Philips N4450

[Michael Maurice]

This has confounded me.

When you switch on the forward play and forward wind buttons light up.

Sometimes the machine goes into play, once or twice the capstans wouldn't start up unless you pressed stop.

I've changed the transistors on both flip flop boards and found a duff CB148 on one of the tension boards which I've replaced using BC547's & BC640's correctly orientated.

No improvement except I can now get the RH motor to turn.

Can anyone shed some light on this?

[TonyDuell]

I'm puzzled too.

Just about the only thing that should stop the capstan motor is selecting 'Amp' mode. I could believe a bad contact on the switch (they are notorious for this!) but that doesn't explain why 'Stop' got the motor running.

I assume all the power voltages are present and correct.

Other than that I think I would check the gating diodes on the mechanism control base board.

[Sideband]

Try swapping the flip-flop boards over and see if the fault changes....at least that will tell you if the fault is actually on one of the flip-flop boards.

[Sideband]

Originally Posted by TonyDuell

I'm puzzled too.


Other than that I think I would check the gating diodes on the mechanism control base board.

These can throw up some very confusing faults if diodes on the main logic board go leaky. Back in the day at Philips, hours could be spent sorting out erratic operation. In very stubborn cases we were able to order a new logic board from the factory....it wasn't available as a normal service part. Of course after 40 odd years this is no longer an option....!

[ben]

Is the output supply to the logic ccts from the rectifier stable? This used to cause erratic operation in some Tandbergs.

[Michael Maurice]

Sorry I haven’t replied, I’m not in the workshop until Tuesday, will report back then

[Michael Maurice]

Progress:

I fitted BC237's instead of the AC128 as they're pin-pin comparable. After sorting out a dry joint they work ok.

However I now have a fault on one of the tape tension units.

If you press <play, the LH motor starts up with very low torque then stops.

If you swap tension boards then the same happens to the RH motor when >Play is pressed.

I think they should both spin but with reduced torque.

I've changed the transistors both BC547's, I've checked the 4K7 preset and its OK and I've checked for dry joints.

I should add that FF and REW both work correctly.

So whats next?

[Michael Maurice]

I found a spare tension board on ebay Germany, bought it and fitted it and it now works.

Someone at some stage had got to this machine. The LH spool was rubbing on the chassis, because there were no shims on the bearing.

There was no RH channel playback in forward because of a broken wire on the switch board.

All the lamps had been replaced and all different types had been fitted.

The mains switch had failed, so I replaced it but had to modify the mounting bracket for the new switch.

Hopefully it will soon be re-united with its owner.

[Ray_Singh]

Good work Michael. I have a feeling that I am going to have all sorts of 'fun and games' with my two machines.

[Sideband]

I was always bemused by the fact that, like you have found, you could replace transistors on the Tape Tension panels, check and/or replace other components and it still wouldn't work but a replacement board cured the fault.....