HP3478A - AD link Fail - FIXED

[prasanna30]

Hi Guys ,

So i got an HP 3478A from Ebay  , the seller shared pictures which depicted the instrument was working well and no major issues. But during shipping or previously, I received the unit damaged. The following were damaged:
 1. Main Power Transformer (Broken bobbin pins ) - found it to be flapping around. Screws that were supposed to connect the transformer to the chassis was missing and not found inside. So must have been removed during a prior repair. [Pic 1].

 2. CR123A Lithium Battery pins broken - found to be rolling around [Pic 2].

Since the seller did not have a return policy, I decided to go ahead the repair it.  I got the transformer fixed and rewound to match the original windings. I installed the transformer and the CR123A lithium-ion Cell. But the device did not start (display was blank). I looked into the service manual and tried debugging, all the voltages were present as per the specifications, but the regulated 5V on the Digital common section was a bit low. I gave an auxiliary 5V through my bench power supply to see if that was the issue and it turns out it was, the instrument booted up but presented with an AD link fail. I then turned to see what caused the low voltage on the +5V rail. After a few hours of debugging, I found the issue to be with a 3000uf 20V Capacitor, this despite appearing good was not filtering the rectified output from the half bridge circuit properly ]. So, I replaced it and the instrument booted up and the volatge was correct  but display shows the the AD link failure. The service manual instructs me to check certain pins with a HP signature analyser, but I do not have it, so I’m looking if anyone knows how I can test the device without a HP signature analyser. If anyone have any ideas as to what could be wrong also, please help.

Thanks in advance

[Kleinstein]

The link from the ADC µC to the display µC is via magnetic couplers.  There could be a problem with the communication, but also the ADC side µC not running at all.  A first point is a visual inspection at the coupling transformers for mechanical damage. Than could check the signal from the display side to the ADC side. after reset there should be some message send and the transforer + komparator part should work like an 1:1 through with only a little delay.

Another point is signs of life at the ADC side (e.g. link back, reset, ADC signals, SPI clock to the hybrid).

[prasanna30]

So a quick update , i checked the pulses as per the service manual , and the pulses on both ends of the isolation circutary were the same . When got me thinking what could be wrong. The one thing I forgot to mention was that I replaced the 5.84Mhz crystal , with a close one of 5.9Mhz as i could not find the exact  value on digikey . I had a tingling feeling that that might have been the mistake, I was able to find the exact value crystal on EBay , it arrived today and turns out the crystal was the issue. The device works well , but needs calibration. Thanks all for your help ... sorry I forgot to mention my goof up.