Marantz SR8100dc, advice needed.

[redrooster]

I hope this post is in the right place? I couldn't find anywhere else to put it! I'm trying to repair this receiver for a friend,it came to me with a burnt out power supply that was caused I suspect from the conductive brown glue they used in those days,I have cleaned up the glue and recapped the amplifier and tuner completely.This model is from late 70s / early 80s and has an MSU digital controller for functions FM, AM, AUX, PHONO (not working) I suspect that the MSU (MN1455LF) has failed! What I need to know is can I replace the MSU without having to program it? I'm hoping that I can solder in a socket and just plug another MSU in! I have only ever repaired analog amps before,I would appreciate any suggestions, advice, ideas ect...

[jaycee]

No, it's effectively a computer on a chip. With no program, it won't do anything.

The only way would be if you were to get a working chip out of another SR8100dc

[Mooly]

Normally the microprocessors are the last to suspect.

My tactics for dealing with anything like that is to have the service manual in front of me and then go along every pin with a scope and DVM and make sure that what you see and read is what would be expected.

As jaycee says, the IC has to be correct type and it's the numbers after the main device number that matter as these are the unique program burnt into the ROM of the chip.

[redrooster]

OK thanks for setting me straight there fellas.

[sregor]

Check the clock and crystal part of the circuits. Also, look for a pull up (or down) reset circuit which could be bad. Need to look at the manual, and as suggested go through all the pins. Good luck.

[redrooster]

Ive changed all dodgy components transistors,diodes ect..checked and rechecked voltages everything adnauseum,so that all voltages where correct, I turned it on and it still didn't work,after I had calmed down a bit I turned it back on and Begorrah! It was working! I left it on for about 4 hrs FM & AM worked in fact it sounded excellent.After turning it on & off a few times with it still working I noticed it was intermittently cutting out for a fraction of a second I had a look and found a connector ribbon had a broken wire and was just resting there,it is the 5v power for the MCU I fixed that and f#@%$# me dead I was back to where I started except when I plug it in now the relay hums and chatters so I give the speaker relays a rap with my knuckles and the amp comes on but not the radio or function side of things. Any ideas I could try next fellas?

[Mooly]

It all sounds as though there is something physically intermitent. So after fixing this broken ribbon it now doesn't work ?

If a relay hums its usually because of a problem with the drive to it so check the supplies feeding it. Maybe there is a problem/dry etc with a smoothing cap on a rail. The relay coil won't hum with pure DC.

[redrooster]

Originally Posted by Mooly

It all sounds as though there is something physically intermitent. So after fixing this broken ribbon it now doesn't work ?

If a relay hums its usually because of a problem with the drive to it so check the supplies feeding it. Maybe there is a problem/dry etc with a smoothing cap on a rail. The relay coil won't hum with pure DC.

Thanks mate! Your advice did the trick,I followed the circuit and found a shorted diode in the power supply that I had replaced before thinking it was blown originally when I got the receiver ,so I looked a bit further and found that when I recapped it I have used a 470uf cap instead of a 330uf cap.Duh! I have ordered the cap and should get it in a week or two,I live in regional Western Australia and it comes from Eastern Australia.I would say that this would be the problem ay?

[Mooly]

The shorted diode will be the problem.

The difference between 330 and 470uf won't make any difference tbh. The tolerance on electros used to be +100 and - 50%

[redrooster]

Originally Posted by Mooly

The shorted diode will be the problem.

The difference between 330 and 470uf won't make any difference tbh. The tolerance on electros used to be +100 and - 50%

Well here I am again, all confused.I went thru all the old caps so I could try the 330uf cap back in it but there wasn't one there? I had recapped cap for cap and it looks like the service manual has got it wrong again! What I have now is 12voltsDC on transistor Q820 its collector its supposed to be 0.38? I'm reading about 9vAC on that collector as well (R915 2W) is actually a wire link, It goes to the power and speaker relays thru j823 and returns thru j829 at approx 20volts.Transistor Q820 is a new one as is Q812.What do you think?