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Old 12th Jun 2008, 7:02 pm   #13
paolo67
Retired Dormant Member
 
Join Date: Jun 2008
Location: Milan, Italy
Posts: 15
Default Re: Racal RA17L BFO behaviour

Here how I decided to temporarily fix the problem while waiting to find a replacement:

I removed the white insulation material and discovered that the tap of the coil - which is made of litz and machine-wounded so impossible to "rework" - was partially disconnected.

I fixed that and then the opposite problem arose: insufficient drive. The coil is definitely defective

Here what I did: I inverted the two ends of the coil. Of course now the valve is over-driven. To compensate that, I replaced R126 with 1.2Megs Now I have approx. 15Vpp after C215 and the correct dc voltages at anode and screen. Waveforms look quite sinusoidal. I found the value experimentally: smaller values produce distortion, with higher values the feedback is insufficient.

One point I noticed is that the alignment to 100kHz must be done when the main variable capacitor is half way. This is actually quite obvious because the oscillator must span +/-8kHz. Anyway, there is another stable 100kHz position when the capacitor is fully open.

Point 5.4 of the manual says "...the meter reads approximately half f.s.d" at zero beat and the system switch in "Check BFO" position, and this condition is met. To me, this means that, when receiving with BFO on, the meter will never read less than half scale.

The listening test is quite satisfactory - although not exceptional: very large SSB signals cannot be decoded, somewhat acceptable because they can be handled using the MAN position.

A special thank to Al for his support. I am now looking for a BFO unit, if eventually someone in the group would have one beyond his needs please let me know.

Paolo
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