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577 VERTICAL (Current/div) selector dead increments


DW
 

On the 177 test fixture if I move the VERTICAL current/div to either 0.1A 0.2A or 0.5A the collector voltage goes to 0 on the display and does not respond when adjusted. If I move the VERTICAL selector outside 0.1A 0.2A 0.5A desired operation seems to immediately resume. It appears these 3 values are dead on the selector, has anyone had an issue like this? Thanks


NigelP
 

I have a 577/177 but not had this problem, but I have discovered that the whole system is quite convoluted in design.

I assume you have the 177 schematics, then you can see that the specific current ranges you refer to are unique in having a connection to R630B/630C (0.09 and 0.9 ohms respectively). It seems possible that the problem is a connection to this (I think, without actually opening up my machine) hybrid resistor network; a 10ohm tapped device R630A,B,C,D.

I think the current switch has a load of wires connecting to a PCB so that might be where the issue is? It looks like it might be section 3 from the schematic. I can't actually see in the manual exactly where R630 is located but I suspect it is on the PCB somewhere. It seems likely that the fault is not with the resistor pack itself (otherwise the whole current range switch system would be upset) but a connection that junction of section B,C. It might just be a dirty switch contact.

Regards

Nigel


DW
 

Thanks for the reply,

Reviewing the 177 fixture manual


DW
 

Recently I took apart the 177 test fixture to look at the current division switch, I found on the bottom of the switch there are small metal finger contacts that make contact with a mechanical bar that moves with the shaft of the switch. From what I could tell initially, at least from the bottom of the switch the metal fingers appeared to be making contact. The switch appears to be several layers thick. I will have to look more carefully as perhaps some of the hard to see areas of the switch might reveal some failed contact fingers.


NigelP
 

A good blast of switch-cleaner/IPA on all wafers while rotating the shaft might help.

Regards

Nigel


DW
 

Nigel,

I will give your suggestion a try, thanks!