?Testing Transistors?

[Ylli]

LED is now on with the jumper installed and the brightness is about right.

And I assume off with the jumper removed. Let's leave Q411 out of the circuit for now.

One resistance check to make: Ohmmeter between ground and the cathodes of D401/D402. Should show near zero ohms with the FM Auto/Mono switch in 'Mono' and should show >10K ohms in the 'Auto' position. Leave in the 'Auto' position. We're checking to see that the FM Auto/Mono switch is not shorted.

Let's disable the noise gate (Squelch) so that doesn't interfere with troubleshooting the 19KHz/38KHz chain. Remove Q409.

Voltmeter on the base of Q410. Tune to a know stereo station. Any voltage there now? (there wasn't before).
Is that 'scope available? If yes please tell me the make/model so maybe I can help with the settings.

 

[rswojo]

And I assume off with the jumper removed. Let's leave Q411 out of the circuit for now.

One resistance check to make: Ohmmeter between ground and the cathodes of D401/D402. Should show near zero ohms with the FM Auto/Mono switch in 'Mono' and should show >10K ohms in the 'Auto' position. Leave in the 'Auto' position. We're checking to see that the FM Auto/Mono switch is not shorted.

Let's disable the noise gate (Squelch) so that doesn't interfere with troubleshooting the 19KHz/38KHz chain. Remove Q409.

Voltmeter on the base of Q410. Tune to a know stereo station. Any voltage there now? (there wasn't before).
Is that 'scope available? If yes please tell me the make/model so maybe I can help with the settings.

Stereo lamp - jumper removed, OFF.
D401/402 cathodes: Mono: short to ground, Auto: about 7Megohms
Q409 removed
Q410: B=0V on stereo station

Scope is ready to be powered up. Tektronics 2465

Would you like a PDF of the service manual? It's in color and about 50M in size. Too big for an email but Firefox has a free file transfer service I believe.

 

[Ylli]

Just needed a picture of the face with the controls and I found that online. Looks very familiar, may have been one of the scopes we had in the lab before I retired 6 years ago.

Let's view channel 1, AC coupled, 1 Volt/Div (if a x10 probe, then 0.1 V/Div). <--adjust as required
Horizontal Sweep 50 uS/div <-- should be right, but you can go one up or down if the display looks better.
Trigger AC coupled, channel 1, auto or auto Lvl <--adjust as required

Q410: B=0V on stereo station

So we don't seem to have any 19KHz getting through.
The composite audio from the FM detector is applied to the base of Q401. That composite audio consists of Left and Right channel information as well as a 19 KHz pilot carrier. The entire composite audio signal is tapped at the emitter of Q401 and passed through a 19 KHz (and a 78 KHz) trap and is applied to the stereo demodulator. The collector of Q401 is connected to a tuned circuit that selectively picks off the 19 KHz signal.

'Scope the collector of Q401 and try to find a 19 KHz sine wave signal. I would expect it to be *at least* 1 volt peak-peak and will probably be accompanied by a bit of noise.

 

[rswojo]

Here it is.

 

[Ylli]

Not quite what I was expecting to see, but there is some 19 KHz there. Appears it must be riding on a larger AC signal and you were able to sync at one point.
Let's look at the base and the collector of Q402 with the scope.

Oh, be sure the "Var" knobs on the channel gain is set fully CW into the detent.

 

[rswojo]

Base Q402

 

[rswojo]

Collector Q402

 

[Ylli]

Why is the trace sloped like that? Do you have the probe ground tied to the chassis?

 

[rswojo]

Why is the trace sloped like that? Do you have the probe ground tied to the chassis?

There is no probe ground on this probe

 

[Ylli]

Well, you will need to add one. Luckily, we are dealing with basically high audio frequencies. You can just tie the scope ground to the receiver chassis. Plug a banana in here and clip the other end to the receiver chassis:

 

[Ylli]

Then lets look at Q401 collector again.

 

[rswojo]

There is no probe ground on this probe

 

[Ylli]

If you connect the scope chassis (that ground connection in the pix) to the receiver chassis, that will provide a signal ground and get rid of the 60 Hz hum you were seeing on the scope.

 

[rswojo]

Then lets look at Q401 collector again.

Q401 collector again at .5V/DIV

 

[Ylli]

Just doesn't look quite right, but lets look again at the base and collector of Q402.

 

[rswojo]

Base, then collector of Q402. 5mV/div, flat line at 1V/div

 

[rswojo]

I can get a somewhat better trace at 2uS/div

 

[Ylli]

Yeah, nothing there.

Earlier, we had Q401: C=9.0V, B=4.1V, E=3.5V. Might want to confirm.
They are low but reasonable and not indicative of a bad transistor. Doesn't appear to be an open L401.

Let's remove C403, the 100 pF cap, and see what happens to the collector waveform of Q401.

 

[Ylli]

I can get a somewhat better trace at 2uS/div

19 KHz should be a cycle time of 1/19000 = 52 us. Don't know what you would be seeing at 2 us/div sweep rate.

 

[rswojo]

Q401 same voltages as before.
When I scope Q402 at 2uS it looks more like a noisy sine wave. Noise?
Can't find C403 on Board Diagram. Trying one more time then will have to get a clue as to where it might be from schematic

Found C403 between 2 Styrol caps. Removing.