Crotech 3132 Scope - no trace.

[Diabolical Artificer]

Not sure if this counts as vintage as I've been unable to find much info or schematics for this scope. It was working OK about a month ago but when I came to use it yesterday there was no trace. I did however manage to get a very brief 'blip' when pressing various function buttons. So it's not the CRT I think. I detected a slight smell; that smell you get when a component is overheating.

When I opened it up the mains transformer was cool and the only sign - ish of overheating I could detect was a small discoloured line transformer with peeling varnish. See pics 1 and 2. LTR is in the foreground.

Not having a clue as what to look for apart from the obvious loose wires ,damaged caps etc, I took temperature readings of the big transistors heatsinks. The only disparity was a reading of 80-90 degrees of the heatsink on the right top PCB. Most of the other transistor heatsinks gave readings of 40 -50 degrees. See pic 3. Brown heatsinks. Hot one on the right.

I'm not sure where to start here apart from checking PSU voltages, but have no schematic to refer to. Any guidance would be most appreciated. There are quite a few presets on the underside of the bottom PCB, pics 3 and 4. Thanks, Andy.

[ukcol]

Hi Andy

I have a Crotech 3131 CRO which has failed twice over the years I have owned it and both times the fault was caused by a tant. bead capacitor going short circuit and an associated fusable resistor going open (hence the burning smell). The tants were 10uF 16v (from memory) and both times there was room to fit a normal electrolitic. The resistors were something like 4R7 or 10 ohm 0.25watt fusable (again from memory).

I have the manual for the 3131 somewhere which may or may not be similar to the 3132. I live not far from you at Harlaxton near Grantham, so if you need any help PM me.

Colin M

[Diabolical Artificer]

Thanks for that Colin. I have a starting point now. If I get stuck I'll be in touch, thanks again, Andy.

[ukcol]

Hello again Andy,

I have just found the manual for the crotech model I have (3131); it uses 25 tants, 21 of which are 10uF/35v (not 16 volts as my memory suggested).

I remember thinking when the second tant failed that I really should be changing all of them, but I never did. Looking at the circuit again, quite a few of them could cause more damage than just a fusable resistor, so perhaps I will change them all.

Colin

[dinkydi]

Hi Colin, Whether or not you should replace all tantalums or only those that fail is a vexed question.

If the tantalums which fail are the same size and from the same batch and fail regularly, they may all be faulty. Perhaps they should all be replaced - but not necessarily.

Tantalums have a bad reputation. I believe that reputation is largely undeserved. Let me explain...

Tantalums are close to ideal capacitors because their series resistance and inductance is small. As a result they perform excellently connected across supply rails, bypassing ac signals to ground, and are often employed in this way. However, their excellence is also their downfall.

Their low series resistance means that when power is suddenly applied a very large current flows which may well degrade the cap prematurely. In a good electronic circuit the current peak must be limited by design. Unfortunately, such design seldom occurs.

It is my contention that even the best electronic designers have used tantalums without proper consideration of the current peaks that can occur in these humble devices. I find that even great equipment from the likes of HP and Tektronix suffers from this widespread oversight. The problem is not fundamentally due to component failure, but rather, equipment-design failure - so simply replacing tantalums doesn't actually fix the cause. Replacing the tantalum may appear to work, however, particularly if the replacement tantalum is a poor one with a bit more series resistance to limit the current!

Readers may have read in my other posts that I am against mindless recapping. This post provides further reasoning behind my position.

Peter

[Diabolical Artificer]

I can find only three Tantalum type caps. Two at the back of the CRT, but these are rated at 3KV Most of the caps are small ceramic disk. Is the fusable resistor any different to a normal resistor? Were the suspect caps/resistor on the PSU or the main PCB or the switching PCB Colin?

Thanks again,
Andy.

[Diabolical Artificer]

I've searched everywhere for a fusible resistor, which as far as I can tell has a colour band after the Gold band. No tantalum bead caps either.

For anyone unfamiliar with this scope, it has two channels but only one Time base selector switch. Would somebody be so kind and suggest where I should start looking? As there is a momentary 'blip/trace' I don't think its the PSU. I'm only a beginner and don't want to do any further damage by powering up the scope and making a mistake by inappropiate probing with a DMM.

Thanks,
Andy.

[ukcol]

Hi Andy

Tantalum capacitors are a type of electrolytic and are only available in low working voltages. Your 3kV rated capacitors will certainly not be tants.

Tants are small and can be spherical or similarly shaped. The ones I replaced were in the 12 volt supply but mounted on the main PCB; they had gone short circuit so you may be able to locate the fault with an ohmeter. If a resistor has failted as well it will be discoloured and probably open circuit.

