Electronics > Repair

Where to repair an HP3478A in Melbourne for cheap?

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gwpt:
Hi,
I just picked up a very cheap HP3478A but it has the error "AD Slope Err" which I have read could be a blown comparitor (or something like that)
Does anyone know a reasonable place in Melbourne (or Australia wide, I could post it) which could fix that and maybe do a calibration?
Any idea what that might cost?
I Am just a hobbist without much cash... :)
Thanks
Guy

alexwhittemore:

--- Quote from: gwpt on March 25, 2014, 09:52:13 pm ---I Am just a hobbist without much cash... :)
Thanks
Guy

--- End quote ---

I feel ya, but that probably implies that your best bet is to fix it yourself! I haven't finished reading the service manual, but you can probably find some test points or procedures in there surrounding the digitization circuitry and maybe you can isolate the problem that way. There's a good chance that if you've got a blown discrete chip, you can find a replacement.

lowimpedance:
An AD slope error is more likely the AD hybrid or its controller (a micro) and rather 'un-obtainium'. As alex. said your pretty much going to have to look into it yourself. Download the manual  and read through the section 7-d-38 and flowchart d (unfortunately they use an signature analyzer to test, but it gives you an idea of whats going on!). Look at the circuit, U403 is the AD and you may find something amiss around here.
Check the DC supply voltages for correct values and no mains ripple etc (you have another multimeter or CRO?), at least to eliminate bad power supplies. Check the AD reference is good U461.
Is the PCB clean, has this meter been abused, since it was cheep.
Does the error message all that happens, no other functionality possible?.
Good luck

PaulAm:
Welcome to the swamp of repairing old test equipment.  You really do have to do it yourself, you can buy a replacement for a tenth of what it would cost to have somebody else fix it.  You probably want to buy a spare anyway so you have parts available.

You'll really want to get a signature analyzer.  HP was really into that during that era.  The prices are reasonable since most people (ie, those without broken HP gear) have no idea what they're for.  I picked up a dead one for $20 US that had a bad PS cap.

Just keep telling yourself you're having fun, you might just convince yourself  ;D

VK5RC:
Repairing old gear can be very difficult,  a 3322 with a bizarre switch on fault has beaten me (for a while anyway)  but the real joy I got when I fixed the ac v range on a 34401 keeps me doing this as a hobby.
I always start with the 'low hanging fruit'  and check out the voltages and power supply (a really common fault area and not too hard to fix),  don't always believe the fault messages. :-)

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