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Old 20th Apr 2020, 7:52 am   #1
MelJon66
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Default Cossor CDU150 rescue

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. Itís pretty compact and there is a lot going on inside spread out over multiple small PCBís identified A to J. No integrated circuits in here, itís 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 PCBís 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 wasnít 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. MPS2369ís 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 itís 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 donít 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. Itís quite small at about 40mm diameter and the few I have seen for sale are much larger. Itís not a problem yet as the 11kv anode voltage probably isnít 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 itís 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 canít 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 donít 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
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Old 20th Apr 2020, 7:55 pm   #2
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Default Re: Cossor CDU150 rescue

Hi Mel and welcome to the forum.

Nice write up of your CDU150 make me want to drag mine out of hiding

I am wondering if there isn't at least some resistance in the multiplier at the transformer end it might give the EHT generator circuit a bit of a hard time?

Those Fairchild U17718 are they matched pairs in a single can?

When I was at Plessey Avionics in the Mid/late 70's they were a thing, I guess improved manufacturing means its easier to match transistors although I have somewhere got a stack of multi transistors in SM packages.

Looking forward to hearing more about your Scope, then whats next I wonder

Cheers

Mike T
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Old 20th Apr 2020, 9:36 pm   #3
MelJon66
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Default Re: Cossor CDU150 rescue

Thanks for your input Mike,

Are you thinking that the resistance in the multiplier may be causing the fading out of the trace? I have measured the EHT voltage at the final capacitor and it may be a tad high at around 11.4 kV and should be 11.25 kV. High resistance would surely lower it. But my measurements are a bit questionable as I have an old Philips HV probe with 1440 Meg resistance (2 x 720 Meg) so it divides the voltage by 145 with my DVM. The DVM reads 78.6 volts = 11.397 kV assuming the probe resistances haven't drifted. I have nothing that can measure such high resistances.

Could the resistance of the old rectifiers increase as the scope is handling signal rather than just idling as it was when I was taking the readings? It does tend to vary at different timebase settings. I'll have a closer look tomorrow and see if the trace fades more at higher speeds.

I should have said that the U17718's are indeed double transistors. TR3/TR4 and TR5/TR6 on board H for channel 1. Perhaps two modern well matched singles would do the job if anyone has a datasheet and a close match can be found. It's not going to be required to maintain calibration from now on. They are working fine at the moment but I would be happier knowing that parts are available if needed in the future.

Cheers
Mel
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Old 21st Apr 2020, 12:24 am   #4
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Default Re: Cossor CDU150 rescue

There are some high value resistors in the CDU150 as part of the EHT regulation circuit. If they go high, EHT goes high.

Had one of these scopes back in the eighties. Not up to the HP1740 on my bench at work, but I developed a liking for it.

David
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Old 22nd Apr 2020, 10:14 am   #5
MelJon66
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Default Re: Cossor CDU150 rescue

It is with great sadness that I have to report that battleship HMS Cossor CDU150 has sunk to the bottom and is probably beyond economical rescue. Unless you know otherwise!!!

Yesterday I checked all the LV supplies and they were spot on. I then checked the EHT and adjusted to the required -1140V. It had been -1170V so it had been a bit out. The anode voltage was then checked at 11.16kV which was also spot on. The trace was sharper and the focus worked well so I was well pleased. I had some trouble with the beam current but that is irrelevant now.

Later in the day the scope was taking a very long time to produce a trace and then it was a bit jumpy. I found that the 50V and 12.6V LV supplies were not producing any output. It was difficult to check why as by the time I got in there it had warmed up and the outputs were back. I suspected a bad joint or a dodgy bridge rectifier.

Today it won't switch on at all and I have found that the transformer secondary to the 50V supply (secondary taps m and n on circuit A) is open circuit at 5M ohm. The 50V supply feeds the regulation circuit of the 12.6V supply so this is reading very low too. Strangely the -50V supply takes input from the +50V but this still works.

So, the failure is just down to old age I suppose and is probably terminal. I will hang on to it as these machines are either given way or very cheap and one might pop up when the lockdown is over. Patience pays with these things.

My next project is a HP 1743A. I know someone has already been messing around with it big time as it came with a bag of bits that were left over from the last attempt so I am expecting trouble! I will not rush into this project and it will proceed slowly as I am also converting a campervan. I will post something when I get started.

Thanks for your contributions.

Cheers
Mel
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Old 22nd Apr 2020, 10:56 pm   #6
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Default Re: Cossor CDU150 rescue

Hi!

