Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site mgweed.UUCP Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxn!ihnp4!mgnetp!mgweed!rjr From: rjr@mgweed.UUCP (Bob Roehrig) Newsgroups: net.ham-radio Subject: Replacement FET for Heath scopes (and other scope comments) Message-ID: <21464@mgweed.UUCP> Date: Thu, 7-Nov-85 10:28:24 EST Article-I.D.: mgweed.21464 Posted: Thu Nov 7 10:28:24 1985 Date-Received: Sun, 10-Nov-85 06:44:42 EST Organization: AT&T Information Systems - Montgomery Illinois Lines: 52 This is particularly intended for anyone needing replacement FET's used in the Heath IO-105 scope. I assume that other Heath scopes use the same device in them: I am referring to the SCF-2912 in the front end of the vertical amps, and other places. Nowhere could I find this device listed and did not want to wait for an order from Heath - I needed the scope NOW! The result was that I tried a Sylvania ECG-452 and it works perfectly. Other type FETs tried may have worked if the mate in the other side of the differential circuit would have been replaced with the same type. But I could not get DC balance just changing the bad one. I do not understand the so-called diode protection circuit in the input circuit. The gate of the device is at zero volts. Why are the diodes connected to a voltage divider rather than ground or at least something "stiffer" than this divider. The divider will allow excessive signal input swings to destroy the input FET. On the whole, I have been pretty satisfied with this scope (It is the first dual-trace Heath scope). I bought it second-hand from a friend. However, This scope went thru 3 power transformers! In one case the scope was sent back to Heath for repair, where they replaced the transformer themselves at least once. After getting it back, the transformer went out again after 24 hours use. The problem each time was breakdown in the high voltage winding of the transformer. I took the last one apart and could see the sloppy winding job and where the breakdown occured. Heath would not admit to having any problems like this. Out of warranty, I could not see spending the money for another one of their bad transformers. I ended up bolting the scope on top of a 3 inch high Bud chassis and rebuilding the entire power supply section in there. I used an old Knight photoflash transformer for the high voltage source and have had no trouble for the last several years. Horizontal triggering leaves something to be desired but otherwise the scope is fine. Sure glad I did not buy it new for $400! Has anyone else out there had any notable experiences with the Heath scopes? Oh yes, Since it would not auto-trigger on lower audio frequencies, I increased the value of one of the coupling capacitors and that solved that problem. Bob K9EUI