| jimB |
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Reply with quote | #1 |
Good day,
I have been trying to restore a Sansui Six receiver. I have problems with one of the channel blocks (F-1439-1).
There is some noise emanating (hiss, pops)from whatever channel the block is plugged into. In addition, the relay that (I assume) turns on the amp power stages does not always function with the offending block plugged in.
I have replaced the 2 large electrolytic capacitors and I am in the process of obtaining the 2 T0-220 transistors.
Am I on the right track?
TIA
jimB |
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| Riaan Louw |
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Reply with quote | #2 |
Greetings,
That hissing pop effect is 99.9% dried out electrolytic capacitors. The big ones you replaced are probably the power supply caps; good to replace, but usually it's not those ones causing the snap crackle and pop. It's usually the smaller electrolytics somewhere in the sigal path. Like the 220uF and 470uF and 10uF etc caps. Do yourself a favour and replace them all. Or one at a time if you'd like to keep as many oiginal components as possible. |
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| jimB |
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Reply with quote | #3 |
Good day,
I am attempting a repair of a Sansui Six receiver
I have isolated the problem to one of the 2 channel boards (pre-amps?) and replaced the only electrolytic capacitors.
One was a 47uF 50V the other was a 100uF 10V.
The problem remains to some degree.
I also have some issue with the same channel board in that it affects the relay that (I guess) passes the signal to the power stage.
Board in- relay closes and opens or doesn't close at all
Board out- relay closes
Swap boards one for the other, noise moves to other speaker
Both boards out- relay closes
jimB
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| Warren |
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Reply with quote | #4 |
It sounds like there may be a bad transistor on that driver board. I don't have the schematic for this model so I can only advise in general. With the board in place, check the voltage on the output of the amp, on the input side of the relay (with the relay open you can't check it at the speaker jacks.) It should read 0V. If it reads higher than 100mV, there is a problem on the driver board. Check the value of any resistors in series with the DC supply to any transistors on the board. Sometimes they go open. If the resistors are ok, then check every transistor on the board. You may have to remove them and check them on a transistor tester or a DMM that has a transistor tester socket.
Warren
Quote: Originally Posted by jimB
Good day,
I am attempting a repair of a Sansui Six receiver
I have isolated the problem to one of the 2 channel boards (pre-amps?) and replaced the only electrolytic capacitors.
One was a 47uF 50V the other was a 100uF 10V.
The problem remains to some degree.
I also have some issue with the same channel board in that it affects the relay that (I guess) passes the signal to the power stage.
Board in- relay closes and opens or doesn't close at all
Board out- relay closes
Swap boards one for the other, noise moves to other speaker
Both boards out- relay closes
jimB
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| jimB |
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Reply with quote | #5 |
Warren, thanks for the input.
I am afraid that your suggestions would take me waaaaay beyond my level of expertise.
Digital Multimeter? I have enough trouble with a soldering iron.
I have (with some difficulty) obtained replacements for the 2 TO-220 transistors so I can blindly replace them. There would remain a few TO-92.
I will also re-flow and generally clean up all the solder joints.
I would assume that due to the fact that the problem is intermittent, resistors could/should be eliminated as they generally are either "closed" or open. Once open, they shouldn't "close".
I would not have undertaken this whole exercise other than I have sentimental attachment to this particular unit and an serious aversion to standing in a big-box store trying to pick out a new one to replace it.
I can buy a working one on eBay for short money.
cheers
jimB
Note: I am a product designer and I used to do a lot of PCB layouts so I am familiar with the components per se but not really the circuits themselves.
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| Warren |
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Reply with quote | #6 |
First off, you can buy a DMM dirt cheap (less than $10.00) at Harbor Freight, so for a minimal investment, you will greatly improve your troubleshooting skill and tools, and maybe learn something. How could that be bad? Next, if you do buy a meter and go probing around inside the thing (since I am guessing you have no clue why you are replacing those transistors and therefore it will probably not fix the problem) wrap some electrical tape around the tip of the meter probe, so that only the vey tip is exposed. This will help prevent shorting things out and causing more damage if you slip with the probe.
Sansui had used some resistors that can and do change resistance and become intermittant, so this is a possibility. If you measure the resistors (with the power off) you can be sure if the resistor measures higher than the marked value, it is bad. Lower readings are probably due to the resistor being in the circuit with other components, and it is probably ok. Hope this helps.
Warren
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| jimB |
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Reply with quote | #7 |
Warren,
Thanks again for the input and the vote of confidence.
FWIW, I do actually have an analog meter. I will seek out a DMM.
Such as it was there was a rhyme to my reason...
In my ignorance (and observation of the failure mode symptoms), I went with the law of averages in that certain type of components are
more likely to fail with age than others hence, I replaced the electrolytic capacitors first. Next would be the transistors and lastly the film
and ceramic caps and the resistors.
I also wanted to make sure that the transistors we still available and reasonably priced so I obtained the transistors as a stop gap. They are Japanese series SA706 and SC1124. There still are some TO92's but I will wait.
If I had to guess (and my visual inspection of the solder side of the board), the problem may lie in the blackening of the leads on some of the
components and/or a failed solder joint.
I am visiting with an associate this weekend and together we hopefully can get it sorted out.
Question: In a previous message, you mentioned checking the output at the speaker jacks (with the relay energized). What would I be looking for. The relay does energize intermittently.
As they say, stay tuned.
Thanks again.
jimB
PS And after this, I need to deal with the antique BOSE speakers I have.
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| Warren |
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Reply with quote | #8 |
As I said, With the board in place, check the voltage on the output of the amp, on the input side of the relay (with the relay open you can't check it at the speaker jacks.) It should read 0V. If it reads higher than 100mV, there is a problem on the driver board. If you are not sure where to check it on the relay, look at the driver board. There should be a large air core coil, one for each board. That connects to the output. Check it there.
Warren
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