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HK 730 FM Stereo Alignment

Hmm, I'm running out of ideas. You have composite audio in to the demux. Lifting R309 should have disable the 'weak signal stereo inhibit'. You have 19 KHz at TP301. Did you run VR301 through it's range? Did you measure the frequency at TP301 or did you just see a signal there?
 
Yes, I ran VR301 up and down. I viewed a sawtooth waveform at TP301 on the 100MHz scope and it read 19KHz on the separate freq counter. I must be missing something simple. I re-capped this board, but the light wasn't working before the re-cap. I inspected all the solder joint under magnification and double checked the path from the lamp to the IC. All looks good. I'm thinking of pulling and testing all transistors that may have an effect on this circuit.
 
The only transistor that would have an effect on this would be Q315, and you removed that from the circuit when you lifted R309. In your first post you said it sounded great. So I assume the signal getting to the de-mux chip is large enough. Does the output *sound* in stereo? If you manually ground IC301 pin 6 does the stereo light come on?
 
Yes, I can hear distinct stereo audio. And no, the stereo light does not come on when pin 6 is pulled low. (grounded to chassis)
 
Wouldn't matter reading back , you have no 0v at the stereo pin. If you have the required 12v at q6 and you ground pin 6 the bulb should light. This makes no sense to me , I've done this on any tuner with a ha1156 and it works.
 
Grounding IC301 pin 6 does not cause the lamp to light.....hmmmm
Should be about +13 VDC at the emitter of Q6 on the Sub Rectifier PC Board. That goes to one lead of the stereo lamp. The other lead of the stereo lamp goes to one end of R302 (180 ohm, 1/2 W). The other end of R302 goes to IC301 pin 6. Grounding pin 6 should turn the lamp on.

Put your voltmeter on the end of R302 that does *not* go to IC301 pin 6. You should read about 13 volts. Now ground IC301 pin 6 and note the voltage on the meter.
 
Ylli said:
Grounding IC301 pin 6 does not cause the lamp to light.....hmmmm
Should be about +13 VDC at the emitter of Q6 on the Sub Rectifier PC Board. That goes to one lead of the stereo lamp. The other lead of the stereo lamp goes to one end of R302 (180 ohm, 1/2 W). The other end of R302 goes to IC301 pin 6. Grounding pin 6 should turn the lamp on.

Put your voltmeter on the end of R302 that does *not* go to IC301 pin 6. You should read about 13 volts. Now ground IC301 pin 6 and note the voltage on the meter.
Click to expand...

I read 12.63 VDC at the emitter of Q6. Grounding pin 6 of the IC does not turn on the lamp. Confirmed the continuity to lamp on both lines.

Note: During the course of re-capping, I had completely removed the preamp board to make it easier to work on. I took numerous pics and kept notes in order to wire it back up correctly. However, there's always the chance of a miswire somewhere. I think I should re-check all that wiring.
I really appreciate your help with this problem. Thanks!
 
bktheking said:
Wouldn't matter reading back , you have no 0v at the stereo pin. If you have the required 12v at q6 and you ground pin 6 the bulb should light. This makes no sense to me , I've done this on any tuner with a ha1156 and it works.
Click to expand...
Makes no sense to me either. I need some more ideas.
 
Here's where I'm at Ylli and All:
Q6 emitter is 12.6V. The stereo lamp lights when R302 away from pin 6 is grounded, but not when pin 6 is grounded. R302 (away from pin 6) measures 12.6V and 11.6V when pin 6 is grounded. Pin 6 remains at 12.7V under all conditions. I replaced R302 and Q315 to eliminate suspects. IC301 is also new and Vcc on pin 1 is 12.2V. I adjusted TP301 to exactly 19.0 KHz. Where else should I look?
 
A Sound Mind said:
Here's where I'm at Ylli and All:
Q6 emitter is 12.6V. The stereo lamp lights when R302 away from pin 6 is grounded, but not when pin 6 is grounded. R302 (away from pin 6) measures 12.6V and 11.6V when pin 6 is grounded. Pin 6 remains at 12.7V under all conditions. I replaced R302 and Q315 to eliminate suspects. IC301 is also new and Vcc on pin 1 is 12.2V. I adjusted TP301 to exactly 19.0 KHz. Where else should I look?
Click to expand...
You either have a bad connection between pin 6 and R302 or are not making a good connection at pin 6 when you ground it. Measure the resistance from Pin 6 to R302.
 
dr*audio said:
You either have a bad connection between pin 6 and R302 or are not making a good connection at pin 6 when you ground it. Measure the resistance from Pin 6 to R302.
Click to expand...
0Ω. And pin 6 to the other side of R302 is 180Ω. It appears that something is forcing pin 6 to remain high.
 
A Sound Mind said:
0Ω. And pin 6 to the other side of R302 is 180Ω. It appears that something is forcing pin 6 to remain high.
Click to expand...
It can't stay high if you hard-ground it. When you say "R302 away from pin 6" are if you are referring to the side of R302 not connected to IC301-6? Be careful, you have a 13 volt supply and a bulb rated at 8 volts.

Maybe lift the end of R302 that connects to IC301-6 and then try grounding the open end of the resistor.
 
Ylli said:
IC301 pin 6 is an open collector output. There is nothing in the chip that could be holding that pin high.
Click to expand...
Ylli said:
It can't stay high if you hard-ground it. When you say "R302 away from pin 6" are if you are referring to the side of R302 not connected to IC301-6? Be careful, you have a 13 volt supply and a bulb rated at 8 volts.

Maybe lift the end of R302 that connects to IC301-6 and then try grounding the open end of the resistor.
Click to expand...

I tried this. No light. Tacked in a 1k pot from the light side of R302 to gnd. Light came on at around 25Ω.
How does the IC trigger the lamp driver? What external components that support this IC can I examine? It has a good power supply and there is 19KHZ on pin 10.

Another thing to consider is the right channel takes 1-2 seconds longer to come up than the left.
 
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