It is worth having a most careful look at the mains transformer from the CDU150 before you condemn it as it may be a secondary leadout wire that's broken away from the connection tag, etc., that might be repairable!

Chris Williams

PS!

One of these transformers with all four secondary sections wired in series would give you about 48–52V unloaded, to replace the suspect m–n original secondary, and you'd probably be able to fit it discreetly inside the oscilloscope somewhere!

https://www.ebay.co.uk/itm/232182622505
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Old 23rd Apr 2020, 7:31 am   #7
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Default Re: Cossor CDU150 rescue

Thanks Chris, your input is much appreciated.

I have looked at the transformer and there are clear signs of oozing varnish and molten solder drops where the +50v supply lead out wires enter the secondary windings. It is inside the windings so not easily accessible and the whole thing is dipped in varnish so it would get messy. This ooze was there when I initially did a visual on it before powering up so it has been heading for failure for some time. When it powered up successfully I thought I had got away with it

I have thought about adding a separate transformer as I have done that before when heater transformers have failed. Unfortunately there is no space inside the Cossor, it is very well stuffed! I did measure the voltage between taps m and n before it went down and it was 56.3v loaded, so I probably need 60 to 70v unloaded. There is another tapping off the transformer, points o and p on the circuit A schematic, that goes to the -50v supply. That measured 56.0v loaded so it is the same and still works so I will measure that unloaded and see what the voltage is.

The -50v supply is fused at 1 amp and the +50v at 500mA. As the +50v is lower rated I was tempted to connect both the +50v supply and the -50v supply inputs to the o and p tapping but I don't want to overload it and blow another winding. It can't get much worse because the transformer is already broken. But, I am puzzled why they would use two tappings with the same voltage when one with a higher rating would do. Is there any reason why they would go to the expense of splitting them in two? Better regulation perhaps?

To get the scope going again, at least for testing, I will run the +50v supply from a bench power supply so I can monitor the current and consider connecting both supplies to the one tapping.

Does anyone know if there any way of assessing the current capability of a tapping?

Cheers
Mel
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Old 23rd Apr 2020, 7:52 am   #8
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Default Re: Cossor CDU150 rescue

The place I worked at in the mid 70s had a CDU150, it was the most unreliable piece of equipment ever and seemed to spend half its life back at Cossor being repaired. They even replaced the CRT after about 3 years. I think the mains transformer was replaced under warranty.

Our accountants regarded it as capital equipment that could not be scrapped and happily kept on paying for repairs. After five years its value had been written down and I bought it off the company for a song. In those days there wasn't much time for hobbies and it probably only got used for a couple of hours a year but that didn't stop it playing up from time to time. The last time it broke down, a large EHT decoupling capacitor had gone short and I successfully used the scope for a while with this disconnected. In the end it went to the local dump and I was glad to replace it with something reliable.

Len
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Old 23rd Apr 2020, 8:06 am   #9
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Of course it's unreliable, it's British!

It was clearly soldered by hand and some of it is very questionable, with little solder splatters liberally sprinkled about. It's ideal for someone who likes to fix things, as long as it is only your backup scope. My main scope is an Iwatsu - Japanese.

Cheers
Mel

By the way the CRT is German (Telfunken).

Mel
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Old 23rd Apr 2020, 9:31 am   #10
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Default Re: Cossor CDU150 rescue

Hi!

Yes, but you need an adjustable electronic load across the rectified output for this!

Alternatively, you'll probably find two 24V 5W vehicle sidelight bulbs in series will approximate to the full load rating of the 50V winding!

(I was lucky enough to get an Iwatsu 6122 at a very good price from Devon there years ago and it's superlative!)

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Old 24th Apr 2020, 11:58 am   #11
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Default Re: Cossor CDU150 rescue

Hi!

I've just had another thought – if you can find a miniature high–voltage supply module (Asian Suppliers or eBay) of about 4–8kV output for the p.d.a. anode supply (1mA current output is adequate), you might be able to dismantle the existing e.h.t. multiplier circuit and it's supporting standoffs/spacers, and use the space freed up to use the "R Core" type transformer of the type I suggested in an earlier post – I think there's enough depth for a 30VA one!

Chris Williams

PS!

