[Ron Ramirez]

Now, while I was down in the basement a little while ago taking the photos in my previous post, I decided to do some further testing. I removed the output and driver transistors, turned the receiver upside down, and brought the unit back up on the Variac.

I had it tuned to an FM station as it turned out, and as I turned up the voltage, the signal meter slowly swung to the right and the Stereo Beacon lamp (converted to an LED already) came on.

I had my signal tracer ready, so I looked for an audio signal at the predriver.

Nothing.

I went back to the preamplifier.

Nothing.

I turned it off, flipped it right side up, turned it back on again, and checked for audio at the output of the IF board.

Bingo - there was mono audio.

To make sure the cable between the IF board and the MPX board was OK, I checked for audio at the input of the MPX board.

Yup, still there. But there is nothing at the L and R outputs of the MPX board, and nothing at Test Point 402 (Mono).

So the signal gets to the MPX decoder board, but is cut off there as there is zero output from the MPX decoder.

Side note: The Stereo Beacon LED stays on whether the set is tuned to a station or not.

This ought to be fun to figure out...not.

[Ron Ramirez]

C O R R E C T I O N

After further testing, I have to revise my previous comments.

A second test proved that I was indeed getting audio at the L and R outputs of the MPX board.

Now, here is where it gets really interesting.

With the function switch in either the FM AUTO or FM MONO positions, audio gets all the way to the inputs of the driver boards, and it is quite strong indeed.

With the function switch in FM STEREO...nothing. Dead. The signal is lost at the preamplifier; very weak at the preamplifier input, nonexistent at the output.

As an aside, I am getting a hum in the left channel at the input to the left driver board that is not present at the input to the right driver board.

One more thing. With the function switch in either FM AUTO or FM MONO, the Stereo Beacon dims somewhat but remains illuminated whether the set is tuned to a station or not. It remains illuminated when the unit is set to PHONO or AUX. In the FM STEREO position, it gets brighter and stays bright, whether tuned to a station or not.

Through all of this, I realize that the load of the driver and output transistors not being present may, or may not, have an effect upon these tests.

Click the thumbnail below to see the schematic for this set.

[Image: http://www.philcoradio.com/images/phorum...ematic.jpg]

[Mondial]

First, as to the resistors. They are 5 Watt, Ward Leonard wirewounds. Very rugged and reliable. Vitreous enamel coated, which will not burn or smoke.
You probably only burned the dust off them! I would definitely leave them as is.

Regarding the stereo light. When you switch the selector to FM mono, it should ground 4D ( TP 402 ) on the multiplex board. Doing this removes base drive from transistors Q404 and lamp driver Q406. If you ground terminal 4D, the stereo light should go out. If it does not, then one of those transistors is probably bad.

Check the voltage at 4D and confirm it goes to zero when switched to FM mono position.

[Ron Ramirez]

I forgot to try grounding 4D. I measured 0.5V at TP402 (4D) in FM stereo and FM auto; in the millivolt range in FM mono.

I tested all six transistors and they all test good. I went ahead and replaced the two 2N2924s and the 2N2925 since I have some of those. No change.

I pulled the board and was going to replace the electrolytics, but apparently I used the 27 uF caps already, or misplaced them. In any event, I shall have to order more. Once that's done, I'll try it again, and try grounding 4D in stereo mode.

One thing I noticed this evening - on the schematic, there is a 200 uF cap (C30) which is switched in when the unit is in the FM stereo position. This also connects to point "AA" (4F on the MPX board, which also connects to the Stereo Beacon bulb). Also connected in this circuit are a few resistors (R65, R67 and R68 ). This does not make sense to me. It is applying the voltage from "AA" through those three resistors to 1E, 1H and 1L on the preamplifier board. Is this why I lose audio when the unit is in the FM Stereo position but not in FM Auto or FM Mono?

C30 is buried within the selector switch, and I haven't replaced it. Not sure how to go about replacing it, either, since it looks like it is nearly impossible to get to.

[Mondial]

Hmm, that's a bit odd. I didn't notice the connection from the stereo lamp to the function switch, but it may have something to do with the low audio.

