HP3325A function generator repair

[trobbins]

I'm nearly finished repairing my 3325A, so was interested to see curiousmarc in his latest video get one of two units going that he just got in.

My unit had intermittent front key activation as well, but that was alleviated by replacing the main 5VDC filter capacitor, so appeared to be ripple/noise related control issue rather than key contacts (but good to see a video of the front panel keys being accessed).  I replaced the other power supply related electrolytics at the same time, to avoid future issues.

My unit then had function and cal problems, that was traced to some DAC levels not getting passed through to the various measurement circuits due to failed LM310 op-amps in the various sample-and-hold circuits.  I had one spare LM310 so fixed the main functional signal, but waiting on another LM310 to come in to allow residual DC offset on the output to be auto-corrected.  On the way to fault-finding, it can be worthwhile checking/trimming the analog circuit bias points as detailed in the service manual.

The heat generated by the main rectifier diodes, and the regulator devices is quite noticeable, and although likely not gross, can do with a check that mica insulators are given a thin grease smear.  I didn't go the path of swapping the diodes out, or paralleling a 100V schottky, but that may be worthwhile for hot ambient use.

[trobbins]

Located a spare LM310, so finished the repair and now the output is pretty much within spec for dc offset.

Checking the harmonic distortion of sinewave showed 2nd was about -46dB, with higher orders reducing monotonically for about 0.8% THD, so noticeably above the -65dB spec level.  But I may leave tracking down that issue for another day unless someone has been down this path before.

The fan gets a bit noisy, so it will get replaced when we can all venture out.

[George Edmonds]

Hi

I am repairing a 3325B where the fan had failed.  This has caused major problems/damage with the power supply as the pass transistor cooling is marginal at best.  Do not use the 3325A without a working fan.

Regards

George.  G6HIG

[trobbins]

I haven't used the unit without the fan connected even though the fan is a bit noisy.  The fan is run from a 115Vac secondary winding off the power transformer.  I looked around for fan replacement options and a Papst 8500N is cheap and available, although I did look at one 'modern' fan option that had a universal AC input but it was more expensive and had a higher noise rating.

I noticed the localised power supply heating during fault-finding, otherwise I would probably have been oblivious to the issue.  I have some 100V schottkys (31DQ10) that could be soldered across the original MR751's as a way of lowering the temp in that pcb region, but haven't done that yet.  I noticed the main Q1 mica insulator didn't have any thermal grease, even though the manual says to use it for any replacement, so gave it a light thin even smear on both sides.  I checked the part temps with a temperature gun, and although they were getting up, they probably aren't excessive with the fan operating and covers shut up.

[SilverSolder]

I haven't used the unit without the fan connected even though the fan is a bit noisy.  The fan is run from a 115Vac secondary winding off the power transformer.  I looked around for fan replacement options and a Papst 8500N is cheap and available, although I did look at one 'modern' fan option that had a universal AC input but it was more expensive and had a higher noise rating.

I noticed the localised power supply heating during fault-finding, otherwise I would probably have been oblivious to the issue.  I have some 100V schottkys (31DQ10) that could be soldered across the original MR751's as a way of lowering the temp in that pcb region, but haven't done that yet.  I noticed the main Q1 mica insulator didn't have any thermal grease, even though the manual says to use it for any replacement, so gave it a light thin even smear on both sides.  I checked the part temps with a temperature gun, and although they were getting up, they probably aren't excessive with the fan operating and covers shut up.

Isn't the main heating coming from the pass transistors screwed onto the case rail?

[trobbins]

Yes the highest power dissipation appears to be via the three regulator parts mounted on a chunky heat spreader bar that is then bolted to the chassis side rail (and the chassis side rail can get uncomfortably hot to touch on my unit). 

On inspection, the MR751 are each reaching quite high package exterior temperatures (from a quick IR gun test), and without forced airflow, would be significantly raising the local ambient of any pcb part nearby, including the electrolytics.  I wouldn't have a concern about the MR751 die temp, but I was concerned about the local ambient air and pcb track temps.

[SilverSolder]

Yes the highest power dissipation appears to be via the three regulator parts mounted on a chunky heat spreader bar that is then bolted to the chassis side rail (and the chassis side rail can get uncomfortably hot to touch on my unit). 

On inspection, the MR751 are each reaching quite high package exterior temperatures (from a quick IR gun test), and without forced airflow, would be significantly raising the local ambient of any pcb part nearby, including the electrolytics.  I wouldn't have a concern about the MR751 die temp, but I was concerned about the local ambient air and pcb track temps.

Yes, that whole board does get hot.  I actually replaced the entire PCB in mine, simply because it looked so "well done" that it could only be a matter of time before something failed, and I was lucky to find a spare board on everyone's favorite auction site.

I also fitted a quieter fan, but with enough airflow to keep temperatures reasonable.

[trobbins]

I just had another look at the sinewave harmonic distortion performance.  The distortion I measure is well above spec level, and I was able to exclude the Mixer stage (H section) and later stages (Function circuits J, or Output Amplifier K), as the Mixer stage has the same distortion levels as the output signal.  The Mixer stage has a balance pot, which only lowered the HD by about 1dB when trimmed for minimum distortion. 

So I will have to check out the Multiplier stage (G section) and VCO stage (D) next, although on a quick read of the theory of operation it's not made clear as to whether the sine waveform itself (and hence distortion level) is derived from the VCO related signal or the 30MHz related signal that are combined in the Mixer.  If anyone has an understanding of this then that would be appreciated.

Luckily I just repaired a vintage Systron Donner 6250A counter/timer, which provides for frequency measurement up to 50MHz, which should allow me to cross-check a nominal 45MHz from the VCO along with scope.