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.

 

[Patrice B]

Hey hey…

I had some time… and energy to pursue the « resto »:

- 1uf electrolytic to replace the one in the MPX. No more howling but stereo wasn’t obvious, almost no difference with FM Mono so, as usual, I had to tweak things. Nothing to loose as I have the option to send the board to our resident expert, @dcgillespie. Anyway, my ears aren’t the most precise tools so…

- Looked in my mess and saw this (the Philips generator, not the big Eico scope!):



I bought this cheap about ten years ago but never thought trying it! I just fired the thing with a BNC cable terminated with two crocodile clips. I didn’t even need to connect it, it broadcasted on 100 MHz with all the stuff needed: 1KC and 5KC, R and L signal with pilot. After around 15 minutes, I had real good separation just going with my ears. Not measured but the difference is now obvious compared to mono!

- However, I fought for more than one week with an annoying misbehavior: sometimes, at power on, no deflection at all in FM Mono and reduced closure in Stereo. After resoldering all connections in V3 and V6 sockets, thorough sockets cleaning, poking with a plastic tool under V6, tube swap… I think it’s solved! Before, just rocking a bit V6 would lead to reduction of tuning eye closure or even loss of!

I still have to keep an eye on this.

————————————-

More to follow…

 

[Patrice B]

This week, when diagnosing, I was tired to have to connect tape outs to another amp to monitor FM progress. Without much thinking, I installed the 7868 quad that came with the receiver and continued investigation with the aforementioned « loss of tuning eye ». Nothing blown, we had sound with the old quad!

Fast forward, yesterday: I soldered 10 ohms resistors to check bias and this quad is almost shot! One tube was registering under 10 mv!!! The three others are varying between 15mv and 30 mv.

Having a quad of 7868 in my Sherwood S7700-III (project never completed, bad FM reception!), I’ve put three of them (one of the Sherwood tube reads 50 mv!!!) mixed with the stronger one in the tired quad and got, @415 vdc plate voltage, between 30.5 and 36.8 mv readings. A bit high to my taste but good enough before I build an IBBA or an IBAM.

- I am tempted to not touch the Ero foil coupling caps, sounds good as is. Your thoughts folks?

Thanks,

Patrice

 

[gadget73]

measure voltage across the 330K grid to bias resistors. That will tell you the leakage on the ERO caps. If you aren't measuring more than a few millivolts, the caps aren't leaky and can probably be left alone.

 

[Patrice B]

measure voltage across the 330K grid to bias resistors. That will tell you the leakage on the ERO caps. If you aren't measuring more than a few millivolts, the caps aren't leaky and can probably be left alone.

Thanks Sir!

Also, should I replace C41, the 8uf electrolytic at the detector/discriminator?

 

[gadget73]

if its an electrolytic I'm usually not real motivated to keep them at this age. Without looking it up I don't specifically know what that does or what effect failure would have but if all of the other caps got changed, that one probably isn't much better.

 

[dcgillespie]

Definitely replace that cap. It is the reason that the ratio detector discriminator circuit is superior in limiting action to the original foster/seely discriminator design. If the cap becomes compromised, the AM rejection of the circuit is gone.

Dave

 

[Patrice B]

Definitely replace that cap. It is the reason that the ratio detector discriminator circuit is superior in limiting action to the original foster/seely discriminator design. If the cap becomes compromised, the AM rejection of the circuit is gone.

Dave

if its an electrolytic I'm usually not real motivated to keep them at this age. Without looking it up I don't specifically know what that does or what effect failure would have but if all of the other caps got changed, that one probably isn't much better.

8uf replaced, thanks guys! FM reception seems to be better as the eye closes a bit more now.

While I was there, the four .047 couplers found the bottom of my trash bin… 4 new ones, two @630v and two @400v.

Also swapped the connections at the headphones jack.

Now, with my four 10 ohms measuring in reality 10.5 ohms and 415 vdc plate voltage, I have:

On left, one at 70% dissipation (354 mv) and one at 72% (368 mv). On the right channel, one at 72% (366 mv) and the only remaining strong 7868 of the original quad is at 60% dissipation (305 mv). Bummer as in either case - one 7868 too hot at 500 mv, from the Sherwood quad - this older original one a bit colder), I’ll have to build and IBAM… for one tube in fact!

