Did My Solenoid Fry in my RCA VDT-350?

[TinCanAlley]

Many years ago members here helped my fix my RCA, and after all these years, it has a new problem. I sat idle for over a year and when I went to play a tape, the reels weren't turning. At first I figured one of the new belts I had installed was bad, but upon inspection, all belts are good, all pulleys are turning, etc. Upon closer inspection I noticed the capstan roller wasn't moving to pinch the tape against the capstan. I manually moved it and moved it and the tape played. So I checked it for binding and all was well. The piston of the solenoid moved freely, but it wasn't engaging.

The SM has schematics and it seems there is a constant 19.86V (I measured 20.5V on both leads from the moment the power is turned on) on the two leads for the solenoid, and from what I can tell, when play is selected, it drops to 17.7V (I measured 17.3V when play is pressed). At no time (20V or 17V) does the solenoid move. I understand the workings of a solenoid, but not sure why there is 20V constant and 17V during play). Doesn't this mean the solenoid is basically charged the entire time and should be retracting? The schematic shows the leads and on goes directly to the lead of the winding and the other one as well, but it has a diode on it as well as a bunch of transistors that are switches, drivers and pause.

Anyway, I really don't know where to begin with this one. I can attach some pics of the solenoid and the section of the schematics if that will help.

Thanks

[Doug]

How many volts across the solenoid when play is pushed?

[TinCanAlley]

Originally Posted by Doug

How many volts across the solenoid when play is pushed?

It drops to 17.7V when play is pressed. I also pressed pause while in play, thinking the voltage should change again extend the solenoid for pause, but the voltage stays the same. I wonder if there is a problem in the pause circuit.

Is the solenoid testable by disconnecting it from the circuit and applying voltage directly to it? I'm really confused by how it is supposed to work by trying to interpret the schematics. If it has 20V on power up, why isn't it moving, and if it isn't supposed to move, how would it by dropping the voltage from 20 to 17? I'm hoping it the circuit that's at fault because I can fix that. If it's the solenoid, that's an almost impossible part to replace as I would need a donor unit and I'd have to figure out how to adjust the tension spring like it is now. The spring plate sits on top of the solenoid and is rotated to provide the correct tension and tightened in place. I'm sure they used some small tension device to measure.

[Doug]

Look in the parts list see if it has anymore info on the solenoid.
Moyer electronics is your best bet for old parts .you will need part number then send
Him an email
.are u staying thee is 17v across it or the power supply is dropping 20 t o 17
Doug

[TinCanAlley]

Originally Posted by Doug

Look in the parts list see if it has anymore info on the solenoid.
Moyer electronics is your best bet for old parts .you will need part number then send
Him an email
.are u staying thee is 17v across it or the power supply is dropping 20 t o 17
Doug

Yes, across the solenoid leads on power up I measure a constant 20V. When play is pressed, it drops to 17V. The schematic shows 19.86 supplied to both leads. From there I'm not sure how to read the schematic as there seems to be multiple transistors for different functions (play, pause, etc.). I'm assuming that based on which transistor is triggered, the voltage is supposed to be changed. On one of the leads in the schematic, it seems there is a diode on one.

I really need to study more on how to read schematics. I seeing the diode on one of the leads with 19.86V and it is pointing towards the source, not the solenoid, so I'm assuming no voltage should be getting to the solenoid until play is pushed and one of the transistors gets triggered and applies voltage, or ground? to the second lead on the solenoid.

[Doug]

D624 is to protect the transistors when the solenoid is turned off .
A voltage comes back from the magnetic field .
Someone else could explain that better.

[TinCanAlley]

Originally Posted by Doug

D624 is to protect the transistors when the solenoid is turned off .
A voltage comes back from the magnetic field .
Someone else could explain that better.

I disconnected the solenoid connections from their respective boards (one on the transport control board and one on the power board). I connected it to an 18V source and it activated. So now I know the solenoid is good.

I'm hoping someone can look at the schematic I attached in a previous post and help me determine where on the transport board to check. I figure I see 19V on it when in circuit because of the connection to the power board and something in the circuit must give it ground to activate, so somewhere in the schematic it just show what is doing that. The only other thing I can think of is there is a small plug on the back for a wired pause button. Maybe that jack is bad and it thinks I'm always in pause.

[Doug]

the tape end lamp is in part of that circuit. its possible if its open the circuit will not turn on the solenoid which would protect the tape.
check that bulb.
doug

[Alex KL-1]

'bout the various transistors for solenoid: some group conducts first fro fast and strong mechanical magnetic force in the solenoid. Then, the others engage permanently with lower voltage. I see it in various cassete tape players and R2R, for not needing overdissipate the solenoit, since when it is already engaged, the needed force for maitain is much lower.

[TinCanAlley]

Originally Posted by Doug

the tape end lamp is in part of that circuit. its possible if its open the circuit will not turn on the solenoid which would protect the tape.
check that bulb.
doug

Bulb is working. I replaced that over a year ago and it is still working.

[TinCanAlley]

Well I believe I have found the culprit. I found a transistor in the transport circuit that is labeled as "Emitter Follower." It is 2SC1847Q NPN and when I test it, no matter what leg combination, I get no readings. I believe this to be the one that keeps the solenoid energized as it is the only one with a special heatsink that I believe is needed as it is energized during the entire play session and must get pretty hot.

As for the transistor, it isn't available anymore and NTE has a replacement (NTE373). I'm sure there are others that will work, but I don't know enough about comparing old to new to make a decision. If anyone with knowledge of better replacement choice wants to chime in, I'm open to getting the best replacement.

Thanks!

[redk9258]

Look at TTC004B. That looks to me like a good sub.

https://www.mouser.com/ProductDetail...eZ2DvzIw%3D%3D