Harman Kardon 730 stereo bulb driver

[robz2000]

I think an old dead bulb shorted the stereo light in a Harman Kardon 730. If I ground directly to chassis it lights up fine. And of course stays lit. Getting @12v from the striped orange cable. Do any of you know what part of the system turns on/off the stereo bulb? Hoping maybe it's one or two transistors I can try replacing.
Thanks in advance

 

[llwhtt]

Looks like the STEREO lamp is controlled by IC301/Pin 6 thru R202, 180 Ohm, resistor. Check the resistor then check to see if Pin 6 changes with the tuning, should go LOW in STEREO.

 

[robz2000]

thanks a bunch llwht. will take a look and see what i find there. appreciate the response.

 

[dlucy]

Plan on refreshing all the electrolytic caps while you're in there.

There stereo light is controlled by the MPX chip, it needs to "see" a stereo signal, there is a trimpot that controls how/where the MPX chip "sees" the signal enough to light the lamp.

Here is the adjustment procedure from the service manual. There are other adjustments you probably want to review there, too.

 

[robz2000]

Thanks. Dlucy. my #1 rule is to fix what is broken first, then move on to improvements like recapping and swapping our noisy transistors. hoping to make progress on this today.

 

[robz2000]

I'm going from my own memory here, but i don't think it should get 24v no matter what. In fact, it can vary depending on the output from the MPX board (as dlucy described). If you move the dial, that voltage should vary a bit and down to zero when not supposed to light up.
never heard of anything beyond 12v on these bulbs either, usually 8. again, this is from the top of my head, and i do not have a service manual for the 930.

 

[chazix]

930

Misplaced reply?

Regardless, voltage measurements on either terminal of a stereo lamp are generally meaningless if the lamp is open. Most such circuits need to be loaded by the lamp to make any sense of the voltage measurements.

 

[robz2000]

Yeah, thought I was answering a voltage question from someone on the 930 stereo lamp. ?!?!? email link took me here. apologies to this thread, and the unknown question i was attempting to answer!

 

[Yused Towork]

The voltage supplied to the lamp & the circuit is measuring 24 volt with 2 resistors in parallel before the lamp. On the schematic, the lamp is rated as 14volt 80 mA. The lamp is blown open, and the other side of the lamp is connected to a darlington pair looking circuit which comes off the IC. If I measure voltage on the pre parallel resistor side, I get the same 24 vdc. SO I am thinking the voltage needs to be dropped for the lamp & circuit to work properly. No?

 

[Yused Towork]

Yeah, I'm not sure how my post got here, but I'm in the ball park, I guess. I think the 730 & 930 have similar circuity for the stereo bulb? Do these bulbs act like the dim bulb tester, where they are in series and run off the current?

 

[Yused Towork]

@robz2000 I'm working on the HK930C, which I think should have a similar bulb circuit as the 730, which is why I'm posting here. @dlucy . Can anyone tell me if this bulb is in series with the voltage & circuitry, based on the schematic? After the chip sees the signal strength, How does this bulb circuit work? Does the chip take the other side of the bulb to GND through TR303? If so would 24v work in the circuit? I'm trying to figure-out what voltage is supposed to feed this bulb. 730 manual is clear at +12v=B3. The 930 manual tells me the bulb is 14v, but does not spec the B5 voltage. Here are both the 930 & 730 lamps-the chip is different & the 930 has different circuitry, but I think they operate pretty similar? I am trying to figure-out what B3 voltage is so I can replaced the fried bulb and not re-fry.