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Old 8th Feb 2019, 7:22 pm   #1
simondhopkins
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Default Technics PR305

Hi there,

I have a dead technics pr305 in my living room wanting some tlc to get it working again.

No lights what so ever on the control panel but the power led on the frond of the piano comes on.

It has 15v at the test point on the main board, but trying to find the 5v is proving a pain.

Also is it possible that the reset is being shorted to stop it powering up (got power led on on front of piano) and if so what is the reset line voltage ment to be and is it just a pulse or continual signal to keep the cpu up?

Any other ideas would be greatly received too to check using just a digital multimeter.

Thanks.
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Old 15th Feb 2019, 11:32 am   #2
boxdoctor
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Default Re: Technics PR305

It's 20+ years since I worked on one of these, but I recall problems with the solderring of the power regulators causing your symptoms.
If the +5v supply is missing, the whole circuit will shut down.
Sadly, all the manuals I had for Technics products were disposed of (in the skip!) before I could get hold of them when the firm closed its service department.
(I was hospitalised at the time.) Tony
I suggest that you check especially the accuracy and purity regarding noise and ripple, of the +5v, as this is fairly critical to the correct operation of the circuit. Tony.
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Old 15th Feb 2019, 8:55 pm   #3
Silvered_Mica
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Default Re: Technics PR305

I've just been searching the net only to find very little so I presume you're working blind fold (no circuit diagram). The next best thing to a diagram is the component data sheets these will give you the pin outs so you know what should be happening at various points in the circuit.

It sounds like you might have a missing voltage rail. From the bits of info I did find you should have two 15 volt main rails one positive and one negative going, you should be able to check this with you multi meter at the output connectors of the power supply PCB. If you're unsure post some good clear pictures of the inside guts, me or someone might be able to advise more.

Good luck
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Old 17th Feb 2019, 12:40 pm   #4
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Default Re: Technics PR305

I've just ferreted out my list of people I knew when I was in the trade.
I knew I'd still got it somewhere.
This chap is of long experience, and should be able to help you if you get really stuck. http://www.mikebarden.com/ He is a genuine, highly experienced engineer, not a dabbler.
I also tried searching for a service manual, and had no more success than Silvered Mica had.
Dry joints / poor connections are still the most likely bet (in my personal opinion). Tony.
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Old 22nd Feb 2019, 12:25 am   #5
Silvered_Mica
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Default Re: Technics PR305

I agree with Tony it could be dry joints. power up and start taping the boards with the plastic or wood handle of a screwdriver and it might burst into life.
It could also be a transistor, FET or regulator gone bad. Is anything getting warm on the heatsink that is strapped to the power supply board?
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