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Old 7th Apr 2019, 6:48 pm   #1
Avid_Nerdlinger
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Default Telequipment DM63 timebase fault

Comrades.

I recently acquired this 'scope to help fix a valve radio (Grundig2043). It seems to work OK except for timebase settings 1ms to 20us/div when there is no sweep, just vertical lines where the trace normally starts on the left of the screen.

I have the service manual from Mauritron but the cct diagrams are a bit hard to decipher around the timebase. I don't see any separate circuit for this range of timebase settings.

I've had the cover off, vacuumed up some spider webs and detritus, reseated both V4s but nothing else, the timebase PCB is a bit more tricky to unplug from the mainframe but reseating this seems like a good idea. Can't see any obvious faulty component.

Before I get too involved can anyone point to a likely cause? Googling this fault hasn't helped so far. Any alternative schematics available?

I retired from a short career in electronics 12 years ago and I'm a bit rusty. I can't say I enjoyed the job but I always thought it would make a nice hobby...
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Old 7th Apr 2019, 8:37 pm   #2
MotorBikeLes
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Default Re: Telequipment DM63 timebase fault

I have that same manual. Fig 2 shows the trigger and timebase diagram, but the timing parts are all in Fig 3. Assuming one common fault, you will have to flit your mind between the links from the fig 3 to fig 4 parts. There is a key to the timebase knob there, with numbers corresponding to the outer connections. BUT, don't overlook the inner connection plates. Find out which terminals correspond to the 1ms/div, and also the 20 microsec/div. What is common? That is likely your fault area.tuddy the inner plate connection, THAT alone may hold the answer.
Les.
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Old 7th Apr 2019, 8:52 pm   #3
mhennessy
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Default Re: Telequipment DM63 timebase fault

I'd check C154.

The timebase switch chooses between 9 values of resistance (S151 2R) and 3 values of capacitance (S152 2F). For speeds between 1s and 2ms, C155 is in play, between 1ms to 20us, C154 does the business. C153 deals with the remaining faster speeds.

The higher resistance values are only used for the slower speeds; once you've moved onto the smaller capacitors, R155 is the highest in use; the pattern repeats every 6 clicks.

So it's possible that C154 has failed short. If it was high-ESR, then the scope would sweep quickly.

FYI, the other parts of the switch deal with ext-X operation (S151 1F) and holdoff (S151 1R).

Getting the timebase module out is easy - just unplug the orange and brown wires from the side of the CRT and disconnect the black earth wire (it will work fine without it for testing purposes). Then unscrew the single screw holding it in at the back. Simple!

Mark
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Old 7th Apr 2019, 10:15 pm   #4
Avid_Nerdlinger
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Default Re: Telequipment DM63 timebase fault

Yep. C154 is short. That is some stunning remote fault-finding. Many thanks Mark and Les.

regards

Andrew
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Old 8th Apr 2019, 11:52 am   #5
mhennessy
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Default Re: Telequipment DM63 timebase fault

Excellent news - another one lives

They are lovely 'scopes - one of Telequipment's best. I still use one daily. Enjoy!

Meanwhile, C154 is a 1% part, so it'll be worth looking around for a decent replacement. The original is polystyrene, and these are still available, but pricey. I found one on the RS website, but it's about £4 each (and you have to buy a pack of 10!). That one is only 2.5%, but if you have the means to measure them, you can pick the best of the 10 that arrive. https://uk.rs-online.com/web/p/polys...itors/0113409/

Alternatively, here's a polypropylene cap that is 1%, and comes in packs of 5 rather than 10: https://uk.farnell.com/vishay/mkp183...-pp/dp/2470602 - they're only £1.23 each, so much more reasonable.

If you end up with a noticeable error between 1ms and 2ms (and 20us and 10us), then the replacement is not close enough to the original. The spec of the timebase is 3%, and 1% components in the timebase switching circuit is key to achieving that spec.

Hope that helps,

Mark
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Old 8th Apr 2019, 1:29 pm   #6
MrBungle
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Default Re: Telequipment DM63 timebase fault

CPC sell polystyrene 1% 10nF. Only 63V which should be fine in there!

https://cpc.farnell.com/lcr-componen...63v/dp/CA07064
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Old 8th Apr 2019, 8:20 pm   #7
MotorBikeLes
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Default Re: Telequipment DM63 timebase fault

I have been struggling with a planning application for many days, so my mind is not working. I managed to think out the likely area, but could not face trying to go further. Perhaps I should send the planning problem to Mark?
Les.
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Old 9th Apr 2019, 8:48 am   #8
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Default Re: Telequipment DM63 timebase fault

Yes I hit that brick wall re replacement caps. I'm reluctant to throw money at hobbies so I'm hoping to get by with a selected 10nF or some combo from an assortment of 10% CBB metal film 630V caps I already ordered. At least until I have a longer shopping list.
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Old 18th Apr 2019, 6:20 pm   #9
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Default Re: Telequipment DM63 timebase fault

Got my Chinese capacitors today and put one in my 'scope. All OK now, I have my tower of power, thanks again.
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