Did you use the same circuit with the shunt and series resistor with the LED replacement? It seems something other than Q406 is completing the circuit and causing the LED to light, because the transistor should be definitely off with only millivolts on terminal 4D. It should take at least 1.2 VDC at 4D to turn on the transistors and make the LED come on. You can confirm by removing the wire from 4F and see if the LED still lights. If it does, then some other circuit external to the MPX board is providing a path to ground for the LED current.

[Ron Ramirez]

Yeah, it stays lit no matter what position the function switch is in - Tape Head, Phono, Aux, any of the three FM positions...it stays lit. And, yes, I did use the same circuit for LED replacement as I used in the 440-T and 4400.

[Image: http://www.philcoradio.com/images/phorum.../led02.jpg]

The only difference is, there is a 2.7K resistor connected from the positive terminal of the 200 uF capacitor in the drawing above to ground in the 700-T.

I have the MPX board out and have partially recapped it. I'm not going to reinstall it until I order and receive those 27 uF electrolytics.

[Ron Ramirez]

So, Mondial...I went back to the bench and reinstalled the MPX board, but did not resolder the wires as I will be pulling it back out to replace the 25 uF electrolytics with new 27 uF units when I order them.

Before I did that, though, I discovered that sure enough, there was a wire which goes from the Stereo Beacon bulb to terminal 10 on section 1 of the function switch. A wire goes from there to the MPX board.

I removed both wires from terminal 10 and soldered them to terminal 9, which is a dummy terminal (does not connect to anything no matter the position of the function switch). That change made my audio return in FM stereo mode. I still do not understand why Fisher wired the circuit in that manner, but it is also shown in the earlier version (serial number 10000 and up) of the 700-T service manual. My 700-T has a S/N in the 35000 range and I am using the service manual for S/N 30000 and up. It does not make any sense to me why they would apply Stereo Beacon voltage to the feedback circuit of the preamplifier and the equalization circuits only in FM Stereo mode.

Well, anyway...the LED still remains lit no matter what position the function switch is in. One difference: it gets just slightly brighter when tuned to a strong station in FM Stereo mode, then dims slightly when tuned off station. I removed the wire from 4F and grounded it, and the LED lit up brighter than when connected to 4F.

Voltage readings:

TP401 (4G): 2.5 mV (mono), 150.6 mV (stereo) - schematic shows 0.8V in stereo mode.
4B: 4.25V (shows 4V on schematic)
24V source - 24.8V
4F (Stereo Beacon): 21.4V (mono), 20.8V (all other positions)

I think I'll go see if I can find where the hum is coming in on the left channel, while I have the signal tracer out.

[Mondial]

I think I an beginning to understand the connection from the stereo beacon lamp to the preamp equalizer. When you are in "FM Stereo" mode, you should only hear stereo stations, mono station will be muted. This is accomplished by biasing the preamp transistors off with the voltage from the lamp circuit. The voltage at 4F should be about 24 V receiving mono, which when applied to the emitters of the preamp input transistors, biases them off and mutes the audio. When receiving stereo, the voltage at 4F drops to near zero and allows the preamp transistors to function normally.

The problem is that your beacon circuit is not working correctly. When receiving stereo the voltage at 4F should drop to less than a volt. You seem to have about 20 V at that point whether receiving stereo or not. So the muting circuit thinks you are receiving mono and it mutes the preamp.

Once you figure out why the voltage at 4F is not switching properly, the "FM Stereo" function should work correctly.

[Mondial]

I took a closer look at the stereo beacon circuit on the MPX board, and there are only a couple of possibilities for the cause of the problem. One is the transistors themselves, Q404 and Q406. Since you already replaced them, they can be eliminated as a cause.

The second possibility is cap C419. Any leakage here will apply current from the +24 V supply to the base of Q406, causing it to conduct regardless of any detected stereo signal. This is a 1 uF tantalum, and although they have very low leakage, the circuit applies 24Vdc to a 25 Vdc rated cap so it will be suspect. Fisher should have used a higher rated cap in this location, especially since low leakage is critical.

You can try lifting one end of C419 and see if the stereo light remains on at all times. If it goes out then the cap is leaky.

