?Testing Transistors?

[Ylli]

Audio signal trace - do you have audio on the inductor?
If yes, then check for audio at that 8.2K resistor.

If OK, then check for audio at the top of R404 (emitter of Q401).

If OK, then check for audio at the top of R415.


Audio level should be very close to the same on all these points. May be a little less on the last point.

 

[rswojo]

At 8.2K resistor - good
At R404 - 330 ohm - good
At R415 1.0K - nothing

 

[Ylli]

So there appears to be a problem in this signal path.

Before I have you pull any parts, let's make a couple of ohmmeter measurements. With power off, measure the resistance on both sides of R415. One side should measure near zero, and the other should read a bit less than 1000 ohms (1K). Confirm that you looked for audio on the side that measured near 1K. (confirm that I did not screw up and tell you the wrong side ). Now measure both sides of R451. One side should measure the same as the top of R451 (bit less than 1K) and the other side should measure 510 ohms higher. Don't think you can get at any other points from the top of the board.

If the above resistance measurements are OK, you will need to pull and test (or just replace) C405, the 100uF coupling capacitor. While it is out, check the DC resistance between ground and the pad where the capacitor negative lead was connected. Should be a bit less than 1.5K. Checking the continuity of the filter coils (L403, L404) here.

 

[rswojo]

R415 - 750, 0.2 Audio was on 750ohm side
R451 - 752, 1250

A 100uF 16V cap should be OK, right? That's all I have unless I scavenge an old one somewhere.

 

[Ylli]

100 uF/16V will be fine Do check the filter coils for continuity when you remove C405.

 

[rswojo]

Capacitor negative pad to ground - Open

 

[Ylli]

Well then there is a open in one of the filter coils or there is a cracked solder connection in that line. Resistance from the end of R451 connected to L404, you measured 1250 ohms. Measure the resistance on the pin of L404 that connects to this resistor. Measure the resistance of the other two pins. Move to L403 and measure the resistance on those pins. Looking for the open. If/when you find it, try reflowing the solder on suspected junction.

 

[rswojo]

L403 is open. Re-soldered L403 connections. No help. I just happen to have another L403 that reads about 6.5 ohms though.

 

[Ylli]

Well then, PUT IT IN! And if C405 is still out, put that back too.

 

[rswojo]

OK!!! LOL

Back in a little while.

 

[Ylli]

How is it that you managed to have a replacement L403?

 

[rswojo]

Sold a parts MPX board to a guy from Michigan on the condition he send his bad one back. I should have kept it, his FM is working fine with my parts board.

Results are distorted, somewhat louder FM mono. Levels very low on FM stereo. Good at inductor.

Signal trace same points?

 

[Ylli]

If you use the signal tracing technique, is the FM audio clean at the top of R415? Is the stereo light behaving itself?

 

[rswojo]

Clean FM at R415.

 

[Ylli]

Stereo lamp behaving? Does it come on reliably on an FM stereo station, and go off when turned off station? Trying to determine if if the 19 KHz/38 KHz chain is working properly.

 

[rswojo]

Test signal at negative of C416? Stereo lamp always on.

 

[Ylli]

Yes, you can check the signal level/quality at C415 & C416. That would be decoded stereo at that point.
Stereo light is always on even when you are tuned to a dead frequency? Look at the voltage on the base of Q410. Should be > 2 volts when tuned to a stereo station and <0.5 volts when not tuned.

 

[rswojo]

Clear FM at C415. Nothing at C416. 0V at the base ofQ410, stereo lamp always on.

 

[Ylli]

Get back to you after dinner.

 

[rswojo]

How about we start again tomorrow? I could use a break.