Telequipment D51 missing trace - UK Vintage Radio Repair and Restoration Discussion Forum

MalphasWats · 20th Apr 2018, 3:29 pm

Hi,

I recently obtained a second hand oscilloscope.

When it powers on, there are two dots on the screen, but as it warms up, one of the dots drops towards the bottom and disappears.

With some fiddling I was able to get a nice trace using input 2, but I can't make input 1 show on screen.

The up/down position knob for channel 1 has a very different feel to it than 2, it feels as though there isn't much resistance to turning it, you can definitely feel the 'pottishness' of the working one.

I wonder if anyone has any ideas about what might be wrong. With pretty much anything else I'd take a screwdriver to it and see if something obvious has fallen off inside, but I'm really wary with this as I know almost nothing about oscilloscopes, aside from the fact that they use(d) CRTs which are capable of holding on to a high voltage charge for ages after power off, so truth be told I'm ever-so-slightly frightened of its insides!

Any advice would be appreciated.

Thanks, Mike.

MotorBikeLes · 20th Apr 2018, 9:38 pm

Before you do anything, have you got a manual. You should find one online FOC easily enough. Must be getting on for 40 years since I touched a D51, I think I only ever had the one. I think there are various versions, so get the correct manual. I have a vague memory that this was a slightly modified S51, maybe different CRT and not much else.
Anyway, consult the manual and SET the CONTROLS exactly as it says. You could simply have a button set wrongly for starters, but obviously the moving trace needs attention. It may be a valve that is slow to heat up. ECF80? You can sometimes get a clue by moving valves to different sections to see if fault moves, but we need to know much more before going further.
Les.

MalphasWats · 20th Apr 2018, 10:20 pm

Hi Les,
Thanks for the reply. I did have a good look for a manual online but have only been able to find one for the S51.

The manual has many words, and I even know the meaning of most of them, but together, in those particular combinations, not so much...

This is the first oscilloscope I've had, and only the second I've used (new audio project needed debugging!)

It's current state will probably serve for what I need, it'd just be nice to get it working properly if I can, plus a great opportunity to learn more about how these things work!!

The manual didn't appear to include any obvious warnings about taking the case off... Ok, so, I took a peek (and survived).
I have a theory, but I'm probably quite wrong.

The potentiometer that controls the x position of Y1 is wobbly, a bit rusty looking and crucially, does something weird when I put a multimeter on it.

Compared to Y2, which goes steadily from min to max resistance when turned, Y1 appears to be more of a bell-curve type affair, so it's high at one end, drops low as you turn it, then starts going back up again.

Does this sound sensible at all? Do pots sometimes fail in this way? (I've never had anything long enough for it to have done this!).

Attached a picture of a grotty looking potentiometer

-Mike

MotorBikeLes · 21st Apr 2018, 8:49 pm

It looks as if there has been some corrosion around, but may not be too bad. First, ensure switched off, then you can squirt a small amount of servisol, or deoxit into the pot and operate it vigorously a few times.
It seems Ch 2 is the better of the two, so concentrate on that and ignore Ch 1 for now.
I don't have S51 and D51 manuals, so just guessing, but treat it as an S51 for now, ensuring any choices between Ch 1 and Ch 2 are set to Ch 2.
Now, go through the settings in the manual. I did see one manual online, but it only had circuits, you need a COMPLETE one. Following that, you should at least get an idea of how controls should be set, which may start it doing something useful.
I always used to go through the calibration procedure, and fix any problems associated with each stage as you meet it. If you don't have calibration gear, you can usually find some sort of source which will help you understand each section.
There may well be some components that I would check or change just by looking (with valve radios, everybody starts with "that capacitor", and there may be similar items that stare at us.
Some decent internal photos may help.
Keep searching for manuals if you don't have "the full words".
Les.

