[Test-Equipment] HP 428B current probe- whats normal

[D. Robbins]

Greetings,  I have a  428B that has  slight fluctuations on the 1ma range 
(less noticeable on higher ranges) of .005ma, 2 to 3 divisions, this is 
also seen with the scope on AC  and reading the 272 volt supply as well as 
other points in the circuit. There also seems to be the occasional  fast 
drift of the meter needle up to .2  to .4 ma marks where it will usually 
remain, this also seems to be noted on the scope as the trace goes off 
screen while the input cap recovers, only slight .5v maybe is noted on the 
272 supply monitored with analog meter ( to note fluctuations easier than 
DVM).
Original problem I had was that the meter needle could not be zeroed, it 
could be either side but would jump to one side or the other. I replaced 
all of the smaller lytic caps (several had questionable ESR)   and all was ok.
(used the "99 cent" ESR tester - google it, on the scope. A very handy 
device I use often, using a square wave instead of sine from function 
generator  may reveal  slight oddities the sine misses. Also different 
frequencies other than  100 k may be handy.)

In the process of this repair, I have swapped out all tubes  except V9, 
ditto for the 3 transistors and checked and cleaned just about everything.
I guess the main trouble in this diagnosis is that the circuit is one big 
feedback loop.

Question, I noted in the manual that it mentions slight meter moment over 5 
sec period on other ranges etc,  however I would like to know if I have a 
problem or is this normal due to probe pickup  of noise etc.  You would 
think  if it were normal that HP would have put it in the manual.
I know that a service bulletin was issued for the 428B for later serials 
than mine that was for a procedure to remedy a intermittent meter 
problem....................perhaps some one knows what this was?
Any help, any clues, any assistance much appreciated.
Dave Robbins