Cossor CDU110 Issue

[vinyvamos]

Hi folks. I was recently given this scope from a good friend of mine. It is a lovely looking piece of gear! Upon first inspection a number of fuses were blown. 500mA which feeds the HT inverter, and two on the "Driver" board; 500mA for 120V and 50mA for 100V. There are no markings for fuse ratings so the last two could be ill-rated. I do know that the inverter one is correct as I found a scheme of the EHT section on this forum. Other than that I have no manual or schematic yet, except a part of a CDU150 scheme which hints at some of their design methods at least.

Anyway, after replacing fuses all is ok (i.e. no smoke!) but I can only get a flat trace with the channel amp module (CAM111) removed. Something is being displayed when CAM111 is present but it is far off-screen. Upon lengthy probing and signal tracing with another scope it seems that everything after the CAM111 is ok. Scope displays ~9Vpp sine wave full-screen when injected into the main Y input on the module connector. Then from tracing signals within the CAM111 module it seems that I am getting severe asymmetrical clipping of some sort, all the way from that dual NPN 6-leg can. There is a tiny sub-PCB sitting between that and the input attenuator with 2 transistors, 4 diodes, a cap and resistor. This must be the first amp, followed by the dual NPN can. Balanced all the way from there? The "position" controls seem to have no effect except to adjust the clipping point (when viewing the Y output of CAM111 on another scope). What is rather strange is that I am getting the same result on both channels. Is this a power supply issue perhaps? If it is not a Power rail issue then I would assume that it would be the final Y amp, seeing as it is affecting both channels, also when I select "CH.1+2".

If anyone has a schematic for the CAM111 or even the whole CDU110 scope then that would really shed some light on my puzzle.

Thanks in advance for any advice you can give!

Vinny in Iceland

[WME_bill]

Cossor CDU110 Oscilloscope and Y Amplifier plug in_CAM111.
This was introduced originally in 1966 with a nuvistor 6CW4 as the input stage. In Jan 1967, a modification kit was introduced to use a FET BFW10-C1 with transistor on a little extra board which fitted into the nuvistor space CAM111A.
Then in 1968, the board was modified to build-in the FET stages; CAM111B.
It sounds as though you have the extra board version CAM111A.

You will have seen my warning about the dual capacitors C10 and C11 shorting between the two parts, posted in Jan 2011.
Your unmarked fuses are correct at 500ma.
I thought I had posted the circuit diagrams some years ago, but apparently not. The CDU150 power supplies are pretty close.
I attach all the circuit diagrams of the CDU110 with the two alternative Y amps. They are what I would call typical Cossor. Practically impossible to follow through the circuit flow. Oh, for manuals by Tektronix or HP or Marconi or Telequipment. The whole manual is too large to post.
Please let us know how you get on and what faults you find. Only in that way can the rest of us learn.
wme_bill

[vinyvamos]

Thanks so much Bill for the schematics! This allowed me to solve the issue. I had checked the multi-caps after reading your other post regarding that issue, and all seemed to be fine. On further checking it seems that the 100V regulator circuit was not regulating at all. VT1 & VT3 had blown, thereby passing most of the unregulated rail through to the 100V rail. 100V was now ~125V, and the 125V rail is stacked on top of this, so that was also 25V too high at 150V. These transistors had been replaced before, so it is not the first occurrence of this fault.

My conclusion is that this raise put a large offset on all channel amp circuits, thereby causing my asymmetrical clipping and off-screen displacement of the beam. All is fine now, apart from a noticeable drift in vertical position sometimes which is a tad annoying. Anyhow, I now have a scope for the electronics workshop in our Creative Centre, so I am very happy