HP8568 guidence and advice sought

[fourtyniner]

Hi Folks, First post in eevblog. 

I picked up an HP8568b RF section with the intent of upgrading my 8568a.
I recently fired it up just to see if it worked at all and it does but has two problems:

1)  YTO unlock, touching R35 pot made it go away so that should be an easy fix.

2) A weird constant  ~39.9 mhz signal that seems to add to any input signal. 
    This one has me worried. I have no idea where to start tracking that down.

Alternatively, has anyone done the 8568a to 8568b conversion that an provide some guidance.

-Tom

[KE5FX]

Start by checking the ESR of all axial electrolytics on all of the plug-in boards.  95+% chance that you'll find a few bad ones, hopefully one of them will make the 40 MHz artifact go away when you replace it.

The next thing I'd do, after replacing the trimpot you already know is bad (or bridging it with a resistor to force the adjustment point away from the bad spot), is to go through the calibration procedure and see how far you get.  If and when you reach a step you can't complete, you can focus your troubleshooting effort on that particular section.  Lather, rinse, repeat until the whole thing works. 

You may need to get creative with test equipment substitution, of course, since many of the specific instruments they expect you to use won't be available.  This is rarely a showstopper.

[fourtyniner]

Right. A bad cap is very much a possibility.  Ive tried to attach an image of the ~39mhz artefact  with a 50mhz 10db input. Its shows up shifted by ~39mhz.

[MadTux]

Is there any relevant difference between A/B, apart from changeover from custom HP-9825A microprocessor over to Motorola 68k in B version? Not much difference in manual operation, as far as I know, unless you want to run BASIC programms on it

[0culus]

The basic RF designs of both the 8568/66A and B are the same modulo the occasional tweak. The 66B, for instance, had some tweaks done ca 1985 that improved phase noise performance in certain offsets, for instance.

[KE5FX]

RF performance will be essentially identical between the 8568A and 8568B, but the front-panel controls will feel much snappier on the 'B'.  The oldest 8568As felt downright sluggish.  I actually have an 8568A now that feels much faster than the first one I had.

In addition to the ability to run instrument-hosted BASIC programs, which isn't normally that useful, the 'B' models had a much larger GPIB command set. 

Given the choice, 'B' is better.  But if you have an 'A' in good working condition, there's not much need to upgrade unless yours is one of the super-slow ones.

[0culus]

Unless the OP wants more GPIB functionality. I have found that leveraging the MATLAB Instrument Control Toolbox with my 66B is awesome; I take advantage of the excellent RF circuits while offloading the fast analysis to a PC. It "just works" and to the person interested in writing code, there is a wealth of on-PC analysis you can do that will give you capabilities that you can only get on modern instruments for a small fortune. And it's way faster than on-instrument BASIC code.

[fourtyniner]

I'm note sure if my A is one of the extra slow ones. The B is definitely snappier even though its has an issue.
The B update also eliminates that large battery.  As to whether its worth updating depends. I've only found instructions for
the 8566.