🚀 go-pugleaf

keating@motcid.UUCP (Edward Keating) writes:
>In  article  <55022@iuvax.cs.indiana.edu>,   commgrp@silver.ucs. 
>indiana.edu (BACS Data Communications Group) writes:
>> 
>>          IBM MONITOR REPAIR NOTES: MODELS 8512, 8513, 3192
>>           -------------------------------------------------

>...  What controls are adjustable within the monitor, i.e. Vlin, 
>Vsize, Hsize ... Also, is there an adjustable focus control (with 
>rectifier  - does it blow?) or are the centering rings  the  only 
>hope for fuzzy screens?

How come they can't make monitors as reliable as consumer TV sets? :-(

IBM 8512, 8513 and 3192 monitors have internal trimmers for Vlin, 
Hsize, etc. but they are not marked.  The rectifiers and controls for 
focus and screen voltages are inside the flyback; look for small 
plastic knobs which take phillips screwdriver.  The glue over the 
controls will break away when you turn the shafts.

Several things cause dim/fuzzy picture, including:

1.  High-time picture tube:  Brightness, contrast and focus may 
degrade after 10000-15000 hours of use.  Playing with the focus and 
screen-grid controls can sometimes squeeze a little more useful life 
from a tube.

2.  Incipient flyback failure.

3.  Failure of an auxiliary power supply (a common problem in all CRT 
monitors, not just IBM):  Several high voltages are taken from flyback 
transformer windings, via half-wave rectifiers and filter capacitors.  

Filter caps operated at high frequencies (including those in switching 
power supplies) tend to get rather warm, leading to loss of 
electrolyte and hence capacitance.  Good engineers select capacitors 
made especially for such applications, but accountants often overrule 
them.  Overheated capacitors are often easy to spot because their 
covering of plastic shrink-tubing is discolored.

The rectifier diodes also run hot, sometimes hot enough to oxidize 
their solder connections.  When testing diodes, check for reverse 
leakage (should be too small to measurable with analog ohmmeter).  
Replace failed diodes with fast-recovery types suitable for high 
frequencies-- the 1N4002 family is unsuitable.  1N4937 is a good 
generic replacement diode for flyback-derived power supplies.  Leave 
the leads long for cooling.

Specifically, failure of C313 (3.3 uf @ 160v) in the 8513 monitor 
(model with separate power supply) will cause a fuzzy picture.

--

Frank Reid     reid@ucs.indiana.edu