Repair of Sony 8F-56W Radio

[vol.2]

I've been working on fixing this radio for a very long time and I feel pretty stuck. I'm not a complete beginner, but
the radio is older than I'm used to working on and the service information lacks the test info and has very few voltages to check.

I'll include a link to the service manual, but I'll also put a couple of snippets of the area I'm testing for reference.

I'm working on the power side of the radio and I'm tracing voltages. I confirmed the main voltage area, which is ~7.5VDC. That voltage seems to be everywhere the schematic actually has it called out. I checked the voltages surrounding the transistors, and they are off, but not missing. There is a 4.8VDC voltage that is missing and I don't know why.

I've confirmed most of the resistors on the board. Everything around the 4.8VDC has been checked. I'm sure all the electrolytics are good and replaced what isn't. I've checked the transistors, and my cheapo transistor checker says they are ok, but I can't be 100% of course. I also took out and checked the rectifier diodes and they test good (but they would because the main DC is okay.

Here's the area I don't see the 4.8VDC (it's also 4.3VDC in FM radio mode) The voltage should be tested from the red "O" to the red "X":


This is the power board schematic:


This is the whole thing:
https://drive.google.com/file/d/1TivmQ7DWOipUwUybtJfXFzpE6GiG163W/view

Other things I've done are: I checked the AM radio by hooking the volume pot up to a scope and looking at the AC as I run through the band. It appear to be working ok (the radio section) I took the speaker out of circuit and confirmed that it's not shorted. I removed both T1 and T2 transistors to confirm that they are not shorted.

I am really at a loss for why I'm not getting the 4.8VDC. If anyone can help I would be grateful.

Thanks

[coromonadalix]

is the thermistor good, r39 good and c45 and c48 good ?

do you have an scope ?

i know you did some tests ...  you must follow the signal path, not just checking the voltages, you have a feedback path with r42 r38 c44

if you can generate an 1khz 775mv - 1v pk signal on  c39  negative leg lifted from the board  and try to follow it

T1  is center tapped on a positive voltage and t2 is tapped to the ground, x8 and x9 are push pull audio driver


I hated sony in the past for their stranges ways to design some stuff, complicated things for nothing  in my sense of view,  mostly tv's and monitors

[Rollin Hand]

The ability to read a wiring diagram and visualise it against what is in front of you is priceless, not many people "have it"

[vol.2]

Thanks for the reply.

is the thermistor good, r39 good and c45 and c48 good ?

Yes, I pulled and verified all of those.

do you have an scope ?

Yes, I checked the AM radio with my scope. I can see the AC current coming from the radio when I switch from FM to AM, but's very low and doesn't change much when I move the dial. I can see a similar level all the way through to the speaker. It's very low though, max 800mV Pk-Pk. Interestingly, the AC signal gets much higher and looks more like an AM signal when the radio power is off than it does on. It gets up to like 2VAC Pk-Pk and looks quite like a normal AM signal.

[coromonadalix]

you need a constant fixed signal  to do some checkups

[vol.2]

if you can generate an 1khz 775mv - 1v pk signal on  c39  negative leg lifted from the board  and try to follow it

I have a signal generator. I'll try to figure that out. Thanks!

[vol.2]

you need a constant fixed signal  to do some checkups

Ok Thanks for the help.

I put a 1k, 800mV pk-pk signal at the negative side of C39 (lifted) and I found that it disappears at D5 on the tuner board. It goes through the band selector switch and comes out the orange wire and goes to R30. The strength of the signal drops over R30 (but I assume that's correct) It then goes over R28, and I still see it, but I don't see it anywhere else on the board. I assume it's supposed to be going over D5 to the IF transformer, but I see nothing there. I guess I'll pull and check D5. Not sure if the logic is sound, but I'm not sure what else to try.




I checked D5 and it seems OK. My tester says it has a forward voltage drop of ~700mV, and the datasheet says it's supposed to be 1V @ 4mA Max. Best I can figure is that is OK. Also, C37, which is across the - side of D5 is ok, but a tiny bit low. It's suppose to be .01M, and it's more like .0077M. Seems "close enough" to me in that it wouldn't cause everything to fail.

Not sure what could be making that signal disappear. If I had to guess, there's a shorted component somewhere dragging things down. I just don't know how to go about finding it without removing everything one at a time.

Let me add that I pulled and tested IF transformer A3 and it isn't shorted. The resistance on the right side is only ~1.1ohms, but it's not shorted.

Also, X5 transistor seems ok.

[vol.2]

Here's the signal as it goes from C39 through the resistor and then through D5. The signal is still "there" after D5, but it turns into a blob, you don't see the wave form anymore.
 

[vol.2]

Still no luck. I thought I found an issue when it started to hum loudly, but after removing the output transistors and checking them, and putting them back in again, it is back to what it was. I think the hum was just a wire touching something wrong on the inside.