?Testing Transistors?

[rswojo]

Voltages across R664/774 were the only ones I didn't write down. IIRC it was 30 something. Speculation - bias too high, crank up to full volume in attempt to hear FM audio results in blown Q802?

I recapped this unit in 2013, cleaned controls and switches, replaced lamps with LEDs, replaced rectifier diodes and power switch. At that time I set bias to specs, 7 years later bias is way too high. It's past time for pot replacement. Back around 2pm.

 

[rswojo]

watching this is like half of my own troubleshooting and repair efforts. Start on one problem, realize part way through there is a new and different problem in addition to the original.

For you people that are experienced and good at this stuff it is just a challenge. For the rookie budding wannabe tech, panic and then despair is the result.

 

[Ylli]

I recapped this unit in 2013, cleaned controls and switches, replaced lamps with LEDs, replaced rectifier diodes and power switch. At that time I set bias to specs, 7 years later bias is way too high.

I was going to ask you about the units history - with the bias set that high, it was almost as if someone was trying to push it closer to Class A.

Why Q802 went out is still a mystery to me. I can not see any set of conditions that would push Q802 out of a safe operating range. And a B-E short is a somewhat unusual failure mode. But then again, having an enclosed inductor (L403) fail open is rather unusual, and we still haven't figured out the misbehavior of the stereo detect switch (Q410/Q411), or why the 19KHz/38KHz chain doesn't seem to be working. Still some fun ahead.

 

[rswojo]

Injected hum at R464 and R465, good results, touching R464 produced hum in left channel, R465 produced hum in right channel. I was going to attach the bottom side of board layout got top instead, I'll attach the bottom next.

 

[rswojo]

Mounting diagram, solder side.

That doesn't help much.

 

[Ylli]

Injected hum at R464 and R465, good results,

Great. And perfect timing, I just finished my Burrito for 'Taco Thursday'.

Reinstall C415 and C416, observing polarity. You should now get FM sound out of both channels, but it won't be in stereo.

I did some number crunching and have come up with what I believe would be needed to properly simulate that incandescent stereo lamp with an LED. When not in stereo, the lamp would be off, and in that condition the filament resistance would be down near 100 ohms. With Q411 turned off (stereo not detected) or removed, the voltage on the high end of C434 should be +12.7 volts. This provides proper bias to other parts of the circuit. When Q411 is on, we want about 3 mA through that LED. To obtain these results.......
Add a 100 ohm resistor (1/4W ok) across the existing R807. Then change the resistor in series with the LED to 1K.

Since you appear to have parts boards, and you already salvaged a new Q802... A 100 ohm resistor can be found on the protection board at R771. A 1K resistor can be found on your spare MPX board at R414 or R480.

Confirm that Q411 is still off of the board. Measure the voltage at the + end of C434. Looking for between 12.5 and 13 volts.

Were you able to obtain a 'scope?

 

[gadget73]

just out of curiosity, if the stereo lamp were out would that cause odd problems? Asking because I chased garbled stereo output on a Sherwood tuner for a while. Couldn't figure it out. Found the lamp was bad and just for S&G I changed it to see if it was even giving me stereo indication. The problem went away completely. The lamp was required to bias that particular section of the circuit correctly and it got really distorted without it. Its an odd lamp and I worked around it with an LED and some shunt resistors.

 

[Ylli]

just out of curiosity, if the stereo lamp were out would that cause odd problems? Asking because I chased garbled stereo output on a Sherwood tuner for a while. Couldn't figure it out. Found the lamp was bad and just for S&G I changed it to see if it was even giving me stereo indication. The problem went away completely. The lamp was required to bias that particular section of the circuit correctly and it got really distorted without it. Its an odd lamp and I worked around it with an LED and some shunt resistors.

When this unit does not see a stereo pilot, Q410 and Q411 turn off. that allows the node on Q411's collector to be driven high by the current through the (not lit) bulb. That current biases the demux diode ring to turn on both series diodes (D403 & D406) and allow the audio (which is arriving at the center tap of the transformer) to pass to the left and right channel outputs. In addition, it raises the voltage on the emitter of Q403 (the 38KHz amp) which turns it off and prevents unwanted noise from entering the diode ring demod.

