Farnell DT1215 Oscilloscope.

[Vintage Engr]

I have one of these dinky little 'scopes and because of its small size it sits on a shelf above my bench. It's only 12MHz bandwidth but still handy for most relatively LF measurements. I've had it for over 20 years and it's been very reliable. I have other 'scopes, with better performance, but they won't fit on the shelf!
Suddenly today, the timebase died. I downloaded a manual from the Farnell archives, & expected I would fix it in an hour or so.
But that was before I discovered that the Trigger & timebase PCB was entirely different from the manual! None of the TR or IC numbers tally with the manual. The rest of the circuits are correct.

This TB PCB is marked 'Ident issue D' All the components are designated 600 series but bear no relation to the original manual.

Can anyone help please?

Thanks,

Dave.

[Vintage Engr]

Hello again,
Have now discovered why the circuit shown in the manual is different, with many more IC's than mine.
The manual from the Farnell archive covers from serial no.1401 onwards.
Mine is s/n 856. Have traced the trigger pulses o.k, but how they get to the sweep generator is unclear.
So now, what I'm looking for is a copy of the trigger & timebase circuits of this early version.
Hopefully someone has one?
Strangely I saw a pile of these 'scopes for sale at a rally about five years ago at £12 each. Hindsight's wonderful thing!

[Radio Wrangler]

Originally Posted by Vintage Engr

Strangely I saw a pile of these 'scopes for sale at a rally about five years ago

About 1,399 of them?

Sorry, couldn't resist. It sounds like something forced them into a redesign.

David

[Vintage Engr]

Thanks for that,

I was thinking just the same. The later version has another 4 IC's!

That said, it worked perfectly for the last 20 years or so, & I'm loathed to bin it, it's in as new condition.
Have just got out another 'scope, but it won't fit on the shelf, it's too deep front to back.
I'll keep looking.

Dave.

[Radio Wrangler]

Good timebases and especially good trigger circuits are not easy to design and there are quite a lot of variants, so I'm afraid you do need to find the right map. Tracing one out would be a large undertaking. You could fry asking Farnell... you never know and being helpful is good PR. I take it you've tried BAMA and Mauritron?

David

[Vintage Engr]

Thanks for the suggestions,

Unfortunuately, the current Farnell (Element 14) is now unconnected with its family-owned predecessor, A.C. Farnell & Farnell instruments. Interestingly, most of their test equipment looks as if it was made with parts from an early Farnell catalogue! (Well, it was).

I've been plagued with 'scopes with trigger/timebase faults. I also have a Philips 100Mhz 4-channel which at least has working timebases, but no trigger. It never worked from new, so how it got out of the factory, one will never know!
I will keep hoping someone may actually have one of the early DT12-5 'scopes that came with a manual.

Dave.

[Vintage Engr]

The plot thickens!

Have obtained via some kind folks a copy of the manual for this model, DT12-5; that purports to be applicable from s/n 0 -.
However it's still not the version that covers my unit! I'm beginning to think that there were a small run of completely different internals, but with the same model number on the front panel. Even the chassis type number is correct.

There must be an answer, if only I could locate the correct documentation. I've likened it to travelling on the Paris Metro, with only a map of the London Underground.

I have traced the trigger pulses to the clock input of a 74LS74, but there it ends. The logic is totally different thereafter; whereas the Q bar output should become a 1 in the presence of a trig input, it doesn't because the D input, instead of being held low ( (literally connected to ground on the cct diagram) isn't, it goes off to some transistors that don't exist on any of the ccts, and is held high; hence no trigger to the sweep circuit!

Probably somewhere out there is a retired Farnell Instruments engineer who has the correct circuit- where are you!?

There, I've vented my frustration!
Seriously, if anyone has an early version of this 'scope, with/without a service manual, there might be some hope, I'll not bin it yet.

Dave.

[mhennessy]

They had these back at school - happy days!

I have photocopies of the schematics, but sadly my diagram is the same one that is available on-line - serial number 1401 onwards.

I remembering studying the timebase diagram when designing a timebase control circuit to replace a failed custom Tek IC in my Telequipment DM63. And like Farnell, I used a 7474 clocked by the trigger circuit after a comparator. There the similarity ends.

Over the years I've got fairly reasonable at reverse-engineering schematics from PCBs, and I suspect that will be necessary here, I'm afraid. But you don't have to trace out everything, and there will be plenty in common with the diagram you have, I'm sure. And what you come up with assist others in future, if that's any incentive...

I suppose an obvious thing to try here: what is actually causing the D-type to not change. Is it the lack of a logic 0 at the D input, or is either /PRE or /CLR being held low by mistake?

On this diagram, the /CLR input is used to lock out the timebase in X-Y mode. Check that it changes from 0 to 5V when switching between X-Y and normal.

The /PRE input is used to disable the triggering during the flyback and holdoff period. Check that is going up and down in sympathy with the sweep, if the sweep sweeps at all...

This gives us one of 3 possible directions to go, so with luck, we'll be able to concentrate on just a small area of the 'scope.

It's good that you have other 'scopes - makes fixing 'scopes so much easier! Oh, and you've tried basics like switchcleaner and percussive maintenance?

[Vintage Engr]

Thanks for the encouragement.

It's crazy really, I have 6 'scopes, all of them different & all much higher - spec than the Farnell.
But.. it performs a very useful function sat above my bench, either being permanently connected to my audio analyzer, or a a display from my wobbulator. For those purposes its more than adequate, and, the only reason I'm determined to fix it is that it is so compact, only 300mm front-to back, & 1/2 rack width. I bought it around 20- 25 years ago from a customer, & it was & is in mint condition. The only thing that could replaced it is a modern slim digital thing, but I don't want to go that route.
I've just found a wire trapped between the crt mount & the front panel, & this went to the TB selector... So having stripped down the front panel & released it, I'm no further forward.
I agree I may have to 'reverse-engineer' it, not easy on this one as the TB board is permanently soldered into the bottom main board, & getting a viewable decent access to the print side of the panel is very difficult.

Its most annoying, it was working perfectly last week, using it to set up the bias/crossover distortion on a large PA amplifier, just got the thing perfect, & then the TB on the scope stopped & I had to turn down the brightness quickly! No smoke, no bang, no obvious catastrophic failure.

Re the circuit, well, I now have one that indicates its' from serial No 0, but it is almost the same as the later ones. All the diagrams have 9 IC's; mine only has 6, but has more discrete transistors. Even the cal generator is different, on all the circuits, it uses a buffer from IC 9, which doesn't exist in mine, neither do IC 's 7 & 8!
I'm wondering whether Farnell used a PCB from an earlier model for the first 856!

I don't give up easily, having spent over 50 years in both the domestic & broadcast TV/ Radio/Audio industry, but sometimes one wonders why we spend so much time on this old stuff!
I'll keep you posted.

Dave.