Not to get in the way of this, but you could also just check the individual voltage drop between the Drain and Source leads to their origin and find out if the connector is actually causing any drop. If it where me, and there is no evidence of problems with that connector I would keep it until at least the scope if back to normal operation, just in case.

On Mon, Aug 10, 2020 at 11:58 AM, Tom Gardner wrote:

I went and picked up a CRT, and the bloke let me have a decent deal on a 485
that took a long time to startup first thing in the morning. Locating the
dubious startup electrolytic fixed the problem, but lead to the problem of
buying more duff scopes :(

Tom,

Laughing here at reading this. Almost exactly how I got into the same predicament here. Some people say you smoke crack one time and you are hooked and people like us fix one scope and we are hooked, we cannot stop buying old blue boxes with a whole bunch of little knobs and switches on the front.
--
Michael Lynch
Dardanelle, AR

After some long posts spanning over 2 threads, trying to figure out why the DC503, then the DC505A did not display anything else than just a randomly placed single “0” at any one time while plugged inside the mainframe, I finally founded out, without understanding why, that Option 12 messed everything up (see topic here, on the extender:

https://groups.io/g/TekScopes/topic/tm500_plug_in_extention/76102597?p=,,,20,0,0,0::recentpostdate%2Fsticky,,,20,2,0,76102597

Also, the DC505A that was working when plugged into the extender did display all the digits, but was not counting at all, is now working perfectly inside the mainframe, after I disconnected option 12.
It turns out that even if no pins are need to power the modules, you have to connect them all in order for some plugins to properly work. I found that out the long way.

I’m sorry if it took 2 threads to finally figure this out.

On Mon, Aug 10, 2020 at 12:01 PM, <toby@telegraphics.com.au> wrote:

Aaarg, sorry, ignore my message about Sphere. It's sold.

There is one on ebay UK though:
https://www.ebay.ca/itm/154-0677-10-TEKTRONIX-CRT/264704023173

--Toby

Toby,

Yes, I found that one. Too rich for my blood. I will just lurk around until I find a parts scope. One of these will turn up sooner or later.

--
Michael Lynch
Dardanelle, AR

Hi satbeginner
Not yet
I will do it tomorrow and check if that improves the situation.
Thanks
Saroj

I tested all the transistors in the Z-axis amplifier and all were OK. The amplifier puts out a square wave which varies from 0 to 35 V with the intensity control, but the beam was still too bright at all intensity settings. I started to suspect the voltage multiplier circuit (C1372, CR1377, CR1379) as the input was at least 40 V p-p. The components tested OK with a multimeter, but I replaced the diodes in case they were breaking down at circuit voltages. No change, but all controls operate as described in the manual with the scope probe on the cold side of C1372. The circuit diagram in my copy of the manual does not correspond exactly to the circuit description as it shows a separate winding on the HT transformer for grid bias, which I cannot locate (transformer on other side of the board). However, a 40-50kHz signal is coming through the 390 kΩ resistor (R1326) from somewhere. I will just live with a bright trace for the moment and concentrate on the Y output amplifier, which looks as thought it can be removed and tested on the bench.
Simon

https://www.ebay.co.uk/itm/254685390375

Is this produced by one our members?

David

I have been successfully using an HP 4815A for in-circuit ESR testing for more than 20 years.

OK so here is the good news update, a massive thank you to Andy Warner who sent me a new pair of ROMS, both Roms had gone, but so had U2092, I managed to get a MC68B02 for only a few UK£'s to test, and it brought it back to life (shame about the SC67127P MIL Spec Processor) but it is alive again. With the limited test kit I have calibration seems quite good, the only oddity is that channel 1 voltage measurement is about 3mv out when I check an accurate 10V reference, ( Channel 2 using the same probe and settings is spot on) Frequency on both channels is as good as I will need. I will post a working photo, and again thank you Andy.

Hi Saroj,

With respect to the voltage drop after a while, did you already removed the Molex connector from the FET?

Over the years, the FET, so also the Molex, worked at temperatures around 70 Celsius, so maybe the spring loaded contacts of the Molex are not so spring loaded anymore.... ;-)

At 1.8A about one ohm is enough to drop 2V.

Curious as always,

Leo

Ok.. Since the only difference between the extender and the connector inside the mainframe is “Option 12” connected to pins 27A’s and 28A’s, I removed all the 6 coax from the back of the connectors to the 6 BNC’s at the back of the unit.

Everything behaves as expected... Baffling to me...

Now I can turn my attention to the DC505A which is counting gibberish if at all, and to the DC503 that, all of a sudden, just stopped counting altogether. Then I’ll turn my attention to the SC502 that came with the TM506.

Thinking out loud: I guess this is what you have to go through when you buy a lot of things at the same time....
A lot of troubleshooting!!
A big thank you to ALL of you for bearing with me.

Worth noting though, this mainframe has Option 12 installed. But there is nothing connected to the BNC’s at the back. And all 6 of them have their leads go respectively and independently to each one of the 6 ports 28A and ground to 27A.
But there’s nothing there, and they’re not shorted.

On Wed, Aug 12, 2020 at 02:32 PM, teamlarryohio wrote:

...and the Common for the variable supplies is tied to 21A, 21B, 34A, and 34B
if the schematic is accurate.

You probably mean 24A/A

Hint: The 5V ground is tied to the chassis of the PS503A. See if that is useful.

Yes, this is exactly what I have.

However, on my extender, none of these are connected. Only every pin, but 6B, from 1 to 13 are.
And, according to another post, that seems to make this unit behave “normally”...
That doesn’t make any sense...
None of this makes any sense. How can things work as intended when on the extender, but not when directly inside the mainframe? This is mind boggling....

On Wed, Aug 12, 2020 at 10:17 PM, Roger Evans wrote:

...
I think the C suffix means ceramic case
...

FWIW: The C stands for commercial grade, which has to do with the operating temperatures and possibly bias and offset values.

On Wed, Aug 12, 2020 at 02:14 PM, <sdferg7@gmail.com> wrote:

Looking at your middle picture, it does seem that a good clean around the IC socket would be a good idea. It looks like Q555 may have been replaced and there are signs of corrosion on the surface of R570. If you get the 7A12 working well then the internals of R570 are probably OK. I would certainly pull Q555 and Q575 and give the leads and sockets some IPA.

Best regards,

Roger

...and the Common for the variable supplies is tied to 21A, 21B, 34A, and 34B if the schematic is accurate.

Hint: The 5V ground is tied to the chassis of the PS503A. See if that is useful.
-ls-

Link to the video with the gerber files linked in the description: https://www.youtube.com/watch?v=LKyo1sXVjpk

This evening the DC503 that was sort of okay and counting when on the extender, just stopped counting altogether... Just showing zeros.

PS: The DC505A I received a few days ago is just randomly displaying numbers.
I must be cursed with counters... or something...

Feeling puzzled 😕

On Wed, Aug 12, 2020 at 11:42 AM, EJP wrote:

That's the correct behaviour. The +/-20V posts are all floating, including the
centre one. You can't measure anything ref. ground on them. If you can in one
configuration something is badly wrong with it, i.e. one of the three is
grounded and shouldn't be.

EJP

That’s what I thought too. Otherwise why have 2 of them...?
But this correct behavior is only showing when the plugin is on the extender.
When it’s inside the mainframe, it behaves differently. Like the DC503 that is not working
when inside the mainframe, but is when on the extender... (although now it stopped counting altogether...)

This backwards behavior is insane... It drives me nuts...