First, make sure you have a working and tuned up radio..........
An oscilloscope and a signal generator or test set.
Optional equipment: Distortion meter, see text below for explanation.
The O scope bandwidth does not have to be very wide.
The signal generator has to be able to deviate a 1 kHz sine wave at 25 kHz.
1) Connect your O scope to the "IF Amp" pin. Attach the scope probe ground to A- .
Set the scope input to DC and set the sensitivity to . 5 volts per division.
Set the trace two divisions below the center division.
2) Connect your sig gen. or test set to the receiver input jack. Make sure the RF out is fully attenuated.
3) Slowly inject an on frequency signal with a 1 kHz sine wave deviated at 25 kHz.
You will start to see a "ramped" 1 kHz wave form start to grow on the scope, then the entire display will start to move towards the top as the DC starts to increase on the IF Amp pin.
Let the wave form rise 2 to 4 divisions on the o' scope. Be careful not to inject too much signal into the receiver or you will start to saturate the limiters. Keep the signal below the limiter threshold.
I have found that 3 to 8 microvolts to be plenty providing the rest of the RF chain has been aligned and the radio is in working order.
4) Tune L 505, L 520, L 521 and C 521 for a peak DC reading on the scope
display. The peak is sharp but well defined.
Keep going back and forth on the tuned circuits until there is no more DC increase.
Be careful with the slugs, some of these may have never been touched since the radio left the factory and may seem stuck but they will move. Go slow when turning the slugs the cardboard threads are delicate and can be stripped, I speak from experience.
5) Readjust the detector according to the service manual procedure or read the note below.
6) That's it!
Note on detector alignment:
I adjust for minimum distortion vice max. audio output. Use a non-inductive dummy load on the speaker output. I tune for maximum output first then tune for minimum distortion.
You can buy non-inductive dummy loads at Radio Shack for very little money.
If you use a speaker for a load it can lead to misleading distortion
readings plus be very annoying to listen to. I peak the audio output
first then adjust for minimum distortion.
The reason why I chose the minimum distortion technique is to squeeze every microvolt out of the SINAD sensitivity, after all it is signal to noise over signal to noise plus distortion.
The modulators in the exciter get the same treatment, aligned for a peak then tuned for minimum distortion. Oscillator "pulling" has not been a problem.
I have found that the audio drops only 0.1 to 0.3 dB when adjusting for minimum distortion.
So after the tweaking what do you get?
The "fringe" seems to be a bit further out. Improved SINAD, improved signal to noise, improved am modulated noise products.
Those signals with real hot transmit audio that are in the noise a bit do not seem to "deve" out of the machine as readily as they use to. Improved SINAD, and improved modulation acceptance bandwidth.
The squelch hysteresis seems to track a bit better or seems tighter. Improved signal to noise, improved limiter action, and improved SINAD.
The audio on the repeater seems to have lost some of that "edgy" sound, it seems a bit clearer. The detector adjusted for minimum distortion, improved limiter action, and less distortion in the exciter if the modulators were tuned for minimum distortion.
Please send me any of your findings or observations on improvements.
Click here to email Pat.
Copyright August 19, 2001 Patrick M. Conway WA6JGM
HTML Copyright August 23, 2001 Kevin K. Custer W3KKC
All Rights Reserved.