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 Post subject: Re: HP 3312A generator - replacement parts
PostPosted: Nov Thu 14, 2019 10:39 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 803
Some time ago I found that I needed low audio distortion to verify performance of high end components. That need no longer exists, so as long as my source is better than about 1% disortion I am okay with it. Still, the function generator seems better than the audio generator, something I wouldn't have expected.

I have two other function generators besides the HP; I should test them as well. One is Wavetek and the other a lesser known brand, International or something. But they are bigger, and bench space has become important.


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 Post subject: Re: HP 3312A generator - replacement parts
PostPosted: Nov Thu 14, 2019 10:40 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5384
Location: Sunbury, Ohio 43074
Chris, some of those transistors run warm, but really not HOT, as long as you have the heat sinks still on them.

Since it's 3 stages of DC coupled push pull amplification, with a huge feedback loop from the final outputs back to the input, troubleshooting it is truly a pain. How low is your output, and is it still clean? Make sure your SYMMETRY control is in the CAL position btw.....

I'll have to scan the page you need ...all I did so far was shoot it with my cell phone. I fully understand your wanting paper copy.... I do that myself. If I want to write on one, I copy it first. But of course having the manual helps ...

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Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: HP 3312A generator - replacement parts
PostPosted: Nov Fri 22, 2019 1:48 am 
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Joined: Nov Sat 09, 2019 2:24 am
Posts: 3
Hi Barry, I have an update. I did replace Q507 and Q508, and as I powered up the generator, with a variac, both of the 15 ohm resistors (R533 and R531) started to smoke, so I powered it down and retraced my steps. (Both resistors still measured 15 ohms, but I will probably replace them anyway.

I checked The original Q507 and Q508 transistors with a DMM and as it turns out, the were fine.
I also removed and tested Q504 and Q506, and they were fine too. (Q509 and Q511 I checked in circuit and they were good).
I unsoldered one leg of each cap and diode, in the area, and they all checked out ok.

Then I removed and checked C510, the 1.5 PicoFarad Capacitor. It measured 162 PF on one meter, and 343 PF on another; but that is the same measurement result for an open circuit, or if nothing was connected to the test leads, so I suspect that it may be open inside. (with an ohm meter it measured in the megohms)

I was thinking it would be odd to have two components fail at the same time instead of one component failure.

The feedback loop may be the reason why I had a increase in output, when I had a multi meter testing the Q507 and Q508 transistors with it powered up, which led me to suspect them. So I will try to find a new capacitor and see how that goes.
TO answer your questions;
The output was clean and low in amplitude at lower frequencies, 5 to 10 millivolts output, and then maybe 1 volt P to P at the highest frequencies with output set to 10. Those transistors (507 and 508)were hot but not hot scalding and they did not have the little metal hats as 504 and 506. I will check the symmetry control as well when I power it up again.

I printed off the pictures you posted and that worked pretty well so don't worry about the PDF. . You may want to post a picture of the A2 schematic, just for the next guy or gal working on one of these in the future.

Thank you for your help with this. I will update again later next week when the parts may be in. Have a good weekend, Cheers Chris


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 Post subject: Re: HP 3312A generator - replacement parts
PostPosted: Nov Fri 22, 2019 2:05 am 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5384
Location: Sunbury, Ohio 43074
Chris, make sure the transistors you used have the same pin assignments as the ones that came out. Sometimes they seem to want to put the collector where the base normally is on some of these instruments. If memory serves, and it's already fading on this repair I did, they will get warm without the heat sinks installed, but should be fine with no load.

Check the last 6 transistors and all the bias diodes in that system.... the entire set of 3 push-pull pairs are all DC coupled, so any open resistor, shorted or open diode, has the possiblity of causing enough imbalance to cause destruction.

Also note that some of the DC power to those stages is not the MAIN power ... they are isolated from the main power rails by resistors and capacitors. (Decoupled is the technical term). You can see that on the schematic as a reference such as +17Vb ... which is shown in the lower left of the schematic as a power source isolated from the main +17V supply.

Happy hunting. It was truly a pain ....

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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