CRT heater 6.3V – Topward 7021

[MelJon66]

Greetings All,

My first scope many years ago was a Topward 7021. Not the greatest of scopes but it was all I needed or could justify for occasional audio repairs. It was sold as “it was my Dad’s and he always kept his stuff in good working order but I am unable to test”. I am sure he was absolutely genuine but it didn’t work and I could get no trace at all.

After some diagnostic work I was puzzled by the fact that sometimes I could measure -2000V at the CRT cathode and on other occasions I couldn’t. I eventually worked out that the -2000V disappeared as soon as I connected the heater leads from the transformer. The transformer secondary was producing the required 6.3V but it was breaking down when connected to the cathode supply and was taking the -2000V to ground.

I bought a small heater transformer with 3kV isolation (Camdenboss brand) which when fitted enabled me to get a bright trace, so the CRT was good, but even a flat DC trace always had a pulsating wave running through it. It was usable but not good enough. I assumed this was interference because the additional transformer was mounted too close to the CRT. There was nowhere else I could mount the extra transformer inside the case so I called it a day and the Topward sat on a shelf in the garage for years. I bought a Hitachi.

Fast forward many years and I got into PM contact with Chris Williams (Chris55000) on here. Chris said “have you ever tried one of those isolated DC to DC converter modules”. I eventually purchased a 240V AC to 6V DC switched mode converter with 3.75kVAC isolation off eBay. There are very many similar units available but very few actually specify the level of isolation and when they do it is sometimes not enough. I have attached the spec sheet for the one I purchased – Inhaos AP008 Series – Part Number ZA005L‐VU0600080‐NO purchased from eBay seller “best-ego”. It’s very small unit in size at only 45mm long so it will fit in almost anywhere. It cost a shocking £2.99 including shipping from China - how can this possibly be done! The seller doesn’t have any listed at present but he does have others from the same series listed so he can probably supply if required.

Anyway, I got around to testing and fitting the module only this week and it does the job perfectly. Standing alone it produces 6.28V and when in circuit it produces 6.03V which is plenty for the Toshiba 150BTB31 CRT in the Topward. No dropper resistors required! The trace is bright even with the intensity set pretty low. The scope now needs all the switches cleaning of course after standing for so long but so what – it works!

I thought this was worth sharing as it might help others who have similar problems with a Topward or potentially any other CRT scope and haven’t yet found an economical solution.

Many thanks to Chris Williams and good luck with the pending op. Chris.

Cheers
Mel

[G.Castle]

Thanks for posting the information Mel.

A young friend has two of these oscilloscopes, and both have suffered failure if the transformer heater winding insulation, one she has brought back to life with a home made isolation transformer that uses the supply provided by the original winding, the other is proving more difficult as the transformer is like your original repair and interacting with the tube. As it is in my care ATM I will suggest your method and get it sorted for her.

Cheers,
Greg.

[turretslug]

Thanks for the heads-up, and worth knowing that these modules are both easily available and cheap. The highly elevated CRT heater winding does seem to be an Achilles' heel with 'scopes and is often part of the mains transformer even with 'scopes that have a separate HF EHT generator. I have a hunch that a fair group of these Far Eastern "utility" 'scopes with a variety of brand names (including component suppliers such as RS, Farnell etc.) all came from a small number (possibly one!) of Taiwanese and/or South Korean sources with more-or-less common circuitry and a bit of front panel control re-arrangement/change of colour schemes for different middle-men. That suggests a common basic PSU arrangement and breakdown susceptibility. Fortunately, the Japanese 8 x 10cm CRTs used seem to have 6.3V 95/100mA heaters, so the power requirement is low. It's certainly something I'll be looking into for the Farnell DTV20 here as a pre-emptive strike (passed onto me by a forum member and a good and useful instrument, one of the more comprehensive basic 'scopes, though I realise that's a bit of a contradictory statement!),

Colin

[MelJon66]

It's great that people are finding this information useful so soon.

I believe Topward 'scopes (I must remember the apostrophe) were popular in schools and colleges so if failed windings is a common problem there may be plenty of them out there in need of rescue. They will never be very desirable but a useful tool nonetheless.

Cheers
Mel

[cmjones01]

Even the mighty Tektronix isn't immune from this problem. My 535A scope (one of the big ones, 60+ valves and all that) suffered breakdown of the CRT heater winding on the mains transformer soon after I got it in the late 1980s. I wedged another heater transformer under the chassis, fixed in with a bit of Meccano. It's still working more than 30 years later - I'll be using it this morning.

Chris