Barlow Wadley 1 Mhz harmonic generator question - UK Vintage Radio Repair and Restoration Discussion Forum

netsmo62 · 30th Jan 2021, 8:54 am

Goodmorning, I have a BW recently buyed not working only thing I heard through loudspeaker is static, no effect if I do the "finger" test touching closed antenna tip. Transistor I have tested on board all seems to be ok. Only thing that seems strange is 1 Mhz harmonic generator behaviour. BEFORE R15 270 ohm resistor I have 1 Mhz regular oscillation, AFTER this resistor there's nothing. I have tested with oscilloscope too. I have looked at circuit boards and resistor value and all seems ok, like toroidal transformer after R-C network. I have substitute C15 and C26 too. Is that normal? I think it no...

Thank you, F.

orbanp1 · 1st Feb 2021, 2:10 pm

BAMA has the service manual for that radio:
http://bama.edebris.com/manuals/bar-wad/xcr30/

The schematics shows "19mV" signal level at the first mixer, though it does not specify "what kind of signal" is that.
The manual has the description of the full alignment of the radio, you should follow that.
It will give indication that what is wrong with the radio.

Regards, Peter

majoconz · 4th Feb 2021, 2:58 am

The harmonic generator for the FRG7 is almost the same except it uses a couple of diodes and a low pass filter as it's output should be a comb of 1MHz spaced spikes from 2 to 32MHz. There are a couple of very well written articles by J. Linsley Hood (is there anything he didn't write....?) in "The Short Wave Magazine"for September and October 1981 on how the FRG 7 works and how to align it.

However, this doesn't solve the current problem! I assume (careful!) that the Ferrox core in the XCR30 acts as the low pass filter. What the centre-tapped toroid L1 and it's capacitor C82 does is a mystery - perhaps that does the same as the diode pump in the FRG7. The OP said that he can't see anything on his 'scope but what did he expect to see? You would only see the string of harmonics if you had a spectrum analyser - I suspect a normal scope may show a 1MHz square-ish pulse but it only needs to be at the incoming RF signal level at the balanced mixer D2 and D3 so not many scopes are going to show anything at those low levels.

IMHO the OP may need to actually find those harmonics maybe with a digital receiver and see if they are actually there. I may have to dig out my BW XCR30 and have a play! I did find using a digital transistor radio a great help in getting the XCR30 aligned when I first got it - stated as 'not working' hence the cheap price - but it took some mind-bending to get the kHz dial reasonably close to calibration.

30th Jan 2021, 8:54 am	#1
netsmo62 Tetrode Join Date: Nov 2020 Location: Nonantola (MO), Italy Posts: 56	Barlow Wadley 1 Mhz harmonic generator question Goodmorning, I have a BW recently buyed not working only thing I heard through loudspeaker is static, no effect if I do the "finger" test touching closed antenna tip. Transistor I have tested on board all seems to be ok. Only thing that seems strange is 1 Mhz harmonic generator behaviour. BEFORE R15 270 ohm resistor I have 1 Mhz regular oscillation, AFTER this resistor there's nothing. I have tested with oscilloscope too. I have looked at circuit boards and resistor value and all seems ok, like toroidal transformer after R-C network. I have substitute C15 and C26 too. Is that normal? I think it no... Thank you, F. Attached Thumbnails

1st Feb 2021, 2:10 pm	#2
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Barlow Wadley 1 Mhz harmonic generator question BAMA has the service manual for that radio: http://bama.edebris.com/manuals/bar-wad/xcr30/ The schematics shows "19mV" signal level at the first mixer, though it does not specify "what kind of signal" is that. The manual has the description of the full alignment of the radio, you should follow that. It will give indication that what is wrong with the radio. Regards, Peter

4th Feb 2021, 2:58 am	#3
majoconz Heptode Join Date: Jan 2008 Location: Ashhurst, Manawatu, New Zealand Posts: 570	Re: Barlow Wadley 1 Mhz harmonic generator question The harmonic generator for the FRG7 is almost the same except it uses a couple of diodes and a low pass filter as it's output should be a comb of 1MHz spaced spikes from 2 to 32MHz. There are a couple of very well written articles by J. Linsley Hood (is there anything he didn't write....?) in "The Short Wave Magazine"for September and October 1981 on how the FRG 7 works and how to align it. However, this doesn't solve the current problem! I assume (careful!) that the Ferrox core in the XCR30 acts as the low pass filter. What the centre-tapped toroid L1 and it's capacitor C82 does is a mystery - perhaps that does the same as the diode pump in the FRG7. The OP said that he can't see anything on his 'scope but what did he expect to see? You would only see the string of harmonics if you had a spectrum analyser - I suspect a normal scope may show a 1MHz square-ish pulse but it only needs to be at the incoming RF signal level at the balanced mixer D2 and D3 so not many scopes are going to show anything at those low levels. IMHO the OP may need to actually find those harmonics maybe with a digital receiver and see if they are actually there. I may have to dig out my BW XCR30 and have a play! I did find using a digital transistor radio a great help in getting the XCR30 aligned when I first got it - stated as 'not working' hence the cheap price - but it took some mind-bending to get the kHz dial reasonably close to calibration. Attached Thumbnails __________________ Cheers - Martin ZL2MC