Greetings all, first post by a new member.
I have registered at last having been searching the forum for some time. Living in Somerset, I am now retired so have more time to spend on old electronics, mainly driven by nostalgia for the old stuff and wanting to learn more about electronics. I am mainly interested in old HiFi but having been given or acquired a few old oscilloscopes I am spending more time on these. I am a mechanical engineer so my electronics knowledge is limited.
My current project is a Cossor CDU150 CT531/3 oscilloscope which had been owned by a radio ham who left it to a fellow ham before he passed away at the grand old age of 102. Nobody at the ham group wanted it so it was on its way to the skip unless somebody like me went for it on Ebay. I really like it as it is military specification machine and is built like a battleship. Cast aluminium front panel and internal frames. All metal controls that slam into position and maintain a stable trace when switching unlike some of my other scopes. It’s pretty compact and there is a lot going on inside spread out over multiple small PCB’s identified A to J. No integrated circuits in here, it’s all discrete components except for a few bridge rectifiers in the LV power supplies. The Cossor calibration label inside says 28/11/1982 but I find this hard to believe that this was the original factory calibration as the technology seems much older (selenium EHT rectifiers!). Perhaps the military just wanted it this way. It runs warm as there are a lot of metal can transistors screwed to the chassis so it will keep the work area warm on winter nights. All the PCB connections are soldered leads but access to everything seems pretty good and many of the PCB’s are on hinges for access. The vertical pre-amp pull out module is not so good as access is limited without desoldering the connections and removing the module which makes fault finding with power on a bit trickier. This unit had been in storage for many years so I checked everything out visually before switching on. A cracked ceramic capacitor in the EHT multiplier and some questionable solder repairs in the input/output connections to the vertical pre-amp module were put right before I brought it up to full voltage on a variac.
Well, channel 2 worked fine but there was no visible trace on channel 1 so there was a problem in the vertical amp somewhere, which was probably why someone had been in the pre-amp module before. The problem was exactly as described in this post in this post by JohnPearson a few years back but the final solution was never revealed in that post.
https://www.vintage-radio.net/forum/...d.php?t=122673
I have attached a pdf copy of the CDU150 circuits for reference. Thanks to WME_bill as these were a great help even with the full manual available as it is great to be able to switch easily between the manual text and the schematics.
So it turns out there was a trace but it was well off the bottom of the screen and only with a large amplitude signal and the position control at the top of its range could you see the top of the trace. I did have CH2 to compare to for diagnostics but this wasn’t necessary as a few prods with a chopstick soon showed up a fault on TR12 on vertical pre-amp board H. Prodding TR12/H caused the trace to jiggle around and occasionally invert but it would not stabilise. It did not appear to be a bad joint so I removed it and it had failed open circuit base to emitter when tested but it must have been working intermittently in circuit. MPS2369’s are easily available and cheap so I replaced it, just about possible with the module still in position. Success, CH1 now works, and with a few adjustments of the pre-sets it’s just as good as CH2.
So, next steps and a few questions for knowledgeable members:
1) Calibrate the machine to check everything works as it should. I do have a pdf of the manual so this is just on the To Do list.
2) Replace the selenium rectifiers in the EHT multiplier (STC K83/150/D). I have a batch of silicon diodes (2CL75) ready to go. I will need some series resistors but where should I put them? One in series with each diode seems the normal approach but it seems easier to insert one large value resistor as only one has to be replaced if the final voltage is not correct. I realise it will have to have a cope with higher voltage drop but is this a reasonable approach or am I missing something? There is a resistor at the end of the multiplier anyway, R37 (4.7M) on circuit B, going to the final doorstop capacitor, so I could just increase its value. Has anybody done this with a CDU150 or anything similar and could advise on the resistor value?
3) Replace all the remaining old ceramic caps in the EHT multiplier and C2, C3 and C25 on board B as one had cracked and they all seem oddly discoloured and I don’t like the look of them. I have the parts and will do this when I know what to do about the selenium rectifiers and can do it all at the same time.
4) Replace a couple of resistors R35/B and R36/B that have gone high (1 Meg vs 680k) in the focus section of the EHT circuit B. The focus pot has to be at full rotation to get a sharp trace and I suspect they are the cause.
5) Check out the trace brightness circuit as it is very bright in the first 1cm of the trace but gradually fades as it moves across the screen. There is plenty of brightness available from dark to dazzling and turning the brilliance pot up brightens the end of trace but the start of the trace is then too bright in dim conditions. Is the uneven trace brightness normal on these machines or is this something worth investigating?
6) Replace the CRT final anode cap because the rubber has perished. It’s quite small at about 40mm diameter and the few I have seen for sale are much larger. It’s not a problem yet as the 11kv anode voltage probably isn’t strong enough to jump much of a gap anyway and there is no aquadag on the outside of the CRT to track along. I have got my hand too near to it for comfort already as it’s very close to the vertical pre-amp board. Any ideas on where to get one?
7) Check out the availability of the Fairchild U17718 transistors used in the vertical pre-amp. I can’t find a datasheet and can only find one supplier in the UK, Ellisons, who specialise in hard to find and military parts so are probably expensive. I don’t need them at this stage but does anyone know if there is a non-military spec equivalent that is readily available? If they are available I will probably keep machine as it seems like a good old workhorse for everyday use.
Thanks very much in anticipation.
Mel