rjr@mgweed.UUCP (Bob Roehrig) (11/07/85)
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
larry@tapa.UUCP (Larry Pajakowski) (11/09/85)
I have had good service from an IO-4510 Heath scope with one exception similar to Bob's. The front end is a FET differential pair which has a bad DC drift with temperature (the 4510 is DC coupled). Once after leaving it on in the upright position for 3 days the DC offset (kinda dumb anyway) the DC offset went too far for the trimmer pot rendering it useless. Has anybody got any ideas about a cheap fast front end, DC to about 25mhz. which I can use to replace the fet front end? By the way the scope is very nice. The bandwidth specs. are conservitive and the trigger works very well. Larry WA9QOO ihnp4!tapa!larry
wb6rqn@yojna1.UUCP (Brian Lloyd) (11/13/85)
> [...] The front end is a FET differential pair which has a bad > DC drift with temperature (the 4510 is DC coupled). Once after leaving it > on in the upright position for 3 days the DC offset (kinda dumb anyway) the > DC offset went too far for the trimmer pot rendering it useless. > > Has anybody got any ideas about a cheap fast front end, DC to about 25mhz. > which I can use to replace the fet front end? > > Larry WA9QOO > ihnp4!tapa!larry The temperature problems with DC balance might be cured by making sure that the two devices in the diff pair are thermally connected. Several of the semiconductor mfgrs make matched diff pairs in a single package. These are very fast and very closely matched. Try Siliconix, they have some. I looked very quickly and they have a dual that is designed for wideband differential amp applications. The device number is 2N5564, -65, and -66. I am sure that there are other vendors, and you didn't specify the parameters needed (gfs, quiescent current, etc.). 73 de Brian, WB6RQN Brian Lloyd ...![bellcore!cp1]!yojna1!wb6rqn P.S. Also look at 2N5911