Vintage Computing & Consoles BBC Micro; Teletext Adapter, Winter Project

Posted by: @crustytv

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This is also an issue 10, let's see if this one works

The answer to that is a resounding yes it does!

That's fabulous, Chris! Just got back from setting up the vintage trains layout at Spalding so this was good to read over a cuppa.

Excellent!

Further Adventures into the realm of the BBC Micro

Since getting all the BBC micro equipment, naturally it would be nice to have the period correct monitor rather than using the Roadstar TV I have been using, or connecting up an LCD, even if the latter would save on space.

Trouble being the metal cased Microvitec monitors seem to command a premium price, (£299) very few sub £100. I did spot one in the week, but it was the HL series in a plastic cabinet, it was working and was priced at £75 but no delivery. I considered this, so sought a shiply quote. It must have been crazy week on there, as I struggled to get a quote under £100, so decided against it.

Then a couple of days later, a 1431MS turned up, metal cabinet, listed as power indicator on but monitor appears dead. At BIN £55, including shipping, I could not resist the challenge and potential bargain. I contacted the vendor and expressed my interest and concern about damage in transit. I was assured it would be packed accordingly, and they had shipped many such items before.

Anyway, 20-mins ago it turned up, safe and sound. Now to get to work and see if I've found a £50 bargain, or a heap of trouble. Can I get it working, time to find out, at the very least it should be fun and a bit of forum entertainment.

So when described as power indicator light on, but monitor totally dead, I wanted to see how dead. As power had already been applied I first checked the three fuses, these were OK and then plugged in myself. Well, it's not totally dead, I have heaters so LT, I also have the 124V rail and I have 24kV EHT, but yes, the screen in blank.

We used to have those monitors at school in the early 90’s! I’m sure there were a couple still hanging around into the late 90’s too. I remember the one at my first school, it had a computer that used cardboard floppy disks that you shoved in the machine and closed a lever over it, that would have been around 1991-92, the school only had the one computer back then!

Tested R235 10R fusible, getting really spurious readings across it. Disconnect TL201, rechecked R235, now reading across it is infinite resistance so yep o/c. Hooked another 10R fusible across the old, powered up, and it lit up cherry red, something not good going on here.  C224 is apparently notorious. Going to need to remove the board now, as you cannot get at anything without doing so.

Edit: Another observation, I hear the EHT rustle but no degauss thump, so is that needs looking at too.

It's spotless and looks to be mostly original, the only evidence I can find of past work is TR2, the chopper transistor and the big can C11, that has a date label of 9.7.89. The can itself is 1984. Possibly R228/R230.

Now for some cold checks to see if I can narrow down why R235 would be burning up. I'll leave degauss for now.

Posted by: @crustytv

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Now for some cold checks to see if I can narrow down why R235 would be burning up

That'll be because there is a short to ground somewhere forward of TL201, and it's not C224. Hmmm maybe this would be an opportune time to have a play with the Toneohm 500A.

Edit: scrub that muppet boy picked the wrong pad, not helped by no screen print on trace side. So, hunt for R235 killer continues.

@crustytv Looking at the circuit, the only thing that can realistically shift that kind of current is the field output chip, IC301. Lift one end of D302 to isolate it and see what happens then and as you've already mentioned, consider C224.

Posted by: @cathovisor

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Lift one end of D302 to isolate it and see what happens then

That cured R235 going full Chernobyl, which is good as my stock of 10R fusibles are low. 👍 

This fella was relieved too after seeing his brother go up in flames. All the rails are OK, clearly I have a fault in or around IC301, a brutal one at that, to R235.

Hi All, My Cub out of the attic, checked a few electrolytics and then powered up. Came on o.k. nice sharp picture. Run for 10 mins and I noticed the E-W correction was starting to fail. Within 2 mins the field had centre compression. Looks like the bootstrap capacitor and field output coupling capacitor were failing before my eyes. Funny I checked these as these would be the first suspects on an older monitor. They obviously didn't like waking up very much. Re-Caped and then all was well as can be seen.

Chance would be a fine thing to have it so easy! 🙄 😉

I'm on the last leg of this long slog to get my BBC micro set up sorted. It seems nothing is easy for me, what with the teletext adapter having a logic fault, at least I've got the replacement that works, so at my leisure I can use that for comparison to fix the other. I tried fitting the Watford Electronics ROM/RAM extension board, it seemed to work for a while, but it did cause two problems. The first was the keyboard would not sit flush due to the WE extension, Then it caused the keyboard to randomly work, not work. For the time being it has been removed so I'm back to basic 5 ROM slots.

Now this monitor is leading me a merry dance. I've checked components around theTDA1170P ic but can't find the fault yet as everything seems OK, unless it is the TDA1170!

Another confusing thing is this monitor, the case states 1431MS, but the screen looks like the High Res dark phosphor screen version, not the grey phosphor. I have the white main board with black screen print, not blue. However, the CRT PCB is blue. I don't have a diode split lopt, but normal and tripler. Also, the main board does not have C225 or the big choke at the back left. I also found a handwritten label which has "121 1451 on it.

Basically I've no idea what version it is, or what cct is correct, it makes me think it's a bitsa set.

Posted by: @crustytv

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I've checked components around theTDA1170P ic but can't find the fault yet as everything seems OK, unless it is the TDA1170!

I think that's exactly what it is - the TDA1170 itself.

Before changing the chip, it would be a good idea to make sure that C304, D302, C311 and R323 are OK. 

That diode capacitor combination C304, D302 will be found on most TDA1x70 and TDA817x field ICs. The capacitor I would always change when replacing the IC faulty or not as it can destroy the IC at switch on or lead to a very short life.

@Crustytv my Microvitec Cub is also a medium resolution and I’m certain the CRT is a Mitsubishi, not sure of the type though.

@jayceebee Pretty sure they're the bootstrap components for the output stage of the TDA1170. Only fault we ever had on my parents' Salora set was that cap failing.

Frame output IC's where a common failure on these packages. The latter  Tab mounted frame output ICs where a lot more robust but suffered dry joints. I think this was caused by the IC getting warm, legs expanding and contracting with use over time there being a circular crack appearing in the soldered joint then turning into a full dry joint. These smaller frame IC's where working hard as as already said the bootstrap capacitor and output coupling capacitor going low would lead to failure. There is a 10uF in there two. Practice for me would be change the IC and three associated capacitors and check the supply decoupling electrolytic usually coming off the line output stage as in this case. This will be the TDA1170S.

The tube on mine is a Mitsubishi 370GUB22-TC01. These monitors were made over a longish time period for a monitor so changes where introduced but I've not had a problem Most of these I have serviced over time had the transformer/tripler combination, yours looks to be the same as mine.  

Forgot to mention, you will be looking for a BBC Master next. The ROM sockets can be paged on some sockets. The Masters ROM in 1Mbyte so contains many 128k ROMS in the single ROM, have I got my bits and bytes correct there? The Master has sideways RAM to. The plug in ROM sockets can be handy to.

I'm working my way through this board, and I'm now at the TDA1170, has anyone figured out, I'm sure you have, how the hell you desolder the heat-sink from the chip wings. I've got a Metcal, so heat should not be a problem, but it's like the proverbial .... to a blanket!