HP 54504 A - Gain Chan 1 Calibration error

[Disciple]

Greetings Gentlemen,

I have an issue with my HP54504A. When I do the self calibration operation for channel 1 I get the error : Failed Gain Cal Chan 1 0000 0000 0010 0000. All other calibration test are passed with success. The calibration is passed with sucess for channel 2.
When I go to the service menu, I have for Chan 1 gain the 0020H error code.

I search a clue on the service manual but I found nothing. Do you have any idea where I can start to search ?

Thank a lot for your help,

Disciple

[Runco990]

The input attenuator may be damaged or just dirty.  Try cleaning it or swap around to see if the problem moves.  You DON'T want it to be a custom IC.  There is some talk about the input attenuators, in particular the 50ohm switch.  Swap them first, PITA I know.

[SoundTech-LG]

Set Chan 1 50 ohm input impedance to ON. then, measure the input resistance. If higher than 50 ohms, likely the relays are problematic, possibly a damaged or open 50 ohm resistor... Sometimes switching them from 1meg to 50 ohms repeatedly may temporarily clear a relay problem. Then try a reCal to see what happens.

[Runco990]

Set Chan 1 50 ohm input impedance to ON. then, measure the input resistance. If higher than 50 ohms, likely the relays are problematic, possibly a damaged or open 50 ohm resistor... Sometimes switching them from 1meg to 50 ohms repeatedly may temporarily clear a relay problem. Then try a reCal to see what happens.

That's good advice.  And simple.  The relays don't get switched much.  A Tektronix tech told me they take the problem attenuators and "buzz" them for an hour to self clean the contacts.

However, they had a special setup for that. 

On the HP scope... yep... try manually switching it if possible.  Otherwise you have to pull the attenuators and manually clean them. 

[Disciple]

Thank you all for your help. I was offline during few month due to a lot of work. I will try during my holiday to finish the repair.

Thank a lot again !