Managed to grab a Heathkit IM-2410 225MHz counter from eBay. Inevitably for the age it has a couple of problems with it but it mostly works. I am however stumped on one of the problems.
The power supply isn't working very well. The schematic is as below. The 12v rail is regulated by a TO-92 78L12 and the 5v rail is regulated by a uA7805 (1.5A variety)
Quick voltage check showed low voltages on both. I expect a volt or two less as this is a 50Hz country and it's a 60Hz design
... The 11v point is reading 9.5v and the 20v is reading 17v. These are within the dropout specifications of the regulators. They were a LOT lower than this but I replaced all the electrolytic capacitors with Vishay/BC 021 long lifes as they were rancid old TI and Nichicon branded things from the 1980s.
The capacitor replacement killed a number of ills. However periodically it refuses to count for a few seconds. I decided to pop the scope on the 12v and 5v rails after the regulators and have a look on the basis that this might be dropout causing the input bias to get screwed up which makes sense for when I have traced it doing it. Both of them have a rather large dip in sync with the smoothing caps charging. See the trace below.
The bottom trace is 1v/div taken from the output of the 5v regulator. This is peak 5v dropping down to 4v.
The top trace is 2v/div taken from the input of the 5v regulator. Peaks at 11v, drops to 9v.
I've never seen R30 used in a regulator topology before and it's a rather large 5W carbon composition 20R resistor in this built version, not a 10 specified in the schematic and it gets quite warm.
Annotated board:
Note: BOTH the 12v and 5v regulator outputs have the bottom waveform which is confusing me.
Note 2: transformer is humming slightly.
Any ideas what to look at next or any diagnostic approaches to try?
Any help appreciated!