Re: 7A26 CH2 Readout


On Thu, Aug 20, 2020 at 12:29 PM, Raymond Domp Frank wrote:


Yes, as a description of the actual situation, It seems to make sense.
However, the BNC Identfy ring (which was shorted to gnd) and the Identify
button are switched in parallel, see dwg 3 (if I'm looking at the right
schematic version).
I’m sure you are.

That means, if *either* is connected to gnd, the other
doesn't matter and has no further influence, unless... the ring-short wasn't
low-impedance enough. That would explain the slight movement when pressing
I suppose it was very low imperfections. It was just, I think, what seemed to be, some kind metal part, less than a 1/10 of a mm long. I was very hard to tell, even with the magnifying glass I used.

The trace jump doesn't need and doesn't work in parallel with the readout
showing either IDENTIFY or voltage scale. That's the darlington circuit's
That’s what I figured, without understanding how it actually works.

The trace jump circuit is completely on dwg 3.

Again, I advise setting up and doing the live measurements. Being able to look
at CH2 and where necessary comparing CH1 and CH2 physically and logically next
to each other should really make this a lot easier.
I will definitely do that, or try to, next time I run into an issue. And since I bought quite a few things recently that need restoring or minor adjustments, it shouldn’t be long before I do.

Do have a look at the junction of CR621, R620, R621 and C621 in CH2 and
compare with CH1.
This is such a simple circuit that finding the fault shouldn't be that
difficult. This remark is in no way meant to criticize you; there's no reason,
looking at what you've achieved with this group recently.
Thank you. I’m doing my best, in my spare time, to educate myself and try to understand all this, little by little.
Since I had the readout board unscrewed to take those measurements, I figured I’d put back the original Q620.
And guess what. Everything is now back to normal, and working properly. I haven’t tried, but I must have mixed E and C on the equivalent I used. Could that reverse the order of things like I experienced?

Just make life a bit
easier, especially if you don't understand the circuit completely, as you
wrote earlier.
That is correct, I don’t.

Just one bit of information that you may not be aware of: The whole circuit of
Q620 and what is drawn to its right in dwg 5 is meant to set currents that the
analog readout control uses for generating its information to be displayed:
Resistors are switched in and out and Q620 is just one of the devices that
with its series resistor R622, controls a current, like the other mechanical
switches to the right of it with their series resistors like R630, R631, R647,
R648, etc. Read the interesting description of how the readout works here:
or in one of the 7000-series mainframe manuals. Many (not all) go into great
detail on this. It's a brilliant circuit, devised by a Brit called Barrie
Will definitely read it. Thank you 😊

Don't forget (in general): It's not completely unthinkable that a previous
owner modified the circuit...
Yes, nobody knows what previous owners did. But in this particular case, the original issue Was just the shorted BNC/Identifying Ring like Eric had experienced.

One last question: Are the voltage scale indications (V/div) correct?
Yes. Even when the order of operation was reversed, this was correct.

Join to automatically receive all group messages.