New to me Fisher 400 - first assessment

[dcgillespie]

I believe you are describing what sounds like an external MPX-70 MPX unit. It is indeed a very similar unit to the MPX-65 used in the 400 receiver, although there are significant differences you would need to address in swapping it into the 400 receiver. In the MPX-70:

1. There is typically only one B+ feed (245 vdc) to the MPX-70 rather than two as in the MPX 65, with an internal dropping resistor included in the MPX-70 to create the second lower voltage B+ source internally within the MPX-70. It is decoupled with the second 1 uF cap you noted. If you leave this feature of the MPX-70 in tact when installed in an MPX-65 setting, it means that the 38 kHz oscillator and 19 kHz pilot amplifier are drawing their current from the same B+ source that the line stages operate from and will drop that voltage source accordingly. So, you may need to eliminate the added dropping/decoupling components in the 70 to then split it's two MPX B+ circuits, and then connect the two separated high and low B+ voltage rails into the 400's B+ connections in the same manner that the 65's B+ connections were made.

2. In the MPX-70, there is no noise detector circuit included, so it has no ability to distinguish the difference between when a 19 kHz pilot signal is present, or when not. As a result,

3. The MPX-70 also does not have an indicator output for triggering the eye tube when a stereo MPX signal is being received.

The components for both the noise detector circuit and the indicator output would need to be swapped over from your MPX-65 to allow the 400's eye tube to work correctly. If only the indicator output is added but the noise detector is not, the eye tube would always remain closed in either stereo setting of the input selector switch whether a station is tuned in or not, since the presence of noise or a 19 kHz pilot tone would act to close the eye tube's beams. Therefore, both of these circuits will need to be added to the MPX-70 for the eye tube to work properly.

4. Due to operating into different intended input impedance levels, there is a slight difference in the value of R222 and R223 in the stereo de-emphasis circuits of the MPX-70. While these don't have to be changed, they would if it is desired to retain the same audio frequency response characteristics as provided by the MPX-65 unit in the 400 chassis.

Good luck with it -- I hope this helps!

Dave

 

[Patrice B]

I believe you are describing what sounds like an external MPX-70 MPX unit. It is indeed a very similar unit to the MPX-65 used in the 400 receiver, although there are significant differences you would need to address in swapping it into the 400 receiver. In the MPX-70:

1. There is typically only one B+ feed (245 vdc) to the MPX-70 rather than two as in the MPX 65, with an internal dropping resistor included in the MPX-70 to create the second lower voltage B+ source internally within the MPX-70. It is decoupled with the second 1 uF cap you noted. If you leave this feature of the MPX-70 in tact when installed in an MPX-65 setting, it means that the 38 kHz oscillator and 19 kHz pilot amplifier are drawing their current from the same B+ source that the line stages operate from and will drop that voltage source accordingly. So, you may need to eliminate the added dropping/decoupling components in the 70 to then split it's two MPX B+ circuits, and then connect the two separated high and low B+ voltage rails into the 400's B+ connections in the same manner that the 65's B+ connections were made.

2. In the MPX-70, there is no noise detector circuit included, so it has no ability to distinguish the difference between when a 19 kHz pilot signal is present, or when not. As a result,

3. The MPX-70 also does not have an indicator output for triggering the eye tube when a stereo MPX signal is being received.

The components for both the noise detector circuit and the indicator output would need to be swapped over from your MPX-65 to allow the 400's eye tube to work correctly. If only the indicator output is added but the noise detector is not, the eye tube would always remain closed in either stereo setting of the input selector switch whether a station is tuned in or not, since the presence of noise or a 19 kHz pilot tone would act to close the eye tube's beams. Therefore, both of these circuits will need to be added to the MPX-70 for the eye tube to work properly.

4. Due to operating into different intended input impedance levels, there is a slight difference in the value of R222 and R223 in the stereo de-emphasis circuits of the MPX-70. While these don't have to be changed, they would if it is desired to retain the same audio frequency response characteristics as provided by the MPX-65 unit in the 400 chassis.

Good luck with it -- I hope this helps!

Dave

Oh yes it helps! I thought it would be a simple swap in case the onboard MPX is too messed up… but no

Anyway, I’ll replace the 1uf when time and energy permits and depending on the conclusion, troubleshoot or declare success!

***Edit: it’s indeed the MPX-70, going in consoles like the Ambassador or similar.
In the first link searching for « Fisher MPX-70 », I also read this ( https://antiqueradios.com/forums/viewtopic.php?t=410863 ):

« Very often a slight (1/4 turn left or right, no more) touch up of the 38 KHz oscillator coil and/or the 19 KHz tuned amp phase will restore the stereo. It's not very technical but if you don't have the right equipment it can be done by ear (better to listening through headphones ) with the tuner tuned on a strong and clean station broadcasting in stereo. You won't get optimum separation this way but it works.
Also remember that if the tuner is out of alignment (especially the detector) the demultiplexer won't be able to recover the stereo information.
Of course, a fault in the MPX section is also possible but in most cases (80% or more) loss of stereo is due to drifting alignment with aging. »

and…

« A stereo FM modulator designed for use in a car makes a good inexpensive signal source for checking and aligning MPX decoders. It allows you to broadcast a left, or right channel only signal, which is essential when adjusting for optimum separation. »

I have a car fm modulator and some audio software to visualise signal amplitude so it’s a quick non-optimal solution if needed.