UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Test Gear and Workshop Equipment

Notices

Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders.

Closed Thread
 
Thread Tools
Old 20th Sep 2020, 6:42 pm   #1
samjmann
Heptode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 645
Default Scopex 4D 10. Display fault

Hello everyone.

I been using this very basic scope for about 6 months or so. It has suddenly developed an odd fault. When a simple sine was is fed into either input, there is a 'retrace' line that appears along with the wanted signal. If you look at the attached clip you can see the fainter 'retrace' along with hum pick up on CH1.

Both V amps appear to be OK. The H timebase stills seem OK. It still triggers as it did before (not very well). It's only been fed with audio signals and has not been overloaded on the V amps.

Have a look at the clip, the actual display is less chopped, there is interaction between the phones shutter and the scope.

Any ideas what this could be? I've never fixed a scope before!

Regards, SJM.
__________________
It's never been right since we've had it...

Last edited by samjmann; 20th Sep 2020 at 6:47 pm. Reason: clip missing unable upload mp4 file.
samjmann is offline  
Old 20th Sep 2020, 6:54 pm   #2
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Scopex 4D 10. Display fault

Hi!

The 4D10 has simple half–wave rectified low–voltage power supplies without any voltage stabilizers, so yakky smoothing capacitors will all spread hum, noise and grot all over the circuits, causing all sorts of problems!

Please PM me if you get mega–stuck, there is one on offer FOC from me at the moment!

Don't forget there were three versions, the original 4D10, 4D10a and 4D10b which all have different circuits & layouts, and it's important you look for the right circuit!

I've just located a copy and attached it to this post – it's only half a meg!

By the way your clip was missing from your query!

Chris Williams
Attached Files
File Type: pdf Scopex 4D10 Oscilloscope OM.pdf (516.5 KB, 52 views)
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

Last edited by Chris55000; 20th Sep 2020 at 7:14 pm.
Chris55000 is offline  
Old 20th Sep 2020, 7:24 pm   #3
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Scopex 4D 10. Display fault

PS!

You MUST be especially careful with these oscilloscopes as the CRT e.h.t. comes directly from a 50 Hz mains transformer fed multiplier circuit that can deliver a lethally dangerous shock, so allow a minimum of 30 minutes after switching off before you investigate one of these instruments internally!

The capacitors you need to check are C605 to C610, the main board diagram will show you where they are – they're all 1000u 40V (modern 50V types are suitable).

Note the negative supply rectifier D603 has been drawn the wrong way round on the circuit diagram!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!
Chris55000 is offline  
Old 20th Sep 2020, 7:35 pm   #4
samjmann
Heptode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 645
Default Re: Scopex 4D 10. Display fault

Thanks for the offer and the Circuit diagram. This one is the original 4D10 with no suffixes. The fault has just happened today as I was using it. The only thing I noticed before was that the spot in the centre of the display after the mains had been turned off, appeared on for a long after switch-off. Would this point to a Z modulation/spot suppression fault?

I'd got the clip as a mp4 file, only 6.4mB. The upload window on the site would not accept this file type. Is there any other way of getting the file onto the site?

SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Old 20th Sep 2020, 7:57 pm   #5
OldTechFan96
Octode
 
OldTechFan96's Avatar
 
Join Date: Jul 2017
Location: Newcastle Upon Tyne, Tyne & Wear, UK.
Posts: 1,177
Default Re: Scopex 4D 10. Display fault

Retrace lines suggest a fault in the CRT blanking circuit. Possibly a bad opto-isolator IC or associated transistor.

Here is a thread from this summer where a similar problem was discussed.
OldTechFan96 is offline  
Old 21st Sep 2020, 1:50 pm   #6
Chris55000
Nonode
 
Join Date: Aug 2007
Location: Walsall Wood, Aldridge, Walsall, UK.
Posts: 2,853
Default Re: Scopex 4D 10. Display fault

Hi!

There is no opto–coupler unblanking circuit in this oscilloscope, a much simpler system using a resistor/capacitor coupling network R534/C611A directly from the collector of the sweep–gate multivibrator direct to the grid of the CRT is used to blank the retrace, this is done by switching off TR504/TR505 at the end of each sweep (TR504/TR505 switch on/off by cumulative Schmitt action), which produces a negative–going pulse across R518 that is directly coupled to the CRT grid by R534 & C611 as mentioned earlier!

It's important that you do not, under any circumstances, try and measure anything at the CRT grid pin itself, as you will damage the tube by driving it's grid positive with respect to it's cathode – the correct way to fault–find this is to switch the scope off, leave it 30 min for all the capacitors to discharge, then test all the CRT chaIn resistors, plus the focus and intensity controls to make sure all their values are correct, check the lead from the main board to the CRT grid pin of the CRT socket is securely connected, then check the blanking–pulse feed components – R534 can be checked with a conventional dmm, whilst C611A is best checked with something like a Chinese TC1 component–tester, or replace it if not sure, but make sure you use a 2000V d.c. working–voltage rated component!

If the various resistors, controls, etc., in the CRT divider chain change value, this will alter the working point of the CRT enough to prevent the negative pulse from TR505 blanking it off properly, which is the reason I asked you to check them!

If you have another working 'scope, it IS safe to check the negative–going pulse directly across R518, but you'll need to temporarily disconnect the Scopex's earth @ the mains plug whilst you test this, as R518 is returned to the –30V LT line, so if both the Scopex and the rest 'scope are connected to mains earth, the Signal lead will short–circuit the –30V line unless you isolate the Scopex from earth first!

If you don't have another 'test scope to look at the blanking pulse, you'll need to remove and test TR504, TR505 and the associated components and diodes separately, but before starting any of this, check the power supply lines first as per my previous post!

Chris Williams
__________________
It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

Last edited by Chris55000; 21st Sep 2020 at 1:56 pm.
Chris55000 is offline  
Old 21st Sep 2020, 8:53 pm   #7
samjmann
Heptode
 
samjmann's Avatar
 
Join Date: Feb 2014
Location: Nottingham, UK.
Posts: 645
Default Re: Scopex 4D 10. Display fault

Chris, thank you so much for your most comprehensive reply. Not only have I learnt a great deal about 'scope fault finding, there's a good chance that you saved the 'scopes CRT from an early grave by stopping me 'dobbing-round' with a meter and damaging the tube!

I'm afraid it will be the weekend before time allows me to have a look at the scope again, but I'll report back with any findings.

I wasn't aware that the cathode/heaters/grids on 'scopes were biased in such a way. Apart from anything else there's a chance I would of got a nasty shock from meter probes etc...

Regards, SJM.
__________________
It's never been right since we've had it...
samjmann is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:53 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.