16-06-2020, 08:24 PM
(This post was last modified: 16-06-2020, 08:24 PM by Mike Watterson.)
Don't think about the loops, they only affects sensitivity.
As a general principle either align MW & LW at these frequencies, or special marks on scale. Always check pointer / cursor is mechanically correct first. Never align at absolute ends of bands, especially the HF end as some caps increase slightly at fully open due to capacitance of mount, i.e. the minimum capacitance is often just before fully open.
Concentrate on the MW LO. Solder shorts across the switch contacts that close for LW as the symptom does sound like the LW LO coil switching isn't perfectly correct.
Referring to circuit I posted:
Part A:
1) T4 and LW secondary LO coil in series with T6 is shorted on MW. Solder a jumper across switch.
2) R4 and LW primary LO coil is shorted on MW. Switch resistance here will not so much detune. Only short switch here if step in one isn't enough.
3) T5 & LW RF coil are shorted on MW, don't worry about that. You can check later when LO is correct.
4) The LO trimmer on tuning cap T1 (furthest from drum) and T3 interact. T1 is HF end, adjust at about 1400 kHz (215 m)and T3 (likely in the IFT style can?) is the LF end, adjust on 600 kHz (500m). Repeat.
5) Remove the shorts. If the HF end shifts at all, then the switches are not Zero Ohms, especially the one in (1). Clean switch till no frequency shift on MW.
6) NOW trim the T2 (RF) for max gain at about 1400 kHz (215 m). Solder a short across the LW coil or the switch contact. Use as little signal as possible so AGC is at max gain (0V, goes negative. You can monitor DC volts on AGC line or AC volts at volume control. The MW loop coil is only "wrong" if no position changes the gain. THE LOOP NEVER AFFECTS TUNING, only gain. The number of turns sets gain at 600 kHz. It's unlikely to be wrong.
7) Remove the short on LW coil. If the signal level drops on 1400 kHz (215 m), then the switch isn't Zero Ohms on MW, clean it.
Only do Part B, LW, if the LO tuning on MW is correct at 600 kHz and 1400 kHz
1) The three switches are open on LW. T6 sets the LF end, about 170 kHz (1750m). The T4 sets the HF end, typically 252 kHz is a good choice unless it's the rarer set that tunes to over 300 kHz. These interact. So repeat. Do not worry about LW coil or T5, those only affect gain, not tuning. Do not adjust T1, T2 or T3, though they all affect LW as they are to set up MW. The T6 and T4 may not set band limits correctly if Part A isn't properly done.
2) Adjust T5 at about 252 kHz on weak signal generator or RTE1. If the LW loop is wrong it only affects the gain, never tuning. If T5 has no effect on gain, re-check MW and T1 at about 1400 kHz. Only consider the LW loop aerial being wrong if all the TUNING is correct, but reception is totally rubbish.
I do get the main UK MW stations here on the 1939 Lissen from dusk and the R4 LW all day. Tuning accuracy is reasonable and the trimmers on the main tuning cap look "normal". See where Limerick City is on a map. I'm 10km further west. The 1945-1946 Model A also correctly tunes but is a lot quieter.
As a general principle either align MW & LW at these frequencies, or special marks on scale. Always check pointer / cursor is mechanically correct first. Never align at absolute ends of bands, especially the HF end as some caps increase slightly at fully open due to capacitance of mount, i.e. the minimum capacitance is often just before fully open.
Concentrate on the MW LO. Solder shorts across the switch contacts that close for LW as the symptom does sound like the LW LO coil switching isn't perfectly correct.
Referring to circuit I posted:
Part A:
1) T4 and LW secondary LO coil in series with T6 is shorted on MW. Solder a jumper across switch.
2) R4 and LW primary LO coil is shorted on MW. Switch resistance here will not so much detune. Only short switch here if step in one isn't enough.
3) T5 & LW RF coil are shorted on MW, don't worry about that. You can check later when LO is correct.
4) The LO trimmer on tuning cap T1 (furthest from drum) and T3 interact. T1 is HF end, adjust at about 1400 kHz (215 m)and T3 (likely in the IFT style can?) is the LF end, adjust on 600 kHz (500m). Repeat.
5) Remove the shorts. If the HF end shifts at all, then the switches are not Zero Ohms, especially the one in (1). Clean switch till no frequency shift on MW.
6) NOW trim the T2 (RF) for max gain at about 1400 kHz (215 m). