Enertec / Schlumberger 2618 counter repair

[TheUnnamedNewbie]

There is another repair of this counter model on the forum, find it here: https://www.eevblog.com/forum/repair/counter-repair-enertec-schlumberger-2618/ . All the schematics I have about this I got from this thread.

So I got a new counter, bought broken from ebay. The counter has three inputs, called A, B and C (Imagine that).

From looking at pictures it seemed like A and B are pretty much identical to each other. C is a prescaler for A. It extends the upper range to 1.2 GHz. This was the main reason for me buying this device, as I don't have anything that can measure that high (but I do have a source that can generate). The seller advertised it as A & C not working, B functional.



When I got the device, it seemed like the seller was wrong. A and B are broken, C seems to work (see picture below). I suspect that they just didn't look at the front panel when testing and went "The second connector must be B".



Channel B does seem to be able to count intervals, which is something. A doesn't do anything usefull, sometimes when I move a switch it will read a short value (unrelated to whatever I'm trying to put in the input).

I'm not sure how much of this is me just not understanding what the buttons do and not the unit malfunctioning, although looking at the schematics has helped me understand some of it. As frequency counting doesn't do anything on A or B, I suspect it is going to be something that they must share, but they don't share with anything to do with input C - I'm going to look at some voltage rails but I would suspect if they are the fault, nothing would work at all.

I also quickly checked the oscilator and it seems to be quite accurate. I'm still looking at getting a GPSDO or GPSRO in the near future, though I have some other stuff I need to sort out before I'm spending money on that. I'll keep updating this as I try stuff out, now first I just need to figure out what it is I am going to start probing.

[TheUnnamedNewbie]

Ok, seems like I found the issue. In channel A, the first differential pair was broken. I removed it, but couldn't measure it because the package was damaged and all the bond wires were broken.
I'm now going to try and find a replacement part to fix this. The part is marked E421 and what might potentially be something like the linear tech logo, but it can't be since it has a datecode of 1980, and LT was only founded in 1981 (at least I think so).

So I'm going to try and look on the internet to see what I can find. If I can't find anything I might just look to get a few similar matched JFET packages and see what I can put together...

[Chris56000]

Hi!

I am familiar with the logo you're referring to – It's Siliconix and the E421 is a dual FET – if you've any oscilloscope remains, you might find a dual FET in the vertical input amplifiers you can use, and all the Siliconix Databooks are on Bitsavers!

Chris Williams