Agilent 33120A repair and teardown

[Chipguy]

Hoiii, welcome to teardown tue.... ok it's Sunday and I still don't have a decent camcorder, so only photos..

I got a Hewlett Packard 33120A 15 MHz Arbitrary Function Generator.
Some 2 years ago (cough cough) it stopped working all of a sudden.

When powered on, it beeped twice which is an indicator that the Main Processor can not communicate with the microcontroller on the frontpanel.

I tried to repair it back then but didn't have enough time and/or patience.
So it went into a cabinet for later.
Now much later I finally found the time and patience to have another look into it.

This time all the yellow evilness (SMD tantalum caps) that are prone to fail were perfectly fine.
All the voltages where there and looked good on the scope.

So I started to probe the actual announced problem, the serial SPI like bus to the frontpanel.
What I found was that there was no communication whatsoever.
Not a good starting point, so I probed the ASIC where the serial bus is connected to.
I probed the RESET and clock signals. Everything seemed to work fine.

Just to be sure I also probed if the ASIC is really connected to the frontpanel board.
I did not expect anything wrong so i was surprised that one trace was not connected anymore.

For those who want to play along, it's the connection betweeen R107 (215 Ohms) and Pin 2 of J101.
So i soldered a wire to bypass the broken connection.



And what do you know : A Bobby Dazzler, it works again


Below are pictures from the inside as a little teardown, enjoy:

[vaualbus]

If you want sell you can sell parts only. I'm looking for a vacum display given the fact my broken....
Best regards, Alberto

[robimarko]

For how much would you sell it?
Here in Croatia it is impossible to get decent price function generator

[Chipguy]

Search for HP 33120A on ebay and include Germany in the search, there are not many around.

[radhaz]

In your picture of the display, one of the electrolytic capacitors looks heat stressed.

[Chipguy]

In your picture of the display, one of the electrolytic capacitors looks heat stressed.

Yes it does, so I took both of them out and checked them.
They have the same good capacity and ESR as per specification.
No idea why that one heatshrink is misaligned.
I left the generator on for a while (a lunch time duration) delivering 10Vpp into a 50 Ohm load at 15 MHz.
Even with the enclosure off and the airflow not being optimal those transistors don't really get warm.
Maybe just around body temperature.

[What_NZ]

Hey good to see you have repaired your unit - well done and nothing more satisfying than doing it yourself.

Have to laugh though, in just about every repair Thread there is someone who spots or mentions about suspect Capacitors.