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Electronics => Repair => Topic started by: Johnny10 on October 17, 2019, 12:54:44 am

Title: HP 35665A Power Supply
Post by: Johnny10 on October 17, 2019, 12:54:44 am
Just received a HP 35665A with faulty power supply.

No power at all.
After watching Dave's HP 35660 Tear Down I believe he said no schematic available for power supply.

Just hoping someone on the forum may have more information.

Thanks
Title: Re: HP 35665A Power Supply
Post by: wn1fju on October 17, 2019, 01:56:40 am
Don't have a schematic, but for what it's worth, I fixed my 35665A power supply solely by inspection.  In my case, the thing blew a fuse immediately
and was already drawing 2A when I supplied 8VAC into it via a Variac.  It's a bit of a pain to disassemble, but an ohmmeter showed shorts in the main
switching transistor, the driving transistor, three 1N4148s and three 5.6V Zeners.  Quite a bit of carnage, but all easily replaced, and it fixed the unit.

So open it up, look for burnt parts, shorted parts, the usual stuff.
Title: Re: HP 35665A Power Supply
Post by: Johnny10 on October 17, 2019, 02:44:35 am
Yes, that is what I started doing.

So far found 3 bad Diodes.
Title: Re: HP 35665A Power Supply
Post by: Johnny10 on October 19, 2019, 01:40:20 pm
I am going to have to do more investigation or just buy a new power supply.

TRW 095-10097


The AC line fuse has not blown on my unit. But
I am finding full 155VDC voltage on VCC pin of UC3842 IC and gate of Power transistor BUW12A, doubtful if much of anything is going to work in the future.

Title: Re: HP 35665A Power Supply
Post by: vtp on October 19, 2019, 05:17:13 pm
Some years ago rebuilt this power supply from my 35665A.

I do not think there was a single good electrolytic in this. I have also at least partial schematics drawn somewhere but they are not in electronic format and likely to be scattered over several pages of notebook. I do remember that there was something interesting about the switching transistor(s), IIRC it was emitter switched BJT with a lower voltage MOSFET. Or some other not so common arrangement.

Will see if I can find my notes.
Title: Re: HP 35665A Power Supply
Post by: Johnny10 on October 19, 2019, 07:45:06 pm
Thanks for your help.

I keep wanting to put this aside but I keep researching!!!
Let it GO !   :-)

I just picked this up Thursday.

Someone else has been in here before me.
Title: Re: HP 35665A Power Supply
Post by: dom0 on October 20, 2019, 05:24:41 pm
Some years ago rebuilt this power supply from my 35665A.

I do not think there was a single good electrolytic in this. I have also at least partial schematics drawn somewhere but they are not in electronic format and likely to be scattered over several pages of notebook. I do remember that there was something interesting about the switching transistor(s), IIRC it was emitter switched BJT with a lower voltage MOSFET. Or some other not so common arrangement.

MOSFET+BJT cascode for switching was sometimes used in ye olde times to work around voltage limitations of old MOSFETs. Basically using the lower MOSFET as a fast switch and the upper BJT to keep the high voltage from the MOSFET. I believe this arrangement was even made in an integrated form for some time.
Title: Re: HP 35665A Power Supply
Post by: Johnny10 on October 20, 2019, 07:55:08 pm
I have been tracing out one of the boards.

It is as you said, UC3842A Current Mode PWM IC to BUZ71A MOSFET driving BUW12A High Power Transistor.

Have a lot of blown diodes, MOSFET, Resistors.