Coutant LB200 - Thermal Stabilty - UK Vintage Radio Repair and Restoration Discussion Forum

Bazz4CQJ · 8th Aug 2021, 12:54 am

It seems to me that of late, the voltage from my venerable LB200 seems to drift upwards as it warms up, and appreciably more so than I ever noticed in that past. It's just an impression at this stage, but a recurring one. Of course these units are fairly elderly now are cooled only by convection.

I wonder if anyone as experienced this problem with a similar unit, or has any ideas about any component changes that might be helpful? The best cct drawing I have is attached.

Thanks
B

kalee20 · 8th Aug 2021, 8:00 am

If the output is stable with changes of load, it's likely that the regulation circuitry is working properly and the problem is in the voltage reference. That's MR1 (5.1V) so stick a DVM across it and see if it varies with temperature, time etc.

Bazz4CQJ · 8th Aug 2021, 6:06 pm

Thanks for that suggestion. I haven't had any sense that V is showing load dependence. Since my first post, I've found the photographs I took the last time I had the case off, and they are pretty well designed for maintenance. MR1 is very easy to get at and could be replaced in a couple minutes if necessary

.

B

Richard675 · 8th Aug 2021, 6:46 pm

I’d look at the elcos. Work back from the output. I have just repaired an old BNOS unit that was tripped with no load. Turned out to be a leaking electrolytic at the output.

Julesomega · 8th Aug 2021, 9:22 pm

You have the important bit of the manual there, but if you want any more I could scan it. However there are no notes about servicing

Bazz4CQJ · 8th Aug 2021, 9:30 pm

Thanks Julian; I do have the whole manual, but the quality is not great.
If you think you could produce a better copy of the circuit diagram than the one I posted, I'd certainly appreciate that

.

I stand by to be corrected, but I don't think there any electrolytics in the regulation/control circuits. Was it the case that zeners were more expensive back when this psu was made? They certainly seemed to be keen on using low-power zeners.

I think I paid £10 for the Coutant back in the mid-90's; seems like a good buy!

B

Julesomega · 8th Aug 2021, 10:49 pm

I've got a spare copy if you'd like to PM me your address

Bazz4CQJ · 8th Aug 2021, 11:37 pm

ooh - many thanks; PM sent.

B

Bazz4CQJ · 17th Sep 2021, 9:47 pm

I am indebted to Julian for sending me an original copy of the manual for this PSU. The original schematic is printed at A5 size, and the quality is not great, but I have scanned it as best I can and I think the PDF I have may be better than any other copy I have seen. So for all lovers of the LB200.2, it's attached

.

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Herald1360 · 19th Sep 2021, 11:45 am

Is the drift now sorted? I don't think you've said whether the problem was the 5V1 zener or something else.

Bazz4CQJ · 19th Sep 2021, 2:47 pm

Re the drift, being keen to stick with "if it ain't bust don't fix it", it did occur to me that the PSU was sitting with virtually no space to the sides of it to assist with ventilation, so I altered that. And I also decided to run it for a minute or two on a resistor, when starting from cold, before using it for the task, just to "wake it up". I think that these changes may have improved things somewhat, and I did not notice any drift last time I used it

So, I won't say for sure this has resolved the problem - I need a bit more experience with it now to see whether or not to think more about the zener.

B

8th Aug 2021, 8:00 am	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	Re: Coutant LB200 - Thermal Stabilty If the output is stable with changes of load, it's likely that the regulation circuitry is working properly and the problem is in the voltage reference. That's MR1 (5.1V) so stick a DVM across it and see if it varies with temperature, time etc.

8th Aug 2021, 6:06 pm	#3
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Coutant LB200 - Thermal Stabilty Thanks for that suggestion. I haven't had any sense that V is showing load dependence. Since my first post, I've found the photographs I took the last time I had the case off, and they are pretty well designed for maintenance. MR1 is very easy to get at and could be replaced in a couple minutes if necessary . B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

8th Aug 2021, 6:46 pm	#4
Richard675 Pentode Join Date: Jun 2009 Location: Braintree, Essex, UK. Posts: 170	Re: Coutant LB200 - Thermal Stabilty I’d look at the elcos. Work back from the output. I have just repaired an old BNOS unit that was tripped with no load. Turned out to be a leaking electrolytic at the output.

8th Aug 2021, 9:22 pm	#5
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,102	Re: Coutant LB200 - Thermal Stabilty You have the important bit of the manual there, but if you want any more I could scan it. However there are no notes about servicing __________________ - Julian It's good here

8th Aug 2021, 9:30 pm	#6
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Coutant LB200 - Thermal Stabilty Thanks Julian; I do have the whole manual, but the quality is not great. If you think you could produce a better copy of the circuit diagram than the one I posted, I'd certainly appreciate that . I stand by to be corrected, but I don't think there any electrolytics in the regulation/control circuits. Was it the case that zeners were more expensive back when this psu was made? They certainly seemed to be keen on using low-power zeners. I think I paid £10 for the Coutant back in the mid-90's; seems like a good buy! B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

8th Aug 2021, 10:49 pm	#7
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,102	Re: Coutant LB200 - Thermal Stabilty I've got a spare copy if you'd like to PM me your address __________________ - Julian It's good here

8th Aug 2021, 11:37 pm	#8
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Coutant LB200 - Thermal Stabilty ooh - many thanks; PM sent. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

19th Sep 2021, 11:45 am	#10
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: Coutant LB200 - Thermal Stabilty Is the drift now sorted? I don't think you've said whether the problem was the 5V1 zener or something else. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

19th Sep 2021, 2:47 pm	#11
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: Coutant LB200 - Thermal Stabilty Re the drift, being keen to stick with "if it ain't bust don't fix it", it did occur to me that the PSU was sitting with virtually no space to the sides of it to assist with ventilation, so I altered that. And I also decided to run it for a minute or two on a resistor, when starting from cold, before using it for the task, just to "wake it up". I think that these changes may have improved things somewhat, and I did not notice any drift last time I used it So, I won't say for sure this has resolved the problem - I need a bit more experience with it now to see whether or not to think more about the zener. B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.