UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Building the 'Minimod' AM modulator-questions!

1100 man · 28th Sep 2018, 9:28 pm

I hooked up mine to the frequency counter this evening to see what was going on.

Initial frequency 1.08534 Mhz
After 5 mins 1.08316
Then pretty stable for 1/2 hr. Last two digits drifted slowly up, then slowly down.
Then after an hour from switch on, frequency decreased to 1.07985
It then started to climb again.

I considered the most likely candidate would be the FET. Sure enough, a squirt of freezer spray sent the frequency rocketing back up to over 1.085. Granted, I can't find a straw for the freezer, so several components got hit, but when I placed a finger on the FET, the frequency started to fall pretty quickly.
I wonder if it would be worth sticking the FET to a small heatsink just to try and stabilise the temperature it runs at?
Maybe there is a more 'electronic' way of achieving temperature stability?

All the best
Nick

28th Sep 2018, 9:28 pm	#47
1100 man Octode Join Date: Apr 2016 Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK. Posts: 1,377	Re: Building the 'Minimod' AM modulator-questions! I hooked up mine to the frequency counter this evening to see what was going on. Initial frequency 1.08534 Mhz After 5 mins 1.08316 Then pretty stable for 1/2 hr. Last two digits drifted slowly up, then slowly down. Then after an hour from switch on, frequency decreased to 1.07985 It then started to climb again. I considered the most likely candidate would be the FET. Sure enough, a squirt of freezer spray sent the frequency rocketing back up to over 1.085. Granted, I can't find a straw for the freezer, so several components got hit, but when I placed a finger on the FET, the frequency started to fall pretty quickly. I wonder if it would be worth sticking the FET to a small heatsink just to try and stabilise the temperature it runs at? Maybe there is a more 'electronic' way of achieving temperature stability? All the best Nick