If you try this then I suggest you retain R37a/C19a and the standoff/p.d.a. lead to provide a safe termination for the output from a replacement e.h.t. supply module!
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Old 24th Apr 2020, 2:17 pm   #12
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Default Re: Cossor CDU150 rescue

Thanks Chris, what an excellent idea

I was planning to replace the selenium rectifiers anyway and I have new diodes and caps. I was going to string out the diodes over the the existing widely spaced standoffs but if I was to make up a compact assembly and encapsulate it it may be small enough to free up space for the additional transformer. Heat might be a problem as the power supply transistors and diodes are bolted to the bottom of the EHT chamber back panel and it gets very hot in there.

One remaining question is what size dropper resistor I would need to compensate for the lower forward voltage drops of silicon versus selenium. I might make up an assembly and see what final voltage I get and calculate the resistor size.

In the meantime I am going to set up a supply from my variac to replace the burnt out secondary that feeds the +50v power supply. It's more convenient to do that than use a bench power supply to provide +50v dc as my variac is always on my bench anyway and I won't have to mess about running bench supplies in series.

Cheers
Mel
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Old 24th Apr 2020, 3:16 pm   #13
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Hi!

Be very careful if you do that because the oscilloscope will be LIVE to the mains supply if you try and use a Variac direct – you MUST also include a suitable mains isolation transformer between the mains and the Variac., or a suitable low voltage Stepdown transformer with isolated secondary windings between Variac and 'scope!

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Old 24th Apr 2020, 7:13 pm   #14
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Yes, my variac is connected via an isolation transformer but thanks for the reminder.

Well, it's powered up again with the variac but it wasn't very successful. I had to set the variac at 77v to get +50v dc on the output and it wasn't stable like when running off the original transformer. It's also quite sensitive to variac voltage. My variac is not too accurate either as it varies +- 3v when you switch on even when the control has not been touched so this will be changing the +50v supply voltage.

The scope operates but not well. Ch2 is best but when grounded input is selected at some timebase settings the trace is not flat and has gaps in some sections. Ch1 is even worse and at some settings the trace breaks up into two curved lines and there is a deflection like ringing toward the end of the trace. It's like looking at a heart monitor with one beat per sweep! I assume it just doesn't like having an ac signal cable routed inside. I have tried rerouting the flex but to no avail.

Overall this won't do so I will have to try the dc bench power supplies tomorrow.

Cheers
Mel
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Old 24th Apr 2020, 8:25 pm   #15
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Default Re: Cossor CDU150 rescue

Hi!

What about one of these?

https://www.ebay.co.uk/itm/272413354752

(Order the £9.32 "Boost Converter Step Up Module)

If you remove the finned heatsink and mount the board to the 'scope chassis, with TO–220 mounting kits between each device and chassis, you might get it in a corner somewhere!

Power the module from 22 (+) and 19 (–) and connect the module o/p to 12 (C4 +), reducing C4 to 100u 100V, and then set the module's output voltage control to give 65V at pin 12 of the PSU PCB A!

Please note the +50V supply does need to be stabilized itself, so this is why trying to power it at 50V from a Variac, etc., didn't work!

These little modules give about 50mV p–p ripple, so if you feed the 65V to pin 12 as I suggest, the existing series stabiliser TR6 to TR10 will remove the "grass" from this module, and you'll still get the tracking between the supplies the 'scope needs for stable operation!

The 12.6V supply rectifier BR3 and winding q–r of T1 hopefully should keep you going until you can get another "parts mule" or get the transformer rewound!

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Old 24th Apr 2020, 10:24 pm   #16
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Default Re: Cossor CDU150 rescue

Thanks again Chris,

Now that looks interesting as the converter might fit in the space above the transformer. There's a ridiculously large snap connector block in there but it may be movable. For my education how have you determined where to pick up the inputs to this converter?

Adding this converter is putting extra load on the q - r tapping so why is this better than just adding the +50v PSU to the o - p tapping which is at the correct voltage anyway? It must surely be more efficient that way as there must be some loss of efficiency in the converter.

Before the secondary failed I measured 56.3v ac at the m - n tapping while it was operating properly and +67.5v dc at pin 12, which seemed to make sense. I believe my DVM and the variac voltmeter both read RMS so when I connected the variac to pins 9 - 11 I was expecting to set it to around 56v, the same as the original transformer output. m - n and 9 - 11 are at the same ac potential, before rectification, so I still don't understand why that's not so. Putting 77v ac in to get 67.5v rectified dc out puzzles me.

Apologies for all the questions but I have a lot to learn.

Cheers
Mel
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Old 25th Apr 2020, 5:15 am   #17
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Default Re: Cossor CDU150 rescue

Hi!

By comparison of the values of the reservoir capacitors!

