Last post by W1DAN - January 30, 2021, 11:31:54 PM
The oscillation goes away when the power is turned off.
Definitely coming from the preamp sections, not the RIAA stages. I lifted R50 to separate the gain stage from the emitter followers. The oscillation appears at the emitter follower section. I confirmed that C103 and C104 are 100nF.
I also am a radio ham and broadcast engineer, and work on RF on both my radio hobby and at work, but there with slightly higher power of 88kW. :-)
Last post by W1DAN - January 30, 2021, 07:49:29 PM
Hi Per Anders:
Thank you for quick replies today. This is enjoyable.
I edited my previous post as you posted, please read again.
Yes I have an older analog oscilloscope that is good for RF. The scope case is grounded to the preamp chassis case, and the scope probe ground lead is connected to the ground pin of J2. The preamp card Chassis ground connection is connected to the preamp chassis.
Both preamp channels oscillate. I checked with a spectrum analyzer too. Good transmitter at 284MHz!
Last post by peranders - January 30, 2021, 07:31:57 PM
Good that you have found a point where the servo doesn't hit the roof. The output offset in TP1 should be the value which the OP07 has, single millivolts is fine. The DC servo doesn't contribute to any noise since it's heavily filtered. Nice that one channel works. Now you have to check the other channel.
Do you have the oscillation in both channels? Do you have an oscilloscope capable of 350 MHz or more? Make sure your ground wire of the probe is wounded around the probe and connected very near the TP1. A long ground wire serves as an antenna. If possible you should use a ground needle on the probe tip.
Last post by W1DAN - January 30, 2021, 05:40:26 PM
More tests on IC5: Add -15v to IC5 socket (no IC inserted) pin 5, TP1 moves from -180mv to -190mv Add +15v to IC5 socket (no IC inserted) pin 5, TP1 no change T -238mv. Shorting IC5 pin 5 to ground, TP1 = -237mv.
With IC1 removed, TP1 is at -300mv. Adding -20v to IC socket pin 6 (no IC inserted), TP1 reduces to -170mv.
Added 22K across R29, added 100k across R27. Bench supply of -9.5v at pin 6 now creates 0v at TP1!! With IC1 (OP07) installed, TP1 is now -7.5mv. Good! Will this increase noise from OP07 to preamp?
Inserting a 1KHz sine wave to the input causes IC1 servo to oscillate at about 40hz. Adding 10uf at the output of R29 eliminates this. Using an oscilloscope, I now see crossover distortion on the test sine wave at the card output, possibly from the RIAA stage-will look at this later.
If this servo change is approved by you, we now need hints on eliminating the 284MHz RF oscillation (this is not from a Radio Sweden International transmitter, but generated in the preamp). With the input shorted, the RF signal is 70mv p-p at TP1.
Last post by W1DAN - January 30, 2021, 05:12:33 PM
Hi Per Anders!
Thank you for your suggestion and guidance.
With input shorted to ground: Added variable 0 to +15v power supply to pin 5 of IC5 socket. No change at output at TP1 (which is at -184mv). IC5 socket (no chip inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv. IC5 socket (MAX420 inserted) pin 2 = 00.00mv. Pin 3 = 00.00mv. Pin 5 = -371mv. TP1 is at -183mv.
Where does input to IC5 come from? Where does IC5 output enter the preamp for DC correction?