Re: And the winner is my "new" 7A13! Was: 7K series - Conceptual question - What's the 3rd most useful plugin

Leon Robinson
 

Check the supply voltage to the relay's for voltage and ripple both cold and warm.


Sent from K5JLR

-------- Original message --------
From: Fabio Trevisan <fabio.tr3visan@...>
Date: 09/24/2018 3:50 PM (GMT-06:00)
To: TekScopes@groups.io
Subject: [TekScopes] And the winner is my "new" 7A13! Was: 7K series - Conceptual question - What's the 3rd most useful plugin

Hello Folks,
Thank you to all your input on my previous topic about the 3rd most useful plugin.
Not only it clearly demonstrated how regarded the 7A13 is among many users, but it also gave me the incentive I needed to pick that one that was selling at the local auction site (Mercadolivre.com).

So I scored this 7A13, in pristine cosmetic condition for a decent 70 dollars mark. Cheaper than what it usually scores on US.

I added an Album "7A13 - Newly scored in Brazil" to the photos section:
https://groups.io/g/TekScopes/album?id=72382

As not everything can be perfect... although it's in pristine cosmetic condition, outside and inside, it has a few problems....

The first one that called my attention was that, after working for some minutes (while cold it would work fine) and pressed briefly the "Identify" button, the square waves would turn into a "differentiated" square waveform (like if going through a small value capacitor).
After some trial and error and swapping parts, I got to the conclusion that not one, but both relays that are associated to the identify function are bad (K10 and K30).
They are 148-0050-00, SPDT, 15V relays (differently from what it says on the 7A13 Service Manual parts list, that describes them as SPST 12V).
Apparently as they warm up a little, their contact fail and they behave like a small capacitor.
Interestingly though... cooling them with coolant spray doesn't improve matters.
I was only able to isolate the problem to the relays by replacing them by - fixed - jumpers and confirming that the "differentiating" effect disappeared altogether, even after warming up and after fiddling with every possible setting.
For those relays I now I have to either source working ones (Do any of you have a couple to part with?), or find new relays that behaves well up to 100MHz.

I've seen the links on TekWiki that points to the 7A13 relay problems, and ways to work around them... But haven't attempted - yet - to open them and possibly repair them.

Besides this problem, this unit is ALSO showing a weird problem, while changing ranges from 500mV to 1V (in x1 Vc mode) or changing ranges from 50mV to 0.1V (in x10 Vc mode).
The problem only happens to the signal in the + signal path.
From the two symptoms above, I narrowed the problem down to the X100 attenuator in the plus signal path (R5A, R5C, R5D, R5E, C5A, C5B, C5C, C5D and relay K6).
I will add photos this evening but I will describe it briefly:
1. While unit is still cool, all works fine and changing ranges produce the expected change in waveform height.
2. After problem sets in, An all positive 0 to 2.5V square wave that displays 5 div at 500mV/div, when I change the range to 1V/div, the waveform overflows the screen up and down (meaning that it turned AC and the attenuator is stopped attenuating as it should... as the net display should have changed to 2.5 div waveform).
3. Connecting a test oscilloscope to the X100 attenuator output (at R5C output), as the problem sets in, the signal changes from a 1/10th of the input signal (as it should) to 9/10th of the input signal... and from an all positive waveform to an AC waveform with 5 parts positive, 4 parts negative. (square wave duty cycle = 50%).
4. Last but not least, I isolated the output of the x100 attenuator from the next stage (I had to cut one trace... Urgh!) to rule out the problem would not be coming from the output and it isn't... The signal at R5C remains showing the same oddities: Almost no attenuation and AC'ed.

In view of the history of the problems with relays in the 7A13, of course the relay is my first suspect... but I keep disbelieving, as I cannot make sense of this set of symptoms...There doesn't seem to be any difference between the signal before and after the relay contacts, both at the input and at the output of this attenuator block.
But at the same time, I cannot point out any other suspect... The attenuator resistors measure good (both cold and at onset of the problem), and the attenuator's voltage divider leg to ground is OK.

Let's see what further troubleshooting brings...I appreciate any suggestions.

Kindest Regards,

Fabio

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