[time-nuts] hp3325B trouble and solution

[Luca]

 Dear Time-Nuts,
       I know this is a little bit off topic, but I think it might be
useful for many of you who have an HP 3325B synthesizer / function
generator. It is a very good tool  and I usually use two units locked to an
external good ref, to generate slightly off frequency signals for test (for
example you can generate a 50 MHz signal with 1 mHz resolution, then divide
it by ten obtaining a 5 MHz tone with 0.0001 Hz steps! i.e. in the 10E-11
region, and you can shift phase in 0.1 degrees steps, and so on....).
My two units recently died almost together after some years of good
operation.
The good news is that it was not so difficult to fix them. The common
problem was a kind of tantalum capacitor extensively used in the series I
own. My units both have 2847A serial number prefix.
These amber yellow plastic cylindrical tantalum capacitors, are mounted on
three of the four major board in the units and mainly they act as power
supply rail filtering (+15V, -15V, +5V). They are all 15uF 20V. The problem
is that many of them turn in a near short circuit state and they do not
explode only because of the protection circuits on the power supply board.
So, the supply rail shut down and the function generator stops or fails.
Not all 3325Bs have this not-so-good series of capacitor. Previous editions
mount good metallic old school tantalum caps.
So, if you have trouble with your HP3325B, first of all take a look on the
boards and check these tantalum cap. If they are of the yellow plastic
type, look for hot ones, burned ones,  or for some little melted metal
drops on them.
I attach some imeges. My first 3325B had one bad cap in the A21 board
(under the top cover). The second one, had two bad caps on a board under
the bottom cover.
I replaced them with very old (year 1973 !!!) metal can tantalum caps from
old boards I have scrapped.
I think HP3325A does not have this problem because it is older then 3325B,
so it is equipped with very good caps.
Enjoy your hp3325B and all hp stuff as well!
Ciao
Luca
iw2lje
Milano - Italy
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