Cossor CDU 150 Y amp fault - UK Vintage Radio Repair and Restoration Discussion Forum

throbscottle · 28th Jun 2013, 10:43 pm

My first post here.

I have just acquired a CDU 150 in a partially working state, no manual. Mostly it just needs some switch cleaner I think. My first decent scope!

However, there is a fault on the channel 1 amplifier - the signal disappears at about the point where it ceases to be easy to compare with channel 2. Alt/chop signal is being mixed in ok, looks like signal goes at some point after the invert switch. Transistors check ok, and I've tried testing components in the general area but things seem ok, so need more educated help

I'm not sure, but I think channel 1 possibly worked at first but channel 2 didn't. I had to replace the invert switch on channel 2, which now works.

It looks like there are some people on this forum with experience of these 'scopes - so can anyone help? Post a schematic perhaps?

maninashed · 29th Jun 2013, 1:16 am

Clean those switches properly using Servisol or preferably Deoxit, please do not use WD40.
Check all the transistors on both channels, using a good tester, including the FETs.
Bill

throbscottle · 30th Jun 2013, 1:23 pm

Thanks for the response

I had not heard of Deoxit until today. Far more than I can afford! I will invest in some Servisol 10 though.

I'm loathe to start pulling components to test out of circuit because it's a hard board to work on even when out of the 'scope due to the amount of dismantling required. Back half is okay - only 12 screws and a shield to take off, front half is hidden by calibrated attenuators and pots. At the moment the module is hanging out on mini croc leads so I can work on it.

So I'll leave off hfe testing for now!

The two transistors which are in the suspect area produce about the same level of signal on their collectors as they get on their bases. No or little loss of symetry. The other channel has a gain of around 10 for the similar pair. So I'm guessing it's part of their common biasing arrangement which is failing. Just don't know enough about how it works.

maninashed · 30th Jun 2013, 10:40 pm

I think you should concentrate on obtaining the manual primarily, as with any complex piece of electronics. Otherwise you are on a wild goose chase, unless its an obvious fault. Deoxit is expensive, but nothing compares with it for switch cleaning.
Bill

throbscottle · 1st Jul 2013, 8:06 am

Quote:

Originally Posted by maninashed

I Otherwise you are on a wild goose chase,

Yes, I've been on a few of those...

I'll let you know how it turns out. Thanks for the reponses

throbscottle · 6th Oct 2013, 9:54 am

Well, for anyone who's interested, I obtained a copy of the manual from BD Research - not the cheapest but a good quality copy from a friendly workshop!

I used a load of Servisol switch cleaner on it, marked improvements in some places, not so good in others.

So, so far I've replaced the transistor (MPS2369) that was causing the original fault, a couple of ZTX325's that had been put in as replacements for MPS2369's and failed whilst I was doing the original diagnosis. So now there is a decent trace.

I think there is something strange going on with triggering - or it's simply not working. Looks like flyback suppression isn't working, and the flyback trace is showing a fair bit of overshoot/ringing, though I think this is due to a further y amp fault since neither trace will go into the top quarter of the screen and gets clipped when it tries to go there. At least (I think) it's no longer in the pre-amp, which is a pain in the bum to work on.

maninashed · 6th Oct 2013, 9:59 pm

If you can see the return trace, generally on a scope, it means that the tube control grid cut off pot needs adjusting according to the instructions in the manual. Ideally before you do this you need to check that the tube cathode voltage is correct, but you must be careful, as this is probably about -2kv, yes that is a minus sign, and you need to use the right equipment.
Bill

throbscottle · 6th Oct 2013, 11:23 pm

Well I certainly hope that is true. The manual has extensive calibration instructions but my meter just isn't up to that voltage (I think it's actually about -1.75KV).

Anyway I found the further y amp fault I mentioned, a failing BSX20 in the 2nd pre-amp board (H2) after channel combining - so still a pain to get at. Pulled the old one but had to go out so not had chance to try a substitute yet - that may have to wait until next weekend.

I'll see if I can build a high voltage divider to check the EHT, since my dmm only does up to 600V.

Thanks for the response

WME_bill · 7th Oct 2013, 6:31 pm

Making a EHT voltage probe. I have started a new thread, as this question comes up every year, and is quite straightforward.
I have a manual for the Cossor CDU150/CT531 if you want a copy. wme-bill

throbscottle · 7th Oct 2013, 10:26 pm

Nice to see you have described what I was thinking of there, wme-bill.

I would have been very glad to take up your manual offer back in June, but as it was I bought a copy (which then gathered dust for 3 months).

By strange coincidence, when I got the 'scope home a new transistor fell out of it, a BC549. Turns out it's the nearest substitute I have for the BSX20, so in it went! Works lovely now, just jittery on the beam finder switch and in need of a proper replacement switch for ch2 invert function (the old one fell apart and the new one is a kludge). Looking forward to cleaning up the case now and straightening out some dents. My wife has christened it "squiggles".

