Problem with Technics SE-CH730/ST-CH730 HI-FI System cutting out

Thread Starter

Drewpicker

Joined Jun 7, 2020
13
Hi Everyone /anyone .... I Have a problem with this amplifier/tuner units.... they both work together to control the 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 ... 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 it is 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 schematics for the two units can be found here

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Technics-SECH-730-Service-Manual.pdf

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Technics-STCH-730-Service-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 for the lcd ...
theres another post here for the technics CH7 and it turned out that it was probably just some bad capacitors

https://forum.allaboutcircuits.com/threads/technics-su-ch7-panasonic-sc-ch7-amplifier-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 good at understanding schematics
.......... if anyone has any ideas for me on this one they'd be most welcome
all the best .... Andrew
 

Thread Starter

Drewpicker

Joined Jun 7, 2020
13
Hi Everyone /anyone .... I Have a problem with this amplifier/tuner units.... they both work together to control the 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 ... 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 it is 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 schematics for the two units can be found here

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Technics-SECH-730-Service-Manual.pdf

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Technics-STCH-730-Service-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 for the lcd ...
theres another post here for the technics CH7 and it turned out that it was probably just some bad capacitors

https://forum.allaboutcircuits.com/threads/technics-su-ch7-panasonic-sc-ch7-amplifier-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 good at understanding schematics
.......... if anyone has any ideas for me on this one they'd be most welcome
all the best .... Andrew
Is there anyone that can take a look at this ... if you could just identify the capacitors In the St-Ch730 that power the display it would give me a place to start as the display isn't functioning properly.... thanks ... Andrew
 

Thread Starter

Drewpicker

Joined Jun 7, 2020
13
This is a photo from under the display ... components seem to be covered with a brown film ... is there likely to be a component that has burnt out ? The area is hard to examine closer as its below the display itself . As i explained above there is a problem with the display ... the information is missing many points when a fonction is selected .... any help from someone would be more than welcome ... Andrew
 

Thread Starter

Drewpicker

Joined Jun 7, 2020
13
I finally sorted this out thanks to PJL on a vintage radio forum ... unfortunately I got 0 replies here ... I'll post the solution I found to help anyone with a similar problem.. after trying various things ... it turned out that replacing two capacitors (C363 and C371) in the fan protection circuit solved the problem... when the fan cut in ... or worked for a short while the bad caps caused the protection circuit to believe the fan was faulty and so it put the amp into standby to protect the power amps ... I still have the problem with the VF display (see photo) if anyone has an idea for that ... I suspect something has blown on the display board but have been unable to find it ... visually nothing seems obvious ... any ideas ? 20200613_143943.jpg
 
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