HP8161A repair Saga

[RFmaster89]

Thank you for reading this post.

First time poster, but long time reader of EEVblog.  For the last couple of month I've been slowly bringing a dead HP8161A pulse generator back to life.  Unit was a complete DOA when I got it in, with only a cooling fan functioning. Based on S/N and components generator dates back to 1989, made in West Germany! 

Firstly I was able to secure service manual which is extensive. Upon initial inspection power supply rectifier board A12 loaded with silver can Tantalum capacitors revealed ravages of time! Along with electrolytic capacitors it required a complete re-capping. Since wet tantalum caps are hard to find I had to resort to a hybrid solution by doubling small tantalum with electrolytic capacitor to keep ESR.   It is an old trick and ripple on supply rails is very low, almost not detectable.  Both A12 and A18 regulator board were recapped. After addressing power supply issues unit came back to live. One small victory!

Checking for functionality I have found that all waveform parameters are functional except for Period. Period is stuck and does not respond to keyboard entered values. All other parameters are responding to user inputs.  So this were I am.  Digital service block is a bit cryptic, but it appear that A23 is responsible for timing, hence Period. Device BUS address table 8-2-1 states FF7E value is the bus address for Period  programming.    But which board is a mystery to me?

[Rollin Hand]

Can you give me the link for the manual ?
Thanks

[RFmaster89]

The only service manual I was able to find is for HP8160A, which is a slightly lower performance instrument on Keysight.com. You have to log-in

https://literature.cdn.keysight.com/litweb/pdf/08160-90002.pdf?id=806426

I've purged A12 of all electrolytic s and Ta's

[fisafisa]

 Hi.
Anybody with a dump of the 8161A ROM?

Many Thanks

[fisafisa]

Hi.
Pinging the forum again for a ROM image.

Filippo

[fisafisa]

Problem solved.

The issue was that the instrument was scanning always the same key and I initially incorrectly measured that the scan address corresponded to a non existing key combination (row,col).
For that reason I started doubting the integrity of the EPROM.

In the mean time I obtained the ROM of the 8160, a new processor and a RAM chip.
The 8160 is 95% the same as the 8161 so I has high expectation that it would work to a degree that would show whether the old rom was ok.

The result is that no individual or combined change would change the behavior.
I ended up changing also some other components involved in the keyboard scanning with the same result.

Only after this I started checking the voltages on the keyboard rows and columns, finally discovering that I had misread the service manual (very poor scan) and the key scanned was in fact number 3.
Key 3 was stuck closed. Un-sticking it solved the problem.

Conclusion:
1) do not jump to conclusions. Always double check
2) I have good dumps of the Roms of 8160 and 8161. If anybody needs them?

Ciao
F