The Fisher 440-T FM Receiver
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I don't have a photo of the top side of the chassis right now, but here is a clarified photo showing the control amplifier board (where the tone controls and balance control are mounted) and the preamplifier:
[Image: http://www.philcoradio.com/images/phorum...e00007.jpg]
The control amplifier appears to only have a few 1 uF electrolytics, but the preamplifier has several electrolytics of various sizes.
It looks like I can easily unsolder, replace and solder any electrolytics I need to replace on the control amplifier board without removing it from the chassis. The preamplifier is a different story - it would have to be removed to replace components, and it does not look like a fun job.
On top of the chassis reside the FM tuner, the IF board, and the multiplex board. Both IF and multiplex boards have electrolytics on them. Those boards also would have to be removed to service them.
I get it...I need to replace those electrolytics, also...right?
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Ron Ramirez
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Well, if you have to place another order anyway, you might as well order all the remaining electrolytics. Even if you don't choose to replace them all now you will have them available as needed.
As I recall, those plastic cased electrolytics in the preamp board were the most problematic. They tended to dry out and develop high ESR. The larger ones are in the low impedance feedback circuit and an increase in ESR will cause a loss of gain, especially in phono mode.
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Acked. I guess it's time that I practice what I preach regarding the old Philcos, and apply it to the vintage Fishers. After all, the 440-T is now 48 years old. The multiplex board does not appear to be working correctly, anyway, as the Stereo Beacon does not light on stereo signals. Could be a bad bulb, but I did not notice any stereo separation at all on FM signals known to be in stereo - they sound mono. My 200-T, which recently had its right channel go bad, has the MPX issue also.
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Ron Ramirez
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Ron
If the cap is not dry AND indicates upon measurement correct (usually it is 15-20% up from nominal) capacitance - you could reform them. And, considering they are from 60s-70s and not from 40s, they should do fine, or at least most of them.
This will save you quite a bit - those are not cheap. Especially in quantities they are used in solid state devices,
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Ron,
If it's like a lot of other MPX sections in receivers of this age, if the bulb dies (and that IS a major probability), the MPX circuit stops working, as the bulb is in circuit with an active part of the decoder. Also, IIRC, those were very picky as to the lamp properties, you pretty much need to have the exact voltage and current ratings. You may be able to sub in a 6 or 12V lamp for testing purposes, but it's not likely to work totally right.
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Is it possible to sub an LED for the original Stereo Beacon bulb? From what little info I've been able to find on this subject, it appears that it is possible. Your thoughts?
I have read just enough about the Fisher MPX circuit (you know they say a little knowledge is a dangerous thing  ) to know that there is a positive voltage at the MPX decoder on mono signals that drops to near zero on a stereo signal. One leg of the bulb is connected to this point. The other leg of the bulb connects to a constant positive voltage that is around 30 volts (23 volts in the 440-T) on a mono signal but drops to around 15 on a stereo signal.
Morzh - Acked, but is it really worth the time and effort, since there are so many electrolytics in a typical stereo receiver? It seems to me that if I was going to go to the trouble of removing electrolytics for testing and possible reforming, that I might as well save a little extra time and just replace them. Replacing just the audio and power supply electrolytics made a big difference in this 440-T's performance, so replacing the rest should make it that much more reliable, perform better, etc., yes?
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Ron Ramirez
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Well, a new is supposedly better than old.
I mostly talk about large caps, not those interstage coupling 10uF types.
In fact Western electric old caps from 40s after being reform often times have better leakage current than new caps. I read of it, and did one (I have it in my Western Electric oscillator) - it indeed has a leakage order of magnitude below the formula that comes with todays caps.
Also if the old caps take longer time to reform, the newer ones will probably take less time.
Besides, we do know where the old caps were made and where the new ones are made today.
Somehow when I see a WE cap I do not have it in me to gut it unless it is dry.
Again, it is time and effort vs cost. If it is a buck a piece...sure.
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[Image: http://www.philcoradio.com/images/phorum...-T_MPX.jpg]
This is the 440-T multiplex board. Notice one end of the Stereo Beacon bulb connects to point 4C on the MPX board.
Now, the 440-T power supply:
[Image: http://www.philcoradio.com/images/phorum...0-T_PS.jpg]
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Ron Ramirez
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You should be able to replace the lamp with a LED with a few conditions.
The transistor Q404 just pulls the lamp to ground to light it, but it also controls bias for the demod driver Q403. When the lamp is off (mono) the voltage at Q404 collector is pulled up to +23v, removing bias from Q403 and shutting down the demodulator.
So you need a series resistor to limit the LED current and a shunt resistor to insure that the off voltage rises high enough that bias is removed from Q403. The original bulb drew about 36mA, but you really don't need that much current with a LED. I would try a 1.5 K in series with the LED and maybe a 10K across the combination as a pullup.
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Mondial - thanks!  Just the info I needed.
Now a dumb question:
Should the cathode end of the LED go to 4C on the MPX board, with the anode toward the power supply?
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Ron Ramirez
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I took the 440 chassis over to where I have a better set of speakers sitting in the basement, and hooked everything up. Yup, the right channel is definitely "muddier" than the left.
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Ron Ramirez
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Yes, anode to the power supply and cathode towards the MPX board.
The LED is far more sensitive than the incandescent bulb, so it may tend to light up on noise even with no stereo signal. In that case, just decrease the value of the shunt resistor so that more current bypasses the LED and reduces its sensitivity.
Because the LED draws less current, the power supply will not drop as much as it did with the bulb (23v down to 15v) so you may only see a couple of volts drop.
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Is this the proper hookup?
[Image: http://www.philcoradio.com/images/phorum...S_rev1.jpg]
Mondial, you mentioned the possibility of the Stereo Beacon lighting up on noise with no signal. My 600-T's Stereo Beacon light does that, but I just ignore it. I noticed my 4400 does that also. The 4400 still has an incandescent lamp, and I think the 600 does also.
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Ron Ramirez
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A judicious adjustment of R401 will set your decoder to show a stereo signal only on high signal levels. I usually set this adjustment so that the lamp just goes off between stations.
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Yes Ron, the hookup on your schematic looks good.
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