Re: TM506


On Tue, Jul 21, 2020 at 12:47 AM, Roger Evans wrote:

U340 is a 7447 BCD to 7 segment decoder, £1.50 plus postage on eBay.
One of my local stores should have them.
Are you sure that U340 is the culprit?
Nope. Just a guess.

U 340 drives the individual segments of all the
7 segment displays in parallel. A fault with U340 will give the same missing
segment or wrong segment on each of the eight display positions.
There are 2 (or maybe 3) positions I could see that actually light up (not all do) with one missing segment. Every time the same one.

Your pictures show only one position on at any one time which looks like U335 is
not switching its outputs in sequence (scanning the eight display positions).
I suggested where to test the scanning circuitry in an earlier post.
Did you? There are so many replies that I must’ve missed something. My apologies.

Anyways, now that the TM itself isn’t working properly, I can’t test anything anymore until it’s fixed.

There is a _lot_ of information in the archives here about the pass
transistors, I would take time to read it before ripping everything out of
your TM506.
I’m just going to replace all of them just to be on the safe side. And one less variable to worry about (hopefully)
Don't despair, I bought a TM503 with three failed plugins and no UK/US mains
voltage selection and ended up with everything working and had a lot of fun in
the process.

Excellent. But I’m neither as skilled nor as patient as you obviously are.
I’m not an EE by any stretch. I’m a computer engineer. But I do work quite a lot on old tube guitar amps, and built a few too. Very basic and simple compared to these things. And that’s where the extent of my knowledge stops. I’m learning though...

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