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Old 12th Jan 2016, 1:35 pm   #1
JohnPearson
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Default Cossor CDU 150 scope large offset on channel 1

Hi, any ideas please;

I have a Cossor CDU 150 scope that has developed an odd fault in that there seems to be a large DC offset on input channel 1.

If I hold the invert switch to the centre a trace appears in the centre of the screen so it looks like an issue before the invert switch.

Also, if I put a large sine wave into ch1 I can see the tops of the trace and I can move the trace tops up and down using the position selector.

I do have the circuit diagrams for the scope but I haven't dabbled with this type of equipment so I am a bit in the dark.

Has anyone had this problem and been able to fix it?

Any help would be greatly appreciated.
Thank you.
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Old 12th Jan 2016, 3:01 pm   #2
John_BS
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Default Re: Cossor CDU 150 scope large offset on channel 1

John:

Can you please post a scan of the relevant input circuit as the schematics for this scope do not seem to to be available on the web.

John
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Old 12th Jan 2016, 5:41 pm   #3
JohnPearson
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Default Re: Cossor CDU 150 scope large offset on channel 1

Hi John,

Thank you for your interest.

I have uploaded a scan of the channel 1 input amplifier which seems identical to channel 2.

The switch SW1 in the middle of the drawing is the Invert switch and if I position it dead center so that it is not connected to any signal the trace is then centered on the screen. When set it to normal or invert the trace is way off the screen. The invert moves it off in the opposite direction.

The amplifier still works though and if I apply a large signal to the input I can see the tops of the sine wave or the bottoms if the invert switch is set to invert.

It seems that there is a DC offset being applied to the circuit prior to the invert switch.

It did work fine when I used it about a month or so ago so something has happened.

Also, when I first switch it on, I get a hissing sound from the back and it seems to be coming from a small transformer which I am assuming is the eht transformer. After about a minute or so the sounds disappears.

Any ideas?
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File Type: pdf Cossor CDU 150 Channel 1 amplifier.pdf (433.4 KB, 139 views)
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Old 13th Jan 2016, 4:48 pm   #4
MotorBikeLes
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Default Re: Cossor CDU 150 scope large offset on channel 1

First things first. The hissing sound is likely a corona discharge from the EHT generator transformer. This could be simply dampness, so it would be a good idea to allow it to thoroughly dry out somewhere. If allowed to continue, the paxolin or whatever can become conductive and you could end up with a dead transformer. It could be worth spraying with a PROPER circuit board varnish ONCE it is DRY. However, it could simply be a "spike" at a soldered joint, or a whisker formed by a single strand of connecting wire at the termination. In this case, a big ROUND shiny glob of solder after cooling down should prevent future hissing.
Regarding the Y-channel displacement off the screen centre, it may simply be a dirty "twiddle pot", "vertical shift balance" or whatever it is called. The easiest way to find out is to start with the calibration section of the Y amp. The very first couple of adjustments will either have it fixed, or tell you you have a dirty pot, or to check early semicons.
Follow instructions exactly, with ac, dc or ground ref on the input coupling switch.
Cheers, Les.
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Old 13th Jan 2016, 10:20 pm   #5
JohnPearson
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Default Re: Cossor CDU 150 scope large offset on channel 1

Hi Les,

Fortunately the hissing sounds on switch on no longer occur, I think it was probably just moisture as you suggested. I had left the scope in an unseated shed for a couple of weeks.

I think the y amp offset is probably a failed component rather than just a dirty potentiometer but I shall try what you suggest just in case. The coarse and fine adjusts on the front panel can't bring the trace onto the screen so I guess I shall have to remove the case and follow the setup instructions in the manual.

Other than this channel 1 problem the scope is great and I have just set up a guitar amp repair service so I do need a decent scope. I do have a PC based Vellaman multi function scope/signal generator/spectrum analyser/bode plotter and whilst it does work quite well it doesn't respond instantly to signal changes like a proper scope. I do find the lag rather annoying.

Thank you for your reply, it has convinced me that I need to get stuck in and diagnose the fault.

Regards,
John
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Old 14th Jan 2016, 9:44 pm   #6
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Default Re: Cossor CDU 150 scope large offset on channel 1

John:

You have Ch2 working, so that will allow you to check and compare DC's with Ch1.

I'd start with Ch1 input grounded, and check the dc's on the gates of Tr1 & Tr2: both should be the same and very close to zero (< 50mV).

Next, there's a pre-set "Invert Balance" (RV4) which, if the wiper gets dirty, will throw the whole thing out. Give that a few full rotations and a splash of switch cleaner and return to centre: check the wiper to each end is c. 500 ohms.

If they are all OK work down the chain looking at the DC's on the bases of Tr3 to 8 inclusive: they're in pairs so they should be similar top/bottom and I think around -5 to -6V (Tr3 to 6 ) and around +9V (Tr7 & 8), but Ch2 will tell you the correct readings.

Please report back if you measure some odd voltages.

John
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Old 14th Jan 2016, 11:58 pm   #7
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Default Re: Cossor CDU 150 scope large offset on channel 1

You can even use a probe on the channel 2 input to go looking around the channel 1 input amp.

Even with channel 2 selected, channel 1 is still powered so that the temperatures are OK ready for whenever you turn channel 1 on

David
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Old 15th Jan 2016, 11:57 am   #8
dave cox
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Default Re: Cossor CDU 150 scope large offset on channel 1

I would look at the collectors of TR5 & 6. The differential output should be proportional to your input signal. If that looks OK then TR7 & 8 (followers) would be next. More likely TR5 & 6 will have a differential offset - since you see some signal I would look at TR2 / TR4 as they are doing the ground ref ...

dc
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