That reading shows T4 is reverse biased.
Yes, pull it out. Make a note of how it fits so you can put it back again if needed.
I'll think some more on this tomorrow but in the meantime...
Leave T1 out of circuit and now measure the voltage across R6 and across R10. Both readings should be almost zero because the AD149 should be off with T1 removed. If the readings are not zero then it looks like the top AD149 might be faulty. We would then have to prove that.
Leave T1 out of circuit and now measure the voltage across R6 and across R10. Both readings should be almost zero because the AD149 should be off with T1 removed. If the readings are not zero then it looks like the top AD149 might be faulty. We would then have to prove that.
Not quite following you on that
What does the good turntable bulb do ? bright or dim ?
and what does the faulty board do ? bright or dim ?
I had one of these MKI tables with the dual rectifier set up, the bulb should be very dim..
Check the transistors with a real transistor checker..
Yes the bulb should be pretty dim in an oscillator like this. It looks like the bulb voltage (at the moment) is coming from the DC present on the amplifier output and via the speed setting pots.
Those readings are all good. That seems to show the AD149 really is turned off as it should be.
Refit T1 back to the board.
Did you check C7 that I mentioned earlier ? You can run the unit without this fitted as a quick test.
Those readings are all good. That seems to show the AD149 really is turned off as it should be.
Refit T1 back to the board.
Did you check C7 that I mentioned earlier ? You can run the unit without this fitted as a quick test.
We need to be more methodical, that's what
You have changed all the electros
You have checked the transistors
You have checked the resistors
If there were a problem affecting just one speed then the others would be OK... that fact eliminates quite a few parts.
Is C9 OK ? If that was leaky it would forward bias T5 and cause problems. The only way to check it is with one leg disconnected and check it for leakage. You must use a 'high ohms range' for leakage checks, a range that would show meg ohms of leakage.
OK. So we need to start afresh with basics.
Make sure all the parts are refitted back in place and then we can do some more voltage checks. When you have refitted the parts do the following.
We are going to test the output stage. This will help prove whether the fault is to the left (the output stage) or the right (the oscillator)
1/ Remove T5 and then measure the voltage on R9 as before. The voltage should be under 1 volt.
Make sure all the parts are refitted back in place and then we can do some more voltage checks. When you have refitted the parts do the following.
We are going to test the output stage. This will help prove whether the fault is to the left (the output stage) or the right (the oscillator)
1/ Remove T5 and then measure the voltage on R9 as before. The voltage should be under 1 volt.
It is the variable resistor R27. It should read less than 5k depending on where it is set to.
That's good. If there is resistance it is leaky.
That's good. And no leakage ?
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