Help! Problem with my Technics Amp cutting out .

[Drewpicker]

Hi Everyone /anyone .... I realise this isn't vintage hifi however I Have a problem with this amplifier + tuner unit... they both work together to control the amp system ( it has tape deck and cd units attached) when on low volume all works perfectly however if i increase the volume to a normal level the system cuts out into standby ....i can then turn it straight back on and the same thing happens... the fan is working well inside the SE-CH730 .... there is a problem with the display on the ST-CH730 ...some of the led's dont light up ... even when in standby (a clock is displayed) ... i suspect that my problem could possibly due to one or more of the electrolytic caps having dried out ... there are no obvious burst ones .. the units are getting quite old now and the clock display has been on for most of its life ....
the service manuals for the two units can be found here

https://www.vintageshifi.com/reperto...ice-Manual.pdf

https://www.vintageshifi.com/reperto...ice-Manual.pdf

I've checked both units for dry solder joints and have found none
i found signs of overheating (darkening of the pcb ) in the Tuner unit around Q505 + c510
and around C615-618+C620 C251
my plan is to change these capacitors and the transistor Q505 if i can find one .... but i dont know if these are causing the problem...
there's a post i found for the technics CH7 which is quite similar and it turned out that it was probably just some bad capacitors

https://forum.allaboutcircuits.com/t...-issue.120179/

for the led the guy says for the ch7
"Then I replaced C702 and C713 and C714 in the tuner. That fixed the problem completely! Instant power-on from cold, no flickering on the tuner display. Everything works fine."

schematic for CH7

http://www.atipica.it/clienti/SU-CH7.zip

ive looked at this but the layouts seem quite different to me ....

I have a soldering iron and multimeter and am fine at dis-assembly re-assembly but not very good at understanding schematics other than locating parts ... points etc ...
.......... if anyone has any ideas for me or can help me solve this they'd be most welcome
all the best

[tri-comp]

Hi,

On some Technics amplifiers and receivers with a built-in fan, which accellerates when increasing volume, the sets may suddenly switch to standby because the fan itself is defective. (SA-GX130, SA-GX170 etc)
In your amp too, there is a circuit that monitors the fans power-consumption and when that increases beyond some preset levet, the set switches to stand-by.
Try to disconnect the fan and see if the problem still exist when turning up the volume (for a short while only as there is no airflow to cool the output integrated circuit)
If the problem is gone you know the cure. If not you'll have to look elsewhere.
Getting hold of a new fan in case the original is bust isn't easy.
If you do replace with a new I believe Panasonic came up with a modification to the fan-circuit to make the fan last longer.
Possibly to add in parallel with the motor a capacitor to reduce induction-spikes generated by the fan.

Good luck with repairs,

/Torben

[Drewpicker]

Hi Torben ... First and foremost thanks for your help with this...I've been looking at the amp today and what you suggested seems very plausible... as I said the amp works perfectly on low volume and effectively the cutting out happens when the fan is stop starting... however when I disconnect the fan and turn it on the amp goes back into standby immediately as the safety circuit realises its absence .... I could try to modify it as you suggested... I've looked on the net and so far haven't found anything... if I put a capacitor parallel to the fan what value would you suggest ? and should I add anything else ... Thanks again for your help.

Here's a photo of the faulty display .... as I have it all open if I can mend this if its something simple and obvious ... I might aswell... though getting the amp working is obviously my main aim .

Andrew

[Drewpicker]

Hi again Torben ... been looking and I just found this
https://www.petervis.com/Amplifiers/...gx370-fan.html


I measured the resistance of the motor and it's 4.5 ohms so below the 5 ohms causing the protect mode .... the fan itself is a Nidec Beta SL model D06T-12BG 01 .... its 12v o.13amp ... if I added a 10ohm resistor would that be a possible temporary solution as the circuit would have a heavier load so wouldn't go into standby.... the fan itself works although it is probably nearing the end of its life .... I,ll look for a fan with as close to 0.13amp as I can find ... I can always use the old mount or build something to hold it ... do you think a higher or lower amperage fan would be more compatible with the circuit ..
I've found one at .12a and .17a
Thanks in advance ... Andrew

[tri-comp]

Hi Andrew,

It's been quite a long time since I've repaired a Technics receiver with this kind of fault.
At least 20+ years so I don't recall precisely what modification was adopted for extending the life of the fans.
I do believe it had something to do with dampening the induction-spikes generated by the fanmotor and that would have been done by simply adding a capacitor across the motor.
My experience comes from models like SA-GX130 and SA-GX170 as I wrote but studying the schematics that goes with your amp I see yours is made differently.
The principle may be the same but the solution is different.
Anyway, go for a download of a better quality schematic here:
https://freeservicemanuals.info/en/s...hnics/SECH730/
On page 18 you'll se the protection circuit as well as the motor control.
Transistor Q371 is the one monitoring what the fan is up to.
Across resistor R358 will be pulses which depends in size on the motor speed and possibly power-consumption.
The pulses are fed to the base of Q371 after having been made negative with reference to ground with the help of D371.
At some point, when the negative pulses are big enough, Q371 turns ON and shuts down the amp through Q354.
To stop this action I suggest to simply short-out D371 and see if the problem disappears.
If it does I would replace capacitor C362 and see if the problem is solved (Don't forget to remove short across D371).
If the problem persists I would double C362 in value and make sure that it's a low ESR-type. If still not cured the fan is most likely defective.
All the above assumes the fan-circuit is causing the shut-down problem.
There may of course be another fault triggering shut-down and if you see no effect of shorting-out D371 that is likely the case.

