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Old 17th Dec 2019, 4:23 pm   #2
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 1,206
Default Re: Marconiphone 538

There was a small number of rubber sleeved cables. The black earth wire from main chassis to the HT capacitor sub-chassis mounted next to the speaker had crumbled as had the wires to the primary and secondary of the AF output transformer. The earth wire was replaced with black neoprene and the transformer wires were re-sleeved. The reworked chassis is shown below. Time to power up and check voltages compared to Trader Sheet. The Trader voltages were measured using the 400v scale of an Avo 7 whereas I was using a Fluke 110 multimeter and so I expected the voltages to be somewhat higher. The results are shown below with the Trader values in brackets:
V1 KTW63 (RF Amp) Va=184 (192); Vg2=120 (88)
V2 X64 (Mixer) Va=275 (207); Vg2/4=173 (127)
V3 KTZ63 (Osc) Va=209 (192); Vg2=244 (187)
V4 KTW83 (IF Amp) Va=275 (197); Vg2=119 (88)
V5 D63 (Det) -
V6 KTW63 (AF Pre-amp, triode connected) Va=Vg2=133 (93)
V7 KT66 (AF O/P) Va=280 (215); Vg2=291 (225)
V8 U50 (Rect) Va=365 AC (346); Vk=368 DC
I checked the cathode voltage of the KT66 and it measured 17.5v across a 224R resistor indicating an anode+g2 current of 78.1mA as compared to an expected 61+4.3mA=65.3mA. Even allowing for the use of a DVM I think some of these voltages are a bit on the high side, particularly V2 and V4 so some further investigation is needed. There was quite a lot of intermittent crackling and rustling, and motorboating occurred only when the "Brilliance" switch was in the extreme anti-clockwise position. Moving the KT66 in its holder made the crackling worse. I switched off and cleaned and tightened the KT66 valve pin sockets and also cleaned those of V6. I also squirted some Deoxit into the volume, tone and brilliance controls. That improved things considerably and the motorboating stopped. I was getting signals on all bands and frequency calibration was excellent. There was a slight double peak on tuning in strong stations so I will need to check the IF and RF alignment. On a strong MW station or R4 LW it certainly sounds good! Cheers for now, any comments or suggestions welcome. Jerry.
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