20th Apr 2018, 3:29 pm	#1
MalphasWats Retired Dormant Member Join Date: Apr 2018 Location: Kent, UK. Posts: 2	Telequipment D51 missing trace Hi, I recently obtained a second hand oscilloscope. When it powers on, there are two dots on the screen, but as it warms up, one of the dots drops towards the bottom and disappears. With some fiddling I was able to get a nice trace using input 2, but I can't make input 1 show on screen. The up/down position knob for channel 1 has a very different feel to it than 2, it feels as though there isn't much resistance to turning it, you can definitely feel the 'pottishness' of the working one. I wonder if anyone has any ideas about what might be wrong. With pretty much anything else I'd take a screwdriver to it and see if something obvious has fallen off inside, but I'm really wary with this as I know almost nothing about oscilloscopes, aside from the fact that they use(d) CRTs which are capable of holding on to a high voltage charge for ages after power off, so truth be told I'm ever-so-slightly frightened of its insides! Any advice would be appreciated. Thanks, Mike. Attached Thumbnails

20th Apr 2018, 9:38 pm	#2
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Telequipment D51 missing trace Before you do anything, have you got a manual. You should find one online FOC easily enough. Must be getting on for 40 years since I touched a D51, I think I only ever had the one. I think there are various versions, so get the correct manual. I have a vague memory that this was a slightly modified S51, maybe different CRT and not much else. Anyway, consult the manual and SET the CONTROLS exactly as it says. You could simply have a button set wrongly for starters, but obviously the moving trace needs attention. It may be a valve that is slow to heat up. ECF80? You can sometimes get a clue by moving valves to different sections to see if fault moves, but we need to know much more before going further. Les.

20th Apr 2018, 10:20 pm	#3
MalphasWats Retired Dormant Member Join Date: Apr 2018 Location: Kent, UK. Posts: 2	Re: Telequipment D51 missing trace Hi Les, Thanks for the reply. I did have a good look for a manual online but have only been able to find one for the S51. The manual has many words, and I even know the meaning of most of them, but together, in those particular combinations, not so much... This is the first oscilloscope I've had, and only the second I've used (new audio project needed debugging!) It's current state will probably serve for what I need, it'd just be nice to get it working properly if I can, plus a great opportunity to learn more about how these things work!! The manual didn't appear to include any obvious warnings about taking the case off... Ok, so, I took a peek (and survived). I have a theory, but I'm probably quite wrong. The potentiometer that controls the x position of Y1 is wobbly, a bit rusty looking and crucially, does something weird when I put a multimeter on it. Compared to Y2, which goes steadily from min to max resistance when turned, Y1 appears to be more of a bell-curve type affair, so it's high at one end, drops low as you turn it, then starts going back up again. Does this sound sensible at all? Do pots sometimes fail in this way? (I've never had anything long enough for it to have done this!). Attached a picture of a grotty looking potentiometer -Mike Attached Thumbnails

21st Apr 2018, 8:49 pm	#4
MotorBikeLes Nonode Join Date: Dec 2012 Location: Kirk Michael, Isle of Man Posts: 2,350	Re: Telequipment D51 missing trace It looks as if there has been some corrosion around, but may not be too bad. First, ensure switched off, then you can squirt a small amount of servisol, or deoxit into the pot and operate it vigorously a few times. It seems Ch 2 is the better of the two, so concentrate on that and ignore Ch 1 for now. I don't have S51 and D51 manuals, so just guessing, but treat it as an S51 for now, ensuring any choices between Ch 1 and Ch 2 are set to Ch 2. Now, go through the settings in the manual. I did see one manual online, but it only had circuits, you need a COMPLETE one. Following that, you should at least get an idea of how controls should be set, which may start it doing something useful. I always used to go through the calibration procedure, and fix any problems associated with each stage as you meet it. If you don't have calibration gear, you can usually find some sort of source which will help you understand each section. There may well be some components that I would check or change just by looking (with valve radios, everybody starts with "that capacitor", and there may be similar items that stare at us. Some decent internal photos may help. Keep searching for manuals if you don't have "the full words". Les.