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Old 22nd Jan 2020, 2:43 pm   #21
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

She's now chooching at full capacity, thanks everyone for all the help
Quote:
Originally Posted by Chris55000 View Post
PS!

Cossor oscilloscope circuit diagrams aren't really that difficult to follow – I've seen a lot worse!

Chris Williams
Other than the typo in the PCB B circuit diagram ( R79 connects to 15, not 14, 14 is a data line and wouldn't like it if you connected a 10 ohm resistor to ground) I'd mostly agree, but due to the nature of the single sided board, with a massive mess of solder tracks going every which way, it doesn't correspond to reality particularly easily.
Quote:
Originally Posted by WME_bill View Post
Cossor 3100 +3122.

You have the main frame manual 3100 and the TB 3122 circuits. The other parts of the manuals do not offer any further help. They are rather typical Cossor, little explanation and some very oddly drawn diagrams. You will wish for Tek or HP or Marconi.
wme_bill m0wpn
Now that everything is working, it does appear that trace 2 is substantially off compared to 1, If you have it it would be nice to have the diagram for the inputs (H?) to better readjust it/make sure nothing is amiss
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Old 22nd Jan 2020, 4:23 pm   #22
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Yeah, channel 2 is definitely a bit dodgy, on certain ranges it also shows what looks like compensation issues that it doesn't on others/on channel 1, I assume that the screw/variable capacitor corresponding to that range needs to be adjusted as well so you can compensate evenly through the ranges? no idea what screw corresponds to which range though :/
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Old 22nd Jan 2020, 5:08 pm   #23
Chris55000
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Hi!

That's correct – however, to set the attenuator compensation correctly you need a square–wave/rise time at least no worse than that of the Oscilloscope you're adjusting – A Tek.106 or the later PG506 in the appropriate main–frame is needed, set to give a 1MHz output equivalent to about 5 divisions for each attenuator.

With this connected to the 'scope via a matched 50 ohm termination, the shunt compensation trimmers are adjusted for the best corners of the display, without any overshoot or ringing, while the series ones are adjusted at a lower frequency, usually!

If you can't get hold of a fast–rise Oscilloscope Generator, a really good function generator will do at a pinch but the compensation wouldn't be as accurate!

(For basic scopes, up to 10MHz, you could probably get away with a F.G!)

In the absence of the full written 3122 Manual, look for another similar British Oscilloscope of the same bandwidth as the 3100/3122 with the same or similar attenuator arrangements, that you can get a manual for, and use the other 'scope's bookwork as a guide!

Chris Williams
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Old 22nd Jan 2020, 5:18 pm   #24
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Quote:
Originally Posted by Chris55000 View Post
Hi!

That's correct – however, to set the attenuator compensation correctly you need a square–wave/rise time at least no worse than that of the Oscilloscope you're adjusting – A Tek.106 or the later PG506 in the appropriate main–frame is needed, set to give a 1MHz output equivalent to about 5 divisions for each attenuator.

With this connected to the 'scope via a matched 50 ohm termination, the shunt compensation trimmers are adjusted for the best corners of the display, without any overshoot or ringing, while the series ones are adjusted at a lower frequency, usually!

If you can't get hold of a fast–rise Oscilloscope Generator, a really good function generator will do at a pinch but the compensation wouldn't be as accurate!

(For basic scopes, up to 10MHz, you could probably get away with a F.G!)

In the absence of the full written 3122 Manual, look for another similar British Oscilloscope of the same bandwidth as the 3100/3122 with the same or similar attenuator arrangements, that you can get a manual for, and use the other 'scope's bookwork as a guide!

Chris Williams
i mean the compensation seems Waaaay off, not the kind of thing you'd need a super high precision reference to notice- seems to be overshooting a significant fraction of the size of the square wave - it's visible even with the scopes internal compensation output (other channel doesn't see it,gives a perfectly flat square wave so i doubt it's a problem with the calibrator as such).

wondering if that channel might have developed a fault tbh :/, gain is also quite noticeable- on the order of 20% or so, with channel 1 seeming to correspond far better to the numbers listed on the calibrator at least
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Old 22nd Jan 2020, 5:40 pm   #25
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

ach, no compensation problem- the selector switch for channel 2 is just a bit dodgy( i can make it show the same by applying a little pressure to the rotary selector/wiggling it about till it stabilizes, and it feels a bit "squishier" than channel 1s.

gain problem still there, and actually more like 25% 4.5vs 6 divisions, not a probes issue, swapping them causes the same readings
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Old 22nd Jan 2020, 7:04 pm   #26
Chris55000
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Hi!

