No fm in my Sony 6055, see pix and please help

RRLegend

RRLegend

Active Member
I love the look and sound of these old Sony receivers and I bought this one for $20 knowing the FM wasn't working.

I removed the cover and after poking around awhile, found R420 looking very suspect (see close up photo, lower left). Can someone help me identify the board this resistor is on (I'm thinking it's the preamp).

Without a schematic I cannot ID the transistor or cap immediatly upstream of the burnt resistor. Any help would be greatly appreciated.
 

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That board is likely the FM board, though I don't have a schematic to verify. I thought the 7055 would be similar, but it's very different.
 
I have a service manual and I will try to get back with an intelligent reply tomorrow. But I am inexperienced. It is definitely a tuner board.
 
A lot of those older japanese units split the FM Front End(RF) and the IF board, along with the MPX board. I'd bet the board in question is either the IF board, or the MPX board, but leaning toward the MPX.

So on Top along the front and Right side, you have an AM board, MPX, FM IF, and the FM RF/Front end boards, but I can't ID them offhand. Power supply, and either preamp or amp is in Left back corner.

If that is the MPX board, you should be able to get FM in MONO, unless there are problems on the FM RF or IF boards.


Larry
 
larryderouin said:
If that is the MPX board, you should be able to get FM in MONO, unless there are problems on the FM RF or IF boards.


Larry
Click to expand...

Not always the case. Burnt resistor says something is shorting. The resistor may not be the problem, but a result of the problem. If its open and it's affecting the supply to the MPX board then you will likely have nothing out, mono or stereo. Problems with the muting circuit can also cause no audio.

If the tuning meters appear to work normally, but no audio then the it is usually the MPX, audio amp/buffer or muting circuits. No meters, then RF, IF or power supply.
 
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The tuning meters do move when I move the tuning knob. I get no FM at all, but I do get AM.

Concerning the burned resistor being the result of a problem and not the cause was exactly what i was thinking and why I didn't rush out to the shack to replace it.

I'm away from home for the next month, gearing up for radiation and chemo and so don't have my trusty can of deoxit or MM at my disposal.

A side note on our great hobby; My wife cannot believe in between the 3-4 appointments and medical tests, I've been going out and finding old audio components. I'm in a new city and it's refreshing checking new places and I have scored a super nice Rotel RX-803 in the origional box, a nice Sansui 661 and the Sony 6055 that we are currently talking about.

I've got a hell of a battle in front of me but sitting here poking around a old receiver helps me get my mind off things and being able to listen to music has kept our spirits up (BTW the 661 has an excellent tuner).

Anyhow, sorry about the tangent and keep the tips coming!
 
Thanks for the scans.

I too am still learning and wish I had a better idea on the next logical step in the troubleshooting process.

Not really sure what is "upstream" and "downstream" of resistor 420, so is it stupid to just start checking caps and transistors that are connected by tracers to R420, starting from the closes and moving outward?

Knowing that a burnt resistor is probably caused by a short somewhere, is there a place/component that is more likely to short that would be a good place to start?
 
Diagram 5 is the one you want to look at.

R420 is driven by transformer T401, on the upstream side and via R421. The loaded side is connected to another transformer? marked MU401 and from that to the transistor Q401. I'd check Q401 and that filter cap C404 for shorting. You could start by checking voltages at Q401 which are marked in the diagram (1.0V, 5.8V, 15.0V).

The other things connected to R56 have higher resistance paths so unlikely to have been able to cause that kind of heat damage.

Only other thing I can see but seemed less able to have caused that would be wrong wattage stereo lamp installed.

John
 
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Second checking C404. Even if the transformer were shorted or the transistor was bad, there still isn't any paths to ground with a low enough resistance to cause the resistor to open. An observation - 16 volt cap has almost no voltage head room - better off using 25 (or higher) voltage cap if it is bad. As far as I can tell, the lamp is upstream so probably not a factor. Good luck.
 
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