Farnell 30-4D Oscilloscope - Help with Repair

[OldTechFan96]

It looks like you are seeing a 'retrace line', which suggests a fault with the CRT blanking circuit.

When the electron beam sweeps from right to left the oscilloscope's blanking circuit activates to 'blank' the beam thus removing the retrace line.

Because of the potentially high voltages around the CRT blanking circuit I'd advise against making measurements here with your DMM.

The culprit could be the opto-coupler marked OC501. Is it in an IC socket? It might just need a clean.

Was your 'scope displaying waveforms OK before it stopped working?

[DonaldStott]

Thanks for that OTF96 - I assumed, possibly incorrectly, that retrace lines were always straight whereas the additional signal trace I have displays as a wave form albeit at a much lower frequency that the main signal trace?

I'll try to locate the opto-coupler marked OC501 (I have the manual) and give it a good clean along with its IC socket

The 'scope was indeed displaying waveforms OK before it stopped working?

[DonaldStott]

I've located OC501 - it doesn't sit in an IC socket but is directly connected to the board!



Any way I can test it in situ noting that I can't use my DMM here?

[OldTechFan96]

When I think of oscilloscope retrace lines I think of the attached photograph. The odd artefacts in your photographs look to be dotted, as if the blanking circuit is being switched on and off, when it should be on during the retrace.

Do you have the same issue with channel 2 on your 'scope?

[DonaldStott]

Agreed, that's what I thought a retrace line should look like i.e. a fly-back line from the end of the trace to the beginning?

My lines have a wave-like structure?

The issue is apparent on both channels.

[ms660]

What does the re-trace look like with the timebase period cranked up to show just one or two cycles of the Y input?

Lawrence.

[RogerEvans]

I don't have the manual for this scope but I have seen similar blanking circuits which use a combination of an opto coupler and a high voltage capacitor to supply the blanking / unblanking signal. The opto-coupler is not fast enough to drive the blanking pulse for the first microsecond or so, the capacitor supplies the 'fast' part of the blanking but doesn't provide the low frequency part which comes via the opto-coupler. Your pictures look like the capacitive coupling is blanking the beam for the first 10% - 20% of the retrace but the opto coupler or Tr602 is not supplying the lower frequencies.

Regards,

Roger

[G8vsjDave]

What you are seeing is typical of a scope with a faulty blanking circuit. The dotted lines are due to the high speed switching of the dual 'Y' channels. If you switch off the unused channel you should just see a fly-back line, not a series of dashes. If you can reduce the X gain so you can see the start and end of the trace, or move the trace left and right, the way fly-back works will become clear.

Dave

[ajgriff]

Looks more like a 'chopped' low frequency harmonic rather than a re-trace to my eyes.

Alan

[DonaldStott]

Wow, thanks guys - a lot of really technical stuff there for me to read and digest.

Starting with Lawrence's suggestion here is one cycle of the 996Hz output from my Heathkit signal generator - 2V/DIV and 0.2mS TIME/DIV. This is with only one channel active 'A'.

Now that looks like a re-trace line/fly-back line to me?



Hope this helps?

[ms660]

That's the point, the re-trace time is fast compared to the trace time and during re-trace the Y plates are still doing their stuff, that's to say still responding to the Y input signal.

Lawrence.

[WME_bill]

Scopex 4D-25 or Farnell 30-D4.
Your picture shows the classic failure of the beam blanking circuit, as it shows the trace flyback. A common fault, as parts of the circuit operate at high voltages which transistors tends to fail at. Be warned, the part around the opto-coupler and TR602 sit very uncomfortably at full EHT voltage, 1.6kV.
The light source in opto-couplers tend to fail, so try replacing that. Lots of options, they are standard commodity parts, or remove from an old computer power supply or even a TV power unit.
There will be square wave pulse from TF506 as the timebase is switched on to create the scan. The front of the pulse via C618, a 2kv capacitor, and the slower rise via the opto-coupler.
Now measuring voltages from the CRT grid to earth with your Avo or even a DVM (10M) is the quickest way of ruining the CRT as the current taken will pull the grid very positive.
Either put your meter floating acrossTR602 collector to emitter when switched off, then switch on. Expect about a 30V to 40V positive pulse when the TB starts to scan. Avo or DVM ok here, but be aware it is sitting at EHT voltage.
When the TB is on, but no trigger /no input, the beam will be off. When switched to EXTN X, the beam will be on. That will show your 40V change.

Otherwise, make up a EHT probe. I and others have posted on this before, try a search under Telequipment D75 or D83.
In simple terms, string say 5 of 10Mohm resistors in a plastic tube with a probe point cemented in the end and with a 0.5M at the bottom end. Earth the bottom of the 0.5M, and put your meter across the 0.5M. You now have a 100:1 uncalibrated probe, ideal for high voltages and CRT cathodes. Calibrate it by measuring the X amplifier feed of 115V. Note the ratio and keep your calculator handy, or adjust the bottom resistor to give a more convenient ratio.
In theory, the probe resistors will slowly change value with age and under voltage stress, so recalibate every time you start using it.
wme_bill

[G8vsjDave]

FWIW, my money is on TR602 being duff.
Dave

[DonaldStott]

Thanks to everyone for another educational roller coaster!

A particular thanks to WME_bill for the detailed exposition of how the beam blanking circuit works.

To keep me safe and far away from any EHT voltages I think I'll just stick at this stage to replacing OC501 (6N135) and TR602 (BC212). Are there any other components I should measure or replace while I've got the covers off, the mains plug disconnected from the wall and all those big caps discharged!!!

Constructing an EHT Probe sounds challenging and it's a project that I can look at later.

[DonaldStott]

A couple of quick follow up questions:-

1. Is it worth fitting an IC socket for the 6N135 - it looks as if this has been replaced before?

2. What are suitable alternatives to the BC212 - 0.3A, 50V, PNP ?

Thanks 😀

[OldTechFan96]

I like fitting IC sockets. It make fitting replacements easier.

Both the BC212 and 6N35 can be found at Cricklewood Electronics for little money.

[DonaldStott]

Thanks OTF96 - I already have an Order in the Basket going off to Farnell with Free p&p and I have added the 6N135 but they don't have any BC212, hence my question?

I'll add some 8-pin IC sockets to this Order.

[DonaldStott]

And just to make things more interesting (!) for me, I've removed TR602 from the board and it's a ZTX502 and not a BC212 - here are the comparisons:-

BC212 50V 200mA 200MHz

ZTX502 35V 500mA 150MHz

So in the context of the EHT voltages associated with the beam blanking circuit, which one is more suitable?

[G8vsjDave]

Hi, can you test TR602 with your test meter?
When I test a PNP transistor with my meter set to the resistance diode setting, I place the black (or common) probe from my meter on the transistor base, and the red probe on the collector. My meter reads about .7 Change the red probe to the emitter and again, a reading of about .7. Reverse the probes so that the red probe is on the base and I get no reading, or open circuit. These results will depend somewhat on your meter. The reading of .7 is the volt drop across the transistor junction. If your meter doesn't have a 'diode' setting, use a low resistance range and just look for conduction or non-conduction. A reading of zero ohms anywhere, strongly suggests a duff transistor.
Hope this helps.
Dave

[RogerEvans]

Collector base junction open circuit is another common failure mode.

Roger