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Old 6th Jun 2007, 11:28 am   #15
Pumpernickel
Retired Dormant Member
 
Join Date: Nov 2005
Location: Germany (in the south)
Posts: 6
Default Re: FM tuning drift - any ideas?

Hello all,

interesting -- I had a similar problem months ago with a Philips B2D33A: During the first 5 minutes it drifted, so that the station became very distorted (when it was clear at the beginning). I assumed oscillator freq drifting, and I was right. It moved about 150 kHz downwards, which was enough for that effect. I measured the frequency with a spectrum analyzer, but I also did the same with an auxiliary quartzstable detunable oscillator.

I also controlled the HT stability, but in my case, it was stable. The exchange of the ECC85 was not successful either.

At the end, I decided to exchange a condenser for the oscillation against one with a high negative temp coefficient. I played with some combinations. Then I tested it again, the drift only was 50 kHz, less enough to accept it.

I know, it is not possible to get zero drift. But I don't believe, that the measured amount of 150 kHz was not the target of the design. I also know, that there are measures to compensate as well it is possible. So, maybe one of the components which were for compensation (mostly caps) had lost its compensating properties a bit. It's hard to find out, which it was.

Dieter
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