Sansui 5050 no FM at all

[jmhanna]

"No FM at all"

Kinda like Steely Dan sang about (...no static at all...) only worse. I've been using this 5050 receiver for several years now, works perfectly with the phono stage (about 98% of the use, for me) and AUX and AM inputs all work as they should. FM, however, plays nothing... No noise, no static, nothing at all. No signal strength, and the tuning meter stays at center, even with a large Radio Shack VU-120 antenna on the roof. It's as if the FM tuner has no power (but the dial pointer does light up in the FM mode). It did work at one time, but it's been a few years since I tried it.

I just grabbed the combined 5050 / 6060 service manual and plan do dig in to it tonight, but thought I'd try asking you experts first, in case someone has experienced this before. I appreciate any help!!

 

[LBPete]

Welcome to AudioKarma Exclusively Sansui Forum. You join a long list of members who's first post is for help fixing a piece of Sansui history.

Sounds like a power supply problem. Do you have 13v at terminal 11 and 12 of tuner board F-2590?

Please post the serial number for the database. Click on the link in my signature block below.

- Pete

 

[jmhanna]

Pete, thanks for the prompt reply!! Here's what I measured at terminals 11 & 12 (referenced to terminal 8 for ground):
T-11 = 12.3 VDC (AM switch position) 0.6 VDC (FM position)
T-12 = 12.4 VDC (AM position) 5.6 VDC (FM position)

I unplugged the connector from the tuner board and measured this (using the same terminal 8 ground):
T-11 = 12.4 VDC (AM) 0 VDC (FM)
T-12 = 12.4 VDC (AM) 12.4 VDC (FM)

The FM switch position dropped the voltage on terminal 12 from 12 down to 5.6, so either there's something in the FM circuit pulling too much power, or maybe there's a regulator on the PS that is shot. Should these 12-ish volt readings be 13 or higher? I also stand corrected, the dial pointer is illuminated all the time, I thought it was coming on only in the AM and FM switch positions (thinking of my Marantz stuff, sorry!).

Does this make sense, and / or should I not have referenced ground on T-8? I'm 8 hours away from being able to take more measurements, so let me know if I should have been measuring ACROSS T-11 & T-12.

I really appreciate your help, I added this serial number to the Sansui Database.
Thanks! John

 

[LBPete]

You are spot on with your measurements. A few tenths of a volt doesn't matter here. T12 should have power all the time. It is troubling that it drops by half when in FM. I don't think it's a power supply problem because it doesn't drop when switched to AM. What's the voltage on T13? Does it change like T12?

- Pete

 

[jmhanna]

Since T13 was in the same connector I measured it last night also...

Connected to F-2590:
0 VDC - AM
2.2 VDC - FM

Disconnected from F-2590:
0.5 VDC - AM
12.3 VDC - FM

 

[LBPete]

Looks like there is some sort of current draw in the FM tuner that is exceeding the capacity of the power supply. The schematic has voltage values at various test points on F-2590. It's time to start running those down. Work out from T11-13 and record the values.

- Pete

 

[jmhanna]

Thank you Pete, you are awesome! I'll try to take these readings tonight, but I'm afraid that my copy of the combined service manual has some illegible voltage readings (in orange ovals) on the first copies of the schematics. Do you know of a better source for this image? If not I'll try to make do, or dig a little deeper online, but this copy seems to be everywhere.

John

 

[LBPete]

Can you read this one better? The circles dropped out in the scan but the voltages are there.

- Pete

 

[jmhanna]

Good heavens, that's much better! Thank you thank you thank you! I'll report back when I locate something noteworthy. I sure do appreciate it Pete!

 

[jmhanna]

OK, I'm inclined to go with your first suggestion Pete (a power supply issue) because ALL of these voltage readings are low, because I don't have that strong 13V going in to the tuner board while in FM mode. So, I took the brown wire out of the molex(?) connector (Terminal 13) and put my Fluke in series to measure the current when in FM mode, that was 108 MA, which doesn't sound crazy, does it? I started poking around the PS board (mine has the F-2701) and I think I may have a leaky C601 (see pic) and possibly it may have corroded a pin of that zener diode (zd601). Then again, could that be the infamous glue that I've read about Sansui using? I did a quick poke of the voltage across the zener and it is 12.84V. The pins of TR601 measured as E-12.3V, B-12.9V, C-24.81V, with the selector in AUX. I wish I checked them in FM position also, but I hastily ended my test session tonight. I'll do that the next chance I get. If TR601 is acting as a regulator, I wonder if it's faulty?

Also, an interesting "mod" that I saw when I looked at the selector switch from under the chassis, was a C1312 transistor tacked onto the selector switch, with one of the pins soldered to that brown wire I mentioned above (Terminal 13). I don't see that on any of the schematics I've seen, I may try to desolder it and check it with the fluke's diode test function.

Getting close, I think!

 

[LBPete]

The picture of the caps is glue not a leaky cap. It doesn't look like it's become corrosive so it's probably not your issue. I've never seen a transistor mounted like that. Is that electrical tape around that brown wire at the top of the picture? That's not factory either. Those modifications need further investigation. Can you determine if that switch wafer is switch for the power to the tuner board?

- Pete

 

[jmhanna]

Right on, I'm glad it's glue! It is not electrical tape around the brown wire, it's some kind of cloth or fiber tape, shiny black with lines in it. The orange wire coming in from the right is terminal 'f' from the power supply, 12 volts, and it is tacked to the switch wafer and branches off upward to terminal 12 on the tuner board. Thanks for taking the time to look at all this stuff!

 

[jmhanna]

OK, here we are more than a year later and I've finally figured things out, if anyone else has a similar issue. Using the fine advice of LBPete (thanks again!) here's the short story: Everything worked except FM (AM, AUX, Phono, Tape monitor). When switching to FM, the 12 Volt supply dropped down to below 6 VDC, indicating a large current draw from the FM tuner. I searched for faulty components, starting with transistors in the FM stage, no luck. I was thinking maybe one of the ICs, but I don't have a way of testing them. Started pulling electrolytic caps in the hopes of maybe finding something with unusually high ESR, and again no joy (this thing is loaded with nice Nichicons which seem to have maintained their values over the years). While poking around and unsoldering & resoldering caps I discovered a really well hidden solder bridge between 2 pads on the board! the bridged joint was poorly soldered, which explains why SOMETIMES the FM tuner had worked years ago. I should have taken a magnified photo of it but in my excitement of discovering it I quickly wicked it off and plugged her in and tested her. I was greeted with the sweet sound of FM static! Now it's time to clean that noisy tuning capacitor and enjoy this machine finally. These forums are invaluable and I appreciate everyone who takes the time to share their expertise and offer tips, hats off to you all!

 

[Oldsansui441]

Nice work, glad you have the FM up and running again.

 

[LBPete]

Gotta love a happy ending. Nice work.

- Pete

 

[Dihan Angelo]

Hi Guys

I too has the same problem!!! but i just replaced the D313 regulator trannys and is registering the correct voltages. I recapped the entire F-2590 board. however still no joy. I checked for readings at the test points using a scope and noticed that theres no reading at test point 2 (TP2) and at test point 1 (TP1) any idea why this is??? oh i replaced the all the ICs too as the legs were corroded. I also noted that the 2SC711 were replaced with 2SC945 some time ago.

When switched to FM mode theres only a slight movement on the centre gauge and no movement on the signal guage and no stereo lock either.

any ideas???

cheers