The way to trace this type of fault is to feed an identical signal into both channels by means of a BNC "T–Piece", set the gain–switch to the same factor in both channels (I recommend 1V/div), adjust your generator for 4 div. above and below the centre, then use a second oscilloscope to see where it's getting lost, but if you've not got one, you can use a DVM with a 1uF capacitor (NOT electrolytic – some of the test points have negative voltages on them!) in series with it's red (+) lead, then feed in enough sine–wave input at 400Hz or 1kHz to give 8V pk–pk on the screen if the good side as before!

You can then use the DVM to trace the signal thro' the amplifier stages, loss of gain, if not due to yakky switch contacts in the attenuator switch circuits, may indicate damaged input FETs or transistors in the early stages!

I'm afraid you'll need the 3102 circuit diagrams from Bill to solve this, unless you can prove the contacts themselves are the culprits!

Chris Williams
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Old 22nd Jan 2020, 7:28 pm   #27
WME_bill
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Cossor 3100 and plug-ins 3102 YAmp and 3122 Timebase.
I am glad you have solved the Time base problem. Now you will want the Y Amplifier details.
I attach the circuit diagrams for the 3102 dual trace unit, and the pages from the manual relating to setting up the attenuators.
Don't be put off too much by this talk of an expensive Tek106 or similar. The built in calibrator will give a pretty effective square wave for setting the attenuators, though the faster the rise-time, the better the result.
Chris 55000 suggestions seems very sensible. It is surprisingly difficult to fault find without a second oscilloscope.
wme_bill
Attached Files
File Type: pdf 3102circuits-wm.pdf (879.8 KB, 12 views)
File Type: pdf 3102 set up-wm.pdf (481.1 KB, 7 views)
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Old Yesterday, 11:43 am   #28
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Quote:
Originally Posted by WME_bill View Post
Cossor 3100 and plug-ins 3102 YAmp and 3122 Timebase.
I am glad you have solved the Time base problem. Now you will want the Y Amplifier details.
I attach the circuit diagrams for the 3102 dual trace unit, and the pages from the manual relating to setting up the attenuators.
Don't be put off too much by this talk of an expensive Tek106 or similar. The built in calibrator will give a pretty effective square wave for setting the attenuators, though the faster the rise-time, the better the result.
Chris 55000 suggestions seems very sensible. It is surprisingly difficult to fault find without a second oscilloscope.
wme_bill
o lordy, what on earth is transistor 1/10? it's some kind of to78 dual jfet package.
(7 leads with one trimmed)

The circuit diagram marks it as spec. 128(?) and its metal case is marked wd111 7235? none of which give very convincing hits on google
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Old Yesterday, 1:38 pm   #29
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

hmm, actually, i wonder if the main contributing factor to the bandwidth, is actually just those tantalum capacitors? specifically c4/c18

if you look at the "dc balance" circuit, you'll see that twiddling the position knob on the front of the unit produces an at most ~0.2v in voltage from one extreme to another to the base of TR1B, and that's enough to more than take the trace off the edge of the entire screen, both ways, so maybe 0.01v to create 2 divisions?.

it's a really big capacitor too, implying that line needs serious decoupling, as the frequency increases, isn't the ability of that cap to decouple going to decrease a lot due to ESL, if that line needs to be stable to within <0.01v ripple under load to stay within 2 divisions,that tantalum might not be capable of that at higher frequencies?

wouldn't take a huge parasitic impedance to cause that, something on the order of 10ohms?

dunno if that's how it actually works just spitballing ,

and i don't actually have equipment good enough to test the frequency response of the scope, so just theoretical for me.
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Old Yesterday, 2:44 pm   #30
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

edit, math wrong, probably an ESR change < 5ohms would do it, maybe as low as 2,
that seems reasonable thing to hit at about 35mhz?

idea being - jfet turns on amount due to the voltage difference between GS, the jfet turning on turns other transistors on, which pulls the voltage of the Jfet source down, which acts to lessen Vgs, which turns the jfet less, which turns on the following transistor less, and acts to oppose the amplification.

Yeah, i think that power line needs to be as stable as possible...
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Old Yesterday, 3:28 pm   #31
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Quote:
Originally Posted by golexica View Post
edit, math wrong, probably an ESR change < 5ohms
I mean impedence not esr, sorry.

to get a 1ohm impedance at 35mhz requires ~5nH, not taking into account trace impedance and the such. with all that, more than 10nH doesn't seem implausible, causing several ohms worth of impedance?

