Farnell 30-4D Oscilloscope - Help with Repair - Page 4 - UK Vintage Radio Repair and Restoration Discussion Forum

DonaldStott · 17th Jul 2020, 6:50 pm

Quote:

Originally Posted by G8vsjDave

Hi, can you test TR602 with your test meter?

Thankfully the answer is YES - one of the skills I have managed to pick up made easier though by the acquisition of a MK-328 Tester!

Collector - Base: 0.791V

Emitter - Base: 0.797V

Double checked with my DMM on the Diode setting - a little higher but in the same ball park.

Not much wrong there ... ?

Remember that TR602 is a ZTX502 transistor and not a BC212?

G8vsjDave · 17th Jul 2020, 7:12 pm

As long as the transistor is PNP. ZTX502 or BC212 should read the same on this test.
As you say "Not much wrong there ... ?" I assume collector to emitter reads open. It's unusual, but I have seen correct readings via the base but short circuit Collector to emitter. Looks like the fault is elseware.

DonaldStott · 17th Jul 2020, 7:47 pm

Quote:

Originally Posted by G8vsjDave

Looks like the fault is elseware.

Agreed, that where I'm at currently.

Moving on to Test the 6N135 (OC501) later.

DonaldStott · 19th Jul 2020, 3:02 pm

Tracked down the Data Sheet for the 6N135 and with help from David G4EBT managed to rig it up for testing - I even included the recommended 0.1μF bypass capacitor between pins 5 and 8: -

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Had my DMM on the low ohms range but no sign of life whatsoever!

Should receive my Order in a couple of days for 6N135s, BC212s and some 8-pin IC sockets - always good to keep up your stock levels?

G8vsjDave · 21st Jul 2020, 9:45 am

I may be wrong, but shouldn't the 4.5 volts also be connected to pin 8 (Vcc)?
Dave

RogerEvans · 21st Jul 2020, 12:23 pm

I think the red and black arrows on the right of the diagram imply that the 4.5V also goes to pin 8 and pin 5. My analogue multimeter has the opposite polarity, on the ohms ranges the black test lead is positive wrt the red lead.

Roger

DonaldStott · 21st Jul 2020, 6:55 pm

Just to clarify the above Posts first from Roger and Dave - there is some detail missing from the diagram as there is a separate 9V DC supply going to Pins 5 and 8.

Moving on quickly (!) today I replaced the transistor TR602 (BC212) and the optocoupler OC501 (6N135), the latter of which was fitted into an 8-pin IC socket.

Good progress, I think, as this has certainly 'improved' the display - here is the Farnell 30-4D Oscilloscope displaying a 1kHz sine wave from my Topward Function Generator: -

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Switching between sine, saw-tooth and square wave also has a good display now.

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Now I don't know enough about oscilloscopes to answer the question, is this next thing a fault or a feature - this YouTube video shows what happens when the Intensity knob is adjusted first to minimum and then to maximum and back to a standard display: -

Oscilloscope Intensity

Now this didn't happen previously as the trace-back lines were there all the time so I suppose it's an improvement so long as I don't turn up the Intensity too far?

My view is that I have a 'working' oscilloscope and the Intensity issue is something I can live with.

ms660 · 21st Jul 2020, 6:58 pm

That looks normal to me.

Lawrence.

DMcMahon · 21st Jul 2020, 8:56 pm

Looks normal to me also, so not sure what your concern is with the intensity now ?

David

OldTechFan96 · 21st Jul 2020, 9:07 pm

With the brightness all the way up you should not be able to see retrace lines.

In the service manual there should be a procedure where you can set the minimum and maximum brightness levels.

EDIT: I've just had a look at the service manual and no such procedure is mentioned.

DonaldStott · 21st Jul 2020, 11:33 pm

Thanks Lawrence and David - good to know that what I have is considered 'normal' but I assumed, like OTF96, that you should not be able to see retrace lines.

I thought that was why it's called a 'beam blanking circuit'?

Not concerned, just curious.

DMcMahon · 23rd Jul 2020, 11:31 am

To my eyes your latest photos do not obviously show retrace lines ?

David

DonaldStott · 23rd Jul 2020, 12:24 pm

This has been a great repair Project for me and yet another educational journey, this time into the mysterious world of EHT voltages, CRT base/circuitry and beam blanking circuits.

Thanks to everyone who contributed and helped this Thread to shuffle along.

I'm very pleased to have my oscilloscope back up and working and it can now rejoin the ranks of my other Test Equipment ready for my next restoration!

So that seems to wrap things up for this Farnell 30-4D oscilloscope Thread ...

Chris55000 · 23rd Jul 2020, 9:48 pm

Hi!

Very well done on it going Donald!

I have checked the manual and can confirm there is no preset intensity control.fitted to the Farnell 30–4D/Scopex 4D–25 and also in my unit!

Chris Williams

DonaldStott · 23rd Jul 2020, 11:56 pm

Thanks Chris - a suitable epilogue to this Thread?

17th Jul 2020, 7:12 pm	#62
G8vsjDave Pentode Join Date: Jul 2010 Location: Nottingham, Nottinghamshire, UK. Posts: 231	Re: Farnell 30-4D Oscilloscope - Help with Repair As long as the transistor is PNP. ZTX502 or BC212 should read the same on this test. As you say "Not much wrong there ... ?" I assume collector to emitter reads open. It's unusual, but I have seen correct readings via the base but short circuit Collector to emitter. Looks like the fault is elseware.

