• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Sony PS-X70 Wobbly Speed 33/45rpm

F

freebee

New Member
Hopefully someone can help me out here. There's a 'wobble' in the speed (both 33 and 45rpm). I started measuring the voltages on the power supply and there's only one value completely off Q406 64.4V instead of 35V. Perhaps someone can give me some pointers what might be wrong?
 

Attachments

Register to hide this ad
freebee said:
There's a 'wobble' in the speed (both 33 and 45rpm).
Click to expand...

I am guessing the wobble you are referring to is a wobble on the strobe indicator itself, not an audible wobble in the reproduced audio.

Q406 is driven directly from the PLL and its supply is half wave rectified and filtered by C408 (2.2uF 250V). What is very common on these lovely decks is the capacitor C408 fails or goes low. This causes the strobe indicator to give erroneous jumps and/or flashes as the strobe is being modulated on and partially off by two sources- the drive from Q406 and its own half wave rectified dc pulsing at 50/60Hz.


upload_2019-5-7_16-37-52.png

upload_2019-5-7_16-39-0.png

Replace the C408 smoothing cap and report back here with your results. It is a 2.2uF 250V rated electrolytic.

PS the voltage on Q406's collector is 85V not 35V with a typical neon drop of around 100V. All up, there is 200V on the secondary to power the neon and about 15V dropped over R411.

Also, maybe post a thread like this in the turntables forum as you will generally get more replies. Welcome to AK BTW.
 
Last edited:
Thanks for the info. I forgot to mention that all capacitors have been replaced and joints resoldered. The wobble is audible and cleaning and adjusting the pots haven't changed that. Problem is that I can't get the wave form on my scope (wave form is unsteady and distorted). Voltage appears to be too low. I was able to dial the offset back to 0 volts using my multimeter. As the voltages on the power supply appear to be ok, the problem most likely lies in the other board. Some of the voltages I measured are 32v instead of 24v on the main board. Attached is the updated file (not all measurements have been taken yet). Any idea where I should look at next?

P.S. I much appreciate your help and I have followed your advice to post a tread in the turntables section.
 
Thanks for pointing out that Q406's collector is 85v. The fact that it's marked 35V in the manual must be a mistake then? Also, the value I measured is 64.4V. Does this mean there is another problem to be fixed?
 
The manual is wrong, the schematic is right. Consider we are turning the transistor on and off every 4mS at 33.33rpm- the average collector voltage will not be 85V. A DMM will not indicate the correct voltage at this point- you need a DSO/CRO. The part schematic above shows your voltages.

Have you set the magnetic stripe pick-up head according to Sony's instructions? Glitches in the speed can be attributed to issues with the magnetic stripe (damage etc) and incorrect spacing.

upload_2019-5-8_7-2-51.png
 
Last edited:
I don't get the waveform as described in the manual on my scope (Rigol). Waveform is really unstable and flickery. The 'most stable' waveform corresponds with the strobe when pot set in mid-position. The head was properly fixed with lacquer, however I tried to move it forward/backward to no avail (when moved back too far the platter spins really fast).
 
You must log in or register to reply here.
Back
Top Bottom