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Sony Receiver STR 7065A, using tuner (only) one channel still(very low noise)

dear friends,
please excuse my low level english, i left my german school hundred years ago! I just bought an old receiver with its mistake. Any chance for me to repair it myself?
brain: ok; two right hands; using multimeter:ok!
Does someone has concrete idea?
thanks so far!



Re: Sony Receiver STR 7065A, using tuner (only) one ...

You say you only use the tuner, so I suspect you are using the record out or tape monitor jacks; there may be bad connections here, or at the 5 pin DIN connector. You could have dirty monitor or tape switches; try actuating them several times. You may need a contact cleaner of some type to clean them.
There may be a problem with the MPX IC.

Re: Sony Receiver STR 7065A, using tuner (only) one ...

Hi Johnnysan, thanks for answer. I think i wasn`t exactly enought in discribing my problem: by using tape or aux, both speakers works correctly! Only - with the tuner is the problem. Whatfor is the MPX IC?

Re: Sony Receiver STR 7065A, using tuner (only) one ...

If you only have one channel in tuner mode, I would suspect a bad MPX (FM) IC, or a dirty function switch (AM-FM, Phono, Aux switch) or bad connections in these circuits.

Re: Sony Receiver STR 7065A, using tuner (only) one ...

Hi Johnnysan, I wrote a question already - seems to be lost in universe. If I change the cmos HA 1156 (manual) what`s about the peripherie as R`s and C`s? Where do I get a cmos like this?
Otherwise, how could I check the other faults?

Re: Sony Receiver STR 7065A, using tuner (only) one ...

The HA-1156 MPX decoder chip will be hard to find. Maybe you will need to find a scrap receiver or tuner to take it from.

There is still a possibility that the preamp behind the MPX decoder fail. Some signal tracing will reveal if this is the case.
An oscilloscoop will a handy tool here, hopefully you can borrow one or obtain a second hand one. For audio repair you don't need a high-tech one. If you invest take a dual channel and don't forget the probe's. But the first checks can be done with your multimeter.

Point to check inside the STR 7065:
1). DC on pin 4 and 5 of the HA-1156 ( IC 301 ): about 11Volt, if 0 check R 304 and R305.
2). DC on the collector of Q302 and Q352 ( preamp and separation adjust circuit ): about 6.5 V, if not check R310 to R317, if OK Q302 or Q352 is likely defect and can replaced by european types like BC549 or BC550 ( note: pin layout is different, check before mounting ) or japanese 2SC945 or 2SC2240, if you need to replace replace both with the same type. The circuit is non critical. ( check pin layout with the service manual and information from the web. japanese pin layout when holding the transistor with the type number towards you and the pins down is mostly ECB and for european mostly CBE. ).

If the DC levels are OK check the following using a signal tracer or oscilloscoop:
1). signal on pin 4 and 4 of the HA-1156, if both present HA-1156 appear ok, if one is missing HA-1156 need to be replaced. ( note: signal is superimposed on DC 11Volt, set oscilloscoop input on AC )
2). signal on base of Q302 and Q352, if one is missing check C310 and C311 and replace if needed. ( note: signal is superimposed on DC 11Volt, set oscilloscoop input on AC )
3). signal on collector of Q302 and Q352, if one is missing check Q302 and Q 352 and replace if needed as described earlier. ( note: signal is superimposed on DC 2.2Volt, set oscilloscoop input on AC )
4. signal before and after C314 and C315, if one is missing check C314 and C315 and replace if needed. ( note: signal is superimposed on DC 6.3Volt, set oscilloscoop input on AC )
5. signal before and after low-pass filter LPF 301, if one is missing check LPF 301, if it need to be replaced it is also a hard to find component. If one channel is still ok you need a somebody who will check it's characteristic and build a new circuit for you.
6. signal after RV303 and RV353, if one is missing check RV303 and RV353 and adjust them ( check the setting of the proper one and set the new ones accordingly.

Hope this will help you.
Best regards
Paul

Re: Sony Receiver STR 7065A, using tuner (only) one ...

Hi Paul
thanks a lot for your advice. I`ll do my very best, but still need some times.
Best regards
Willi