Help! Problem with my Technics Amp cutting out .

[ajgriff]

It still seems to me that this problem has got something to do with the change of speakers. For example the protection circuit will kick in at higher volumes if the speaker impedances are too low, as described in the manual.

Alan

[Drewpicker]

Thanks again for your help PJL ... i dont know what you mean by check the power rails ? I can follow the schematic to a certain extent .... do i have to test across things or just with the positive and the negative to a ground point ? Im sure that with some help on this ill learn some more about electronics than just knowing a bit about what some components do and being able to spot one and change it when it looks like toast... ive looked at the schematic but cant see any obvious "power rails" ... are these just the transfo outputs or the different voltages circling around the board ? ....

[Drewpicker]

Hi Alan ...sorry...nn i was replying to PJL when you posted . So I just saw your post ..
. Im up for looking everywhere and anywhere... the shut off definitely comes in at higher volumes... at low volume it plays indefinitely.... the speakers are the original ones ... and my wiring to them seems good to me ... maybe if a tweeter is failing or failed or something that could cause what you're saying ... is that a possibility? Have you any ideas I can test for ? Thanks again for your help ... all ideas are welcome.

[Drewpicker]

To clarify ... the speakers are the original speakers ... I just dont have the bass bin and two mini speakers that make it surround sound . (There's a switch to turn that on or off) ... and as I said it worked fine with these speakers for many years .

[ajgriff]

Thanks for clarification. Probably not to do with the speakers in that case.

Alan

[Chris55000]

Hi!

I've a feeling that all this business of fan speeds, ratings, pulses, etc., etc., is barking up the wrong tree in part, and it's very important to make sure there isn't an amplifier defect first!

The mid–point voltages from pins 1 and 4 of each of the SVI3102D power ICs is summed by the four resistors R569 to R572 and noise bypass capacitor C571 and then fed to the protection sense transistors Q571 and Q572, one connected to sense positive d.c. offsets and the other connected to sense negative d.c. offsets.

Connect a d.m.m. across C571 on the 2V range and turn the volume to zero.

Make sure your speakers are correctly connected and in good order, then gradually increase the volume.

The d.m.m. reading should stay very close to zero when the volume is increased. Watch the d.m.m. at the point where the unit cuts out on the louder volume signals, and see if a voltage, either positive, or negative, is registered on the d.m.m. when the unit cuts out.

If there is a voltage registered on the d.m.m at the moment of cut–out, then one of the four output amplifiers is responsible for the fault, and it's necessary to find out which.

To do this, lift resistors R569 to R572 at one end, one at a time, gradually increasing the volume and watch the d.m.m. connected across C571.

If the reading across C571 falls to zero when one or more of the resistors R569 to R572 is disconnected, then the ref. no. of the resistor which makes a difference is the one connected to the faulty amplifier.

If all four of the amplifiers are producing a d.c. voltage across C571 when you increase the sound volume, then there is most likely a common cause!

The most common reason for amplifiers tripping off at medium to high volumes is either unbalanced positive and negative power supply voltages to the power amplifiers, or defects in the power amplifiers themselves.

Switch off the unit, select a 100V range on your DMM, connect it's negative lead to the metalwork and the positive lead to the + terminal of C701 in the PSU circuit, and switch on the unit, and and note the reading – should be about +38–39V d.c.

Turn the volume up until the cut–out occurs, and note the d.m.m. reading – it will probably be about +35–36V.

Next, repeat the test again, this time connecting the red lead to the – connection of C702 and repeat the no volume and "cut–out" readings as before – they should both be about the same magnitude as the positive voltages across C701.

If either the voltage across C701 or C702 is unbalanced due to these capacitors being defective, an offset voltage will tend to occur at pins 1 and 4 of the SVI3102D's output pins when the sound volume is increased, which will cause a trip to occur even tho' the amplifier devices are in good condition.

If the above test suggests there's an unbalance in the power supply voltages, then I recommend replacement of C701 and C702 together as a pair.

If disconnecting R569 to R572 shows only one of them is causing the cut–out at higher volumes, then the fault lies in either or both of the SVI3102D i.c. modules themselves, and the device producing the unwanted d.c. offset(s) should be replaced.

