Deutsches CPC-Forum > Off-topic

Good forums to help fix electronics?

(1/13) > >>

tjohnson:
Massively off topic for Amstrad but thought someone might know some good forums where I could seek help on fixing a double cassette deck.

The original issue I started to diagnose was the recording deck played fine but the recording would intermiitently distort on one channel.  I have access to a scope and observed the bias signal voltage on the affected channel changing very erratically.  However in my effort to diagnose I seem to have introduced a playback issue on that deck which plays back at massive amp and hugely distorted however it still records.  I think it may be in a multiplexer IC which takes the inputs for deck A and B then outputs them.  Deck A seems to play back more or less ok.  Same IC and outputs but seperate inputs.

I have the service manual and it has a very good circuit and block diagrams but with my little experience struggling to pinpoint this new fault but being a hobbist and complete amateur I'm very much trying to learn as I go along.

Bryce:
Send me the schematics and I'll see if I can help.

Bryce.

tjohnson:
Hi Bryce, thanks for the offer of some help.

I've attached the main schematic and a block diagram.  The original issue appeared to be a bias issue, i hooked up the scope to capacitors C130 and C230 and could see on record the bias on the right channel C230 bouncing up and down alot in voltage.  Anyway I was trying to determine the cause of this when I then noticed the playback on deck B had gone completly distorted and pushing out massive amplification where previously it had worked fine.  I've attached a sound clip with playback on deck B and then playback on deck A which still seems to works.  I was suspecting an issue in the multiplexer IC301 on in that region of the circuit as playback A still works, however I'm really clutching at straws here and could really do with some guidance on where might be suitable places to check and understanding the circuit digram, it would be really appreciated.
Trevor

Bryce:
Ok. Obviously the first culprit would be a bad electrolytic capacitor, but I assume you've already given the board a visual inspection. The next step would be to disconnect the heads and connect a signal generator to each channel at the head connector and compare the signals after the bias traps (L104 and L204) with a scope.

Bryce.
 

tjohnson:
I should say that playback on deck B was fine until I started probing around, I thought I was very careful not to short anything out but I'm not exactly sure when it went wrong but I think it was after I connected the scope up to the legs of C129 and C229.
I had previously checked playback by playing a sinewave 1000hz into the inputs, recording and then playing back.  I hooked the scope up to the legs of C135 and C235 to see the tone and it was fine on both deck A and B, now B is fubar.

On a further visual inspection of the board I noticed that the large 4700uf cap on the powerboard C909 has a small bulge in it.  I had previously dropped the board out and resoldered a few points that looked a little iffy, nothing else looks bad from what I can see.  Annoyingly I don't have a spare but could order one in, fingers crossed it arrives in all this madness.  Unfortunately I don't have a function generator and getting one at the moment could be a challenge!

Navigation

[0] Message Index

[#] Next page

Go to full version
Powered by SMFPacks Media Embedder
Powered by SMFPacks Alerts Pro Mod
Powered by SMFPacks Mentions Pro Mod