In this unit, that 1K resistor across the stereo bulb acts as a 'fail safe' and provide sufficient current to do the switching even with the bulb open. Other receivers/tuners may not have this feature, and yes, the MPX section might malfunction simply because the bulb blew.

By doing the resistor change I mentioned above, the current path through the LED looks more like the current path did though a good incandescent bulb.

 

[rswojo]

Great. And perfect timing, I just finished my Burrito for 'Taco Thursday'.

Reinstall C415 and C416, observing polarity. You should now get FM sound out of both channels, but it won't be in stereo.

I did some number crunching and have come up with what I believe would be needed to properly simulate that incandescent stereo lamp with an LED. When not in stereo, the lamp would be off, and in that condition the filament resistance would be down near 100 ohms. With Q411 turned off (stereo not detected) or removed, the voltage on the high end of C434 should be +12.7 volts. This provides proper bias to other parts of the circuit. When Q411 is on, we want about 3 mA through that LED. To obtain these results.......
Add a 100 ohm resistor (1/4W ok) across the existing R807. Then change the resistor in series with the LED to 1K.
View attachment 1815717
Since you appear to have parts boards, and you already salvaged a new Q802... A 100 ohm resistor can be found on the protection board at R771. A 1K resistor can be found on your spare MPX board at R414 or R480.

Confirm that Q411 is still off of the board. Measure the voltage at the + end of C434. Looking for between 12.5 and 13 volts.

Were you able to obtain a 'scope?

FM sound both channels. + end of C434 = 0V. That jumper is still in place. stereo lamp is out.

 

[Ylli]

Forget about the jumper. Lift it for now.

 

[rswojo]

Forget about the jumper. Lift it for now.

12.29V

 

[Ylli]

OK, that's good. Is the LED out with that jumper removed? Should have asked before, but was the LED on with the jumper installed?

 

[rswojo]

OK, that's good. Is the LED out with that jumper removed? Should have asked before, but was the LED on with the jumper installed?

Stereo LED is out with jumper removed and was out when the jumper was installed.

 

[Ylli]

Should have been on with the jumper. I guessed at the nominal voltage drop on the LED you used. I may have guessed low. With the jumper installed, please measure the voltage drop across R807. My calcs pegged it at about 3.6 volts, should have been enough to turn on the LED. May need to increase the value of that 100 ohm added across R807. Perhaps give 220 ohms a try. Want the LED on with the jumper installed and off with the jumper lifted.

 

[rswojo]

I tacked on a 120 ohm resistor before I put the jumper back, so with R807 being 1220 ohms now I read 1.64V across it. Forward voltage on my LED is 3.4V.

I'm getting close to the end of my rope for the night.
Yes, I have a scope and I am glad you are a VERY patient person. The last time I used one was about 12 years ago.

 

[Ylli]

Two things - I did use too low a LED voltage drop. I put an LED diode in the sim, and thought the sim would use an 'average' LED voltage drop, but it actually used the drop of a standard diode. After fixing that, it looks like a better value for the added shunt resistor would be 470 ohms.

That 470 ohm resistor needs to be added *in parallel* with R807 (1K). That creates an equivalent resistance of 250 ohms. Then the LED with the 1K series resistor needs to be connected in parallel to this combination.

Just for completeness of my explanation, here is what the relevant DC equivalent circuit looks like:

 

[Ylli]

The above sounds confusing, even to me. The resistor you put in parallel with R807 needs to be just small enough that the stereo light is off with the jumper removed - not too much lower. Then the resistor that is in series with the LED needs to be selected for desired LED brightness with the jumper installed. Hope that is clearer.

Leave it at that for tonight?

 

[rswojo]

I understand that except for connecting the LED with the 1K resistor in parallel to the revised R807. I need to look at the schematic and I need to be fully awake to attempt that. I'm ready to shut down for the night.
My LED is "cool white" if I had a warm white LED I wouldn't need such a high resistance to tone it down.
Hasta manana.

 

[Ylli]

I think you originally had it wired right, just need to add the shunt and maybe change the series resistor.
Sometimes the simplest things are the hardest to explain.

 

[rswojo]

Tacked on a 470 ohm resistor in parallel with R807. R807 reads 3.27V. LED is now on with the jumper installed and the brightness is about right. Perfect!