Another A823 Conundrum

[Mikey405]

Hi everyone.

On trying to fix yet another A823 chassis set, I came across a bit of a puzzle. The set in question (a Bush CTV1122 using the A823AV chassis) had low brightness. The A1s were all fine, the beam limiter circuit seemed fine, and the clamp pulse from the scan panel seemed fine too. The only voltages which were a bit high were the outputs from the decoder board, and having the luxury of another set I swapped the panel over to prove that the problem was indeed the decoder.

As the panel is very difficult to work on unless you have extension leads I performed quite a lot of cold checks on the panel, and in fact replaced anything at all which was outside tolerance. I replaced over a dozen components and nothing seemed to be having any effect at all.

As I was blankly staring at the panels in the vain hope of divine inspiration I noticed that the 3 resistors which were connected to the clamp pulse were 6.8K on the working panel and 4.7K on the not-working panel. I decided to try some new 6.8K resistors in the not-working panel and - hey presto - it worked and I got a lovely bright picture back. Now, the two panels are both labelled Z582 (both being the later 2-chip decoder with the chroma sub-panel) but why did one have 4.7K resistors in and the other have 6.8K in?

There was a version of A823AV called the A823B (the BS415 compliant version) and the resistor values listed for that are 4.7K so presumably an A823B panel had been fitted at some time, however why was the panel number exactly the same between the two sets, and was there some other modification to the A823B chassis to compensate for the different value resistors in the decoder? The resistors in both panels looked as though they were originals in both cases.

Or perhaps I'm missing something and there is a completely different reason for this change in value. Can anyone shed any light on this?

Thanks everyone.

From Mike.

[Mikey405]

Sorry, the resistor numbers on the service manual schematic are 3R65, 3R66, and 3R67. I've just found an A823B diagram and it says the decoder panel number for the A823B is Z971, although mine is labelled Z584 (Not Z582 as I said above). Perhaps the wrong values were inserted from new(?) Also my component list for the Z971 lists the resistor values as 4.7K but the schematic shows them as 6.8K... I'm very confused.

From Mike.

[Tazman1966]

Originally Posted by Mikey405

Perhaps the wrong values were inserted from new(?) Also my component list for the Z971 lists the resistor values as 4.7K but the schematic shows them as 6.8K... I'm very confused.

From Mike.

Hi Mike.

Well this won't be the first time that there are differences like this. In the Decca 30 manual the PCB layout shows the connection to the LOPT from the width circuit as TP403 whereas the diagram states TP402. It SHOULD be TP402 as well

I reckon that maybe it was like it from new and it was only really when the CRT started to get a bit tired that the symptom became a pain. By the way, is this the 20" set with the intermittent green?

All the best,

[Mikey405]

Hi Tas.

Well, the tube looks almost like new in the set - I can't imagine it ever working correctly with those values in. The picture was unwatchably dark even with the A1s right up to full. Perhaps someone has swapped the panel over from another set, but off hand I don't know of any mods that would change those values. I put the panel in my big acoustic deluxe, and the picture was then unwatchable on that too. All very strange.

The set is yet another A823AV Tas, a little 22" model.

Thanks Tas.

From Mike.

[Tazman1966]

Ah well. I look forward to seeing some pictures of the set in its glory...or in the flesh when I'm next in your neck of the woods.

Cheers,