Technics su-v4x no +14.7v voltage on ic201

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi,
Could someone please point me in the right direction, I have no +14.7v on the input selector ic201, causing distortion, I traced it and it should be coming from the emitter of Q701, checked with a voltmeter and there is 0v at the emitter . I have 34v at the collector of Q701 which is correct. But no 16.8v at the base. Ground on q703 seems good, and also c707 cap measured ok.

I replaced q701, npn channel transistor with slightly different specs as I don't have the correct p/n, observing the correct pin layout But this did not fix the issue.

Any input would be greatly appropriated, could it be q704 or q703?

Please see attachment for details.
 

Attachments

  • Capture.JPG
    Capture.JPG
    83.9 KB · Views: 118
Thanks so much for your help and quick response, my electronics are a bit rusty,

I will replace the electrolytic cap first, as I have most done already(due a change). I will pull out the suspect transistors /FET's you mentioned to measure them in diode mode, if the capacitor does not fix it.
 
1st time I’ve seen a V4X pull this one...

Kinda interesting timing - just had a friends V9 that has a similar setup cause hell

I simply got rid of the FETs and used resistors and zener diodes to drive regulator transistors.. 5.6k resistors replace FETs Q704,Q705 / MA4160 to replace Q703 (in this setup)
 
Hi, Yes checked on both sides of R701 and the voltage seems good, the only place the voltage is missing is the 16.8v at the base of the fet / q704
Forgot to mention, first thing I did was measure all the resistors (in circuit) and capacitors and they seem ok. I have a few spare k3313 N channel FET's (pins probably different) Otherwise the resistor with diode (I presume in parallel to stop reverse current) would work permanent for the FET as suggested by SVI2004A Thanks so much everyone for your help. Only kind of starting out as a hobby doing a few repairs, think I got the bug....Built a nice audio probe with a copper nail and film capacitor, going to a small amplifier, very handy indeed in pin pointing the distortion I had on this.
 
Administrator
Joined 2007
Paid Member
All the clues will be in the voltage readings. Have you tried the resistor across the FET yet ?

You have to be certain you have +34 actually on the collector lead of Q701.

Also do a resistance check in circuit (with the amp OFF) from the base of Q701 to ground. It should not read short circuit.

Do the same for the emitter of Q701. Again it should not read short circuit.

That doesn't leave much else. Do not run it without Q703 fitted because doing so will cause the 17 volts to rise to 34. We don't want that.
 
Sorry for the delay in replying, I only got to take another look at it yesterday. I checked all the resistors again last night and they read ok. Replaced c707 just in case. Took out (npn) Q703 and tested it in diode mode using voltmeter and it read ok in all directions for n channel. Hope this test is sufficient.

So looks like Q704 fet is the main suspect alright. Placing a 3.3k or 5k resistor across the drain and gate with or without the fet still soldered in gets me about 2.1v at the drian. Checked the ground connections on c707 and q703 and they are grounded to the chassis. Could it be that I have a short somewhere on the output drawing excessive current?



When adding a zenor diode as suggested is this in series or parallel with the 3.3k resistor, and which way around cathode or anode to + side?

And sorry one final question if I leave this in place if it works what wattage resistor should I use so it does not over heat?

Sorry again for all the questions . Like i said my electronics is very rusty, its been a while.
 
Hi Mooly,
Thanks for helping out.


collector Q701 +31.6volts
With amp off Base of Q701 to ground is not short circuit. (792 ohms)
Emitter to chassis 92ohms

I did observe Q703 is an c2631 (presume 2sc2631) not the 2sc1815, maybe replaced at some stage or changed during manufacturing process.


With resistor across the fet getting 2.1volts
 
Administrator
Joined 2007
Paid Member
Yes, it will be a 2SC device. Any suitable small signal device will work in that location. So no short circuits either. Although the readings look low they will be caused by interaction with other parts.

Have you replaced Q701, the series pass transistor?
 
No I have not replaced 701 yet, but it did test good to out of circuit, I will order some transistors, looking at the data sheet for 2sc2631 this looks like an pnp transistor and its calling for an npn, this could well be the problem. Looks like someone already tried to repair this one, even though the solder joints did not look fresh to me. It may have been in storage for a while. It will take me a few days to get new component, the joys of living in the middle of nowhere :) will keep you all posted on the outcome. I will order a new fet regulator, 701 and 703 and replace the lot. Presume the specs for the FET will have to match closely.
 
Administrator
Joined 2007
Paid Member
2SC's are normally NPN as are 2SD's
2SA's and 2SB's are normally PNP.

Be careful what you order as there are so many fake parts around. The 2SC1815 can be replaced with a BC546 (different pin out so be careful) and the 2SC2592 could replaced with a TIP41C (same pin out)
 
Administrator
Joined 2007
Paid Member
The FET is just used a constant current source... a bit better than a resistor in that location but I bet in practice (and in measurement) you wouldn't tell the difference.

The FET can be a general purpose N Channel device and I would think even a 2N3819 would work OK there. The FET is unlikely to be faulty though.
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.