Kenwood KR-7070 FM muting issue - photocell lamp

[spaghetti]

I am having issues with the tuner section of my Kenwood KR-7070 receiver.
After a long troubleshooting and other minor fixes I realized that something is wrong with at least one electronic component that was unknown to me, a lamp photocell marked HTV P388-W10-380, see the attached image:

There's a total of 3 of them in the receiver, one on the auto tuning board, the other two are on the MXP board. One of the latter seems to have failed, the lamp wirings read open, while the other photocell on the board reads about 50 ohm.
I think at least one of them is related to the muting circuit, the MPX board schematics is here:

The receiver currently plays FM music very quietly in mono, seems like muting is always on, while stereo stations are a bit louder but sound is not right anyway.

I tried to short circuit what in the schematics looks like a resistor on CDSC2 and I got perfect music, but the muting did not work anymore, so I think I need to find a replacement for the failed component if I want both music and FM muting.

I tried to source a replacement for the photocell but I could not find any, or any information about it for what it matters, I suppose we're talking vintage technology that is long gone.

What are my options today? Should I replace it with a reed relay, which I have seen already used in other vintage tuners muting circuits?

 

[amptramp]

It may be possible to replace the lamp with a LED and series resistor. A reed relay may give you a popping noise every time it activates whereas the photoresistor has an inherent time delay which prevents that. The LED - photodiode device is called a Vactrol and clones are made by Excelitas and they are carried by alliedelec.com and coolaudio.com also has a version of the original Vactrol design. They usually run with 40 mA into the LED.

 

[rcs16]

Those old st decoders are confusing, this one is definitely different, a new one to me, I have not got my head around how it all is to work, need to look at it some more. I was learning to tie my skates when this one was designed
Not sure if a reed relay may do the job.
As said above you could see if you can try to make it work again or sub a modern day replacement if it fits the function. It is a lot of fiddling, if you are into it. if you can figure out the lamp specs it could be replaced as another option. Figuring out the response curves to match the original I think would be too time consuming vs replacing the st decoder with a chip and be done with that except figuring out or leave the mute as is since you will have spare lamp/photocells. All work one way or the other if you can not find the original part (would have to come from a old donor unit) or reproduce an equivalent ( make yourself a component EE)

Good Luck

 

[spaghetti]

Thank you both for the suggestions and the insight. Since I had nothing to loose today I cranked the photocell open, as expected I found a miniature light bulb inside, see the picture:

Of course the bulb filament is open, I replaced it with a 6V lamp and will see if it works fine in the auto tuning circuit. Meanwhile I moved the working auto tuning photocell to the MXP board, so now I have good FM sound and muting. Auto tuning is something I can surely live without in case I can't fix the photocell or find a good replacement of it. I'll keep you updated.

 

[avataar]

Did you manage to get this working with a new lamp? I got a KR-7070 recently and one of the issues is no stereo decoding. I identified a faulty bulb in one of my opto-isolators too. I still have to fix the main amp before getting to that but I'm curious how it went for you. Also, if you still haven't soldered yours in could you measure the resistance with and without light applied? It would be good to know if one wants to look for a more modern LED-based replacement.

 

[spaghetti]

Hi, yes I managed to fix the optocoupler with a miniaturized light bulb. I can't check the resistance, the receiver is now stored somewhere in my collection.

 

[Bert Beukema]

I have the same receiver on my bench. Sadly on mine the two photocells on the MPX board are completely busted.
Which means I will have to rebuild them from scratch.
Problem is, I don't know squat about these things, they are rather exotic (only found them in B&O beomasters).

Would anyone care to join in to design a new circuit?
I am thinking I would need an LED and a photocell (resistor)? But what values?
Because they are busted I can't get any readings of the existing resistors.

 

[Bert Beukema]

Oh, the third one is probably gone as well. I am not getting consistent readings on them.
But that could be my inexperience with reading it.

 

[aileenamegan]

I do have the same Problem with my 7070...