As for the sound, impressive with my two Minimus 7 test speakers and a simple 300 ohms dipole.

As for MPX stereo, a new run with the Philips PM 6455 FM stereo generator gave (tell me if my maths are ok please):

Right only: 2.478 AC volts measured at R tape out and .040 AC volts measured at L tape out. 36 dB, right?
Left only: 2.45 AC volts measured at L tape out and .039 AC volts measured at R tape out. Also 36 dB.
Measurements done only at 1 kc and , this time, with positive crocodile clip attached to a tube shield just floating (not grounded) over 1st RF tube. Other black crocodile clip attached to the 400 chassis.

 

[gadget73]

what does your meter read with the leads shorted? About 0.5 ohms wouldn't be unusual for lead resistance, so those resistors may be closer to 10.0 than you think.

 

[Patrice B]

what does your meter read with the leads shorted? About 0.5 ohms wouldn't be unusual for lead resistance, so those resistors may be closer to 10.0 than you think.

Well, how I didn’t thought about it before… You were right:

DVM #1, Mastercraft: probe shorted: 1 ohm!!!

DVM #2, Uni-t : .4 ohms.

Unfortunately, the Uni-t was damaged a few years ago when measuring milliamps directly across OPT B+ and plates on a Traynor bass amp. It still measures fine resistance and voltage (but no more milliamps) but I guess I’ll be better buying a better one! I also busted my first Craftman, the same way! Since these « events » (I think it touched two adjacent pins on a power tube in milliamp mode!), I strictly measure bias with bias probes for my EL34, 6L6GC, KT-88 amps or adding 10 ohms for my « hifi » amps!

 

[Patrice B]

Just ordered a B&K 2709B, a better DVM is a must!

 

[gadget73]

see if the meter that doesn't read current has a blown fuse. If thats good, the current shunt is probably burned open.

I did a lot of work for a lot of years with a very cheap meter. Still have it actually, it lives in my garage toolbox now. Don't always need fancy tools, just have to know the limitations of what you are working with. The current meter I have is decent but the test leads weren't so hot. Upgrading those made a nice difference.

 

[Patrice B]

Bias tolerance:

Having a bunch of 7868’s (three « quads » of different tubes), I ended up with a somewhat matched quad, fitting the original circuit (no IBBA or IBAM).

On one side, tube #1 idling at 29.5 ma and the other one, at 31.5.

The other side, one at 30.5 and the other, at 33.5.

If we were in the 60’s, we wouldn’t probably talk about this « mismatching », right?

Honnestly, when trying different matches with what I have, I couldn’t hear any difference, neither hum, with 10 ma mismatch.

This configuration roughly gives 63 percent dissipation for the coldest tube and 72 for the hottest.

Sure, I could have them matched with added bias circuit but even then, isn’t there some inherent imbalance in the two halves of an output transformer (winding resistance)?

For the record, this quad is made of three Sylvanias (the fourth one isn’t even reaching 20 ma) and one ooooold Fisher that came with the receiver.

I have another almost closely matched quad, a Fisher branded one that came with a Sherwood 7700-III. Three are in the 35-36 ma range (a bit too hot with stock circuit) and the fourth one at almost 50 ma!!! My instinct tells me that it was a quad in a Fisher amp/receiver back then, obviously with a tube drawing way more current! Tried this one at first and my old ears didn’t discern anything strange. However, I didn’t like that they were a bit hot with one, too hot!!! Perfect candidates for an IBAM!

 

[Patrice B]

see if the meter that doesn't read current has a blown fuse. If thats good, the current shunt is probably burned open.

Got my B&K friday so the Uni-t is now relegated to the drawer. The Craftman is my backup! Fuse was replaced and no go! Shorting current to ground wasn’t appreciated it seems!

 

[gadget73]

On one side, tube #1 idling at 29.5 ma and the other one, at 31.5.

The other side, one at 30.5 and the other, at 33.5.

I'd swap the 31.5mA and the 30.5mA so its 29.5 and 30.5 in one channel and 31.5 and 33.5 in the other. That will match the tubes within each channel a bit closer.

Fuse was replaced and no go! Shorting current to ground wasn’t appreciated it seems!

probably fried the current shunt inside. meh, if it works other than the current range its still got some use. Current is probably the function I use the least anyway so not the end of the world.