Technics SU-V3 - suspect AUX in over-voltage - protection circuit not kicking in

[sparkyjf]

Hi all

I have a Technics SU-V3 that has just failed - I suspect that it had too high a voltage range applied to its line-level input on the AUX. Now although the amp still powers on, the relays are not clicking in so the protection circuit thinks something is wrong.

From studying the block diagram, my immediate suspicion is that this might be because it's sensing DC - the PSU I think is fine (lamps, FL power meter all illuminate OK).

The first IC that the line input hits is Q301/Q302, which are a pair of 2SK109's - I've seen these come up a lot in repair stories as I've tried to hunt down the problem this evening. The DC detection for the protection circuit also seems to be fed off the input of the differential amp formed by Q301/Q302 so again I suspect if they've been blown there could be DC being fed to the protection circuit.

However I'm also aware there's a lot of other circuitry, especially transistors around them that could potentially have gone. I've also read on the SU-V4 about some resistors that were designed to go in the event of a failure.

Does anyone here have any experience repairing these amps and their kind? Is there an obvious part to replace (e.g. Q301/302), or any test points to check to try and evaluate where the fault is.

Thanks!

James

[unitelex]

Hi James
Does the same fault exist on both LH and RH channels?
Worth checking the +42 and -42V is present on both amps.
Then check the output voltage on the each of the output inductors with respect to ground

Regards
Chris

[unitelex]

The schematic I have shows the expected voltages at various points on the power amps. See the figures in the rectangular boxes. Recommend you check each of those

[sparkyjf]

Hi Chris,

Thanks so much for your quick response - I've only just had time to strip the amp down this weekend so apologies for my delayed response.

Findings so far are:

Power supply looks ok - I've got +/-42V across C601 and C602, so input voltages look to be within spec.

With respect to ground (I'm using the amp chassis for ground), I'm seeing -20V present on both the output inductors (L301 and L302). These appear to be -20V DC so perhaps this DC presence on the output is the cause?

I have the same schematic as you I think, with numbers in little rectangular boxes as you described. I've started working round the circuit, but I'm not clear what these figures are measured against. I started off using chassis ground, but if I do that then I get some pretty wild numbers.

For example Q603 which looks like the pass transistor for part of the regulator circuit - I get 50.8V on the collector, 35V on the emitter and 35.5V on the base (well outside the 26.8, 15.3 and 15.9 specified in the service manual).

However if I measure relative to the + pin on C602 (which should be the midpoint or 0V rail between the +42V an -42V rails), then I get:

Q603: B 16.8, C 33.1, E 16.1

These are a bit outside spec but not too wildly - not sure how close they have to be (the circuit diagram says the emitter of Q603 should be 15.3V, and as this is a regulated power supply I can see that it's important it should be within spec.

Voltages on Q604 (negative side of the same regulator circuit) are about as close to spec as those on Q603 (i.e. a bit out, but not wildly).

Just wanted to check that I'm measuring this right before I go and probe the whole circuit, as the voltages don't seem right if I measure with respect to chassis ground.

Thanks for your help!

James

[sparkyjf]

Ok further news - so probing against the midpoint between C601 and C602, it did seem that many voltages were "close" (re my comments on the measurements around Q603 and Q604 above) so I went ahead and did some more measurements.

The right hand channel went first, so I've focussed on the right hand channel components, but I can happily take measurements from the left as well for comparison.

First of all, I've confirmed that the output formed by the emitter pair of Q332 and Q334 is at -40V, and not 0V as suggested by the schematic. Some other measurements of note include:

There's a potential divider formed by R336 and R338 - the mid-point of this is supposed to be at -1.3V - I'm getting -42V.

This in turn means that the collector of Q320 is at -42V, and not -1.9V as specified. The voltages around Q302 are especially out of spec - the commoned sources of the FET-pair are supposed to be at 0.2V according to the schematic, but I measure -18.4V. The gates are at -19V (vs 0V), and the drains, which are supposed to be at +5V, are reading -6V.

I can post an image with my markings on if it helps. I'm a bit rusty - studied as an electronic engineer 20 years ago and have been in IT more or less ever since. However, what I can see is:

There's a possibility that Q320 has gone short circuit. The collector on this is supposed to be at -1.9V, and is at -42V, which would suggest its hard on (or short). This would account for the R336/R338 potential divider being at -42V instead of -1.3V. This in turn would pull a lot of other components out of spec - however this is in my total honesty an educated guess.

Any help would be gratefully received!
Some pictures attached as it probably helps

[PJL]

Q320 has nothing on the collector because Q318 does not seem to be conducting, DC coupled amps like this are notoriously difficult to diagnose. You should first of all compare the left and right channels so repeat your measurements for the other channel.

[PJL]

Originally Posted by sparkyjf

First of all, I've confirmed that the output formed by the emitter pair of Q332 and Q334 is at -40V, and not 0V as suggested by the schematic.

All voltage measurements should be taken from chassis as this is 0V, both the transformer centre tap and signal ground are connected at the back panel.

You have already checked the +42V and -42V but you should also check the regulated supply of +15.3V (Q603 emitter) and -16.2V (Q604 emitter) before you continue.

There is something odd around Q318, can you measure the voltage across D308 (should be 1.2V) and then across R330 (should be around 40V).

After this you need to work backwards from the output stage, so measure the ebc voltages on Q330, Q328, Q332 and Q334.

[sparkyjf]

Thanks to everyone who offered help on this! Today I managed to find the root cause - it was nothing to do with all the transistors and was actually a broken track on the PCB. Turns out a large portion of the PCB was connected to 0V through a single track near the Phono RCA input. That track was cracked, and the lack of a ground on a large portion of the input circuitry threw everything out. Soldered a wire to bridge the broken track and it all works perfectly again. I've also learned to check my grounds before going down rabbit holes...