Turntable repair involving photoresistor

[Babyspark]

Hi, I'm a newbie who likes to fiddle with turntables. Recently I tried to fix a Sony PS-X6 which has a little more electronics than I can handle. The problem is that the auto-return/stop function has stopped working. It is triggered by a photoresistor which is supposed to react to a sudden increase in light from a lamp as the tonearm moves into the lead-out groove. (A shutter attached to the tonearm pillar passes across the CdS photoresistor and eventually exposes all of it to the lamp. I replaced the CdS with a gl5516 unit and now get the correct specified voltages at two points near the end of the record as per the service manual but the auto-stop doesn't kick in. I notice that if I manually lift push the tonearm quickly over the end of the record the auto-stop works. This might indicate that the circuit isn't responding at normal speeds (33 and 45 rpm) but I have no idea why. There are two transistors 2sc634a/ksc1845 in the circuitry that controls this function, is it possible that they need replacing to fix things?.... (I have replaced the associated capacitors).
Or can anyone suggest other possible solutions.
Attachments show circuitry with sections with relevant Q417, Q418 transistors.
Thank You.

 

[rjenkinsgb]

I'd say either C417 is failing or the new LDR has a rather higher resistance or lower sensitivity than the original.

It needs a fast enough change in resistance to cause the collector of Q418 to pull low and momentarily turn off Q417 via the capacitor.
As you say you have replaced the cap, that leaves the LDR as the suspect.

Or, a larger value cap should make it trigger at a lower rate-of-change.

 

[Babyspark]

Thanks for your response, you are clearly knowledgeable in this field!
For info - i have changed c417 (tantalum) for an aluminium as I read that may be better, though I noticed the original was still within spec. Also c416 replaced (alu for alu), the original one was almost twice it's rated value.
Could I ask if you have any suggestions for a higher value for c417 ?
Also would you say there is no point replacing q417, q418 ?

Sadly the original LDR seems unobtainable and I don't know the spec (not in service manual) so I am stuck for an exact replacement........so I fear I may never get it to work again. Thanks again.

 

[Ylli]

So the collector of Q418 properly goes from 1.6 volts (with LDR fully exposed?) to 5.1 volts(LDR fully shadowed?). Are you doing that adjustment under conditions such that ambient lighting does not affect the LDR? Good to that point.

Why did you change C417? Can you put the old one back?
Confirm the voltage at the junction of D415 and R444 is zero when the play switch is on. For the voltmeter negative lead, you can use the same point the negative lead was connected to for your 1.6v/5.1v measurements (that is system ground).
C416 - same question as C417.

 

[Babyspark]

Hi, Voltage testing done with everything in place and tt in position as normally used so ambient light shouln't come into play.
As I recall I dismantled quite a bit of the tt so that I could clean off hard grease on the main gear spindle which was stopping auto-start working. This was successful, but to be honest I can't remember if I replaced the caps as part of this op or later! I suspect I did it all in one go. Reason - seemed like a good idea at the time while I had things in bits. OK, I see this opens up a lot of possibilities for the cause, I will go back over my work and check it as best I can and perform your additional test re D415/R444.
Just wondering though is there any comment on the fact that I can force the auto-stop by moving the tonearm quickly over the lead-out groove? Does that point to any component?
( by the way the old LDR seems a bit erratic but the tiny wires on it make it awkward to test, any idea where I might look for a replacement, see picture below)
Thanks for your help, much appreciated.

 

[Ylli]

When light hits the LDR, its resistance drops and the current through Q418 increases. This causes the collector to drop for its nominal voltage of 5.1 v (no light) to 1.6 v (full light). This 3.5 volt dip in voltage is coupled through C217 to the base of Q417. The *length* of this pulse is dependent on the value of C217.

Q417 is normally held on by the base current through R443. The negative going pulse from Q418 turns this transistor off. When it turns off, it allows C416 to charge through R442. When the collector gets high enough, it couples a positive going pulse through R441 and a diode into the circuitry that controls the solenoid.

If the run switch is not in the 'play' position, or the switch is bad, then the base of Q417 is pulled high harder with the 15K resistor R444. This has the effect of reducing the width of the pulse coming through C417 to a value too low to produce the required output pulse from Q417.

So, I can see a number of scenarios where manually moving the tone arm quickly will cause the solenoid to engage, and moving it slowly will not:

1. Something incompatable with the replacement LDR. Its resistance may be changing too slowly with the change in light due to mechanical variations or orientation.

2. Capacitance of C417 too low - this will reduce the pulse width passed on to Q417.

3. Capacitance of C416 is too high or it is leaky. This will increase the length of time Q417 must be off before the collector reaches the 'trigger point'.

Confirm any changed caps are installed with the correct polarity. In the schematic, the cup shape is the negative end of the cap. So negative end of C417 to base of Q417 and negative end of C416 to ground.
Confirm that the value of C416 is 3.3 uF and not more.
Confirm C417 is 22 uF. You could try increasing this. I would try a 47 uF (or just parallel the installed 22 uF with another 22 uF).

 

[Babyspark]

Thank you so much for taking the time to reply in such detail Ylli. I have re-instated the original LDR which seems to be behaving (I did think it was faulty earlier) and will go over the circuit board bearing your comments in mind. It may take me a little while but I will post an update in due course. I may have to settle for manual operation if my 'skills' aren't up to it but at the moment I am optimistic!

 

[Babyspark]

I think this problem may be solved... I tested the voltage as advised at junction of D415/R444 and it was not zero but about 1.3V. I removed and checked the switch which tested OK but when I put it back it occured to me that the lever on the switch may not be fully pushed into the ON position by the mechanism (another bit of a lever) designed to do that. I fastened a thin bit of plastic to the mechanism to push the switch fully ON and now the auto-return seems to work. I say 'seems' as I have not resoldered the tonearm wires and tested by playing a record but hopefully that will work too.
I can't explain the switch problem but I did do a major dis-assembly so perhaps something is slightly out of place. Maybe I need to get a new switch, do they wear out ? If the tt works with a disc I will probably live with it as is.
Again, Thanks to the members that have helped me I would NEVER have sorted this on my own. (Please keep an eye out in case of further mishaps.)

 

[Ylli]

Great! Hope you found the root cause and it keeps working for you.