HK330A STILL Doing The Mono Thing

[DreadPirate]

I spent a couple of hours tooling around with this unit. I'm getting lots of practice dealing with lifted pads. I followed latala's advice to inspect the frequency doubler transistor TR-303. I took it off, it passed the diode tests. I also subbed a 2SA49 from the AM section, still same problem.

MPX Section: I have a Dick Smith meter, all the caps checked out. I tested all the diodes, those appear okay also.

Any suggestions as to what to do next would be appreciated.

********PREVIOUS POST*******
This H/K receiver is acting funny FM-wise. The signal strength meter is dead in FM, but only in FM, it works fine in AM. In addition, channels come in fine in FM mono and when you switch to FM stereo, the signal still comes in on mono (no stereo) and I have a good antenna. The FM beacon glows dimly and continuously when the unit is on and in all modes The schematic can be found here:

http://www.eserviceinfo.com/downloa...ardon_330A.html

Scroll down a bit for the link just under the "Download Restrictions" box.

Using a good viewer like Brava, the schematic is readable, I have found other viewers are finicky with .TIF files.

The only measurement I have made so far is at the stereo/mono switch, I get 104K Ohm in stereo and 0 Ohm in mono.

Any comments appreciated.

[DreadPirate]

New Link to Schematic:

http://www.eserviceinfo.com/download...rdon_330A.html

[rpapps]

Hi DreadPirate
The clue here is the low reading on the tuning meter.
The feed for it in FM mode comes from the base of TR104 so your problem lies somewhere back from that point. The sensitivity seems to be way down which forces the receiver into mono to minimise noise. Check TR101, 102, 103 & 104. It's also possible the fault lies in the front-end module. You say you have a good antenna. Is it external and did you have any thunderstorms around the time it failed?
Cheers
Rob

[DreadPirate]

I recently got this receiver, so I don't know what might have caused the problem. I mention that I have a good antenna because these older HK receivers have poor FM sensitivity, its tough getting stereo in the first place.

So the next step is to pop those transistors out and test them if testing in-circuit is inconclusive? Toolwise, all I have is a meter and the esr tester.

I thought the continuously lit dim bulb (FM indicator) would also be a major clue, does that say anything?

Thanks for your help.

Hi DreadPirate
The clue here is the low reading on the tuning meter.
The feed for it in FM mode comes from the base of TR104 so your problem lies somewhere back from that point. The sensitivity seems to be way down which forces the receiver into mono to minimise noise. Check TR101, 102, 103 & 104. It's also possible the fault lies in the front-end module. You say you have a good antenna. Is it external and did you have any thunderstorms around the time it failed?
Cheers
Rob

[rpapps]

There is always a small current flowing through the Stereo lamp via R323 and R324. This is done to preheat the filament so that TR304 does not initially see a very small resistance when it switches on the lamp. If it's a bother to you, you could probably safely remove one of the above mentioned resistors and cut the current in half.
Cheers
Rob

[rpapps]

I've just noticed an error in my original post.
The line which reads:
"The feed for it in FM mode comes from the base of TR104"
should read
"The feed for it in FM mode comes from the base of TR105"
Cheers
Rob

[rpapps]

Additional thoughts
Check the AGC circuit - D102, D103, TR106 & TR107
Cheers
Rob

[DreadPirate]

Originally posted by rpapps
Hi DreadPirate
The clue here is the low reading on the tuning meter.
The feed for it in FM mode comes from the base of TR104 so your problem lies somewhere back from that point. The sensitivity seems to be way down which forces the receiver into mono to minimise noise. Check TR101, 102, 103 & 104. It's also possible the fault lies in the front-end module. You say you have a good antenna. Is it external and did you have any thunderstorms around the time it failed?
Cheers
Rob

I finally got around to tackling this one and, sure enough rpapps, the culprit was TR103, it is dead. The transistor is a 2SA234 with a good cross at NTE160, but these run about $5 each. Since I pulled off the others while troubleshooting, it makes sense to go ahead and replace these also, I'm guessing, but I'd like to find a cheaper sub. 2SA342 seems reasonable, they run at $3.36. The thing is a small metal can type, 4 lead Ge PNP lo power BJT and was used in the FM IF section.

I have the $5 or so multi-tester from harbor freight that allegedly checks hfe, I got values ranging from 19 to 52 on the different ones I pulled. I have not remounted them.

Thanks again, good call.