Thandar SC110A oscilloscope repair success

[MrBungle]

Thought I'd document this in case anyone else comes across one of these. Bit of fun this afternoon.

Obtained one of these tiny portable 10MHz oscilloscopes for the cost of postage a few days ago in "it's broken" condition. Arrived, powered it up on the bench supply and indeed it was broken. I am generally disappointed when things do actually work these days or just reqiure a bit of a clean which is becoming increasingly common. Not today - this one required some debugging! The timebase appeared to work but the vertical amp was shot entirely with the trace being just about adjustable onto the screen and showing no deflection from the input signal.

Obtained service manual, had a read and followed the calibration steps until I got stuck. Reference voltage was set up properly as it was way out, HT fine, deflection fine. Still no banana - trace was stuck. I could adjust it into the middle of the screen but the horizontal shrunk to a dot. Clearly something up with the amp.

I measured the Y plate deflection voltages and they were at -52v which was in line with it being wedged. So this was a problem with the amplifier. Always start from something you know works in these situations so I so I stuffed a function generator on the input and walked back from the input to the CRT plates.

All was fine until I hit the first follower as it turned out when adjusting the DC offset calibration control, nothing happened. I checked the schematics and the voltages across the trimmer and it was -4.9v at both ends. Suspicious - something open circuit. Traced it from the control to the capacitor which was right on the other end of the board and found a single bad joint on the end of a resistor that stopped the current source working in the input follower.



Popped the iron on it for a couple of seconds and flooded it and hey presto it works. Terrible soldering, not what I expect from Thandar:-



Did the cal steps as best as possible with limited equipment and we're in business.



Obligatory photo of the workspace in the living room that SWMBO has now informed me needs to be tidied up. I'm trying to work as minimally as possible so all the new but nice kit I spent a small fortune on over the last two years that has zero service possibility has been working its way onto ebay over the last month or two. You learn more with less.

[WaveyDipole]

I have never seen an analogue scope that small! Apparently it is made by Sinclair and uses the same CRT as their micro TV, which may go some way to explaining the soldering....

[SiriusHardware]

I've owned the Sinclair version of this (and its similarly styled companion digital storage box) from new.

The only problems I ever had with mine were the rotary switches going very intermittent, and to be honest the units aren't really very good scopes even when working as well as they ever can. That said, when I was doing a lot of field work on CCTV systems, mine did prove very useful.

Like WaveyDipole, I believe they used the same tubes which were used in the original Sinclair 'Microvision' tiny portable TV - as much to use them up, as anything else.

[MrBungle]

Agree with your assertions that they're not very good. You could feel the Sinclair coming off it (usually the ambient warranty return frustration). 10MHz bandwidth was questionable even after calibration. I think that must have been old Sinclair marketing and it was the 20dB point not the 3dB point they were selling

It was a novelty purchase for me really and only purchased on the basis that it fit my criteria for toys: broken, less than or equal to £10 and a service manual available. I buy these things for the repair challenge as they are better than the crossword in the paper. Some of them stick around, some of them move on. This has now departed to a new home for considerably more than I paid for it so I can buy more things to fix and build.

Switches were in good condition. I suspect this had been wonky since fairly new and abandoned shortly thereafter. I had switch problems with a Thandar function generator, a TG102 about a year ago which is based on the same construction principles. I found exactly the same switch, a Lorlin one, still being sold by RS which was nice. I was hoping that I wouldn't have to replace any on this and I was lucky.

Oh and the trigger is awful. Even my old Telequipment S61, which was as economical as they could have made it was better. Ok as an indication, light debugging tool or audio monitoring device but not serious electrical engineering work. Best bit was it was floating.

[Lloyd 1985]

I like that! I'm sure I saw one in an old electronics magazine, can't remember which one it was.

I have a tiny scope too, a Philips one, can't remember the model number, I think it's 5Mhz 2 channel. Very handy scope, works from mains or a 6v lead acid battery (both dead now). Very pleased I bought it as I've never seen another since!

Regards,
Lloyd

I worked for Sinclair in the 80's, a lot of money was spent making things cheap. This was at Milton Hall on the outskirts of Cambridge, free canteen, free filter coffee machines (bring your own mug) and a free bar with one Greene King IPA pump (Clive liked his beer, I had a chat with him one afternoon over several pints, I didn't fall of my bike on the way home). The stuff was cheap, it was designed to work (didn't always!), made in the UK and gave us the worlds best computer programmers.

I left when Mr Sugar bought it, great times, learnt a lot.

[McMurdo]

I read somewhere that Sinclair, Thandar, and Blackstar Instruments were somehow connected by staff, I know Blackstar were taken over by Thurlby-Thandar (TTI) but this was before that all happened.

[Sean Williams]

Had these at school! - can't remember ever being allowed to use on in anger though - might look for one just for a laugh now!

[MrBungle]

We had a couple of old Heath scopes at school which were assembled by one of the physics teachers. Fully transistorised ones, I think IO-18U. They were both broken. I tried to repair one of them in technology class but parts availability was dire and my knowledge was terrible. They wouldn't have bought Thandar; too expensive!

You can see the similarities across Sinclair, Thandar and Blackstar over the years. I'm a big fan of Thurlby products, particularly power supplies, but not necessarily the ones derived from the above companies. My first function generator was a Black Star Jupiter 500 and it was, how shall we say, dodgy and it was new then and that put me off. Got a cheap 1980s American one now (Exact) that I paid about £20 for and it's marvellous.

[Nymrod121]

If memory serves, Cirkit (Ambit as was) advertised the SC110 in one of their catalogues ... I'll have a look, being as how I've scanned them all (!)
Best wishes
Guy

[MrBungle]

Interesting- would love to see that. Cirkit was my regular haunt in the 1990s

[Nymrod121]

Here it is

[flyingtech55]

I used to use one of those SC110 'scopes when I was a TV 'engineer' on field service. As you say, they weren't great 'scopes but I did use it from time to time. Although they had a graticule and calibrations on the knobs they were more of an indicator 'scope then a precision measuring device. The display was also of course tiny. As ex TV field service engineers will know, time in the house was very limited but I always hated lifting sets and always tried to fix 'em in the house.

Happy memories

TimR

[MrBungle]

I certainly wasn't a fan of moving large CRTs around so I sympathise there

Originally Posted by Nymrod121

Here it is

Wonderful - thanks for that. So £139 in 2016 money is £480. I remember my father paying over £900 back then for an oscilloscope (think it was a Tek one to replace the Telequipment once he had) so the Thandar was probably a bargain for someone who wanted a functional device and wasn't too worried about specs. I can see why so many people homebrewed their own right into the 1980s with war surplus CRTs now.

[Nymrod121]

Did you see my Dropbox shared link to all of the Ambit catalogues that I'd posted earlier this year?

https://www.vintage-radio.net/forum/...66&postcount=2

Cheers
Guy

[MrBungle]

I hadn't seen that. Will take a scan through those in the morning. My eyes have given out for the night now unfortunately. Thanks for the link