A friend got this Sony tuner and brought it to me to repair the “frozen” frequency display, which is a known problem and fairly easy to solve.
After the repair, the tuner was working the whole evening until a brief power grid blink, for a couple of seconds, so when the power was restored the tuner remained silent. A 15,3V circuit breaker went off and a transistor Q805 on counter PCB had to be replaced. But now the frequency display behaves weirdly: between 90 and 104 MHz it shows the correct number. However, under 90MHz it will display almot double the frequency (not exact multiplication), whereas above 104MHz it will restart counting from 88.4MHz. Otherwise, the tuner operates just fine.
I have narrowed down the problem to the counter PCB. Further details:
- transistors Q801-Q804 test OK
- Q801, Q802 and Q805 show correct collector voltages as indicated in the schematic, however only in the 90-104MHz range, otherwise they show the opposite values
- IC801, IC802 get considerably warmer than other chips on the board, but this may just be normal
- I did not check all voltages on IC pins as referenced on the schematic because the manual does not indicate the measurement condition (i.e. the tuning frequency) and so they are sometimes there and sometimes not, depending on the tuning frequency
I am kindly asking for help to identify the IC on this board which is responsible for erroneous frequency reading in some ranges. Thanks in advance.
After the repair, the tuner was working the whole evening until a brief power grid blink, for a couple of seconds, so when the power was restored the tuner remained silent. A 15,3V circuit breaker went off and a transistor Q805 on counter PCB had to be replaced. But now the frequency display behaves weirdly: between 90 and 104 MHz it shows the correct number. However, under 90MHz it will display almot double the frequency (not exact multiplication), whereas above 104MHz it will restart counting from 88.4MHz. Otherwise, the tuner operates just fine.
I have narrowed down the problem to the counter PCB. Further details:
- transistors Q801-Q804 test OK
- Q801, Q802 and Q805 show correct collector voltages as indicated in the schematic, however only in the 90-104MHz range, otherwise they show the opposite values
- IC801, IC802 get considerably warmer than other chips on the board, but this may just be normal
- I did not check all voltages on IC pins as referenced on the schematic because the manual does not indicate the measurement condition (i.e. the tuning frequency) and so they are sometimes there and sometimes not, depending on the tuning frequency
I am kindly asking for help to identify the IC on this board which is responsible for erroneous frequency reading in some ranges. Thanks in advance.
Attachments
This is a tough one.
Since basic digit driving seems to work fine, I would rule out IC801, 802 for the time being.
I guess below 90 MHz it continues with 189.9? Then the 100 digit is not being controlled properly.
The weird wraparound at 104 MHz worries me. Some sort of counter problem. Together with the 100 digit control issue, that makes me suspect either IC803 or IC804.
Thankfully all the M532xx ICs seem to xref to 74LSxx.
How your +15V issue even managed to take out one of these 5V logic chips is another question entirely. Q805 may have been degraded from old age to give up this easily.
Maybe one of our old-timers with closer relations to 74 series logic will be able to chime in.
Since basic digit driving seems to work fine, I would rule out IC801, 802 for the time being.
I guess below 90 MHz it continues with 189.9? Then the 100 digit is not being controlled properly.
The weird wraparound at 104 MHz worries me. Some sort of counter problem. Together with the 100 digit control issue, that makes me suspect either IC803 or IC804.
Thankfully all the M532xx ICs seem to xref to 74LSxx.
How your +15V issue even managed to take out one of these 5V logic chips is another question entirely. Q805 may have been degraded from old age to give up this easily.
Maybe one of our old-timers with closer relations to 74 series logic will be able to chime in.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.