The +12.6V series stabiliser is fed from BR3 and winding q–r of the mains transformer, with a 2000uF reservoir, so that implies winding q–r is designed to provide a much higher current than the other three, which only have 500uF reservoir capacitors!

I estimate the current taken from the +50V series stabiliser TR6 to TR10 is going to be in the region of 50mA or so, (you can use a bench power supply to determine it exactly), so if you use the module I suggest, the current taken from the 12.6V rectifier BR3 should be an additional 200–250mA, which winding q–r on T1 should be able to supply!

The output from BR3 would need to be about 15–18V for the +12.6V series stabiliser TR11–15 to work properly, so assuming it's 18V and 50mA is drawn from the +50V line, equates to about 4W of supply power, assuming the step–up converter is 75% efficient!

Therefore the approximate supply current needed to provide 50mA at 65V is about 4/18 or about 220mA!

The reason I didn't suggest using winding o–p and BR4 to supply this module is because it's almost certainly going to been wound of the thin wire used in winding m–n and attempting to draw another 220mA from it would almost certainly have damaged it!

That bring me to your final question – why was the original winding m–n 77.5V a.c?

The reason is because of the voltage drops in the two 2 ohm series resistors, the first between the rectifier BR2 and the fuse, plus the second after the fuse, means that in order to provide "headroom" for the +50V series stabiliser TR6–TR10, Cossor determined at design stage that the stabiliser requires +65 to +67.5V, therefore allowing for the ripple voltage across C4 plus the voltage–drop in the relatively high–resistance secondary m–n, and two diodes, and the two series 2 ohm resistors, about 75V r.m.s. off–load is needed for the circuit to work efficiently, so this is what Cossor designed their transformer to provide!

As to why winding m–n failed, the answer is too much iron and too little copper! Cossor designed these transformers to run at very high magnetic flux densities:–

e = 4.44 ◊ f ◊ B ◊ N ◊ A where;

e = induced secondary voltage; f = supply frequency, B = magnetic flux density in core, A = cross–sectional area of core!

By using a very high magnetic flux density the turns/volt figure for the windings can be maximised, but this unfortunately leads to a higher voltage stress between individual turns!

What happens is that inevitably at some point, the enamel insulation will start to flake off the winding wire in localised spots deep in the windings, which results in local large circulating currents between turns, causing further overheating and even more heavy current flows, and the whole thing becomes self–perpetuating until the winding burns out, as happened in your case!

Chris Williams
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Old 25th Apr 2020, 8:52 am   #18
MelJon66
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Default Re: Cossor CDU150 rescue

Thanks Chris, for taking so much time to help.

That's an excellent explanation and the fog clears a little. I now understand that when I was taking the voltages across the windings, even with the fuses pulled, there was still resistance in the circuit that meant I was not getting an off-loaded reading.

Later today I will remove the variac and connect a dual bench power supply in series which will give me up to 60Vdc. I was going to pull fuse FS3 and connect the +50V to pin 15 and the -ve to the scope ground. So, I will be applying +50V to the PSU output pin. That way the other PSU's will still get their +50V feed. Is that okay or should I be putting +65V on pin 12? That will require an additional 30V power supply so will be very cumbersome but is it the right way to do it?

I just want to check that I get a good steady trace before I take any further action.

Cheers
Mel
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Old 25th Apr 2020, 10:22 am   #19
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Default Re: Cossor CDU150 rescue

Hi!

You should always try and replicate the conditions provided by the original transformer and rectifiers, so the correct course of action is to use two supplies (if you have them) to give a total of 62.5–67.5V, limited to about 250mA on the PSU settings, connect +ve 62.5–67.5V to C4 via a 1N4001 diode (cathode marked end to C4+) and negative to scope chassis!

You need to be sure the +50V stabiliser is still working correctly, hence my recommendation of feeding the supply to C4(+) & pin 12!

Yes it is combersome but the only means of making a valid evaluation of the rest of the 'scope circuits before you decide on a repair strategy!

Chris Williams
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Old 25th Apr 2020, 11:04 am   #20
MelJon66
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Default Re: Cossor CDU150 rescue

Chris,

You say connect the bench supply +ve to C4(+) & pin 12. If I connect to pin 12 only that's the same as C4(+) & pin 12 isn't it? Just checking that my understanding is correct.

I only have 1N5404 diodes to hand, which are a bit chunky to fit inside the case, so I will fit one on the bench supply end of the lead.

Cheers
Mel
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