HP 3326A dual output function generator

[tech_builder]

Hi,

I recently acquired an HP 3326A dual output function generator off of eBay. When I fired it up for the first time it went through a short series of self tests which it passed, after that it showed "error 171". This means that channel B is overloaded according to the user manual. However, nothing is connected to the output.
Channel A seems to work fine from the limited tests I have run, but when I connect channel B to a scope the output is wrong. I do get some output, ie for the sine wave option I get a sine wave that is in the millivolts range and if I remember correctly it is dc offset up a bit too. When I run a square wave the output is not square at all, it appears more like pulses at the start and stop point of where the square wave should be. (I'll try and post pictures later.)
When I run both channels together and combine them into channel A they seem to add up correctly. Ie. if I add two waves together I get an output that looks correct.
I have also run all the internal self tests (beyond the start up tests) and it passes all of them. I'm hoping it's something simple that someone on this forum might be able to give me a head start on. I managed to buy a service manual for it, and I will be going through it more thoroughly when I get some time soon. I would appreciate any head start I can get though!
I do have to note that this thing is huge! So much bigger than it looked in the ebay pictures :-) it weighs around 70lbs and when I opened the top I think that most of the structural internals are cast and machined aluminum! It is really cool looking inside I must say, and I'm looking forward to taking it apart.

Thanks!

[c4757p]

It kind of sounds like the output is overloaded. Have you checked for conductive debris, etc.?

And... wow. That looks not insignificantly larger than my 3325A, which is already a huge freaking tank of a signal generator!

[echen1024]

It might just be a short inside of the gen.

[tech_builder]

Thanks for the replies. I was unsure what it meant by output overloaded. I thought that someone may have applied a voltage that was too high to the output and fried it  . Although the present tense of the warning did confuse me a bit.
So shorting the output is what they are referring to when they say "overload" the function generator? I hope it's that simple.

Thanks!

edit: try to make myself sound more intelligent   

[tech_builder]

I managed to get into the beast tonight for a bit and have isolated the problem somewhat to the high voltage card for channel B (option 2 High voltage) when I swap it with channel A's high voltage card (there are two identical channels inside - see attached pic) the error is "channel A overloaded". The machine runs fine when I remove the offending card, so I will probably just use it for now without the card, but it would be nice to get the option working eventually.

[tech_builder]

For the sake of helping anybody else who may run across this particular piece of equipment and have this particular problem with it I will record the solution to the "error 171 - channel X overloaded" on my HP 3326A function generator.

Being relatively new to the repair game it took me some time to work through everything and get a sense of what was going on, and what was causing the failure. As I mentioned earlier, I narrowed it down to the high voltage board (opt 002) for channel B.

The symptoms showing on the board were:
A: Zener diode CR117 had failed
B: Pico fuse F502 was open
C: Capacitors C113 and C114 were shorted to ground

It took me a while to get see that the capacitors had failed as I was playing around with the fuses and zener diode (which I found first), thinking that they were somehow at fault (rather than a symptom). Pico fuse F502 is not shown in the attached diagram, but is in series with the -28V circuit coming from the power supply.

In hind sight it is fairly easy to guess what was may have happened. Somehow someone overloaded the output and this shorted the caps on the output (C113 and C114). This internal short caused the perpetual error on the generator.

Of course as everybody has already mentioned, the answer was a short on the output. Just took me a while to find it  . Attached is the relevant part of the schematic. Hope it helps someone.

[MadTux]

Hello everyone

Does anyone happen to have a PDF version of the 3326A service manual that he could email me?

Would be very nice, since I'm currently trying to repair a 3326A. I've a working 3326A as a reference, so I can swap modules, but without a service manual, it's a bit difficult to guess what the custom HP marked chips really do.

Thanks