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Old 22nd Jun 2012, 1:29 pm   #6
Malcolm G6ANZ
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Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default Re: NAAFI receiver problem

Hi more information. With the input tuned to 800 KHz as set by another radio, resonance measured by a scope on the top cap (G1) of the frequency changer the LO is running at 1475 KHz. This is measured by the frequency counter on the DVM. This gives a difference of 675 KHz. The IF is 465 KHz so it is way off. To get an IF of 465 KHz the LO needs to run at 1265 KHz for an input of 800 KHz
Measuring the tuning range of the LO with the DVM counter gives 940Khz à 2.08Mhz.
Touching the top caps of the frequency changer or the IF give no noise from the output,. Touching the grid of the detector gives a lot of noise. Injecting a signal ,at IF frequency, on the detector gives a demodulated output. Injecting a signal at the anode of the IF valve or at the anode of the frequency changer gives no output, even if the frequency is swung over a wide range.
It seems as though the RF tuning is way out. I have tried adjusting the coil cores and the padding caps to no avail. They are now back where they started as I counted the turns I moved them!

Voltages:
Mixer. 6K8G
3 Anodehexode237
4 Screen Grid 167
5 Grid Osc 87
6 Anode osc 137 oscillating
8 cathode 96
t/c grid hexode -0.23

Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
6.3
0.3
250
100
-2.0
3.0
3.0
1.3M
0.65


IF. 6K7G
3 Anode 238
4 s/g 166
8 cathode 96
t/c grid 0.0

Vh
Ah
Va
Vs
Vg
mAa
mAs
ra
gm
6.3
0.3
250
125
-3
10.5
2.6
0.6M
1.65

The screen grid volts seem a little high and the grid volts too close to 0. Could there be something in the AVC line cutting the valves off?
I did try removing the detector valve to stop any AVC but it made no difference.
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