More on Melford DU1-12 monitors

[ppppenguin]

The BECG has been given a number of these 12" video monitors. 15 came from a BBC site, about 30 from elsewhere, though these were also ultimately ex-BBC. I've written previously about converting them to 405. This is about making sure that they are good for service as ordinary 625 monitors in our ABC-Thames truck. It will need about 18 of them in total.

https://becg.org.uk/projects/abc-thames/

I've had the 15 that came directly from the BBC sitting at home for for over 6 months, looking at me reproachfully for neglecting them. Late last year I gave them all a cleanup and removed laods of labels. I also powered all of them for a first assessment. I finally got back to them a few days ago. Here are a few on soak test on my bench.

They were in pretty good nick. All the CRTs woke up nicely after an hour so running. On each one I replaced a capacitor (110V rail reservoir) that leads a hard life and has been known to fail. I also bridged out the internal/external sync switch which is nearly always intermittent. A squirt of Servisol in all the presets, general tweak, and most were good to go. One had a blown 2N3055 series pass transistor in the main power supply. Still worked but with bad hum and everything a bit stressed. Another had intermittent contact on a connector to the field scan coils.

The worst offender worked nicely much of time but failed the smoke test sometimes. Eventually I caught it in the act. An intermittent connection to the line scan coils. I ground away the burned bit of the PCB, which was only obvious from the underside of the board. There's a lot of power circulating in a line scan circuit, sveral hundred watts, but almost all of it is recovered by the efficiency diode. When it gets loose, such as via a bad contact, it can do real damage.

They are all now good working monitors, ready to go up to our new base at Hemswell Cliff, the building that will become the Broadcast Enginering Museum. See photo of front of main block as we bought it last year. It's looking better now but still a long way from being ready.

[Mark Hennessy]

Brings back memories of fixing them back in the mid/late '90s. Always quite loud in service, but with the covers off, they were really bad. When not synced, they were deafening - and there were many faults that led to a unsync fault condition. Many faults were caused by the blue axial Philips caps - the ones that are so very reliable in transistor radios. They had a hard life in those monitors.

Of course, some 25 years on, they're no longer deafening. We have a small number left, and use them as a reference during an exercise in visual colour matching. It's fun to watch the new recruits suffer the pain I did back in the day. If I'm feeling cruel, I'll hit the external sync button and watch them clasp their hands over their ears. They're at least 10dB quieter to me now - probably more...

I'll try to remember to look in the card index of faults when I'm back in the office. They won't be for the ones I fixed - those were in a different part of the company - but I'd guess the team here had much the same problems we did. On the whole, given how long they were powered for, they weren't bad. Did quite a few CRT changes. I remember buying a couple of new ones in about 2001, and the design hadn't changed significantly. I'm not sure when they were designed, but would guess the 1970s. I'll try to remember if we still have copies of the manuals...

[ppppenguin]

The drafting of the circuit diagram is horrible. The cap I replaced on each unit is C12. The 110V supply is rectified off the LOPT so C12 is taking high frequency ripple as well  as getting fairly warm. I used Nichicon 22u 200V parts which are good and inexpensive. Not worth looking for axials of the right rating.

[ppppenguin]

Managed to upload schematic. Had to reduce the file size so may not be very legible.

[ppppenguin]

After the success on the 15 that were directly ex BBC I decided to look at the one from the other batch that I'd condemned to "spares only". It had obviously been damp so I plugged it in with some trepidation. After replacing some physically damaged capacitors and open circuit preset pots it crawled into life. Horrible picture with lots of foldover at the left side.

A few measurements showed that everything was struggling. The LOP transistor was coming out of saturation during part of the forward stroke which made the LOP draw too much current which pushed the power regulator into current limit. This in turn reduced the voltage to the line drive transistor so the LOP wasn't driven hard enough. Very chicken and egg faultfinding. The -25V and +110V rails derived from the LOP were inevitably low, as was the CRT heater, also from the LOP.

I tried powering the line drive circuits from an external PSU which helped a little but clearly wasn't the whole answer. Then a resistor on the separate line scan board smoked. The -25V rectifier had gone short circuit. Replace those and the -25V smoother and was only back to where I was before. Swap line scan boards with a known good monitor. "Good" board works badly in "bad" monitor, but differently to its own board. "Bad" board works perfectly in "good" monitor. So there's a fault elsewhere. Possibly something dragging down one of the -25V or +110V rails from the LOP.

Getting annoyed with it now. It will be obvious when I find it....

[ppppenguin]

I am now very annoyed with it.

Out of desparation I changed the LOP transistor. No cure. Then I swapped a good set of PCBs from a working monitor into the offending one. Still got the same problems with the LOP transistor coming out of saturation during the forward stroke.

Not many other bits left. A couple of transistors on the back panel which doubles as a heatsink. The LOP transformer which I suppose could have a single shorted turn and still work after a fashion. The scan coils likewise. The CRT must be OK, as must the mains transformer, rectifier and main reservoir cap.

[ppppenguin]

Put the "bad" boards into the "good" monitor and they work just fine. Only the height control preset was a bit intermittent so I changed it.

Time to check all the bits on the chassis. I really hope it isn't the LOPT or scan coils.

[ppppenguin]

If you have eliminated everything else then it must be what remains. The LOP transformer If I can be bothered I'll swap them between monitors to prove the point but it's quite a lot of work. I think the CRT has to come out to do it.

[ppppenguin]

The CRT didn't have to come out. Put the good LOPT in the bad monitor. The bad monitor has a dead LOPT. I don't think it's worth trying to fit the bad LOPT in the good monitor.

If there's another DU1 series monitor with a fatal fault that isn't the LOPT it can be a donor. I think the BECG has at least another 20, some of which have been stored badly, so there's a chance.

[peter scott]

You could take the overwind off and try generating EHT with a doubler or tripler.

Peter