View Single Post
Old 22nd Nov 2019, 12:36 am   #28
WaveyDipole
Nonode
 
WaveyDipole's Avatar
 
Join Date: Mar 2013
Location: Leicester, UK
Posts: 2,505
Default Re: What is it?

I am working my way through reverse-engineering the circuit in this unit and have so far completed three of the four boards. While working on the third board, which is positioned above the left hand meter (current functions), I came across this (see pictures). One wonders whether the circuit worked at all. At the very least it must have behaved erratically! The joint connects pin 3, which is the non-inverted input of the LM308 op amp. There does seem to be another input via the resistor at the far left of the board that connects the white wire. This might just explain the weird behaviour of the left had side of the unit.

I have yet to map the various wired connections between boards and various panel controls. That will be the next step after I complete reverse engineering the fourth board. I will then post further details. In the meantime, does anyone know how I might be able to find more information regarding the vernier control? It is marked as follows:

Colvern 2402/13 SKOJ 7825

There seems to be no resistance indicated and resistance measurements with a DMM don't seem to make sense. I get about 3.1k across two of the pins and a bit more than 2k across the other two combinations.

With regards to the right hand side of the unit, since there was no output and not obvious fault, I figured that it must require an input. It turns out that when a small voltage is fed to the two terminals at the top right, I do get a reasonably close voltage reading on the right hand meter. I checked several DC ranges at different voltages and there all seemed to correlate. I haven't checked the AC ranges yet and am not sure what the vernier dial is for, or the bias settings. There are also the two 'sense' jacks to consider. I had rather hoped that the unit would produce reference voltages but that seems not to be the case.
Attached Thumbnails
Click image for larger version

Name:	board-3-error-1.jpg
Views:	132
Size:	84.1 KB
ID:	194119   Click image for larger version

Name:	board-3-error-2.jpg
Views:	137
Size:	112.3 KB
ID:	194120  

Last edited by WaveyDipole; 22nd Nov 2019 at 12:59 am.
WaveyDipole is online now