285 volt data? That you're intention was to state that the 310 volt point is indicating 285 volts? If so, that too would be entirely consistent with the lower overall B+ from the power supply.

Question: What is the voltage that the EFB board is supplying to the screen grid terminals of the output tubes (pins 1&7)?

Dave

The 390v point is between R40 & R41 on the schematic.
Yes i must have inverted the 310 and 285,
The voltage to pins 1&7 are all within .5 of 297.5v

 

[notdigital]

Just sliding in here for a quick question: Dave, in post 12, point 4, you mention changing out a ceramic capacitor from .0027uf to .0016uf in order to correct the de-emphasis value. I'm finishing up the recap of my FM-200-B and since I'm in there, I thought I might as well. What component number is it? The unit is 64,XXX. Thanks!

 

[dcgillespie]

Street -- OK good. It sounds like everything is normal. The reason I asked about the 390 point is that if that is the point between R40 and R41 (I don't have any 400 receiver schematic showing that voltage level at that point), then with the installation of EFB, it is now irrelevant, since it doesn't power any portion of the receiver anymore.

Not -- C84.

Dave

 

[notdigital]

Damn.....looking through the junk box: the closest I have is .0018. Can do??

Never mind. C-84 in my unit is a polystyrene. Oh well....

 

[larryderouin]

Not: IIRC, changing R111 to a value of 27K will give you 72.9us and 27.5K will give 74.25us. You won't be able to hear the difference tho. 27k would be easier to implement.

 

[notdigital]

Not: IIRC, changing R111 to a value of 27K will give you 72.9us and 27.5K will give 74.25us. You won't be able to hear the difference tho. 27k would be easier to implement.

That's for mono??

 

[larryderouin]

yeah, mono. stereo would be on the mpx board.

 

[notdigital]

Gotcha! Thanks Larry.

 

[streetwise]

sorry to jump threads dave, The question was meant for this one. What Im asking about is the MPX modification switching from 12AX7 to 12AT7 with the other parts you had mentioned for the B+ voltage. Does the MPX need to be realigned if I switch from 12AX7 to 12AT7?

Also side note did you ever get to the dancing eye in the Fisher 400 I love your modifications and always looking forward to possible future steps!

 

[dcgillespie]

Ah! OK. This thread sort of supercedes the first thread, as I had done more developmental work with the adapters by the time this thread was generated.

So to answer your first question, the answer is still no -- no alignment of the adapter is required with the tube and B+ change.

As for the dancing eye tube -- I'm happy to post that information as well, but I guess I was waiting to see how much of a "problem " it really is. I live out in the country, with most of the big Atlanta transmitter sites being located typically 50-60 miles away. There are closer ones for smaller (i.e. lower power) stations. But so often their programming is a talk format, so that stereo reception matters little. Also, my lab is in the basement at the underground end of the house, and in it, I only use a couple of simple elevated dipole antennas on a PVC form that don't even reach to the ceiling (so they are actually underground as well). As a result, reception is hardly ideal in my location, with many a (properly operating) Fisher tuner/receiver displaying the dancing eye tube syndrome when set for stereo reception. For the non-eye tube units that employ the FM Automatic stereo switching feature, the dancing eye tube issue manifests itself as the unit having a hard time (if it even can) locking itself into the stereo mode (Stereo Beacon indicator lamp ON). But, I don't know if others AKers are so disadvantaged reception wise as to even have the problem that I have. Without knowing, that's why addressing this issue fell so low on the to do list -- but it might be interesting to hear some responses from those so interested if they do.

I do know however that if you look carefully, virtually every Fisher 500C/800C receiver I see listed on the auction site that shows it operating with a strong station tuned in (good tuning meter indication) and has the Stereo Beacon lamp lit, virtually always has the selector switch turned to the FM Stereo Filter position -- a position that forces the unit into stereo mode, disabling the automatic switching feature, and therefore forcing the Stereo Beacon lamp to stay lit. Maybe there are others out there with the same problem?

Dave

 

[streetwise]

My late 400 has a dancing eye on any stations with music other than the public radio stations that have a much stronger transmitter than the general clear channel and other privately owned stations. I live in the adirondack mountains in northern new your and use a cheap 300 ohm wire dipole antenna and am between 20-50 miles from any radio stations. This arrangement is as good or better receptionwise than any car radios i've experienced with there limited bandwith so I would say the tuner is working well but that little eye really likes to dance. Im also considering building or buying a folded dipole antenna to see if there are any big improvements, if anybody knows of a great antenna to build or buy let me know!