Imogen test card generator.

[chriswood1900]

I have just had a problem with my Imogen test card generator. The tantalum capacitor on the input decided to go short circuit and go up in smoke fairly spectacularly burning the supply lead wire and singeing the corner of the case. At the time I was using the 625 test card to test a LCD projector and running it from a 9V DC unregulated wall wart which had been fine for some 5 years.

Symptoms were picture break up followed by sound of sizzling and smell of burning I pulled out the plug on the back and the top of the unit was very hot.

I can’t read the values of the original Tant and there is no fusing between the wall wart and the input circuit so I will repair it probably with a 25v Electrolytic and add a fuse to the input. I will get back on the final details but it may be useful to others to just to watch out for this as these are now about 5 years old.

Regards, Chris.

The original thread on these is here: https://www.vintage-radio.net/forum/...ad.php?t=77624

[buggies]

Might be an idea to check the ripple voltage from the wallwart as I have had electrolytics in them failing (and tantalums do not like ripple...)

[Argus25]

Most likely it was a 4.7 or 10uF 16V Tant and the ripple or peak voltage from your plug-pack exceeded that. Tants do this if over-voltage or reverse polarized, but it cannot have been the latter as it was running when it failed.

It looks like there are 0.1uF bypass caps in the region. You could replace it with a 10uF 35V Tant to prevent this in future, these have a higher ESR than a 16V one of the same capacity though, but with the 0.1uF bypass caps there it wouldn't matter.

Likewise a 25V electrolytic would be ok, that is if there are definitely 0.1uF bypass caps on the regulator's input and output to ground. Sometimes if there is not and the capacitors on the regulator input & output are too high ESR, the regulator, probably a 7805, will oscillate.

One helpful thing with devices that run off plug-packs, which are isolated from earth, is to put a small bridge rectifier, like a WO-4, on the power input to the PCB, that way it is also protected from reverse polarity and works with either polarity presented to it by a plug-pack (wallwart). It prevents reverse polarity accidents. An input fuse never hurts too but often they are not there in many designs, one probably would have saved the capacitor from total burn up and the mess you now have to clean up. (A single series schottky rectifier diode works too for a DC output plug-pack, but will only work with the correct polarity).

[volte-face]

Hello Chris,

Oh dear, that's a first, sorry

From the original B.O.M the capacitor details are (Farnell sourced) -

9708340 CAPACITOR, 10UF, 16V MULTICOMP - CB1C106M2BCB

It may be worthwhile replacing the 1N4001 polarity protection diode too, if it has suffered potential stress like that.

If you need a new casing, I still have some and can send you one FOC. I should also still have all the bits to make up a new input power connector assembly, should you need one of those. Just let me know.

I've always used a better quality ("Mascot" brand from RS) 9V regulated "wall wart" supply, and that is what I used to supply with the unit for people who also wanted the PSU included.

Let me know if there is anything else I can help you with.

Graham.

[chriswood1900]

Graham, thanks for the reply, it has cleaned up quite well, I replaced the Tant with a 22uf 25v I had in stock branded RS so should be OK, the diode still tested out OK so I left it in place cleaned up the case which is unmarked externally. I did add a fuse of 250ma slow blow as shown in the picture. The ripple voltage was OK on the wall wart but as always it was 9v 500ma non stabilised so it was nearer 13v with no load so maybe a 16v might be a bit close.
Argus 25 the supply is well designed with bypass caps as well as the electrolytics on both side of the regulator and a protection diode, so I think I was just a bit unlucky with the Tantalum cap.
Regards
Chris

[volte-face]

Hello Chris,

I'm glad to hear that you've got it all sorted without too much trouble.

One other point just worth noting, regarding creating the most favourable operating conditions, is that the higher the DC input voltage from the wall wart the hotter the regulator's heatsink runs and, thus, the components close by subsequently have to live in a warmer environment.

I will aim to include some input over-current protection in the new version, although I will probably lean towards something like a polyswitch over a traditional type fuse.

[jayceebee]

Originally Posted by volte-face

I will aim to include some input over-current protection in the new version.

Wow, that's great news, any idea when you will be taking orders?

John.

[Maarten]

Originally Posted by chriswood1900

so I think I was just a bit unlucky with the Tantalum cap.

Tantalum is just critical like that. Vishay/BC used to have excellent solid aluminium caps that you could use instead, but they discontinued them (there may still be some stock left).

[volte-face]

Originally Posted by jayceebee

Wow, that's great news, any idea when you will be taking orders?

Hello John,

I'm impressed by your enthusiasm Truth be told, there should have been a follow-up model years ago. There are working prototypes, but it has always been hard to gauge if there was sufficient demand to justify proceeding. Recently I have decided that I would quite like a new model myself (whether anyone else does, or not!), so I've been fiddling with the whole project again. There are two delaying factors at this point, which are :- a decision on included features vs build cost (hence price it will cost to buy) and, currently, that the user re-programming software is written for Windows PC only and I'm not sure I want to (or can) gear up to support every platform/device people use nowadays. So I cannot give you an expected date at this time, but I will be sure to let you know when there is one.

It's not really appropriate to discuss it in this thread (or forum), but if you would like to drop me a PM, or message via the website, as a research exercise I would be interested in your own "ideal" preferred feature set (405, 625 Mono, 625 PAL Colour, Static/Live Teletext, Other standards - 525 NTSC, 819 Mono, or even 405 NTSC). Every little helps!

[jayceebee]

PM sent.