B285 schaltplan (DC fehler)

[davor]

Hello
Mein Deutsh ist nicht gut...
Is there any good Revox user with complete scan of B285 receiver shematic (circuit) diagram or maybe only part of it where could be a source of problem - DC message on LCD?
I have downloaded shemes of B251 (same generation like 285 but only amp.) from revox.name but not shure that will be helpfull.

thanx
Davor Zubcic

[hobby1304]

AW: B285 schaltplan (DC fehler)

Hello Davor,

This message "DC" has been mentioned several times in this forum (you can search for threads in this forum, use "Suchen" and search for "Gleichstrom" since the word "DC" is not accepted because too short...).
Unfortunately I have not seen postings with actual description of source of problem.

But the comments are unambiguous:
- very serious failure condition
- "DC" indicates that (too much) direct current is detected on speaker output
- action to be taken: give the device professional service

Without this protection mechanism your speakers would be damaged by the direct current.
I don't know where (between input, preamp, power amp) the source of the problem and the detection mechanism must be looked for.

Best regards, Jürg

[davor]

AW: B285 schaltplan (DC fehler)

Danke Jürg
I have just pick up my B285 out of service after 2 weeks. Nobody send me a shematic for B285 but problem has solved somehow.
Guys in service just said that they have never seen so difficult thing to rapair!
I responed to it: maybe designers believed that this product would never need a repair!
Changed parts are:
1. Transistors SANKEN 2SA1295 AND 2SC3264
2. Transistor T A968 Y 5A
3. Resistor (burned)
and 500kn (68eur) out of my pocket.
Now I`m going home to warm up machine and check speakers (are they survive DC stroke)

[hobby1304]

AW: B285 schaltplan (DC fehler)

Hi davor,
Glad to hear that hopefully everything is settled.
I must correct myself:
DC does not indicate direct current ON speaker output but rather BEFORE output.
So your speakers should not have suffered since the device has this feature precisely to protect the speakers.
Have a good time with music and information,
Cheers Jürg

[davor]

AW: B285 schaltplan (DC fehler)

Yes, you are right, speakers are OK.
Now I plan to make some coolers on transistors, like showed in this forum, that are really very very hot short after power is on. It is strange that engineers didn`t solved this.
Anyway, I think this machine will survive long enough after digital wave come to our shores.

thanx and herzliche gluckwunsche aus kroatien
davorzubcic@net.hr

Measured temperature on first transitor next to SanKen MosFets: 90 celsius 1 minute after power on!!!

[davor]

AW: B285 schaltplan (DC fehler)

Yesterday - DC error again

[davor]

AW: B285 schaltplan (DC fehler)

I have repaired receiver by myself
Problem was in bad contact of one of transistors around Sanken MOSFETs on PCB after I upgraded it with aluminium coolers. I have cooled down first trans. to the receiver cover for 30 celsius, from 90 to 60!!! (I didn`t measure others)
Bad contact is result of overheating of transistors for years of use. Heat was transferred via elements pins to PCB and was so high that green colour has changed to gray (burned). Lines on PCB can now be very easy damaged. After one night of searching for problem as final attempt measurnig resitance between identical points (chanels) I found critical spot and make bridges (jumpers) of thin wires on some others that was suspicous and possible source of new problems.
I need do say that forum member Peterpan was very helpfull.

regards