Author Topic: Repair of an HP8904A signal generator  (Read 1998 times)

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Offline douardda

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Repair of an HP8904A signal generator
« on: March 04, 2016, 12:36:46 am »
Hi,

I have a couple of HP 3904A signal generator. One of them had a failing PSU and I recently took the time to work on it.

I've made a description of the problem and the repair on a small blog I'm beginning there https://whatever.sdfa3.org/hp8904a-multifunction-synthetizer-restoration.html

Long story short, the culprit was a resistor on the small "secondary" board of the PSU (labeled R7, a 20k 1% resistor involved in the current limitation portion of the voltage regulator

I've made a (very) partial schematic of the +16.3V rail of the PSU, if it may help someone:

 

Now, I have another (small) problem? I've discovered that the 10MHz ref output is flat on the other HP8904A unit. I've begun to probe a bit around the 10MHz output (which is near the 10MHz xtal), and it looks to me the transistor Q106 is dead.



But I cannot find this reference on the net. It looks it's a Motorola, but the numbers (30659) does help so much to find same specs.  Having a couple of other same transistors on the board, I think it's a simple PNP bipolar transistor. I wonder if I can use a basic ref like a 2N3905 instead there.
Anyone have an idea where I can find some specs of this transistor?

BTW, does anyone have the schematic of the A2 (logic) board? It's not in the service manual...

David

Last edit: PNP, not NPN
« Last Edit: March 06, 2016, 01:37:11 pm by douardda »
 

Offline SAUL BRITTO

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Re: Repair of an HP8904A signal generator
« Reply #1 on: March 04, 2016, 01:13:06 am »
Ins't 2n3905 a PNP?
« Last Edit: March 04, 2016, 01:32:54 am by SAUL BRITTO »
Thank You, for all earth.
 

Offline rf+tech

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Re: Repair of an HP8904A signal generator
« Reply #2 on: March 04, 2016, 04:47:53 am »
douardda,

The complete HP part number is 1853-0659, one has to know the proper prefix to prepend to these mysterious part numbers.  :P

The HP-JEDEC cross reference lists this as a 2N3906(SEL), which indicates it is selected for a specific parameter. We have no way of knowing what the selection criteria may have been but I suspect the single digit 3 on the top line might be a clue. Seeing how this transistor sits in a schematic might provide another hint.

For now, pop in whatever 2N3906 happens to be lying about.  :-+

RF+ Tech
 

Offline brob

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Re: Repair of an HP8904A signal generator
« Reply #3 on: March 04, 2016, 04:58:01 am »
It looks like the transistor has a broken wire on the one leg. might be able to patch it back together...
 

Offline douardda

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Re: Repair of an HP8904A signal generator
« Reply #4 on: March 05, 2016, 01:31:56 pm »
Ins't 2n3905 a PNP?
Indeed. I've corrected my post. Thanks


« Last Edit: March 05, 2016, 01:38:43 pm by douardda »
 

Offline douardda

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Re: Repair of an HP8904A signal generator
« Reply #5 on: March 05, 2016, 01:33:28 pm »
douardda,

The complete HP part number is 1853-0659, one has to know the proper prefix to prepend to these mysterious part numbers. [emoji14]
The HP-JEDEC cross reference lists this as a 2N3906(SEL), which indicates it is selected for a specific parameter. We have no way of knowing what the selection criteria may have been but I suspect the single digit 3 on the top line might be a clue. Seeing how this transistor sits in a schematic might provide another hint.

For now, pop in whatever 2N3906 happens to be lying about.  :-+

RF+ Tech
Thanks a lot for pointing me to the  exact reference number.

These  custom part references  are really a pain in the ...

David
« Last Edit: March 05, 2016, 01:39:32 pm by douardda »
 

Offline douardda

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Re: Repair of an HP8904A signal generator
« Reply #6 on: March 05, 2016, 01:38:14 pm »
It looks like the transistor has a broken wire on the one leg. might be able to patch it back together...
Amazingly you are right! I did not notice this (impossible to see  it by direct sight, it's obvious on the  macro picture.) I've made a quick fix  by putting a solder drop on the  wire  and my 10MHz signal is back on the  BNC socket.

Thanks

David


 


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