IO-4510 scope restoration - calibration

[Alex [Temple-Heath List]]

Well, I've been calibrating the scope for the past few days (thanks 
to Gary that sent me scans of the calibration procedures). So far 
I've been able to go through all steps, but a few didn't work out as 
well as I anticipated. I would like to ask anyone that has also 
calibrated a IO-4510 scope in the past if the quirks that I found 
also happened to them. Maybe I'm just asking too much from a old 
Heathkit scope?

I still need to replace the HV paper caps and resistors, maybe some 
of the problems I'm experiencing are related to these being old and 
out of tolerance. I was not able to read the -1000V supply, which is 
the highest range I have on any of my DVM's, so for now I'm assuming 
it should be in the ballpark figure. BTW, the HV being present on the 
small control board to the left already managed to bite me twice! The 
LV 5V and 15V supplies are dead on and rock steady, with very little 
ripple on them. The 195V supply, which actually is reading a bit low 
at 179V, is also clean, so caps should still be ok too.

Soldering technique used by the builder was very good, all clean and 
no cold solder joints at all so far. The only intermittent problem 
related to sweep was caused by IC408 741 Op-Amp chip (on the Time 
base board) which had some kind of corrosion on its legs, and most of 
them completely broke off the chip when I tried to pull it out of the 
socket. I looked as there happened some kind of electro-chemical 
reaction between the metal of the pins and the socket, very strange 
indeed. Replaced the socket and IC, now all seems well, and no more 
intermittent sweep.

Some cal problem (?) examples bellow. Other then these few, the rest 
of the calibration procedures went pretty well.

- Vertical Amplifier Balance (DC Balance and Step Balance adj) is 
somewhat unstable, even after a good warm-up period. Once I'm done 
adjusting Y1 ch, then proceed with Y2, and then after a few minutes 
recheck Y1 I find that once again I need to go over the adjustments 
on both channels. I've done this adjustments many times in the course 
of several days, and always find that proper adjustment always seems 
to been lost, and never a perfect balance can be achieved and maintained.

-The DC balance trimmers seem very touchy and react to the slightest 
pressure from the screwdriver, specially when trying to do the DC and 
Step balance adjustments on the 1-2mV scales, but in general the DC 
balance trimmers feel kind of "cheap", or maybe these ones are worn, etc.

It is almost impossible to avoid trace drifting on the lower input 
ranges with the covers off. The manual states this is due to air 
currents flowing through the sensitive circuits. It seems to me that 
they should have also provided access holes for the step balance 
adjusting trimmers so one could do them without removing the covers, 
as in the case of the DC balance adjustment. Anyway, very complicated 
to achieve with the covers off.

- XY calibration fell short, the manual states that it should be 
adjusted for 5 divisions between the two dots on screen, but at max 
setting I was only able to get 4.2 divisions. I checked the input 
amplitude with my other scope and it was fine.

- Low capacitance probe calibration: I was not able to properly 
compensate my X1/X10 250MHz probe, either with the external (probe 
adjustment) or internal trimmer caps. The probe will compensate 
perfectly on my Tek 2213A scope.

Not really a problem of calibration:

- I've noticed that baseline traces drift downwards 1-2 divisions on 
the screen between a cold start and the first 10-15 minutes. This 
seems specially apparent on the Y2 trace. After that initial period 
of time the Y1 trace pretty much remains stable, but Y2 always 
continues with its very slight downwards drift (about 0.1~0.2 div 
every 30 minutes or so).

- The Y1 and Y2 Vertical position adjustment pots still feel very 
"loose" even after I've cleaned them (improved a bit), but I believe 
one could replace them with better quality parts to improve the feel 
to the touch of the adjustment. Besides, these are obviously been 
placed way too close to the input range selectors, as it is very easy 
to accidentally bump them while changing ranges. I believe this 
design was changed in later scopes in order to avoid this problem, 
but not much one can do on this model.

- Focus control: a properly focused baseline setting doesn't always 
coincide with a properly focused waveform, as visibility of a 
waveform sometimes improves when re-tweaking the focus control during 
the observation.

- Sync Lock: when displaying two waveforms on screen at the same time 
(Y1 and Y2), only one of them will stay "sync'ed" (depending on the 
Trigger select position Y1+/-, or Y2+/-), even if they are the same 
frequency and waveform type. For example, if I feed the internal 
calibrator signal to Y1 (which is about 1KHz and 1Vpp, and then feed 
a 1KHz SQ wave signal at the same level from my IG-18 to the Y2 
input, only one can be sync'ed at a time with the trigger selector. 
The only way of locking in both Y1 and Y2 at the same time is by 
feeding the same signal simultaneously to both inputs. On my Tek 
2213A there is never a problem sync'ing both channels even if fed 
different frequencies, waveforms, or levels.

Thanks for reading this long post. Any suggestions welcomed.

-Alex

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