Well we are still making progress. You now know its not the cap or the transistors!
I have a feeling that part of the problem is the LED conversion. If you connected an incandescent bulb to your MPX board, the lamp probably would not light all the time because it would have only a few volts across it. The LED needs only 2 volts or so to light so its much more sensitive to marginal conditions.
Are Q 404 and Q 406 mounted in sockets so you can unplug them? If so, completely removing them would only leave R414 as a path to ground for the LED current. This resistor may be passing enough current to light the LED but would never be a problem with the original bulb.
Another thing you can try is to connect a 1K directly in parallel with the LED itself. This will reduce its voltage sensitivity so it will take at least 4V across the circuit to light it instead of just 2V, which will make it behave somewhat more like the bulb.
(This post was last modified: 04-08-2014, 07:10 PM by Mondial.)
As I stated before, some of these old MPX decoder boards needed a particular lamp because of a specific current flow being required. I haven't tried to do the math on the LED conversion to see how close the total current draw is, but I know from experience that some of these are VERY particular.
Acked...Well, I guess I could go back to an incandescent lamp in the 700-T. But I really hate to, considering that after I worked out the bugs, an LED replacement is working well in my 4400; and a recent conversion to an LED stereo lamp in a Fisher 170 worked perfectly - it acts just like an incandescent, no delayed dimming, on when tuned in, off when tuned out.
I'll try pulling Q404 and Q406 (yes, they have sockets) this evening and will report back. I might try replacing the 390 with a 1K, as well.
Given how fussy these MPX circuits are regarding the lamp, it might be a good idea to use the proper incandescent at the beginning to at least check if the MPX decoder is functional. After confirming proper operation with the lamp, then proceed with the LED conversion.
Right now, we don't know if its an actual MPX problem or an incompatibility with the LED causing the problem. A further complication is that it seems each Fisher model used a different variation of the MPX circuit, so what worked for the 4400 may be incompatible with the 700-T.
Part of the difficulty with the LED conversion is that the characteristics of the incandescent vary depending on whether the filament is cold or lit. It is basically a nonlinear resistor. When lit up at normal operating current, it may exhibit a resistance of say 400 ohms. When cold however, the resistance drops to around 50 ohms or so, which presents an entirely different load to the MPX driver circuit. Unfortunately Fisher engineers decided to use the lamp as part of the circuit to switch the decoder to stereo mode, which is what makes it finicky. Instead of using a separate transistor to drive the lamp directly, they combined functions, saving the cost of a transistor, but making the circuit problematic regarding lamp replacement.
That makes sense. The 170 uses an IC for MPX decoding, even though it used a "grain of wheat" 8 volt lamp for the Stereo Beacon. The LED replacement worked perfectly in the 170; you could not ask for a better conversion. Now, the 4400 uses an older MPX board than that used in the 700. My 440-T uses the same type of MPX board as the 4400, although there is a 440 on eBay now that uses the same MPX board and, it appears, the same driver boards as the 700. But I digress. I'll try to find a 15V bulb that is at, or close to, 30 mA and will try it. I took a quick look on Mouser's site on my lunch break but did not see any 15V lamps - only 14V and 18V, and neither of those were rated at 30 mA filament current.
I just realized something, looking at the 700-T schematic - voltage at the junction of R46, R47 and C17 is 25V in mono, 17V in stereo. Looks like the ideal lamp is 17V?
No, the measured voltages at Q406 do not make sense - I'll have to remeasure that transistor to be sure.
But the lack of voltages at Q402 puzzled me.
I decided to look into things further. I found that PC401's terminals 2-3 (3.3K) and 3-4 (2.7K) were both open!
So PC401 is bad. With no voltage at Q402 as a result, I'm surprised any audio is getting through the board.
Now, here's the $64,000 question: Do I replicate PC401 with individual components, hoping the board has no other major issues, or do I hoist the white flag at this point and start gathering parts for that PLL MPX decoder?
Wow, now you are really making progress! PC401 looks real easy to duplicate. Its only 3 resistors and a cap. Why not give it a try?
Q402, Q403, Q405 and PC401 have nothing to do with the audio itself. These parts serve to regenerate the 38 kHz subcarrier from the 19 kHz pilot signal transmitted along with the audio, and therefore only function with a MPX stereo signal. So its not surprising that you still have a good mono audio signal with a bad PC401.
The actual audio passes through Q401 (emitter follower), then through the double bridge demodulator. When in mono, DC bias from R416 passes through the bridge diodes, turning them on and allowing mono audio to be fed through to both channel outputs. In stereo, the regenerated 38 kHz subcarrier from transformer Z403, alternately turns on each bridge, separating the audio into right and left channel outputs.
What makes no sense is the voltage at the emitter of Q406, which should be grounded. Something is definitely funny there.
(This post was last modified: 04-09-2014, 10:58 PM by Mondial.)
I'll duplicate PC401, yes. But I am not going to devote a lot more time to it. I need to order some parts, so this receiver is going to be set aside for awhile. I'll include a low voltage .047 uF cap in that order. I know I could use one of my .047 uF 630 volt caps, but there is not a lot of room on the board where PC401 is located - not enough room for a .047/630 plus the three resistors. A .047 uF, 50 volt radial cap will leave plenty of room for the resistors.
I did nothing to the 700-T tonight. Maybe Sunday, I will remeasure the voltages of Q405 and Q406. I will also try to fix the 700-T's off-on switch, which is currently not working.
