Philips EL3302 motor controller - UK Vintage Radio Repair and Restoration Discussion Forum

TonyDuell · 21st Jun 2019, 5:05 am

I've just had a nasty-ish fault in a Philips EL3302. The machine was the early version with germanium PNP transistors in the amplfier, so +ve earth.

I'd reassembled it after cleaning the mechanical bits and fitting new belts, but the motor didn't run. Now the motor controller has an AC128 transistor in a heatsink clip to drive the motor itself. Emitter is earthed, collector to the motor (via a low value resistor) and base to the other transistor on the control board.

Connecting a milliameter between the collector of that AC128 and chassis got the motor turning, too fast of course. So the motor is fine, so is the mechanism.

Removed the control board (2 screws) for tests. Powered up, motor runs fine (on the controller). Put the control board back, it doesn't run.

No, not a bad connection or a cracked track. It was a tin whisker short from the base of the AC128 to its casing, and thus to earth when the PCB was fitted (but not of course when it was removed). This effectively shorted the AC128 base-emitter so it was cut off, motor didn't run.

I've 'zapped' the short with my bench supply and it seems to be fine. I'll replace the transistor sometime I guess...

'LIVEWIRE?' · 21st Jun 2019, 7:57 am

I haven't owned or worked on, an EL3302 for many years, but I remember that two-transistor motor control circuit-variations of which were used by Philips, Blaupunkt, and no doubt other manufacturers 40-50 such as the N2202 years ago. Never come across a 'tin-whisker' fault in one of those. I guess because the metalwork of the recorder functions to some extent as a heatsink for the AC128, it would be difficult to insulate the mounting clip electrically whilst maintaining thermal contact. IIRC there was a n all-silicon version of that control circuit used in later models of Philips Cassette recorders such as the N2202, which didn't suffer from that problem.

TIMTAPE · 21st Jun 2019, 11:24 am

Decades ago I worked in an office technology company and we serviced a lot of Philips dictation gear. Some of them used AC128's and the "base to case" short occurred quite often. It's so long ago now that I don't recall whether we made up some sort of isolation of the case or whether we just replaced the transistors, which were much easier to come by back then than now.

Then years later I found the driver transistors in some Tandberg reel tape machines such as the 12 has the same "base to case" short problem. I modded the sheet heatsink by cutting through the centre so that the cases and therefore the bases were electrically isolated. I think I then had to strengthen the attachment of the (now two) heatsinks to the PCB with glue.

Tim

21st Jun 2019, 5:05 am	#1
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,222	Philips EL3302 motor controller I've just had a nasty-ish fault in a Philips EL3302. The machine was the early version with germanium PNP transistors in the amplfier, so +ve earth. I'd reassembled it after cleaning the mechanical bits and fitting new belts, but the motor didn't run. Now the motor controller has an AC128 transistor in a heatsink clip to drive the motor itself. Emitter is earthed, collector to the motor (via a low value resistor) and base to the other transistor on the control board. Connecting a milliameter between the collector of that AC128 and chassis got the motor turning, too fast of course. So the motor is fine, so is the mechanism. Removed the control board (2 screws) for tests. Powered up, motor runs fine (on the controller). Put the control board back, it doesn't run. No, not a bad connection or a cracked track. It was a tin whisker short from the base of the AC128 to its casing, and thus to earth when the PCB was fitted (but not of course when it was removed). This effectively shorted the AC128 base-emitter so it was cut off, motor didn't run. I've 'zapped' the short with my bench supply and it seems to be fine. I'll replace the transistor sometime I guess...

21st Jun 2019, 7:57 am	#2
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Philips EL3302 motor controller I haven't owned or worked on, an EL3302 for many years, but I remember that two-transistor motor control circuit-variations of which were used by Philips, Blaupunkt, and no doubt other manufacturers 40-50 such as the N2202 years ago. Never come across a 'tin-whisker' fault in one of those. I guess because the metalwork of the recorder functions to some extent as a heatsink for the AC128, it would be difficult to insulate the mounting clip electrically whilst maintaining thermal contact. IIRC there was a n all-silicon version of that control circuit used in later models of Philips Cassette recorders such as the N2202, which didn't suffer from that problem.

21st Jun 2019, 11:24 am	#3
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,971	Re: Philips EL3302 motor controller Decades ago I worked in an office technology company and we serviced a lot of Philips dictation gear. Some of them used AC128's and the "base to case" short occurred quite often. It's so long ago now that I don't recall whether we made up some sort of isolation of the case or whether we just replaced the transistors, which were much easier to come by back then than now. Then years later I found the driver transistors in some Tandberg reel tape machines such as the 12 has the same "base to case" short problem. I modded the sheet heatsink by cutting through the centre so that the cases and therefore the bases were electrically isolated. I think I then had to strengthen the attachment of the (now two) heatsinks to the PCB with glue. Tim