IO-4510 scope restoration - further update

[Alex [Temple-Heath List]]

Yesterday I got a chance to do some more work on the scope, and I 
noticed that it was not possible to get a clean display of any 
waveform, basically the timebase was unable to sync. I traced the 
problem to a faulty U405 High-Speed comparator in the time base 
board. There where appropriate input signals to the comparator, but 
never an output. So as a test I swapped it with U412 which is the 
also same type IC, and now the waveform will sync perfectly, although 
now the swapped-out faulty IC is causing problems in the blanking 
circuit, but this obviously confirms it as bad. This comparator is a 
uA760 according to the manual, but I found that at some point both 
IC405 and IC412 where substituted by someone with LM360N's, an 
updated version of the 760 (760 is good only up to 25nS, the 360 is 
faster at 20nS). The pins on both these ICs are clean, and the are 
National brand, while all the other ICs on the board have the Heath 
p/n and/or the pins look rather aged.

Unfortunately at this time of the year all the electronics outlet 
stores at my location are already closed for the holidays until the 
second Monday of January, so I'll have to wait to get a replacement. 
But since can't even be sure this part will be in stock, I might 
place an order with Futurlec.com today and it should be arriving 
around mid-January anyway. I was also considering substituting all 
the TTL 74xx chips in the scope with modern 74LSxx types, so might 
order those too.

Spoke to the seller the other day about the missing Ops/Cal manual, 
he told me didn't recall the scope had two manuals, but "if" he's 
pretty sure it should be stored away with the rest of his Heathkit 
manuals (quite a few). But he's an older person presently with some 
health problems, so I'm anticipating it will take him some time to 
check and let me know if he found it.

Also wanted to thank everyone that has replied to my query on the 
availability of this manual, so if I end up needing to buy it, I now 
have several reliable sources.

Thanks.
-Alex

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