Uher CR 240 BD241 collector grounding confusion - UK Vintage Radio Repair and Restoration Discussion Forum

Uncle Bulgaria · 4th Jul 2019, 11:39 am

Dear all,

I have been working through this recorder on and off for some time. Not understanding transistors does not help, but methodically working through the circuit diagram and dealing with dry joints has worked wonders.

I'm very confused by this PSU connection though. There is a large BD241 transistor, the collector of which is electrically connected to the heatsink. There is an area on the chassis that is free from paint, and corresponds to the BD241 mounting. As you can see from the crop of the circuit diagram, the collector is not grounded, and if it is just screwed down as the chassis connections suggest then the collector is earthed and everything stops.

Any thoughts?

vidjoman · 4th Jul 2019, 11:45 am

It looks as though it's missing a flat mica washer. There is an isolating bush in the hole for the fixing screw so there should also be that washer. It's probably fallen off when the screw was undone.

Skywave · 4th Jul 2019, 2:24 pm

In your first photo., I can just see an insulating washer in the mounting hole of that transistor. If its collector was intended to make a zero Ohms contact with the heat-sink, that washer would not have been necessary. Hence, a mica washer is missing - which was there once.

Al.

Welsh Anorak · 4th Jul 2019, 2:24 pm

Agreed. It shouldn't be electrically connected to the heatsink, just thermally. The missing washer will either be mica or that thin grey insulating material. Could be stuck to something with grease? If not you're bound to find one on a piece of scrap equipment; one with transistors in, of course....

Uncle Bulgaria · 4th Jul 2019, 7:45 pm

Thanks all, that clears it up. I was concerned I'd got the circuit entirely back to front as it wasn't working when put back together! I own I also was confused by the insulating washer on the transistor as I couldn't see why they'd bothered!

What it is to have some knowledge. I didn't know these washers were available, and that it was possible to have some thermal contact without electrical contact - presumably that bare metal area is for heat transfer.

There was no washer that I've found, though the deck came to me with a broken motherboard which I've repaired with jumpers, so that corner had had a serious whack at some point. Perhaps it got broken and lost then.

Skywave · 4th Jul 2019, 11:36 pm

Quote:

Originally Posted by Uncle Bulgaria

What it is to have some knowledge.

Quite: but as is the case with everything in Life, useful knowledge is nearly always acquired the hard way. Education is never free, but unlike a lot of things, once acquired, it can never be taken away.

But just to add one small - but relevant point - you may well come upon designs where the insulating hardware is deliberately omitted. There are two legitimate cases for that:
1. The cct. design is such that the collector is required to be at 0v.
2. The entire heat-sink is insulated from the chassis / 0v.

Al.

Uncle Bulgaria · 9th Jul 2019, 7:07 pm

Thanks Skywave. It's a real education having decent fora with people willing to share their knowledge. As I imagine I'm on the young side of members here, with no employment or background in electronics there's always an auto-didactic curve with many a dead end.

However, everything gets stored away and I'm equally chuffed with each project that gets a little closer to completion, or that I can look at and diagnose without help as it shows something's going in.

Learning on audio equipment is particularly good as the high-end of yesteryear is cheap, the parts are large enough to see and if it works then good music is the result!

I've got some insulating washers on order (how do RS make money when I can buy a 38p capacitor which arrives overnight and is packaged in a jiffy bag and paper? Instead of buying any packing material, I just do an RS order and repurpose the bag!) and am fettling the contact area on the new switch to work. I misinterpreted how long the switching action was, so I think the contacts are overlarge and are shorting the wrong terminals.

4th Jul 2019, 11:39 am	#1
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Uher CR 240 BD241 collector grounding confusion Dear all, I have been working through this recorder on and off for some time. Not understanding transistors does not help, but methodically working through the circuit diagram and dealing with dry joints has worked wonders. I'm very confused by this PSU connection though. There is a large BD241 transistor, the collector of which is electrically connected to the heatsink. There is an area on the chassis that is free from paint, and corresponds to the BD241 mounting. As you can see from the crop of the circuit diagram, the collector is not grounded, and if it is just screwed down as the chassis connections suggest then the collector is earthed and everything stops. Any thoughts? Attached Thumbnails

4th Jul 2019, 11:45 am	#2
vidjoman Dekatron Join Date: Jun 2013 Location: East Sussex, UK. Posts: 3,326	Re: Uher CR 240 BD241 collector grounding confusion It looks as though it's missing a flat mica washer. There is an isolating bush in the hole for the fixing screw so there should also be that washer. It's probably fallen off when the screw was undone.

4th Jul 2019, 2:24 pm	#3
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Uher CR 240 BD241 collector grounding confusion In your first photo., I can just see an insulating washer in the mounting hole of that transistor. If its collector was intended to make a zero Ohms contact with the heat-sink, that washer would not have been necessary. Hence, a mica washer is missing - which was there once. Al.

4th Jul 2019, 2:24 pm	#4
Welsh Anorak Dekatron Join Date: Apr 2005 Location: North Wales, UK. Posts: 6,921	Re: Uher CR 240 BD241 collector grounding confusion Agreed. It shouldn't be electrically connected to the heatsink, just thermally. The missing washer will either be mica or that thin grey insulating material. Could be stuck to something with grease? If not you're bound to find one on a piece of scrap equipment; one with transistors in, of course.... __________________ Glyn www.gdelectronics.wales

4th Jul 2019, 7:45 pm	#5
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Uher CR 240 BD241 collector grounding confusion Thanks all, that clears it up. I was concerned I'd got the circuit entirely back to front as it wasn't working when put back together! I own I also was confused by the insulating washer on the transistor as I couldn't see why they'd bothered! What it is to have some knowledge. I didn't know these washers were available, and that it was possible to have some thermal contact without electrical contact - presumably that bare metal area is for heat transfer. There was no washer that I've found, though the deck came to me with a broken motherboard which I've repaired with jumpers, so that corner had had a serious whack at some point. Perhaps it got broken and lost then.

9th Jul 2019, 7:07 pm	#7
Uncle Bulgaria Nonode Join Date: May 2016 Location: Cornwall, UK. Posts: 2,335	Re: Uher CR 240 BD241 collector grounding confusion Thanks Skywave. It's a real education having decent fora with people willing to share their knowledge. As I imagine I'm on the young side of members here, with no employment or background in electronics there's always an auto-didactic curve with many a dead end. However, everything gets stored away and I'm equally chuffed with each project that gets a little closer to completion, or that I can look at and diagnose without help as it shows something's going in. Learning on audio equipment is particularly good as the high-end of yesteryear is cheap, the parts are large enough to see and if it works then good music is the result! I've got some insulating washers on order (how do RS make money when I can buy a 38p capacitor which arrives overnight and is packaged in a jiffy bag and paper? Instead of buying any packing material, I just do an RS order and repurpose the bag!) and am fettling the contact area on the new switch to work. I misinterpreted how long the switching action was, so I think the contacts are overlarge and are shorting the wrong terminals.