Sony WM-DC2 static noise on right channel

Hello all,

First some context as this is only my second post. Please forgive my grammar, I am not a native speaker (French).
I'm in my mid 40's so I did have the usual run of the mill walkmans in my youth.
I am a bit of an audiophile (own nice headphones and IEMs) and as odd as it seemed at first, recently I got the itch to handle cassettes again and got my first walkman.
I'm quite handy with small things mechanical (watch maker hobbyist) and I'm not afraid of a soldering iron, so as many things I kinda went all in: Sony 702, Aiwa PC202, Panasonic RQ-S80, TC-D5M, and most recently D6C and finally DC2.
Really enjoying restoring these beauties and I am really in love with 80's Sony!

I restored my DC2 according to the guide and using parts from fixyouraudio and it is running beautifully except for one small issue: there is a faint static noise heard on the right channel only. It is mainly heard when playing Dolby C tapes, as off and Dolby B have sufficient hiss to mask it almost entirely.
Noise is also present using line out.
Also heard when head is desoldered. Noise reacts to volume but seemingly nothing else.
walkman works very well otherwise and sounds fantastic (I’d say on par with my oldest 1st gen D6C) but this is bugging me between songs and on quiet passages.

I have done quite a lot of troubleshooting without success:
- Complete electrolytic recap (even replace the 47 uf chip by an electrolytic). I have triple checked polarity and replaced the output caps twice with different brand with no change in behavior.
- I have reflowed all the right channel audio path but all solder joints looked fine. Reflowed everything around right channel Dolby chip.
- Cleaned 2 spots of glue thoroughly and reflowed the area.
- Checked ground all over (chassis/motor/PCB)

still no change after all this, so I turn to the gurus here that might steer me in a new direction?

thanks for your insights!

 

Thanks for the insight, I should have mentioned, I did clean all contact / switches as part of my normal service / restoration routine

 

The term "static" is a bit generic, would be useful to describe in more detail: is it a hum, a crackling sound or just the noise floor of R channel is higher than L ?

Given the volume potentiometer changes the level of static, it means the source of this is before the volume potentiometer: line amp, Dolby amp or head preamp.
As I assume you probably don't have an oscilloscope, simplest thing to do would be to start isolating the 3 stages one at a time and see when the static disappears.
The head preamp is powered through a resistor (R302), so would start by removing that resistor from circuit and test again.
For the Dolby IC, simplest thing would be to cut the trace going to pin 21, before R303. And the line amp can be removed entirely.

 

Thanks for chiming in Valentin! I’ve read a lot of your posts in the last months and gained quite a lot of knowledge and fixes from them!

The noise is definitely a crackling sound, on the right channel only, on top of the noise floor which sound the same on both channels.
Just for additional info yesterday I have swapped opamp IC301 with same behavior. I have also jumpered the head and the static noise stayed on the right channel.
I will try your troubleshooting steps and report back!

 

So head preamp is ruled out (removing R302 didn’t change anything) and 4558 also (swapped out earlier).
Now I’m a bit wearing of cutting into traces to the Dolby chip
Any other troubleshooting step?

 

That chip was not that common in portable units. For some WML models I keep track of common parts, like ICs, here is the Dolby one (actually 2):

 

So I decided I am going to get an oscilloscope, hopefully a small digital one will be ok for the purpose and I will learn to use it to trace the signal with the aim to isolate the fault.

 

To eliminate the possibility this crackling being motor hum, would desolder the motor and see if the problem goes away.

Dolby IC is very unlikely to be causing the problem. Even if the problem does come from that circuit, it's more likely to be something around the IC than the chip itself.

In regard to oscilloscope, small digital one is fine as long as vertical amplifier's sensitivity is good enough to be able to see the signal (should go to at least 5mV/DIV, prefferably lower).
It is to be kept in mind on some oscilloscopes amplifier sensitivity is different than the minimum vertical range, as the smallest V/DIV setting can be a software magnification.
Having AC coupling is also a must, although vast majority of scopes do have it.

 

I did try desoldering the motor, with no success.

with regards to the oscilloscope, I think I just did an oupsie, ordered one on Amazon but having a look at the manual, it seems to have a sensitivity of 10mv / division… guess I’d better cancel and shop some more!
Thanks for the heads up

 

Hello Ben,
Yes, a digital oscilloscope will work great and jut don't waste your time on one those cheap little super tiny handheld get a small digital benchtop I have early Panasonic boombox with built in phonograph that developed a noisey channel too. this winter I will use a scope and signal generator .
Good luck Ben !!

Sincerely Richard

 

So I had ordered a 10mV/div oscilloscaope, and of course I couldn't see a thing with such tiny signals. I am now ordering a Rigol 1102 (1mV/div) and will report back!

So basically I guess the idea is to just trace the signal (with no signal applied) from head to headphone out and see where it gets noisy, right?

 

Yes, with no tape inserted, EQ set to 70μs, Dolby C (so you have the lowest noise floor) start measuring the output of head preamp (emitter of Q203), pin 1 of Dolby IC, pin 27 of Dolby IC, etc.
To limit the noise floor of the oscilloscope, enable the 20Mhz bandwidth limit since you don't need the full bandwidth.

Given what we know so far, you should see the signal of interest on pin 27 of IC201.
But of course this will be just half of the diagnostic, as you will need to find what exactly causes this.
For this you will need to start measuring the supply rails (pin 20, 21, 22), inverting input of input amp (pin 2), etc.

 

Hello ben,
You, will enjoy that Rigol 1102 good scope !
Sincerely Richard

 

Thanks all for your words of encouragement. I received the scope this morning and tried to play extensively with it after watching tutorials to use it.
Unfortunately I can't seem to be able to see anything on the scope. vertical scale is set to 1mv/div and applying the probe (with alligator clip connected to circuit ground) to pin 27 of IC201 doesn't get me anyplace. I seem to see nothing different from the noise generated by the scope itself.
I am using auto trigger and it all seems to be garble. It doeasn't look like there is any difference between the IC101 and 201. Even connecting to the 4558, I do see more amplitude on the signal but can't make much of it and certainly can't see a difference.

Do you have pointers on the kind of settings I should be applying / best practice to try and capture this low level crackle?

 

This problem happened to me 3 times and each time the problem was from the Dolby system, the ceramic capacitor in the IF transformer was burnt.
Try to enter signal after the heads or with a recorded tape with a signal and visualize the signal with the oscilloscope before and after Dolby

 

Are the IF transformers CP101 and CP201? Did you replace them or is there a way to repair them?

 

Excuse me for the fact that I don't speak English very well, I'm Greek.
I wanted to say that I did not repair your model, I was talking about other cassette players.
I studied a little the diagram from your tape recorder and it could be CP201, 101.
Try to unsolder a leg from the R113 resistor and stick a 390 pF ceramic capacitor in place of the coil. If the noise disappears, then you have found the defect. If the noise continues, the problem could be from the IC
It is very difficult to replace them, where to find them? Maybe if you have another identical wackman.
I fixed them.
Unsolder very carefully and at low temperature, with a tin pump, in order not to melt the legs and not to break the coil. The capacitors are usually outside under the IF.

 

Thanks! I have taken both CP101 and 201 out to compare them and the capacitors below measure both at around 28nf. Unfortunately swapping them does not solve the issue

 

It seems you could try to bypass the Dolby circuitry entirely by lifting the - end of C135 and connecting it to the lifted end of R112, and see if the noise goes away.