Philips N4520 Solenoid Voltage Issue
#1
Hi, I was hoping someone might be able to offer advice regarding a problem with a Philips N4520/00 

The problem is the machine will not stay in any mode, play, fast forward, rewind. The solenoids activate and tape travel begins, then the solenoids drop and the depressed mode switch releases. 

The counter is counting both up and down.

I am finding that the solenoid initial voltage of around 36v is correct, but this should reduce within a few seconds to around 18v but this doesn't happen. The 36v drops at the solenoid and the machine stops. I believe the circuit on panel 4C is at fault. I can make the machine run for longer by substituting capacitor C210 for a 10uf but the machine will still stop after around 10 seconds, once the transistor TS211 turns off. 

I am finding that the 18v at the purple wire to each solenoid increases to around 36v then reduces to 0v momentarily, then returns to 18v once the solenoids have dropped.

Any help will be very gratefully received. 

Thanks, Vic
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#2
Hi Vic,

did you check whether the voltage at point (A), i.e. at the anode of diode D226 , stays close to 18 volts and whether D226 is ok ?
I guess the supply voltage of the solenoids should drop from 36 V down to 18 V without "touching" 0 V in between.
36 V will be good for fast activation, 18 V should be enough for keeping the solenoids activated.
Dropping the current in coils momentarily will let them generate reverse voltage, driving them down to zero.
It might help to let the voltage at the common wire to all three coils drop slower by adding a parallel capacitor to ground,
if this would not interfere with the the function "Tape Travel Start pulse" connected to  node 39.

Regards Kai
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#3
Many thanks for your reply Kai, so I measured 36v - 0v - 18v at the purple solenoid wire when the set is in play mode. There is a constant 18v at point A and at D226.

As a precaution, I decided to pull one leg of D226 and run 18v from my bench PSU to point 34 on PCB 4C and the machine works perfectly, I can see 36v - 18v at the solenoids in play mode. I substituted D226 for a 1N4007 and put the power back as it should be, and it works. I can only guess that the diode must be at fault, its an odd one too SR1K-2, whatever that is! I hope the 1N4007 will be sufficient (1A).

Thanks for your advice Kai.
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#4
Hi Vic,

The simplest explanation would be that the diode has (?developed ?) too much series resistance to be able to keep the voltage at its cathode from dropping to zero. My other suspicion was that the storage capacitor in the power supply from which the 18 V come might have lost much of its capacitance such that it is quickly discharged by some load current.
You could check the current drawn through the diode to get an idea whether a 1A type is sufficient.
I wish you much joy using the N4520.
I wish I had one too ...

Regards Kai
Addendum: The "datasheetarchive" has a 1993 Diode Databook, which lists the SR1K-2 diode as a 80V/1A type.
The 1N4007 is a 1000V/1A diode. I guess a 1N4002 (100V/1A) would be sufficient, possibly even a 1N4001 (50V/1A).
The diode just serves as a "one way street", letting current flow out of node A into the solenoid circuit when the 36 V connection is switched off, but blocking any current flowing from the activated 36 V into node A.
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#5
Hi Kai,

Thanks again for your help - although the diode checked out ok, the set continues to work just fine with a 1N4007 in place. Like you say, it seems overkill to use a 14007, so I will replace it with a 1N4002. I've checked the ESR of the large PSU caps and they are looking good, so no problems there.

This machine joins a late N4522 and two plastic N7300's.

Kind regards, Vic
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