28th Jun 2013, 10:43 pm	#1
throbscottle Retired Dormant Member Join Date: Jun 2013 Location: Nuneaton, Warwickshire, UK Posts: 10	Cossor CDU 150 Y amp fault My first post here. I have just acquired a CDU 150 in a partially working state, no manual. Mostly it just needs some switch cleaner I think. My first decent scope! However, there is a fault on the channel 1 amplifier - the signal disappears at about the point where it ceases to be easy to compare with channel 2. Alt/chop signal is being mixed in ok, looks like signal goes at some point after the invert switch. Transistors check ok, and I've tried testing components in the general area but things seem ok, so need more educated help I'm not sure, but I think channel 1 possibly worked at first but channel 2 didn't. I had to replace the invert switch on channel 2, which now works. It looks like there are some people on this forum with experience of these 'scopes - so can anyone help? Post a schematic perhaps?

29th Jun 2013, 1:16 am	#2
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	Re: Cossor CDU 150 Y amp fault Clean those switches properly using Servisol or preferably Deoxit, please do not use WD40. Check all the transistors on both channels, using a good tester, including the FETs. Bill

30th Jun 2013, 1:23 pm	#3
throbscottle Retired Dormant Member Join Date: Jun 2013 Location: Nuneaton, Warwickshire, UK Posts: 10	Re: Cossor CDU 150 Y amp fault Thanks for the response I had not heard of Deoxit until today. Far more than I can afford! I will invest in some Servisol 10 though. I'm loathe to start pulling components to test out of circuit because it's a hard board to work on even when out of the 'scope due to the amount of dismantling required. Back half is okay - only 12 screws and a shield to take off, front half is hidden by calibrated attenuators and pots. At the moment the module is hanging out on mini croc leads so I can work on it. So I'll leave off hfe testing for now! The two transistors which are in the suspect area produce about the same level of signal on their collectors as they get on their bases. No or little loss of symetry. The other channel has a gain of around 10 for the similar pair. So I'm guessing it's part of their common biasing arrangement which is failing. Just don't know enough about how it works.

30th Jun 2013, 10:40 pm	#4
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	Re: Cossor CDU 150 Y amp fault I think you should concentrate on obtaining the manual primarily, as with any complex piece of electronics. Otherwise you are on a wild goose chase, unless its an obvious fault. Deoxit is expensive, but nothing compares with it for switch cleaning. Bill

6th Oct 2013, 9:54 am	#6
throbscottle Retired Dormant Member Join Date: Jun 2013 Location: Nuneaton, Warwickshire, UK Posts: 10	Re: Cossor CDU 150 Y amp fault Well, for anyone who's interested, I obtained a copy of the manual from BD Research - not the cheapest but a good quality copy from a friendly workshop! I used a load of Servisol switch cleaner on it, marked improvements in some places, not so good in others. So, so far I've replaced the transistor (MPS2369) that was causing the original fault, a couple of ZTX325's that had been put in as replacements for MPS2369's and failed whilst I was doing the original diagnosis. So now there is a decent trace. I think there is something strange going on with triggering - or it's simply not working. Looks like flyback suppression isn't working, and the flyback trace is showing a fair bit of overshoot/ringing, though I think this is due to a further y amp fault since neither trace will go into the top quarter of the screen and gets clipped when it tries to go there. At least (I think) it's no longer in the pre-amp, which is a pain in the bum to work on.

6th Oct 2013, 9:59 pm	#7
maninashed Rest in Peace Join Date: May 2011 Location: Preston, Lancashire UK Posts: 955	Re: Cossor CDU 150 Y amp fault If you can see the return trace, generally on a scope, it means that the tube control grid cut off pot needs adjusting according to the instructions in the manual. Ideally before you do this you need to check that the tube cathode voltage is correct, but you must be careful, as this is probably about -2kv, yes that is a minus sign, and you need to use the right equipment. Bill

6th Oct 2013, 11:23 pm	#8
throbscottle Retired Dormant Member Join Date: Jun 2013 Location: Nuneaton, Warwickshire, UK Posts: 10	Re: Cossor CDU 150 Y amp fault Well I certainly hope that is true. The manual has extensive calibration instructions but my meter just isn't up to that voltage (I think it's actually about -1.75KV). Anyway I found the further y amp fault I mentioned, a failing BSX20 in the 2nd pre-amp board (H2) after channel combining - so still a pain to get at. Pulled the old one but had to go out so not had chance to try a substitute yet - that may have to wait until next weekend. I'll see if I can build a high voltage divider to check the EHT, since my dmm only does up to 600V. Thanks for the response

7th Oct 2013, 6:31 pm	#9
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Cossor CDU 150 Y amp fault Making a EHT voltage probe. I have started a new thread, as this question comes up every year, and is quite straightforward. I have a manual for the Cossor CDU150/CT531 if you want a copy. wme-bill

7th Oct 2013, 10:26 pm	#10
throbscottle Retired Dormant Member Join Date: Jun 2013 Location: Nuneaton, Warwickshire, UK Posts: 10	Re: Cossor CDU 150 Y amp fault Nice to see you have described what I was thinking of there, wme-bill. I would have been very glad to take up your manual offer back in June, but as it was I bought a copy (which then gathered dust for 3 months). By strange coincidence, when I got the 'scope home a new transistor fell out of it, a BC549. Turns out it's the nearest substitute I have for the BSX20, so in it went! Works lovely now, just jittery on the beam finder switch and in need of a proper replacement switch for ch2 invert function (the old one fell apart and the new one is a kludge). Looking forward to cleaning up the case now and straightening out some dents. My wife has christened it "squiggles".