Technics SU CH-7 repair

Thread Starter

Astrocurious

Joined May 27, 2020
4
Hi all,

New member here.

I have been reading through several posts on this form regarding the repair of a Technics stereo system. I thought it was only fair that I joined and created my own post detailing my experience.

This will be a slow post as I have lots of projects going on at the moment, these include,

classic cars
classic bikes
work
family
house

I work in the world of Railway Signalling and have experience in everything from lever locking through to solid state interlockings at a maintenance and fault finding level. I’m currently working with control systems from Server to PLC. Programming experience is very limited as most of our design and install is carried out by specialist companies. My electronics experience at component level is also limited to college work almost 30 years ago! Most things these days are card level changes which then get sent away to the manufacturer.

So that’s the level I’m working at - here is the patient.

E58CEB25-DEF7-42D1-A93C-65A72320DB5C.jpeg

Purchased by me in about 1991 for £560 and used ever since. It’s been relegated to the bedroom now as apparently the speakers are too big and black ash went out of style years ago.

Its working well and sounds amazing but has four issues.

1, CD stuck in the drawer - plays perfectly but the mechanism won’t eject. I expect something has popped off somewhere. In not in a rush with this as all my CDs are in the loft and the one stuck in there is Blondie’s greatest hits.

2, The vacuum fluorescent display on the amp and tuner units flicker and are much dimmer than the CD & Tape decks. Reading through your forums this could be solved by replacing capacitors on the power supply boards.

3, The amp seems to get really hot even though its only played at low volume (young children in the house). There are some comments regarding this but of all the problems this will be investigated last as i think it will be a tricky one to solve.

4, The unit won’t turn on from plug in. You have to wait a few minutes for it to charge up before the power button works. This has been spoken about on here so im not too concerned about this one.

This will be a good project to keep me out of trouble and Help with my soldering skills.

I will keep you posted on progress.

feel free to comment and make suggestions - I’m here to learn

thanks

SEAN
 

Thread Starter

Astrocurious

Joined May 27, 2020
4
Well. Things have started moving.

I found the service manuals to the stereo over several sites online and managed to take the unit apart. Some of the connectors were challenging. 30 year old plastic can be very brittle.

there are several areas of heat damage and one capacitor looks as though it’s been leaking. All of these areas have been highlighted in previous forums so it looks like they are common.

here are some pictures along with the circuit diagram of the area.

55FA0957-5434-44E7-8321-E1A3324E1105.jpeg016FF70A-43E1-42D5-9F69-B336AC4FD341.jpeg
8F142843-61E2-438D-BB9D-04FF170483CF.jpeg8D08BB4E-596C-438D-B839-61DDCF667B12.jpeg
A4C17FA4-588B-4144-AED5-0DEFBBF2E27C.jpeg486D2271-4701-443A-B5E0-DC723B8266B0.jpeg
The problem has been to identify the components and cross reference the old serial numbers with modern equivalents. This has been straightforward for the resistors and capacitors but im struggling with the diodes.

any suggestions would be welcome.

thanks

Sean
 

Thread Starter

Astrocurious

Joined May 27, 2020
4
Would any be able to advise me on which Diodes to look for?

I have spent the afternoon looking intospecs for the two I would like to replace But the Panasonic numbers are not used anymore.

The diodes im looking for are;

old number - MA4033TA
new number (I think) - MAZ4033
new component (I think) Farnell 1097230

Old number - MA4062MTA
new number (I think) - MAZ4062
new component (I think) Farnell 9844651

any advice you could give would be appreciated

thanks

sean
84C41E01-084F-4749-8C22-7F6CE55DC32C.jpeg
 

Thread Starter

Astrocurious

Joined May 27, 2020
4
Thanks Ylli,

I will place an order and await the parts.

I do have some good news. I have fixed the CD eject problem. Taking the cover off, looking at the mechanism and working through everything I noticed that the CD had stuck to the soft cushion that spins it (sorry, I don’t know the technical term). This meant that after playing the CD wasn’t released back into the tray ready for it to be ejected.

it may take a while to update as I now have a motorbike to fix.

thanks

Sean
 
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