Vintage Sony Walkman repair - model WM-10

[steve30]

Someone sent in this old Walkman which unfortunately doesn't work. I fixed a couple of faults but I can't figure out this last fault. Maybe someone here can help?

When you put audio into the tape head, it comes out of the headphones extremely quiet and distorted. If I feed in an appropriately sized sine wave, it comes out as a smaller triangle-ish wave.

The audio path is as follows: Tape head --> Pre-amp IC --> Dolby IC --> Power amp IC --> Headphone output.

The signal is fine until the power amp IC. The input signal looks OK but the output is bad. I don't know if the chip itself is bad or if it is another component. I checked a few of the surrounding components and couldn't see anything that had gone bad. I've attached a photo of the power amp area. The chip is marked 20089 3165 which doesn't mean much to me.

Any ideas?

Unfortunately I couldn't find the service manual anywhere on the web. I found a 'troubleshooting manual' which has been incredibly useful, but this lacks the full schematics etc that a proper service manual would have.

Thanks
steve30

[rcowboy78]

Is the bad audio coming out of both the left and right channel? Are both channels coming out of that IC?

[steve30]

Yeah, one IC for both channels. Good audio goes in on both channels, bad audio comes out on both channels.

I was thinking, if it is the IC that is bad, it might be possible to bodge in a modern headphone amplifier IC.

[mzacharias]

Looks like some corrosion there. That is where I would start. Perhaps a failed foil run, associated with a capacitor? The chip does require external support components after all.

[tooki]

The service manual clearly shows the power amp on page 7 as a CX-20089. That's your part number. That said, it's a Sony part number (Sony chips usually start with CX) so it's probably a custom part. My gut feeling is that mzacharias is correct, that it's not the chip that has failed, but an external support component or a PCB trace (what he's calling a "foil run").

[Rasz]

what is in the lower left unfocused part of the pic? that rusty poop colored thing? is that a connector? is that corrosion?

[rcowboy78]

Tooki,

Can you tell us where you found the service manual?

[stj]

you probably have dried-up capacitors that are used as audio filters.

[steve30]

Thanks for the replies.

Tooki, do you have a service manual you could share? I looked all over the WWW and could only find the 'troubleshooting manual'. I know a service manual exists though as I saw a few expired ebay auctions listing it.

I don't think there is any corrosion. Sorry for the bad photo. I've attached a much better one. The bits which look like corrosion in the first photo are just bits of PCB with no solder mask, and flux residue.

I tested a few of the caps and the ones I tested seemed fine.

EDIT: I just saw the power amp schematic in the troubleshooting manual, probably the bit tooki was referring to. Don't know how I missed that  .

[Alex Eisenhut]

Maybe it's  just me but it looks like the ground braid from the head cable isn't connected? What is that solder pad?

[stj]

braid is soldered o.k.

btw, how will you replace the belt & pinch-roller??

[rcowboy78]

Steve,

It sounds like you've done some good testing. If you suspect CX20089, replace it. In my opinion, you have 3 options. 1) Find a donor WM-10 for the same amp. 2) They are on ebay for sale, but are expensive. 3) Substitute for a different amp if you can find one that is comparable.

Good luck with it. Hope you get it working. 

[steve30]

Maybe it's  just me but it looks like the ground braid from the head cable isn't connected? What is that solder pad?

It looks a bit iffy, but there is a pad right underneath it which it is soldered to.

Pinch roller looks OK. The belt is easy to change. I just need to get one ordered.

rcowboy78: Thanks. I'll test some of the surrounding components. If it is the chip that's bad, my idea was to simply build a similar circuit with a modern chip and stick it in its place. Probably do-able with the right tongue angle.

[amyk]

According to this huge list of amps (see page 59), the CX20089A is pin-compatible with quite a few others:

Code: [Select]

FAN7000D
FAN7000DTF
KA7000
LA4533M
LA4534M
LA4535M
LA4536M
YD4533
YD7000
Page 370 has the specs: Uccmin=0.9V Uccmax=4.5V Poutmax=2x40mW Ri=16R Icc0=8mA Bw=20Hz-20kHz THD=0.5% Gv=32dB

[tooki]

The "troubleshooting manual" is the service manual. The version on elektrotanya is the one I looked at. (That's a great site btw. Just register to be able to download.)

[stj]

you can download 2 per day without registering.

[steve30]

Thanks for the list of chips. I'll save a copy of that book for later.

I've looked at every possibility I can think of and I can only think it is the chip that is bad. As we know now of alternatives, I've managed to order an LA4533 off ebay for about £3.

[steve30]

The new LA4533 chip finally showed up.

Firstly, it is in a slightly different package to the Sony CX20089A. It is slightly smaller, so I had to bend a couple of pins slightly to make it fit.

Secondly, it's mute function is different. On the Sony chip, the mute function is active high, and on the LA4533, it is active low. A PNP transistor pulls the mute pin high for a second when you first turn the Walkman on to prevent any click noise. To make this feature work with the LA4533, I simply replaced said PNP transistor with an NPN.

I'm pleased to say the Walkman now works .

[jopa7]

Hi,

Got one of these walkmans. Initially I thought it was only the belt since the the walkman looks literally like new.

After changing the belt, I only had sound (very, very low from the left channel, no sound from the right channel).

Was searching over the internet and found this post. Steve's problem looked similar to mine. So I ordered a couple of power amp ICs (same model Steve got) and replaced Sony's proprietary chip. All work was done professionally since I have a long experience with small electronics (not walkmans)- the result is as this. Right channel is working perfectly (had no sound), Left channel is working but with constant buzzing and lower volume sound. Buzzing/humming is constant. Manual adjustment of pots near pre amp for right channel increases volume but also buzzing and sound gets even worse (crackling).

What I've done so far:

- substituted right channel re amp pot with left one- no result
- replaced power amp chip with other one - no result.

So this leads me to the conclusion that the power amp IC was indeed faulty but there's an additional issue.

Do you think it can be an also faulty pre amp IC (BA3304F ROHM)?

Thanks in advance

[steve30]

If it is amplifying the sound OK but there is a lot of buzzing, it sounds like it might be a bad connection somewhere, possibly, something not grounded properly.

I could be wrong, but I'd be inclined to double check the soldering of everything as a first step.

Good luck .

[R.J.]

There is a service manual you can download for free.
Look at;
https://elektrotanya.com/sony_wm-10_20.pdf/download.html#dl

[floobydust]

When I owned a few Walkmans,  a constant problem was the switches oxidizing. Especially the auto-reverse tape head switch and play (motor) switch. I would use DeOxit and switch them back and forth a dozen times and that fixes it. Make sure the switches are not just dirty.