Electronics > Repair

repairing a HP 3325A

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carbon dude oxide:
hello :D

i bought a HP 3325A synthesiser / function generator and i was doing some testing on it earlier and have realised that they only waveform that it is able to output is a square wave, even if i set it to any of the other functions.

how would i go about finding the cause of this and maybe even repairing it? would there be any documentation about problems like this and how to go about repairing them?

c4757p:
If you set it to the other functions, does it still output a square wave, or does it output nothing? And do you hear relays switching when you select the functions?

These pages of schematic contain the function switching circuitry.

carbon dude oxide:
the buttons on the front pannel are loud 'clunking' buttons so i cant really hear anything like a relay inside flicking. i have tried all the functions (sine, square, triangle, rising sawtooth and falling sawtooth) but all of them output square waves

the sine wave function can output to 20MHz and the square wave can output up to 10MHz, i have get a 20MHz wave out of the sine generator but its still quite square wave like (however it is getting closer to a sine wave)

c4757p:

--- Quote from: carbon dude oxide on December 03, 2013, 10:47:49 pm ---the buttons on the front pannel are loud 'clunking' buttons so i cant really hear anything like a relay inside flicking.

--- End quote ---

When the relays click, you will hear them. I promise. But they don't always click - it doesn't use them to switch functions, but it does occasionally flip the attenuators around when switching functions, depending on the set output amplitude and offset. I was just wondering whether it was responding at all.

You do see the LEDs on the buttons changing, right?

If you don't realize at first looking at the schematic, they are using a somewhat unconventional way to switch the functions in and out. Each signal goes into a voltage-to-current converter, and the current outputs are all summed together. Then the individual amplifiers are biased on and off. U28 on the first page decodes the bias signals; you'll have to search a bit to see where each goes.

Start by probing U28, to make sure the bias signals are being properly generated. If they are, follow them to their amplifiers and check the inputs and outputs and bias points of each.

carbon dude oxide:
yes the LED buttons changing, i can hear a relay clicking in the case when i change the amplitude of the wave above 2V on the generator (however on my scope it does not seem to increase) but i cannot hear any relays clicking when i change the function type

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