Hickok -385X Frequency Counter

[W3SLK]

I used to make a living, (in a lifetime long ago and far away), repairing digital equipment. So when I saw this sitting on the 'Free' table at a hamfest in May, if figured I'd give it a go. When I first opened it, the prescaler was toasted. I wanted it as a frequency monitor for when I am on the air so I am not worried about that (it is 10Mhz~512Mhz). I concentrated on the primary input which is 1Hz~80Mhz. I replaced a few transistors, 12V regulator, some decade counters and LED drivers to get it working somewhat. The problem I have now is that as I approach 15Mhz, the output of the ECL cascade amp diminishes. In the prior 2 stages, gain seems to stay roughly the same. So in essence I'm not having enough to trigger the gate at a TTL voltage. I did some wholesale swapping of the tantalum and ceramic disc caps figuring it was somewhat frequency related, (and it is). But that didn't resolve the issue. I'm open for ideas before I send this thing into the infamous bit bucket. I'll upload the schematic. It seems that everything drops off at pin 2 of Z203. Q202 and 204 have been replaced and I swapped them for good measure.

[WB6NVH]

Have you considered replacing Z203?  There doesn't appear to be much left.

[W3SLK]

WB6NVH said:

Have you considered replacing Z203?  There doesn't appear to be much left.

As a matter of fact, I did just that last month. I had to wait 4 weeks for the chips to come from 'overseas'. They are 10H116's which is an exact replacement for them. The twig is that in the 1st and 2nd stage of the cascade amp all is good, even up to 30Mhz. But at about 10Mhz, the signal gets some distortion and becomes smaller in amplitude until it is beyond detection mode.

[W3SLK]

To recite an old Batman & Robin quip: "When is a door, not a door? When it's ajar!"
So to put that in line with this thread: When is a 2N4258 not a 2N4258? When its listed as its substitute is a NTE 159!
Probably 95~99% of the time you can get away with a 'general cover-all' transistor but this was one of those times when I needed the real McCoy! I bought 2 from Radwell and installed them for Q202 and Q204. Ran it up to 30M without a hitch. What was happening was as the frequency increased, the zero reference would go higher. To the point the TTL circuits couldn't trigger because the reference went as high as 4 volts! Another lesson learned for the books!