Best regards,

/Torben

[Drewpicker]

Thanks for that .... ill try that and post the results .... have a great week-end... stay sage... Andrew

[Drewpicker]

HI Torben.

i shorted out D371 by disconnecting one side, I hope that is what you meant, instead of shorting across the terminal to effectively take the diode out of the circuit. if you meant the second one let me know and I'll try that.

When I increase the volume and the fan kicks in the amp goes immediately into standby and then turns itself back on then off and stays off. I could change C362 from 10uF to 20uF or do you reccomend trying another 10uF cap before trying that?


I found a similar fan on eBay, same make runs at 0.12A instead of 0.13A, but I cant find out how many watts. I imagine that could be important for the spikes and security circuit. I could order it and that way I could mend the amp or at least maybe eliminate the fan being the problem. Maybe I should double C362 and if that works change the fan? How do you think I should proceed? There's an electronic components shop here where I live (Montpelier France) so going to get a few capacitors is easy for me, although very sad that they're retiring in mid July, their shop has saved my bacon many times. I hope someone takes it over.

As I mentioned in my post in the ST-CH730 tuner Q503 runs very hot, see pics. I presume this is normal as it is a regulator though I would have thought at that temperature it should have a heat sync, too hot to touch for more than a few seconds and the regulators Q601, Q603, Q602 and Q614 also give out alot of heat. I realise this has nothing to do with the fan problem but figured I'd mention it. I can't figure out from the schematic what these two areas of the PCB are used for other than regulating voltages and that the second one works as a pair.

Thanks again for taking the time with this, it's a decent hi-fi system when it works and I hope it can be rescued from a sad end.

Stay safe (the autofill changed it to sage in my last post, sorry about that).

Andrew

[Drewpicker]

Id orderd a replacement fan which finally got here today it's a 0.12a ... the original was 0.13a ... installed the fan and no luck ... the amp is still cutting out when I increase the volume ... I'd already tried doubling the value of C362 from 10uf to 22uf and that changed nothing ... still cutting out .... if anyone has any ideas how to get this amp back up and running.... I'm at a bit of a dead end now .... cheers

[Silicon]

Your photos show areas of the circuit board that have been affected by heat.

The heat was generated by the transistors and resistors.

It is possible that some of the components are faulty. Heat is bad for electrolytic capacitors.

I would also look for 'dry joints' using a magnifying lens.

[Drewpicker]

Thanks for the suggestions ...I've looked for dry/cracked solder joints already but found none ... I'll take q503 off the board and test it ... I've dismantled the display in the St-ch730 tuner as there is an obvious problem with that ... see earlier photo ....and under the display are signs of a component having burned out (see photo attached) the other components are covered with a brownish film ... upon inspection the only thing i can see that may have burned out is some sort of bulb in the display part itself ... see photo ... could this be causing the display problem.... and maybe even the cutting out?

[PJL]

Originally Posted by Drewpicker

i shorted out D371 by disconnecting one side, I hope that is what you meant, instead of shorting across the terminal to effectively take the diode out of the circuit. if you meant the second one let me know and I'll try that.

The theory goes that the speaker output is rectified on to C353 and that is used to increase the fan speed when the volume turns up. Your cutout is then caused by the fan increasing speed and triggering the fan fault sensing circuitry Q371.

To prove the theory you could try shorting out R353 (22K) but only for a quick test as it will stop the fan speed increasing at louder volumes.

If that does not fix the fault then the cutout will not be due to the fan fault detection and we need to look elsewhere.

You should really start with the power supply as I notice that in the aboutcircuits link you posted that the guy fixed the cutout by replacing PSU electrolytics.

At the end of the day this may not be worth trying to save!

[Drewpicker]

Hi PJL ... thanks for your help with this ... I shorted out R353 and now immediately as the fan turns on the amp goes into standby.... with R353 in place the fan comes on at a certain volume level with a stop start intermittance and above that volume level came on full it would go quickly into standby at either of these stages .... the thread I posted talked about a similar amp/tuner
system ... the display being a problem that caused a standby issue ... I'll go and get the electrolytics for the display board and see if that does anything .... there's only 7 of them ...
maybe theres a way to eliminate the cutoff protection and wire the fan to another source of 12v from the main board to be on permanently at all volumes .. thanks again .... Andrew

[Drewpicker]

Forgot to mention that the fan is a two wire fan and appears to be either on or off ... maybe the board supplys more power to increase its speed at higher volumes but as it cuts out quite quickly after it is turned on I couldn't say... just thought I'd mention this as it could be relevant.

[PJL]

Did you short R353 or disconnect it? Short means putting a wire link across it.

It is unlikely to be the display that is causing this problem.

[Drewpicker]

Ah ok ... thanks for that ... no I disconnected one side thinking it was providing the base to turn on the transistor.... I'll short across it and see what that does ...

[Drewpicker]

Shorted R353 and it's still cutting out once I turn the volume up .I presume this means it's not the fan ..... the display shows F61 which is apparently an error code for polarity in the speakers ... I've got the positive and negative connections correctly connected so there's no error there .... could it be something on a board in one of the speakers maybe ... it's a surround sound amp however I only have the two large original speakers ... it didn't use to cut out before ...

[ajgriff]

Sounds like the protection circuit is detecting a marginal short especially if you've only changed the speaker arrangements. I'd carefully check the wiring to the speakers as a next step.

Alan

[Drewpicker]

Thanks Alan ... I'll check that out now ...

[Drewpicker]

Checked the speaker connections... I've shortened the cables aswell in case there was a problem in the cable but it's still cutting out when I turn the volume up ... I opened one of the speakers and there doesn't appear to be any extra circuits inside ..... none of the capacitors have any bulging or leaking ... and as I said I haven't found any dry/cracked solder joints ...

[PJL]

'Wrong polarity' may mean something has been lost in translation!

You need to start at the beginning and check all the power rails.