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SX-750 Bulb Trouble

You have 7v on pin #14 on the power supply(PS) AWR-100 the board with the fuses ,its underneath ,right ? That Pin #14 goes to pin#4 on the tuner board. This 7v is for the Aux #8, Phono #7 ,FM #6 ,AM #5 these pin# numbers are on the tuner board .The ground for these is on the other power supply board AWR-99 ,(with the relay) pins #20,21. Is that how you have it wired? Put your Sx 750 on its side with the tuning gang pulley facing up ,so you acess the bottom and top boards.
 
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Alright, definitely made progress today. Calman, I checked the wiring as you have illustrated, and yes this is how the board is wired. The takeaway here is I can get ALL of the lights working, but the voltages are now a large concern. It appeared that current was not flowing through I believe the FU2 fuse, thus not powering pin 14, power on one side, none on the other. I went ahead and cleaned all metal contacts of all old fuses, as they appeared pretty dirty, and alas, power flows. Following the current, at pin 14, we are not getting 7V, which is what it was prior to the cap and resistor(?) and both sides of the fuse. At pin 14 the voltage reads 14V, which is now what it reads at pin 4 on the tuner board. Thus, lights tied to 5,6,7,8 light up! But at 14V, probably bad news. And with further testing, the wires tied to the power light continue to read about 7V, and we are now getting power via pin 22 (board 099) for the stereo light! This one reads in the range of 25V, very problematic. In addition, the light will not turn on when connected to pin 24 on the tuner board, but WILL when connected to a chassis ground. Tried wire bypass of pin 24, front and back of tuner board, no success. In addition the unit shut off after a few seconds of the light being on, but came back when detaching from chassis ground, so we are best leaving this alone until the voltage is more reasonable. The news here is I think enlightening, although might be spelling trouble elsewhere.
 
the stereo light is from #22 relay board should be 13v.. I think you have the wires mix up . Look at the foil side pictures in the service manual pg 56 60 . Do one light at a time Re check the wiring
 
Yeah, rechecking the boards tied to the stereo lights. Strangely, the voltage I'm getting for pretty much all of these lights is roughly double what you're saying they should be. #22 on the relay board is pulling 27V, which is also tied to #9 on the tuner board, also 27V.
 
Just to confirm, on the tuner board, pin 5,6,7,8 all pull 15V, pin 9 pulls 27V. In addition to this, on 100 board, voltage starts as 5V, but jumps up to 15V after crossing what I'm thinking is a transistor, a small black resistor-like device.
 
This black device is not a transistor, but not really sure what it is. Not a resistor, maybe a capacitor, but not like one I've seen. Edit: Using wiring diagram, this appears to be labeled as D1, 1S1885 (diode?)
 
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Do you have the meter's black probe on the chassis ? Don't go probing around keep to just the pins . On tuner board , pin#4 and a good chassis contact with black probe should around 6v
 
Yes, all pin tests have been red to pin, black to metal chassis ground. Getting about 15V to pin 4 on tuner board. Is it possible the diode or capacitor has failed? Following the voltage, this area is where trouble seems to be lying.
 
Check that the C1 cap is in correctly did you replace it? , the negative to #4. Also look for solder bridges between #13 &#14 that may have happened when you recapped it.
The diode and the C1 make it DC , its AC on fuses
 
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I will certainly check the negative to 4, but I didn't replace this capacitor. Given its behavior, should I be replacing it?
 
That cap might be shorted ,you probably have 15v on the positive side . Yes #4 is ground. Pin13 &14 traces are not touching ,right?
 
Yeah, it looks like I have 15V on positive for that cap. I didn't check the 13 and 14 pins yet, just because I'll need to remove the board to get access to the back, but it seems quite unlikely they are touching, as it once worked prior to going into storage, and the cap hasn't been touched since it was first assembled. I did go ahead and order a replacement cap, so it should show up tomorrow and asap I'll check to see if the replacement takes care of it within the next day or two. If not, I suppose attempting to replace the diode will be next on the list, just harder to get sooner.
 
Alright, the work of replacing both the capacitor and diode has been done. All said and done, the voltages did not change. It is possible these components aren't exactly what they should be, but with some research it seems that they should work, and given the voltages are exactly as they were before replacement, I think they work, but something else is causing this voltage problem. There is no solder bridging on the underside, at least from what I can see. I am currently at a complete loss of what is going on with this section, any ideas?
 

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I'm not a tech but if you at the schematic that 6v comes from a secondary winding in the transformer, through the fuse ,the diode , that cap . I read your #22 post you said you had 7V on Pin#14 before you "cleaned" the fuse ? Check #4 ground . The5.5V on Pin#2 is the AC so the winding is good but check pin#2 and ground with meter on AC . Not the many components in the circuit check all the wires you cut and look for broken wire connection ,those wire wrap connection can break but still look connected. .
 
Watthour: I have replaced both the cap and diode on board 100 that follow the FU2 fuse. So far, no issues, but the originals are kept in case of unforeseen problems.
I am mistaken. User error, voltages on this board are solid. I failed to check both AC and DC voltages, getting pretty much all the correct values at each of the pins. Board 100 pin 4 ground is 0V, pin 14 is 7.4V (as is pin 4 on tuner), re-checked all pins and routes on board 100, everything is looking pretty good. Progress can be made. As such, all bulbs are working, with one exception. Stereo light still fails unless tuner pin 24 is bypassed, and voltages for this bulb are (board 99) pin 22: 12.9V (good), (tuner board) pin 24: 1V. If I recall, pin 24 on tuner is supposed to be a ground, right? If this is the case, my instinct is possibly one of these resistors is bleeding voltage through to it, going to take a look at the foil diagram for it and continue to follow voltage for this segment.
 
I've spent some time looking over the foil diagram for 073 and the board itself around pin 24, and not really sure what to think. I am leaning towards the HA1196 FM logic board that precedes pin 24 and the resistor being the culprit, but its clear I don't have all of this machine figured out, so this is only preliminary. I am attaching the diagram with original intended voltages in red, and what it is reading in this unit in blue to compare. Some match, some don't. It might be important to note that the FM signal is still killing the relay and looping a shut down cycle, so maybe this is causing a few issues? Can do more testing if necessary.
 

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You are correct that Pin 24 on the Tuner/AF board provides the ground for the STEREO lamp. This only occurs when the a strong FM signal is being received and decoded into stereo channels.

Incidentally, any received signal (AM or FM) does not directly affect the protection relay. The protection circuit monitors only the audio output circuits and opens the relay if conditions exceed an unsafe level. You reported earlier that the unit would produce a loud buzz when powered on. That can indicate problem in the audio outputs and/or power supplies. More testing is necessary, including resistance testing of the outputs and drivers, and AC/DC testing of main and regulated power supplies on both the AWR-100 and AWR-099 boards.
 
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