LT23S front-end FET fault. MGF1200 – ATF10136 replacement, substitute. Repair.

The LT23S first stage MGF1200 GaAsFET had definitely blown (T6 in the manual’s diagrams). When I put a probe on the second stage, I could hear the noise level rise in the IF transciever, so it looked hopeful for the following stages. The LT23S still worked fine on transmit.

Searching around on the internet, I found KM0T had mentioned that WM0TEM had suggested an ATF10136 as a replacement for an MGF1200. I managed to obtain a couple of ATF10136 devices from utsource.net. The connecting leads are shorter than on the MGF1200.

I used a magnifying lamp and my best close-work spectacles. The ATF10136 package was marked only “101” with a spot toward the Gate connection. It was a little smaller than my soldering iron tip. I removed the MGF1200, taking care not to lose the little ferrite ring which was around the Source lead. With static wrist-band in place, & touching the soldering iron to the case, I soldered in the ATF10136. I completely forgot to re-attach the ferrite ring. The connections appeared to be the same as for the MGF1200.

The datasheet says the maximum VDS (voltage Drain-Source) is 5V. Measuring the VDS, I brought up the LT23S supply voltage gradually from zero to 13.7V. VDS came up to about 4V. Nothing popped!

Turning on the transceiver for the 2m IF, I was astonished to find Martlesham GB3MHL chirruping in merrily! This was received via aircraft scatter & GB3MHL was not audible when the ‘planes went away. There was about 0.5V across the 47ohm Source resistor, so the FET must have been drawing about 10mA.

Perhaps this is a bit low for the ATF device, so some re-biasing may be necessary? Also, perhaps VDS should be lowered to about 2V?

9-June-2013 update.
I re-biased the ATF10136 by changing the source and drain resistors, RS & RD respectively. Originally these were both 47ohm. I increased RD to 110ohm (2x220ohm in parallel) so as to drop the drain voltage. Then I decreased RS to only 4.7ohm. This left VDS at about 2V and IDS at about 27mA, which felt safer to me and also nearer to the test data on the datasheet.

The datasheet charts suggest a gain of about 18dB and a noise figure of 0.5dB. I haven’t tried measuring these. I did not try re-aligning the 3-pole filter which follows T6.

For good measure, I added a block of insulating foam over the local oscillator crystals in order to try to improve the frequency stability.

With the screening & box re-assembled, it all seems quite stable & GB3MHL (now GB3MHZ) still chirrups in merrily (via the Wimo yagi & and the Bob Platts pre-amplifier, positioned near the mast head).

Note: if you try this yourself, you are on your own!