Forgot to take into account the DC balance, but it still applies - The capacitor is connected to both, so the ripple would effect both roughly equally, and there's a differential amplifier, so the difference would change by a similar factor?

someone who's better at analog circuitry than me help me out here XD

Last edited by golexica; Yesterday at 3:54 pm.
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Old Yesterday, 5:33 pm   #32
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Quote:
Originally Posted by Chris55000 View Post
Hi!

The way to trace this type of fault is to feed an identical signal into both channels by means of a BNC "T–Piece", set the gain–switch to the same factor in both channels (I recommend 1V/div), adjust your generator for 4 div. above and below the centre, then use a second oscilloscope to see where it's getting lost, but if you've not got one, you can use a DVM with a 1uF capacitor (NOT electrolytic – some of the test points have negative voltages on them!) in series with it's red (+) lead, then feed in enough sine–wave input at 400Hz or 1kHz to give 8V pk–pk on the screen if the good side as before!

You can then use the DVM to trace the signal thro' the amplifier stages, loss of gain, if not due to yakky switch contacts in the attenuator switch circuits, may indicate damaged input FETs or transistors in the early stages!

I'm afraid you'll need the 3102 circuit diagrams from Bill to solve this, unless you can prove the contacts themselves are the culprits!

Chris Williams
oh no, not the gain issues, but the weird overshooting does go away if i wiggle the switch a bit (while keeping it on the same range, wiggling the "play").

I'm a little confused what the capacitor is for here? wouldn't 1uf be almost transparent at 1khz?, reactance of 160ohms, isn't that too low?, at least compared to the input of the multimeter, which should be 1Mohm or so on voltage?

Last edited by golexica; Yesterday at 5:46 pm.
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Old Yesterday, 7:16 pm   #33
Chris55000
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

H!

Many of the inexpensive DVMs will give nonsense results or go haywire on d.c. with an a.c. component superimposed on it, so the purpose of the 1uF capacitor I suggested is to block the d.c. potentials w.r.t chassis that will be present in the amplifier circuit!

Compared with the impedance of the meter input, the reactance of 1uF at 400Hz or 1kHz will be negligible, so the meter (switched to a.c.) should show a reasonable representation of the signal voltage!

Chris Williams

PS!

Many of the much more expensive Flukes, HP/Agilents, Keithleys, Siglents, etc., (mostly bench instruments) do have an (AC + DC) function that allows you to select each component and display them separately, but I write my fault–finding notes for use by members who may only have basic instruments!
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Last edited by Chris55000; Yesterday at 7:27 pm.
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Old Yesterday, 7:59 pm   #34
golexica
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

Quote:
Originally Posted by Chris55000 View Post
H!

Many of the inexpensive DVMs will give nonsense results or go haywire on d.c. with an a.c. component superimposed on it, so the purpose of the 1uF capacitor I suggested is to block the d.c. potentials w.r.t chassis that will be present in the amplifier circuit!

Compared with the impedance of the meter input, the reactance of 1uF at 400Hz or 1kHz will be negligible, so the meter (switched to a.c.) should show a reasonable representation of the signal voltage!

Chris Williams

PS!

Many of the much more expensive Flukes, HP/Agilents, Keithleys, Siglents, etc., (mostly bench instruments) do have an (AC + DC) function that allows you to select each component and display them separately, but I write my fault–finding notes for use by members who may only have basic instruments!
Mystery solved , probing across 7 and 8 ( set gain for ch1) gives 162 ohms (250 pot), probing across the equivalent for ch2 (24/25) gives...294, on a 250 pot.

how did it get so far out of wack?

looking at the adjustment pot, it seems to be attached to a long shaft...that goes through the middle of the shaft for the range selector,with a tiny screw in the middle of the rotary selector, that you use to turn that shaft, The ickyness on the rotary selector, must have caused a snag on the inner shaft, messing with the range calibration.

That seems like an unneccesarily complicated design, how often do you really need to adjust the calibration pots?

well at least i know where all the position calibrators are now
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Old Yesterday, 11:24 pm   #35
MotorBikeLes
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Default Re: Cossor CRO 3100 Timebase unit repair (model #3122)

WD111, 7325. This sounds like a dual FET, type WD111. The 7325 is week 35 of 1973, prodn. date od component. Many of these FETs and Dual Fets had in house numberring. many had UC, meaning manufactured by Union Carbide. I don't have any data right now, but the WD111 does seem to ring a bell.
Les.
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