If it is bad, I would replace it with a 35 V or 50 V tantalum, or even better a mylar. A regular aluminum electrolytic will have too much leakage to work properly in this application.

[Ron Ramirez]

OK, Mondial, thanks for the explanation. That makes sense to me now.

Now as to what to do about the malfunctioning MPX board. I was writing this post as you were posting, so thanks again - that is something I can check while I prepare a parts order (27 uF caps plus some caps for the next patient, which will be a Fisher 170 that I intend to sell once I fix it up).

I have three Fishers here (440-T, 700-T, 220-T) with malfunctioning/nonfunctioning MPX boards. I have some LM1310 ICs and could take the easy way out on the 440-T and 220-T and just build new MPX boards utilizing the LM1310 (No more coils! Only one adjustment, a trimmer pot), but since the board in the 700 serves to mute the receiver on non-stereo signals...??? I hope that 1 uF cap is the answer.

Oh, and guess what...my ECG 162 transistor order finally emerged from the black hole it had fallen into and is scheduled to be delivered today. It left South Bend, IN on 03/31, then disappeared until it showed up yesterday in Louisville. Then it went through Evansville and was then sent here to Ferdinand. So...where were my transistors for five days? Probably went from South Bend to Louisville by way of San Francisco, Bangor, Maine and Austin, Texas.

[Mondial]

What are the problems with the 440-T and 220-T MPX boards?

There is not a whole lot to go wrong with these circuits, so it should be easier to repair them than to design and build a replacement with an IC PLL demodulator. Then there is also the issue of maintaining originality....

Glad the output transistors finally showed up. Now you can get back to the power amp repair while waiting for the remaining parts.

[Ron Ramirez]

FM plays very well indeed on the 440-T and the 220-T. Only it's mono. The MPX boards on both will not kick into stereo, the Stereo Beacon LED will not come on unless grounded. Acked, and agreed, on maintaining originality if possible.

Yeah, tonight I'll install four of those ECG 162s in the 700-T and then continue troubleshooting using my old Pioneer speakers instead of a signal tracer.

Yesterday, I found where the hum starts in the right channel in the 700-T - it occurs at the collector of the first transistor in the chain of two in the left channel of the predriver. I'm at a loss to explain why, though - the caps and transistors are all new.

I know we seem to be jumping around from problem to problem on this receiver (and others) - sorry.

[codefox1]

Remember that earlier MPX receivers (before 1970 or so) required a fairly stong signal (like a rooftop antenna,) or they just wouldn't work. Many times the mono was OK though even with a marginal antenna. Alas, many stations are no longer properly maintaining their equipment or have dialed down area served, and some are relying on internet connections for a substantial portion of their market. Of course if a $20 little modern set will pull in the station you are interested in for identical placement, yes, something is not correct.

[Ron Ramirez]

Signal strength is not a problem - there are a few nearby FM transmitters that send the tuning meter over close to a full 5 on the 1 - 5 scale.

Well, the 700-T lives! But not without its anxious moments...

I tested all eight of my new ECG 162 transistors and selected the four with the closest beta to one another. I installed them...brought the receiver up slowly on the Variac...and had nothing in the left channel at first.

After supper, I thought I should check that B+ fuse for the left channel. Yup, it was open. It probably opened when the resistors smoked upon initial power-up, because of the shorted output transistor that has now been replaced, along with the other three.

I installed another fuse, and tried again. Now I had sound in left and right channels. Only the right was distorted pretty badly - at first. After a couple minutes, though, it straightened itself out and now sounds good.

Too bad the FM is in mono. Hopefully that will get resolved soon. But, hey, this is probably the first time in years that this receiver has been up and running.

I'll try lifting that 1 uF cap in the MPX board tomorrow, I won't have time to do any more to it tonight.

Oh, forgot to mention - that hum I was getting in the right channel at the input to the driver board with the signal tracer - it's gone. No hum in left or right speakers.

[Ron Ramirez]

Okay, Mondial, Codefox, and anyone else:

This evening, I removed the 700-T's MPX board long enough to lift one leg of C419, and guess what? The Stereo Beacon lamp is still stuck on.

Now, tell me again why I shouldn't just build a new MPX board based on the LM1310? Oh, yeah - originality.