Colin

[ukcol]

Hello Peter

Thank you for your intreresting post, I have opened another thread to avoid taking Andy's off topic.

https://www.vintage-radio.net/forum/...961#post493961


Colin M

[Diabolical Artificer]

I finally got some time today to have a look at this scope. As I mentioned in my first post a small transformer looked heat damaged. See pic#1. I can't get any resistance readings at all, which I take to mean all windings are OC. Apart from one (the black plastic insulated windings), which is closed circuit. One of these transformer pins goes to a resistor and cap in series (28 ohm + 0.01uf ceramic), which in turn goes to IC. It looks like a non original fix. The other winding which is shorted goes to the emitter of the transistor. The only other physical damage I can find is a 4.7 meg resistor (yl/purp/gn/yl) which has chipped outer 'paintwork'. I think this goes across the supply rail along with a 1uf/3Kv rated blue cap. Also on this PCB I noted little blobs of solder spatter. This is the PCB picture two.

This PCB has a 12V feed from the main transformer, with two wires going to the CRT.

Colin, I can't find ANY tantalum caps. I've checked all the electrolytic caps with an ESR meter. No duds. Pics 3, 4, 5 - spot the Tantalum cap?

Lastly I've checked the main, big, transistors and diodes briefly but cant find any problems. I apologise for the awful picture quality. my camera is broken too. hey ho.

[ukcol]

Hi Andy

I can't see any tants in your pictures either. The 3132 must be very different to the 3131 that I have.

Sorry to have taken you down the wrong path. You will probable need a circuit so that you can fault find properly. Unfortunately I don't know of a source.

Colin m

[Diabolical Artificer]

Don't worry about the Tants Colin, yours was the only clue I had. I've been unable to find a schematic anywhere.

Andy.

[dave cox]

2nd photo, blue blob near the fuse.
3rd photo, blue blobs near the disc ceramic.

If they're not tant then I blame Dr Wobble's wobbly photos!

dc

[Diabolical Artificer]

Aha Dave, I thought they were and said so way back at the start of this thread. Colin has more tants in his 3131 (which were duff) but these were discounted because they are rated at 3Kv. This is the only PCB I can find a fault on. There was a R with chipped outer next to these "tants".

I'm trying to get a schematic at present. They are difficult to get hold of. Have emailed Crotech but there seem to be several Crotechs. One in reading (website under construction) one in LA, USA and a CRO-TECH.

Andy.

[ukcol]

Originally Posted by ukcol

Hi Andy

Tantalum capacitors are a type of electrolytic and are only available in low working voltages. Your 3kV rated capacitors will certainly not be tants.

Just repeating this Andy. If you are right that they are rated at 3000 volts they will not be tants.

Colin M

[WME_bill]

Andy DrWobble.
Don't be discouraged. Even without a circuit, a power supply is relatively easy to sketch out. From that you can check the voltage rails. You can expect one at about +12 to +20V, one at -12 to -20V, one at +100V, and another from the EHT transformer around -1 to -2kv. What is the tube type.
The 3kV capacitors will surely be ceramic if disc form, or polythene film if a block form. Usually either OK or dead short.
The replace-electrolytics-on-sight school of thought is useful advice for commercial grade equipment like radios, but often industrial equipment like oscilloscopes are made with higher grade components, and engineered for lower operating temperatures, so I believe there is less need to blindly replace.
Though tantalums almost always need replacing. Thank you to Peter(dinkydi) for his comments upon tantalum capacitors. One learns something every day.
Fusible resistors. Necessary for production models, in case it catches fire and you sue them for damage when your house burns down. RadioSpares do actually list "fusible axial flame proof resistors, 1/4W,1/2W,2.5W". In values from 0.22ohm to 1kOhm. But you will have to buy 10 at a time, + minimum order value.
But for your home use, try a link of copper wire, or a low ohm low power resistor. It will serve sufficiently well to allow you to get the machine working.
Sounds as though the power pack oscillator transformer has gone. Which means the driver transistor in series has gone. Maybe it failed first, and took the transformer with it. You say the Mains transformer feeds in 12v and is working.
Can you post a picture of the complete scope. I am not sure if Crotech made their own or badge engineered someone else's excellent scope, in the way that Avo or the schools and laboratory suppliers Philip Harris or Griffin & George did. Is it Japanese or Korean do you think, from the components. This might give us some ideas. Bill

[Mikesblunt]

Hi all,

I'm new to posting on this site but I see that Ukcol (Colin M) has a manual for the 3131.

I tried to restore one of these scope a few years back and was plaqued by tants and other components failing one after the other, so eventually gave up.

Colin, I would very much like an electronic copy of the manual for which I'm happy to pay for your time etc.

I don't know the policy of the site regarding personal email addresses, so I shall see if you can help me.

Regards,
Mike

[Darren-UK]

Mike,

Forum Rule B13 covers personal email addresses.

As this thread is now fairly old, you might be better contacting Colin via PM (Private Message) wherein you can exchange email addresses or whatever. To do that, click on Colin's name to the left of any of his posts and then select the PM option from the resulting dropdown menu.

[dragonser]

Hi,
I know this is an old thread.
I have a Crotech 3133 scope so I thought I would post the diagram that shows the CRT and EHT generator. I know that this may be different from the 3131 and 3132 mentioned previously, so please be careful with the high voltages.
I'm sorry about the quality of the diagram.
regards Peter B

[chrisdonne]

Hello all,

To Peter B, dragonser, I note that you have/had a Crotech 3133 manual, if you still have it I would love to purchase a copy of it from you.

Best wishes,

Chris