I haven't looked at my rather poor condition 5248M for a few years, last time I had repaired the high frequency decimal counter board (A17) that drives the first display board, which had a dead tunnel diode.
The remaining faults were a suspected fault with the function control board (A21) and a problem with the regulated +20V supply for the crystal oven.
I've started on the crystal oven supply fault today, in my notes I had found the +20V supply was down to around +16V, so time to have a look at the A25 oscillator power supply board and hope it's not a problem within the oven itself.
Here is the A25 board with a heck of a lot of cobwebs & dangling spider carcass, the previous owner managed to clean nearly everything apart from this corner.
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
The chassis side frame is bent in this corner too.
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1116478)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1116482)
Here is the A25 board layout & circuit diagram (only used in the 524xM counters).
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1116486)
![](https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/?action=dlattach;attach=1116490)
So far I've disconnected all the output wires and added a temporary 470uF 35V capacitor in place of the external one, this was quickly replaced with a 50V one after finding it had 36V across it with no load.
Measuring the two zeners (CR5A/B matched pair) as the manual suggests revealed the 'B' zener has around 20V & the 'A' zener had 0V. I unplugged the mains and checked for shorts, sure enough there is one in the area of the zener, this was pulled along with the capacitor C3 but all was OK with them, I have run out of time tonight but will be checking Q3 & Q4 tomorrow.
David