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Old 20th Dec 2019, 5:20 pm   #4
cathoderay57
Octode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,206
Default Re: Marconiphone 538

Just done a full days work on the Marconiphone 538. Rather than opt for my somewhat unintelligent suggestion in Post #3 re: dropper resistor to lower the HT, I decided to do detailed tests on all the anode, screen and cathode resistors (all originals) and I got some surprising results: R27 2.3k (1.5k), R19 950R (3.5k), R5 895R (350R), R3 45k (5k). Having changed these the voltages changed to (in my view) acceptable levels except for V3 the KTZ63 local oscillator screen voltage so in the end I inserted an additional 22k in series with the existing 15k screen resistor to drop the voltage. The revised voltages are shown below:
V1 KTW63 (RF Amp) Va=235 (192); Vg2=103 (88)
V2 X64 (Mixer) Va=261 (207); Vg2/4=150 (127)
V3 KTZ63 (Osc) Va=218 (192); Vg2=198 (187)
V4 KTW83 (IF Amp) Va=258 (197); Vg2=102 (88)
V5 D63 (Det) -
V6 KTW63 (AF Pre-amp, triode connected) Va=Vg2=118 (93)
V7 KT66 (AF O/P) Va=268 (215); Vg2=281 (225)
V8 U50 (Rect) Va=366 AC (346); Vk=364 DC
I then plugged in an aerial and powered up. Well, it worked but now I had distortion on strong signals and the double-peak tuning snag was still there. Re: distortion I checked the AVC voltage on R4 LW and it only measured about -2v so something was wrong. I changed C35 the AVC diode coupling capacitor and it brought the AGC down (i.e. more negative) by a bit but distortion was still there. I had previously replaced the AVC decoupling capacitors. All of the AVC decoupling resistors (500k mostly) were reasonably close to tolerance so I left them alone and paused for thought. Putting the meter probes onto the grid of V1 and V2 cleared the distortion (shunting some of the input signal?) so it looked like not enough AVC was being developed to control the RF amp and mixer. This together with the double peak tuning meant that the IF was out of kilter which would have reduced the AVC output. That proved to be the case. The full IF and RF alignment took a good couple of hours but it was well worth it. It has made a world of difference with the distortion gone and the sensitivity dramatically improved. I would say the performance and sound quality rival any other set of this period that I have tackled. Thanks to TowerRadio I now have a much better magic eye and tomorrow I'll post a pic of the reassembled cabinet. Cheers, Jerry
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