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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 1st Nov 2019, 4:42 pm   #1
Uncle Bulgaria
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Default Uher CR 240 solenoid does not disengage.

My original Uher thread got bogged down in my misapprehension of the lighting circuit, so here's a new specific problem to start afresh as only long-suffering Lawrence was able to follow my ineptitude. Everything was working until a short burned out the dial lights. Now I have no playback, though the mechanics appear sound in principle.

I've been wrestling with this for a while so it's time to throw it out to the floor.

Problem

The solenoid that engages the pinch roller not disengaging on 'stop' or 'pause'. It has a 500R and a 15R coil, both of which measure pretty close to nominal. It doesn't help that the circuit diagram I have is not the same as the boards. The infra-red 'end of tape' sensor is actually a Hall-effect transistor, and some diodes are missing from board 700, presumably as a result of this change as there is now no LED 'D801'.

Circuit diagram and board layouts are available here. (9MB)


Symptoms


Parts of the circuit are working. On starting up, the yellow 'on' is lit. 'play' is unresponsive as the motor is not running. It starts with switch K5 (just above board 700) when the cassette transport is engaged. Then 'play' causes the pinch roller to snap into position, the spindle rotates, and the green 'play' light comes on while the 'on' goes off.

With no cassette inserted, one can stop the magnetic collar on the spindle as if reaching the end of the tape. The unit recognises this, and the 'play' light quickly goes off and the 'on' stop light turns on. However, the pinch roller is still engaged and the spindle is still turning on removal of the finger.

On FF/RW the motor stops with the collar stopped.

My Deductions

I've been applying my recent transistor education, and discover that grounding the base of T704 disengages the solenoid. I was surprised as it seemed the 15R coil on the collector should be the 'pull in' not the 'hold' coil, and shouldn't be passing current.

Am I correct in saying that on 'start', there is a current draw from T705 as the base of the PNP has a circuit completed to ground through R709 (from the +9.5V rail), R711 and D709? The emitter, being connected to the +9.5V rail by K5 when the motor is engaged, turns on T705 and subsequently T704 through coupling capacitor C701.

I can't work out how this creates a latching circuit, nor why grounding T707 with the momentary 'stop' switch does not cause the pinch roller to disengage. There must be some relation to the motor board as the tape sensor part is acting to stop the motors, but if the latch is supplied through K5 the only way to remove the supply is by taking out the cassette, so that can't be right.
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Old 1st Nov 2019, 5:54 pm   #2
TonyDuell
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Default Re: Uher CR 240 solenoid does not disengage.

Consider the transistors as switches, either on (saturated) or off (cut off). It will be pretty much what should be happening here.

I think....

We start with T706. When it turns on, it supplies base current to T703 (via D705 and R703). This energises the 'hold' winding of the solenoid. It also applies a current pulse (due to C701, initially discharged, the current is the charging current of that capacitor) to T705's base. T705 turns on, turning on T704. This briefly energises the 'pull in' winding on the solenoid. So the solenoid pulls in, and holds.

When T706 turns off, T703 will turn off, removing the hold winding current. There will be a -ve pulse from C701, but this will be clamped by D704 and will have no effect. So the
solenoid releases.

Now, you say that the pull-in winding is being energised. So T704 is getting base current. Is T705 leaky? Is T705 getting base current and thus being turned on. If so, where from?

When the unit is stopped and the solenoid should be released, what voltage do you measure on the bases of T704 and T705? What about the collector of T706?
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Old 1st Nov 2019, 7:06 pm   #3
Uncle Bulgaria
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Default Re: Uher CR 240 solenoid does not disengage.

That's very clear, Tony. Thank you. I hadn't considered that application of C701, which was why I couldn't see how T704/5 wouldn't just stay on. I suppose that once C701 is charged, the DC is blocked so there is no current?

(I should add that I've been pestering Tony with PMs because of his previous experience with the CR 210, but thought I should share the load rather than prevail overmuch on his generosity!)

Situation:

On; stopped; K5 engaged; motor running (as if a cassette is in).

T704: Vb=690mV
T705: Vb=8mV
T706: Vc=273mV

T705 is a new BC548C. C701 is a new electrolytic. T706 is a new BC558B.

Play engaged, then stopped. Pinch roller still engaged, 'play' LED off.

T704: Vb=640mV
T705: Vb=9mV
T706: Vc=1.2mV (with joystick in 'stop'. Returning it to 'neutral' gives 9mV).
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Old 1st Nov 2019, 8:06 pm   #4
ms660
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Default Re: Uher CR 240 solenoid does not disengage.

According to your measurements it looks like T704 has a turn on voltage on the base and T705 doesn't, first suspect would be T705 or the way it's been connected, if no joy then remove T705 and measure the base voltage of T704, if it's still at turn on voltage then either T704 is duff or there's some leakage from a +ve source to the base.

Lawrence.
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Old 1st Nov 2019, 9:31 pm   #5
Uncle Bulgaria
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Default Re: Uher CR 240 solenoid does not disengage.

That's done it!

I had disregarded T704 as having had it out and done a diode test, it passed, and appeared to function in the circuit. However, once I'd replaced T705 I assumed the problem must be somewhere else - perhaps with there being no playback it was something bizarre further away in the connected audio board.

As I always tell everyone else, 'assume makes an ass of u and me'.

...

T704 (S7302 on board, S7267 on diagram) replaced with a BC639 and everything engages and disengages quite happily.

Some more tips for troubleshooting absorbed, thanks both.

Now to trace why there's no playback. Perhaps another thread is gestating...
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Old 2nd Nov 2019, 3:31 am   #6
Uncle Bulgaria
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Default Re: Uher CR 240 solenoid does not disengage.

I found it! Tracking back with my newfound oscilloscope skills, it turned out R66 was pulling the input to ground because of a short, probably from the point when the smoke escaped.

It now plays on both channels and I'm going to go through the service manual to check the operation points.

Off topic, and perhaps the subject for another thread: what could cause the hiss in the headphones when there are no audio boards inserted (so nothing behind the volume control is in circuit) and how could I ameliorate the +2.5V thump spike on switch on, and the -2V decay spike on switch off? I've replaced the coupling capacitors C35 and C36, also C31 and C33 from ICs 1 & 2. Perhaps it's something to do with IC1/A and IC1/B that have some role behind the volume control? Are they some pre-amp circuit for auxiliary inputs that are injecting noise further down the chain?
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