?Testing Transistors?

[rswojo]

I'm not in front of the receiver yet but here is an old AK post related to this MPX problem.

https://www.audiokarma.org/forums/index.php?threads/need-a-part-for-my-sony-str-6060fw.56340/

I took a look at the parts list, L401 and L402 are the same part number.

Also, you didn't comment on my theory as to how the bias got so high, Post #170. Please comment.

 

[Ylli]

Instead of lifting C406, you could just remove Q402 and scope the top of R406 or R409.

 

[rswojo]

Instead of lifting C406, you could just remove Q402 and scope the top of R406 or R409.

C403 put back. Q402 removed. Nothing at R406 or R409, either side.

 

[Ylli]

https://www.audiokarma.org/forums/index.php?threads/need-a-part-for-my-sony-str-6060fw.56340/

Good research. Yes, L402 was on my suspect list.

I took a look at the parts list, L401 and L402 are the same part number.

One is shown schematically as just an adjustable inductor, the other shows as a transformer. I suppose they could be using the same part as L402 for L401 and just not connecting the secondary.

Also, you didn't comment on my theory as to how the bias got so high, Post #170. Please comment.

Yeah, I noticed I forgot that. It depends on the type of variable resistor installed. You need to be careful that you don't get any paint/glyptal/nail polish/other sealant on the carbon path of the resistor. Doing that would make it rather hard to adjust in the future if necessary.

Three parts on my suspect list (other than the caps). Q401, L401, and L402.
The voltage readings on Q401 do not indicate a bad part, but it could have a low hFE.
L401 is not open, as there is a voltage on the collector of Q401 that is higher than would be there if only the resistor was connected.
L402? Since the coils are capacitively isolated you could measure the coil resistances to determine if there was an open. That might be the best next step.

 

[rswojo]

OK, I'm measuring 14.8Kohms between the points where C404 and C406 connect to the board on either side of the solder line that looks common to both coils..

How do you put the red circles on your board diagrams, it would be helpful if I knew how to do that.

 

[rswojo]

I am also measuring 22.9 ohms between where the end of C404 connects to the board and the solder line and 14.8K between where C406 connects to the board and the solder line that looks common to L401 and L402.

 

[rswojo]

See my edit to above post. So one one side of the coil it looks like I am seeing only the 15K R405, not the coil short, the other side is good.

 

[Ylli]

See my edit to above post. So one one side of the coil it looks like I am seeing only the 15K R405, not the coil short, the other side is good.

That looks like a good diagnosis. Can you pull a new part off the parts board?

For the circles, I just use the built in windows Screen Snip/Snip & Sketch.

 

[rswojo]

I have the coils off the parts board. L402 is open one side. L401 is a good coil. Removing L402 from MPX board in receiver and replacing with the L401 from parts board.

I'm using a Mac.

 

[rswojo]

"New" coil is in. Any further tests or instructions before I put back the 3 transistors?

 

[Ylli]

Leave Q409 & Q411 out for now. We can make some measurements before we reinstall them.

 

[Ylli]

I have the coils off the parts board. L402 is open one side.

L402 from your parts board has an open winding? Wow, Sony must have had a bad batch of coils back in that time period.

 

[Ylli]

I'm using a Mac.

I thought a MAC had a better photo editor than the windows one

 

[Ylli]

When you fire it up, I'd like you to measure the voltage at the base of Q410 when tuned to a stereo station and when off tune. Should be >2 volts when tuned to a stereo station and <1 volt when off tune. If good, you can reinstall Q411 and confirm proper operation of the stereo indicator light.

Then as for Q409 and the 'noise gate'. I saw some data a while back that looked not quite right on this circuit, which is why I had you remove Q409. This circuit listens for interstation 'hiss' and if it hears it disables the 19KHz/38KHz chain. To test:
Measure the voltage either side of R457 to ground. When tuned to a clear station, that voltage should be near zero. If tuned between stations so all you hear is the interstation hiss, then that voltage should go to >1 volt. If good, you can put Q409 back in.

 

[Ylli]

"New" coil is in. Any further tests or instructions before I put back the 3 transistors?

What is the third one that is out? If it is Q401/402/403, yes put it back in before doing the above couple of voltage measurements.

 

[rswojo]

Leave Q409 & Q411 out for now. We can make some measurements before we reinstall them.

Q402 back in. Q409 and Q411 still out I believe, double checking. Yes, Q409 and Q411.

 

[Ylli]

We may be seeing the light at the end of of the tunnel.

 

[rswojo]

I thought a MAC had a better photo editor than the windows one

Don't know, never used it.

 

[rswojo]

When you fire it up, I'd like you to measure the voltage at the base of Q410 when tuned to a stereo station and when off tune. Should be >2 volts when tuned to a stereo station and <1 volt when off tune. If good, you can reinstall Q411 and confirm proper operation of the stereo indicator light.

According to board diagram Q410 Base is 0V when Mono and 2.75 Stereo. Base=0V when tuned to a stereo station or off station. Maybe there is no light at the end of this tunnel.

 

[Ylli]

So we are still not getting signal through the 19KHz/38KHz chain. Well, dig the scope back out and look at the collector of Q402. You are looking for a 19 KHz signal/ so sweep rate 50 uS/div.