It's quite a lot to describe in words and read I appreciate, but if you know how to use a d.m.m., and you're careful with the testing, it's not as difficult to do as it looks!

Try the d.c. checks across C571 and C701/C702 first and let us know what you find!

Chris Williams

[Drewpicker]

Wow thanks Chris .... I'll get onto all of that tomorrow as it's getting late for my neighbours .. I've just been watching videos explaining power rails ... I think I understand the basic concepts now though when i went back to look at the schematic i realise I'd have trouble finding and testing the different voltages ... your explanation seems clear to me ... if i have any problems understanding something I'll get back to you and post it here ... i know how to use a DMM ... so that's cool ... thanks again for your time and help with all of this .

[Drewpicker]

Just re-read your post Chris and realised that I should mention that the first thing I did when this problem arose was change C701 and C702 as I was guessing that being the main power capacitors they were failing and causing it to go into standby as they could no longer do their job ....this had happened once on a guitar amp I had I'll do the tests and report back tomorrow

[Drewpicker]

HI Chris .... tested across C571 ... it read around 0.8v and there was no change when it cut out ... tested C701 ans C702 .. they both read around 41v ... when it cuts out they dont drop ... if anything they both go up .1 to .3v

[Drewpicker]

Hi Chris ... ive been looking at the amp again today and i noticed my multimeter was doing strange things like giving volt readings when it wasn't connected to anything it's a turbotech TT3341 ... so I dug out my old meter ... I re-did the tests and got 38.7v on C701 and 39.2v on C702 at 0 volume .... as I turn the volume up the volts fluctuate around these figures and when it cuts out the voltage fades downward towards zero ... sorry for the false readings above ...

[Drewpicker]

I tested across C571 and got 0v and no change of voltage when it cuts out ...

[Chris55000]

Hi!

If the voltages across C701 and C702 are collapsing when the amplifier distorts/cuts out, there's either an obvious dry–joint or defective connection between the mains transformer h.t. secondary and the rectifier diodes, or one of the rectifier diodes is going intermittently open–circuit!

If the rectifier diodes are suspect, use 4 × 1N5404 to replace them, and make sure there's no dry–joints or poor connections to the mains transformer h.t. secondary connections CN701, CN702 and CN703, and also pay particular attention to the switching contacts of the main power on/off relay RL701 – these contacts are in series with the a.c. feed to the main h.t. rectifiers and yakky contacts will cause no end of problems!

The simplest test for yakky contacts in RL701 is to temporarily bridge them across – use the PCB layout diagram to locate them!

If bridging the relay contacts makes any difference to the fault, bin the old relay and get another – I think it's a standard pin layout with a coil of about 5–9V rating.

The 0V you measured across C571 suggests there's no serious problems with either of the SVI3102D power amplifier modules!

Chris Williams

[Drewpicker]

Thanks again for your help with this Chris .. ill look at all you've said and get back to you .... all the best .. Andrew

[PJL]

Does it trip with no speakers?

[Drewpicker]

Hi PJL .... ive just tested it without speakers and it does cut out when i increase the volume ... i would say even quicker than with them connected... thanks again for your help with this ...

[Drewpicker]

Hi .... do i short 1 to 2 and 3 to 4 to test the relay (see photo) ... cheers

[PJL]

As far as I can make out, the trip disconnects the amplifier from its power supply.

It's worth testing to see which channels make it trip. What are you using as a source? Can you use an external source? If you can use an external source then try with just the left then just the right inputs connected. Alternatively you could use the balance control to only deliver sound from one channel.

[Drewpicker]

I've been using the tuner but i could try using an external source plugged in through the aux input on one side only ... there isnt a balance control on this system.... I'll try that and see if one side is tripping out ... and report back ...

[Drewpicker]

Tried with an external cassette player ... either side causes cut out ...

[Drewpicker]

I tested the rectifier diodes ... they seem good but can't roule out intermittent fault of course ... i re-touched the solder joints of the diodes and cn701 cn702 and cn703 no change .... i'll try shorting the relay if its 1 to 2 and 3 to 4 as question above ... waiting for confirmation on that ...