Ok, I'm replacing those two voltage regulators now, and the way the heat sinks were mounted from the factory has been bugging me. It seems as if they intended there to be some thermal tracking between the two transistors, but the surface area of where the sinks touch is so minimal that I question whether it's worth recreating. I've checked photos of this board on other PS-X70's, and they're all the same way. Or was it just a way to afford them more physical stability? Thoughts?
Almost certainly the latter and probably to try to prevent microscopic movement and fatigue of the soldered joints.
Could perhaps someone point me in the right direction? My Sony PS-X70 has fluctuating speed and the platter turns a bit back when you press the stop button. So far I have checked the voltages of the power supply, replaced IC CX193 and crystal. All the other functions such as auto start/stop, return, up/down, 33/45rpm, repeat are working.
Fluctuating speed can mean a lot of things 🙂
Do you mean a little W&F with it running at basically the correct speed, or is the speed varying a lot (say 20rpm to 50rpm) on its own?
If the oscillator is OK and the FG pulses of good amplitude as they reach the servo chip then perhaps you have a motor drive issue or perhaps a problem with the hall sensors in the motor.
You might be able to break the servo loop and inject a control voltage to set the speed manually (from a multi turn preset) and see if the motor behaves OK in that state.
Do you mean a little W&F with it running at basically the correct speed, or is the speed varying a lot (say 20rpm to 50rpm) on its own?
If the oscillator is OK and the FG pulses of good amplitude as they reach the servo chip then perhaps you have a motor drive issue or perhaps a problem with the hall sensors in the motor.
You might be able to break the servo loop and inject a control voltage to set the speed manually (from a multi turn preset) and see if the motor behaves OK in that state.
Hi Mooly,.I mean a little w&f that you hear when you listen to piano music. (When you look at the strobe it's quite regular)
I have a scope, but am a complete amateur. What do you mean by FG pulses? Thanks for your help.
I have a scope, but am a complete amateur. What do you mean by FG pulses? Thanks for your help.
FG = frequency generator, the tacho pulse fed back to the CX193 (pin 8).
Based on your description of the fault I suspect the issue lies elsewhere. In the first instance the motor has to be 100% mechanically, no sticky bearings etc.
Electrically I would begin by looking at all the voltage rails on a scope to make sure they are clean and then start looking at the filtered DC control voltage from IC3 and see if any fluctuation shows up there.
Has this TT got a hidden history? In other words was it working correctly in the past and has now developed an issue, or was it some aquisition that has a problem 🙂
Have any of the presets relating to motor gain and offset been twiddled?
Based on your description of the fault I suspect the issue lies elsewhere. In the first instance the motor has to be 100% mechanically, no sticky bearings etc.
Electrically I would begin by looking at all the voltage rails on a scope to make sure they are clean and then start looking at the filtered DC control voltage from IC3 and see if any fluctuation shows up there.
Has this TT got a hidden history? In other words was it working correctly in the past and has now developed an issue, or was it some aquisition that has a problem 🙂
Have any of the presets relating to motor gain and offset been twiddled?
The turntable was working properly when I bought it. From one day to the next it started to develop problems with the automatic functions. This is when I decide to do a complete recap. Now all functions are fine, but there is the 'wobble' and the platter turns back a bit when pressed stop. The presets relating to motor gain and offset have been twiddled with (I tried to adjust it using DMM and scope). Bizarrely I never got the example waveform (used external power supply to feed it 1.5V)
Regarding the scope, I have a Rigol DS1054Z, and it might be my inexperience that I can't get the right waveform (have no technical background, but am eager to learn 🙂 ).
The scope check must be done without that diode/resistor/cap network as that is just for using a voltmeter as it removes all AC content and just gives the average DC level.
If you were connecting the scope as shown om page 18 then you should see the AC waveform. You need the scope DC coupled and set to a slow timebase speed.
If the W&F only appeared after a recap then I think you have to go back and carefully check all the work you have done. A lot of the small caps are going to be very critical in a circuit like this as it is much more 'analogue' than digital in how the servo's all work.
If you were connecting the scope as shown om page 18 then you should see the AC waveform. You need the scope DC coupled and set to a slow timebase speed.
If the W&F only appeared after a recap then I think you have to go back and carefully check all the work you have done. A lot of the small caps are going to be very critical in a circuit like this as it is much more 'analogue' than digital in how the servo's all work.
Hi Mooly, After a long search I found that C2 and C3 were out of spec (ESR too high). C15 measures an ESR of 50, which seems way too high. It's a tantalum cap and I don't know what the usual ESR is supposed to be (0.22uf 35V). Should I replace all the tantalums for new ones from e.g. Kemet?
I think older tants (older production processes) can have a higher ESR than a comparable electrolytic tbh but I've no idea on actual values for tidlers like these. '50' sounds very high and what I would do first is just try a similar value electrolytic or even a film cap to see what your meter reads for those.
Have a look at figure 6 and 7 here.
http://www.kemet.com/Lists/FileStore/APEC.pdf
Tants usually fail by going leaky... and they were notorious trouble items in commercial equipment of years ago (I'm thinking early CTV's mainly and also the odd VCR servo) where they caused a lot of faults.
Have a look at figure 6 and 7 here.
http://www.kemet.com/Lists/FileStore/APEC.pdf
Tants usually fail by going leaky... and they were notorious trouble items in commercial equipment of years ago (I'm thinking early CTV's mainly and also the odd VCR servo) where they caused a lot of faults.
I found an old chart and apparently old tantalum capacitors have a higher ESR than electrolytes. However, 50 is way more than Kemet's 17 for the same value. Will measure the other caps again to make sure the values are within spec. What's weird is that C2 and C3 were Panasonic caps that I had installed about a year ago (could something cause these caps to go bad?).
You might want to check the VD-12xx diodes. My ps-x70 speed issues cleaned up after replacing them
Also make sure the magnetic speed detection head is in the proper position. The service manual outlines this procedure. Make sure to be real careful not to scrape the magnetic coating on the inside of the platter. I have resolved speed/ cogging issues by getting the head in the correct position.
Thanks for the tips. Had already replaced the VD-1221 diodes with two diodes in series. The head seems to measure fine. Will swap around platter with an other one from a spare tt. Perhaps the magnetic strip is a bit damaged.
As there are doubts I think that you have to first prove your meter is telling you what it should as a first step. Try other similar value (new) caps and maybe try and get some different brand tants of smaller values like your 0.22uF.
My first and best piece of advice: Listen to Mooly.
My second piece of advice: Go through the service manual procedure for properly setting gain and offset. And do as ebk231 suggests and double check the position of the head per the instructions in the service manual. I wish I still had the one I was working on when I started this thread and could refer to it for you, but I returned it to the owner. I do, however, recall experiencing that slight platter reversal when pressing stop at some point, though it eventually resolved.
My second piece of advice: Go through the service manual procedure for properly setting gain and offset. And do as ebk231 suggests and double check the position of the head per the instructions in the service manual. I wish I still had the one I was working on when I started this thread and could refer to it for you, but I returned it to the owner. I do, however, recall experiencing that slight platter reversal when pressing stop at some point, though it eventually resolved.
Will definitely follow your and Mooly's advice 🙂. Will adjust the head again to make sure and order some replacement for the tantalums. I use a EVB ESR meter which measures the other caps within spec. However it's difficult to retrieve the old Sony tantalum specs to verify if they measure okay.