I am not sure why there are no voltages listed for Q405, but from a look at the circuit it seems there should be about 3.5 VDC on the base and 24 VDC on the collector. The emitter voltage will vary depending on whether a stereo signal is being received. With a mono signal and stereo beacon off, there should be about 5.6 VDC on the emitter. With a stereo signal and stereo beacon lit, the the emitter voltage should be around 2.8 VDC. The difference in voltage is produced by R414 which supplies voltage from the beacon lamp circuit (through the lamp) to Q405's emitter, shutting it off in mono.
If your beacon is still lit after removing Q404 and Q406, then the only apparent path to ground for the LED is through R414, which may be passing enough current to light the LED. The R414 current would not be sufficient to light the original bulb, so the LED conversion may be at fault as far as the always on stereo beacon.
I still have not ordered any parts. I was out of town all day yesterday, so nothing happened at the workbench yesterday.
This afternoon, I decided to see if I could get into the off-on switch to try shooting it with DeoxIT. I could not get the switch off the back of the dual volume control, so I drilled a tiny hole in the side of the switch and sprayed some DeoxIT in that way. It didn't help. So it looks like I will have to use a control from a junker set to get a working switch.
So...while the 700-T was on the bench, I removed the MPX board, decided that I could get a .047 uF, 630V metalized film cap to fit in part of the space formerly occupied by PC401. It fit by folding one of its leads over and installing it as a radial mount cap. I also installed individual resistors to replace the three that were built into PC401. I didn't have a 2.7K so I took a chance and used a 2.4K. Oh, and yes, I replaced C419 with a new 1 uF cap. I only had a regular electrolytic on hand so that is what I used. I'll replace it with a new mylar cap later.
I hooked up the speakers, plugged it in, and turned the thing on.
I now have a working MPX decoder!
Now, here's the odd thing about that LED light. Prior to trying it out today, I pulled the 390 ohm resistor out that I had in parallel with the LED, and temporarily installed a 1K as Mondial had suggested. Now, the LED gets bright on a stereo station, then dims when tuned off station...but remains lit dimly when off station. It also remains dimly lit when switched to PHONO, TAPE HEAD or AUX.
So I guess I'll have to go back to an incandescent bulb. Darn it. I really wanted to use an LED here, but it doesn't look like I can in this particular set.
The original was open, so I'll have to find a bulb. (Hmmm...I wonder if that 440-T junker has a good bulb?)
Yes - the 440-T junker did have a good incandescent Stereo Beacon bulb!
It is now in the 700-T - and now the Stereo Beacon behaves as it should; going out completely when tuned off station (or when switched to TAPE HEAD, PHONO or AUX); lighting up only when tuned to am FM stereo signal in FM AUTO or FM STEREO.
It still beats me why the LED conversion did not work in this receiver, but if it must be incandescent, then it shall be incandescent.
I'm just glad the MPX decoder is working again.
I think that once I carefully and gently clean the faceplate, I can button this one up and move on to another. I may still replace the 25 uF electrolytics in the MPX decoder, but I think I'll leave the IF board alone for now. It works very well, and only has two electrolytics in it, anyway. I don't want to press my luck.
Ron, I think you misunderstood what I said regarding adding the 1K across the LED. I did not suggest removing the 390 ohm shunt resistor which was placed across the series combination of the LED and the 1.1K. What I meant was to add the 1K in addition, directly across the LED, not across the series combination. This will reduce the sensitivity of the LED so it should go out when not receiving stereo. Before you button it up, you might give it a try.
In any case, congratulations on getting the MPX board and the rest of the receiver working!
(This post was last modified: 04-13-2014, 05:43 PM by Mondial.)
Mondial Wrote:Another thing you can try is to connect a 1K directly in parallel with the LED itself. This will reduce its voltage sensitivity so it will take at least 4V across the circuit to light it instead of just 2V, which will make it behave somewhat more like the bulb.
Yup, you're right. Speed-reading is not always a good thing.
Nevertheless, I've reverted back to incandescent, having removed the series and parallel resistors under the chassis needed for LED operation and reconnecting the lamp wires direct as original, and incandescent the 700-T Stereo Beacon shall stay. Should I experience this problem in the future, though, I will try the 1K across the LED itself at that point.
I have a few questions I wish to pose, how come you decided to change the 3A120's, how do you replace C30 (200uF) at the main switch, why do you wish to replace the TR1009 output transistors, and why do you have issues with NTE?
I began the resurrection of my 700T, this week.
To hold the surface mount capacitors, I am using sectioned PVC tubing and thin foam rubber, to hold each cap. The reason is, I don't believe receivers as such, need to adhere to aesthetics, over ability to see the replacement capacitor. My belief is, that they should be completely serviced than, restored.
(This post was last modified: 04-17-2014, 11:14 AM by Omer.)
Well...don't hold back, Omer...tell us how you really feel! You do have an interesting way of holding your electrolytics in place. I chose to restuff most of my electrolytic cans to keep them looking more or less original, plus to retain the same can size to fit in the clamps.
To answer your questions...
how come you decided to change the 3A120's
Because I could.
But seriously, after over 40 years, I felt it was time to retire the original rectifiers to prevent the possibility of them shorting out at an inopportune time. Since they're new, they should be good for another 30-40 years.
how do you replace C30 (200uF) at the main switch
I haven't yet.
why do you wish to replace the TR1009 output transistors
Please read this entire thread. One TR1009 in the left channel was shorted, both TR1009's in the right channel were replaced by TR1007s. I replaced all four because one was shorted and two were incorrect replacements.
and why do you have issues with NTE?
Overpriced rebranded transistors which are not consistent in their characteristics.
One more thing. Don't forget to replace that white .01 uF 600V firecracker capacitor in your fourth photo with an X/Y rated safety cap - that connects from one side of the AC line to ground, and is an explosion waiting to happen.
So it looks like I will have to use a control from a junker set to get a working switch.
