HP 3325a repair - Page 1
Forum section: electronics / repair

author: cncjerry
author_rank: Unknown
timestamp: December 25, 2023, 02:33:23 am
content: Hi,I'm working on an HP 3325a function generator that I've neglected. Power is fine. I found one problem in that the sine works (up to a point, see later) but the square, tri, ramps, don't at all. I get an "A Cal Fail" error. This looks like a U32 issue as U31 pins 6 and 8 (outputs of an inverter into U32) are low all the time. This is a posted problem in the manual (repair group J). I ordered a replacement sn74ls290, about the only 74LS IC I don't have.Later:Setting aside the problem with U32, sine wave frequencies under 5Mhz look ok. Above that they start distorting until at 20Mhz, the lower half of the sine wave is pretty much gone. But, if I set the DC offset to anything above zero, no matter the P2P voltage value, it clears up. Setting it to 1/2 the amplitude in P2P is the cleanest. Having typed that, I'm going to dig around in the offset area but was wondering if anyone had bumped into this? I found this after ordering the 74ls290 at a ridiculous price from Jameco. I ordered the 74ls290 as well as the other ICs after U32 just to make up part of the minimum. So it looks like I am going to have another minimum order to make up.earlier:I found a burned IC in the output attenuator. Complete toast. It is the size of a 1/4w resistor and looked like one. It is just to take the spikes off the relay coils. This could be another minimum order once I have the other two issues fixed.I studied the offset and everything up to the output amp, checked voltages, etc, they look OK but something is clearly wrong as it should be floating the output unless a DC offset is applied. I know I'm not giving much input here, but if you've worked on a 3325, then some of this might sound familiar, I hope.Thanks,Jerry

author: p.larner
author_rank: Frequent Contributor
timestamp: December 25, 2023, 05:34:37 am
content: post a schematic.

author: cncjerry
author_rank: Unknown
timestamp: December 25, 2023, 07:29:53 pm
content: I'm sorry, but posting the schematics isn't practical as the problem spans two areas and that would be 6 sheets total that overlap. Just putting them together would be a pain, you either have the 3325 schematics and know the box, or not.I did run the amplitude flatness test and found that as the frequency goes up, the output goes more negative DC offset. Didn't see that before, so that adds another three sheets to the schematic list. But they give a hint that this problem is an MC14xx (can't remember the number) 4-quadrant multiplier could be bad. I'm going to swap out the divide by 10 counter that is loading down the inverters and also could be impacting the current amp stage though it isn't obvious from the schematic. I'd be surprised that this thing blew two ICs so I'll fix the first I know is bad and then shoot the negative offset. I can see why it would be doing this, because in order to maintain and AC signal, they amplify it to account for the frequency increase, and then take it negative to zero-align it. So it is being zero aligned but not amplified. I'll look at that after I swap out the known bad part.So thanks for offering to take a look, P.larner, but it just isn't practical. The schematics are easy to find online.Jerry

author: trobbins
author_rank: Frequent Contributor
timestamp: December 26, 2023, 10:55:23 am
content: All I can offer is that I had LM311's fail (sample and hold amps located after U20 and U24 analog switches). They pass through subtle levels that I found were a pain to initially fault find, and got me going around in circles in the service manual, but once made obvious are somewhat simple to check that they are functioning as intended.I agree that it is a big task to offer insight unless you've been buried inside a 3325/A due to previous fault-finding.

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 28, 2023, 03:55:46 am
content: Hi!. . .What does the OP find so difficult about posting the relevant circuit and layout sections of this Function Generator?. . .It took me only 15 minutes to locate them on k04BB, in high–quality full–page scans, ready to upload straight away!. . .(I admit I don't think much of Hewlett Packard's "primary school" diagram lettering but that's neither here nor there as regards the OP's fault symptoms!)Chris Williams