HP 5340A frequency counter repair.

[PoDuck]

I picked up an old HP 5340A frequency counter for a song, and when I got it, all the digits displayed 4s or multiple numbers.  After a little troubleshooting, I tracked that problem down to the 4s shift register and replaced it.  When I turned the counter back on and didn't see 4s, I was excited, until I realized that there were other problems.  I now saw zeros, but where a 1 should be showing, I see an 8, and where digits should be blank, I see a 9.  Further, the third from the left digit doesn't display when it is supposed to be a zero.  At this point, because I had a terrible time trying to remove the shift register, and the board looked a little hammered when I was done, I assumed I had screwed up the display board by getting it too hot, and so I ordered a new (salvaged) one.  The new one came and has the same exact problems.

Before I continue, the 10Mhz output from the counter seems to be spot on.  There seems to be a little right hand lean to the waveform, but I think that may be the fact that I have it plugged into an ungrounded isolation transformer, so everything looks a little wonky.  It also doesn't seem to respond to external frequencies.  I connected frequencies from 10Hz to 1Ghz, and I saw no change in the display.

I don't see what other things might have been damaged by the shift register, nor what other things might have destroyed the shift register that would cause these problems.  Because of this, I decided to just look at it as though the shift register never was blown and think about what might cause the symptoms I'm seeing.

The first obvious thing to look at is that leading zeros should be completely blank, and not showing 9, I decided to check the blanking assembly.  All transistors test out fine, and I found nothing obviously wrong with any of the resistors tested in circuit.  From what I can gather, whether or not a digit turns on is a combination of the blanking assembly and the display board.

As for showing an 8 where a 1 should be, I have no clue where to look for that problem.  In fact, I don't know if it's actually trying to show a 1 at all, because even when I don't have it set to chk, it shows the same output.  Considering that no matter what I do, the third from the end tube not showing a zero seems to be consistent, so I'm going to assume that is a bad tube for now.

I have the physical service manual, and there is a copy of it here:  http://ftb.ko4bb.com/manuals/68.39.186.246/HP_5340A_0-18_GHz_Counter_Service_Manual.pdf

The portion I've been working on most has been boards A24 and A26, which start on PDF page 254 of that manual.  I'm beginning to think that the problems may start before the display board now though.  I'm just not sure what to test, especially given my limited amount of test equipment.

The equipment I have to work on this that I think is useful at all is a DMM, an oscilloscope, and a very limited signal generator that doesn't seem to be capable of creating any of the frequencies the manual asks for, and the ones it does generate, I wouldn't trust to be accurate.

Does anyone have any suggestions of where I should look next for my problem to be?

Thanks.