Philips N4520 tape transport problem

[sennsai]

The problem is that all functions work but only when the button is held down, as soon as released it goes into stop mode. The electronic counter is working ok, so is the solenoid.

The manual is in German (PDF),100 pages long but only a few pages are relevant, ie p61, 62. Unfortunately no working voltages are given apart from supply. The solenoid connection points are 34-37.

http://elektrotanya.com/philips_n452.../download.html

So, something is triggering the safety-stop function but recorder switching circuits are complex!

[jamesperrett]

Are you sure the tape is threaded correctly? This behaviour is common if the machine thinks there is no tape loaded.

[sennsai]

Yes, I'll check again according to the users guide. It plays ok via the 'phones but play button needs to be held down. Tape tension seems normal, could be a missing or incorrect signal from the RH tension roller which operates the shut-off. There is no optical system on this machine although the RH guide and its adjacent one cause shut off with contact with foil tape .

[Jimmyhaflinger]

That could also be due to a faulty stop button (sticking, shorted or leaky)

Seen this more than once in smokers tellies

[sennsai]

Yes, good point, I've investigated that but I've found that the STOP button is not connected to anything as far as I can see, this is confirmed on the diagram of the button bank (P 97) next to the PLAY button SK 204. So, where's the STOP button!

[julie_m]

If the transport controls are mechanically interlocked, there may well be no contacts associated with the STOP button; this will be obvious from all the other switches in the interlocked set being in their default positions.

[Jimmyhaflinger]

maybe the mechanical interlock has failed and is no longing holding down the buttons?

[sennsai]

No, the tape travel controls are the "tip-touch" type, it is electronic logic control, solenoid operated.

The STOP button is the one at the far right, unconnected. The photo of the panel is when off the machine.

[TonyDuell]

I may be missing something, but if the stop button has no electrical connections, and thus no electrical function, I think it must have a mechanical function (for example to release an interlock). Or it would do nothing at all.

[julie_m]

Sorry, but if the STOP button operates a dummy switch with no contacts, then there must be some sort of mechanical interlock between them. Otherwise, there is no way for the machine to know when it is in STOP.

Those switches look like a kind that were very common in 1970s and 80s audio equipment. The sort of switches I think they are, are modular; available with up to 6 changeover contacts, either PCB pins or solder tags (for use with the switches mounted on a special frame); and can be used in multiple modes. To use them in "push-push" mode, a U-shaped piece of wire is inserted between a slot and a hole, so it rides in a specially-shaped groove in the spring-loaded plunger. The groove directs the bent wire across and backwards as the plunger is pressed in, and it catches into one of two stable positions. To use in "push/release" mode, the wire is removed so the plunger is always pushed out by the spring as soon as the button is let go. And to interlock several switches, the U-links are removed and a metal strip with pins is placed across all the plungers, so one pin rides in each groove. When any one switch is pushed in, it latches in and all the others are released. The dummy switch has two grooves in its plunger; one just like the real switches, so it stays in when pushed in, and one without the catch points, so it always releases leaving no button pushed in. It can simply be inverted to select the desired behaviour.

If one switch had the groove worn out, it would fail to latch but all the others would still latch correctly. It's unlikely for them all to have failed; more probably, something is fouling the interlocking bar, or it has moved out of position.

[sennsai]

You are all correct - now I've exposed the switches, you can see an interlocking bar in the channel, a spring is adjacent to the STOP switch. however, depressing any of the 5 buttons does not move the bar, so I presume this needs to be investigated.

[sennsai]

After a bit of fiddling, and re-setting the associated solenoid extension bar which also controls the latch-bar, the switches are now clicking in positively.

Initially, as I was unfamiliar with these switches, I thought that the light touch of them (in the fault state) was normal. just need to re-assemble and test.

[julie_m]

Nice one I can't see the release solenoid in your pictures (which are otherwise pretty much classic examples of that sort of modular switch system); but it almost certainly works just by displacing the latching bar, so popping out any of the switches, when energised. It's a useful intermediate position between fully mechanical operation (where the controls physically move the idler into contact with the supply and take-up reel tables) and fully electronic operation (using flip-flops for the latching function and solenoids to engage the mechanism).

Originally Posted by sennsai

Initially, as I was unfamiliar with these switches, I thought that the light touch of them (in the fault state) was normal.

That's the other nice thing about this sort of switch; the action does not become any heavier just because you add more switches to the interlocking set. (If it does, by the way, that's a sign of severe wear.)

[sennsai]

Hers a pic of the solenoid, also the recorder.