Technics su-3500 fault search

[OldDIY]

Part of the amplifier is powered by a stabilized voltage. The terminal transistors and drivers are powered from another, unregulated, power supply.

 

[llwhtt]

Remove the left fuse and leave both out until we figure out the power supply DCVs. Measure 341 and 342 again.

 

[HenriFred]

I shut down the amp and turned it on again i heard the relay click. Now 342 measures - 39,4 DCV.
Both fuses are out now

 

[llwhtt]

341? You need to post the polarities of the DCV also.

 

[OldDIY]

The protection circuit and relays are powered from the main unregulated power supply. Operates from constant voltage at any output. The outputs are additionally protected by fuses.
The power supplies are bipolar with respect to earth.
The relay contacts disconnect the power to the output transistors. Check contacts.

 

[HenriFred]

Will do
341 is 40.4 DCV
342 is -39.9 DCV

 

[llwhtt]

OK, power supply is good. Put the right fuse in and the measure 341 and 342 again.

 

[HenriFred]

With R fuse in
341 is 40,4 DCV
342 is -39,8DCV

 

[llwhtt]

OK, the right channel should play music. Can you try it?

 

[HenriFred]

It doesn't neither on the headphones.
There is a humm coming from the trafo though

 

[llwhtt]

OK, left channel is probably loading the power supply. Pull the right fuse and install into the left channel and measure 341 and 342 again. They will probably read 0VDC as the protection will be activated again.

 

[HenriFred]

Ok
Switched the fuse to L
Now 341 measured 40,8 DCV and
342 -1,3 DCV

 

[llwhtt]

Now we know the left channel is the problem channel. Do you know how to test transistors? We need to test TR409 and TR410 to start.

I'm out 'til Monday morning.

Craig

 

[HenriFred]

I'll Google how to measure the transistors. Thank you very much so far!
I'll let the dog out and of to bed afterwards.
Have a great weekend!

 

[bosedtobosed]

are the relay contacts ok ?

 

[HenriFred]

I measured the transistors in diode test setting on the multimeter. I didn't solder them out of the circuit board like some suggested. I guess the values are in mV ?

On the R pre amp

TR 409. TR 410

b-e 068. 069
b-c 521 1
e-b 068. 069
c-e 1. 566

On the L preamp

TR 409. TR 410

b-e 068. 004
b-c -1. 0
e-b 068 004
c-e 1. 004

 

[HenriFred]

The layout got a bit scrambled here.
TR 410 on the left side:
b-e. 004
b-c. 0
e-b. 004
c-e. 004

 

[HenriFred]

are the relay contacts ok ?

Hi,
I hear the relay clicking when the left fuse is in. It doesn't when both fuses are out so i guess the relay is okg

 

[sidsparks]

The layout got a bit scrambled here.
TR 410 on the left side:
b-e. 004
b-c. 0
e-b. 004
c-e. 004

That's failed short circuit.

Sid

 

[bosedtobosed]

if its taking the - supply down to 0v i would have expected the mains fuse to go