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Technics su-v4x no +14.7v voltage on ic201
Technics su-v4x no +14.7v voltage on ic201
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Old 11th October 2018, 04:39 PM   #31
stofbj is offline stofbj
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Yes R706 was open circuit, I did check this a few days back as I remember the awkward spot it was in behind the SVI chip, something must have blown it in the mean time because it was definitely good, I have now replaced it with 15k resistor, took R707 out of circuit measure ok.
R705, R701 and D706 are also all good. C707 & c708 replaced with 47uf as I did not have 33uf. C709 and c710 already replaced with panasonic Fc's The old caps all measure good out of circuit.

I have already fixed all the dry joints as there were a few, nearly all the caps have been replaced on the main board except main caps. Also tried to disconnect the input board at J11 & j4 encase of shorts, makes no difference.

With 3.9k resistor from the collector of Q701 to the Gate of Q704 (with it removed) I am now getting 8.4v at the collector of Q701 and 7.2v at the gate or base of Q701.

Getting there but still something wrong. Maybe the replacement for Q703 (original 2sc1815) with D592A

If I had some spare transistor it would make it easier, I will have to order some new ones tomorrow. Will have another read through the post later when I have more time.
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Old 11th October 2018, 04:45 PM   #32
stofbj is offline stofbj
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Yes the SVI chip has already been re soldered first thing I did, and every joint on the main board for that matter. Mosfet Q704 has been bypassed with 3.3k resistor, and Q 703 and q704 tested good on multi meter out of circuit
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Old 11th October 2018, 05:26 PM   #33
stofbj is offline stofbj
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On checking the mosfet k246 Q704 (presume 2sk246) that I took out of circuit , source on left, drain middle, and gate on right, I checked in diode mode positive on drain and negative on source getting about 600ohms, the other way around nearly the same, does this mean the internal diode is bad?
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Old 11th October 2018, 06:01 PM   #34
Mooly is offline Mooly  United Kingdom
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Technics su-v4x no +14.7v voltage on ic201
All those resistors would have to be checked out of circuit to get an accurate result. DVM readings are thrown by interaction with other components and even a few millivolts of residual voltage floating around.

Its extremely unlikely that those resistors would be open circuit, but theoretically they could generate the fault symptoms you see.

FET's are tricky to test (even out of circuit ) because being a super high impedance device means the gate can literally float and acquire some voltage... and that can turn the FET on and make it look like it is faulty.

When you use a DVM in 'Diode' mode you are not actually reading resistance but are in fact reading the voltage between the test probes as the meter generates its own test voltage. The '600' is actually 600 millivolts and is what you would expect across a forward biased PN junction (such as a diode).

If you want to satisfy yourself that the FET is good then just swap it with the other one, although having it bridged with a resistor guarantees the FET itself is OK.

I can't really suggest any more I'm afraid. If the parts are good and the print good then it has to work. Anything amiss after the reg and drawing excess current would just pull the 34v on the collector down... and that's not happening.
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Old 11th October 2018, 08:15 PM   #35
stofbj is offline stofbj
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Hi,
I replaced Q703 (2sc1815) with a new BC182B yes swapped the pins around so they match, but still getting 8.2v at the collector of Q701 and 7.1v at the base. The fet is still bypassed with 3.3k resistor between gate and drain.

Checked continuity between

- side of c707 to chassis
Emitter of Q703 to chassis
+ side of c707 to gate of Q704
+ side of c710 to chassis

- side of c709 to chassis



all ok. will order a new fet and have a look around for a new Q701, I have a spare BC182B going to check data sheets to see if it would be a suitable.



Regards.
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Old 11th October 2018, 09:22 PM   #36
stofbj is offline stofbj
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Finally sorted it, replaced both transistors, and installed original FET, but it was still pulling the voltage down, I started to disconnect jumpers one by one, starting with the RCA input board, first j11, then j4 one at the time, then disconnected the volume pot board and the +15v came back, checked the board and there was a short on the board, so happy was nearly going to give up. But this short was not the only problem on this thing, I would say all the dry joints did some damage too. Luck this is a hobby, as the hours put into this, but suppose its good for the environment anyway. Too much stuff being dumped these days. Now to cleaning up all the flux and put it all together, anyone recommend respraying new lacquer on the boards?

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Old 12th October 2018, 06:49 AM   #37
Mooly is offline Mooly  United Kingdom
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Technics su-v4x no +14.7v voltage on ic201
Pleased to hear you have fixed it It sounds like you have had a few things randomly going on with that one.

I usually re-apply lacquer but find applying it with a soft brush can actually be visually better and avoids overspray.
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Old 12th October 2018, 10:06 AM   #38
stofbj is offline stofbj
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Hi, ok will apply with a soft brush, yes this was a difficult one, usually re-soldering the dry joints and a spray of switch cleaner does the job. Anyway learned a lot from all this.

Thanks so much for every ones help.
Regards.
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