19th Jul 2020, 3:02 pm	#64
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Farnell 30-4D Oscilloscope - Help with Repair Tracked down the Data Sheet for the 6N135 and with help from David G4EBT managed to rig it up for testing - I even included the recommended 0.1μF bypass capacitor between pins 5 and 8: - Had my DMM on the low ohms range but no sign of life whatsoever! Should receive my Order in a couple of days for 6N135s, BC212s and some 8-pin IC sockets - always good to keep up your stock levels? __________________ BVWS Member

21st Jul 2020, 9:45 am	#65
G8vsjDave Pentode Join Date: Jul 2010 Location: Nottingham, Nottinghamshire, UK. Posts: 231	Re: Farnell 30-4D Oscilloscope - Help with Repair I may be wrong, but shouldn't the 4.5 volts also be connected to pin 8 (Vcc)? Dave

21st Jul 2020, 12:23 pm	#66
RogerEvans Hexode Join Date: Oct 2014 Location: Wiltshire, UK. Posts: 381	Re: Farnell 30-4D Oscilloscope - Help with Repair I think the red and black arrows on the right of the diagram imply that the 4.5V also goes to pin 8 and pin 5. My analogue multimeter has the opposite polarity, on the ohms ranges the black test lead is positive wrt the red lead. Roger

21st Jul 2020, 6:55 pm	#67
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Farnell 30-4D Oscilloscope - Help with Repair Just to clarify the above Posts first from Roger and Dave - there is some detail missing from the diagram as there is a separate 9V DC supply going to Pins 5 and 8. Moving on quickly (!) today I replaced the transistor TR602 (BC212) and the optocoupler OC501 (6N135), the latter of which was fitted into an 8-pin IC socket. Good progress, I think, as this has certainly 'improved' the display - here is the Farnell 30-4D Oscilloscope displaying a 1kHz sine wave from my Topward Function Generator: - Switching between sine, saw-tooth and square wave also has a good display now. Now I don't know enough about oscilloscopes to answer the question, is this next thing a fault or a feature - this YouTube video shows what happens when the Intensity knob is adjusted first to minimum and then to maximum and back to a standard display: - Oscilloscope Intensity Now this didn't happen previously as the trace-back lines were there all the time so I suppose it's an improvement so long as I don't turn up the Intensity too far? My view is that I have a 'working' oscilloscope and the Intensity issue is something I can live with. __________________ BVWS Member

21st Jul 2020, 6:58 pm	#68
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Farnell 30-4D Oscilloscope - Help with Repair That looks normal to me. Lawrence.

21st Jul 2020, 8:56 pm	#69
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Farnell 30-4D Oscilloscope - Help with Repair Looks normal to me also, so not sure what your concern is with the intensity now ? David

21st Jul 2020, 9:07 pm	#70
OldTechFan96 Octode Join Date: Jul 2017 Location: Newcastle Upon Tyne, Tyne & Wear, UK. Posts: 1,178	Re: Farnell 30-4D Oscilloscope - Help with Repair With the brightness all the way up you should not be able to see retrace lines. In the service manual there should be a procedure where you can set the minimum and maximum brightness levels. EDIT: I've just had a look at the service manual and no such procedure is mentioned. Last edited by OldTechFan96; 21st Jul 2020 at 9:16 pm.

21st Jul 2020, 11:33 pm	#71
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Farnell 30-4D Oscilloscope - Help with Repair Thanks Lawrence and David - good to know that what I have is considered 'normal' but I assumed, like OTF96, that you should not be able to see retrace lines. I thought that was why it's called a 'beam blanking circuit'? Not concerned, just curious. __________________ BVWS Member

23rd Jul 2020, 11:31 am	#72
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	Re: Farnell 30-4D Oscilloscope - Help with Repair To my eyes your latest photos do not obviously show retrace lines ? David

23rd Jul 2020, 12:24 pm	#73
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Farnell 30-4D Oscilloscope - Help with Repair This has been a great repair Project for me and yet another educational journey, this time into the mysterious world of EHT voltages, CRT base/circuitry and beam blanking circuits. Thanks to everyone who contributed and helped this Thread to shuffle along. I'm very pleased to have my oscilloscope back up and working and it can now rejoin the ranks of my other Test Equipment ready for my next restoration! So that seems to wrap things up for this Farnell 30-4D oscilloscope Thread ... __________________ BVWS Member

23rd Jul 2020, 9:48 pm	#74
Chris55000 Nonode Join Date: Aug 2007 Location: Walsall Wood, Aldridge, Walsall, UK. Posts: 2,870	Re: Farnell 30-4D Oscilloscope - Help with Repair Hi! Very well done on it going Donald! I have checked the manual and can confirm there is no preset intensity control.fitted to the Farnell 30–4D/Scopex 4D–25 and also in my unit! Chris Williams __________________ It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

23rd Jul 2020, 11:56 pm	#75
DonaldStott Octode Join Date: Aug 2015 Location: Glasgow, UK. Posts: 1,842	Re: Farnell 30-4D Oscilloscope - Help with Repair Thanks Chris - a suitable epilogue to this Thread? __________________ BVWS Member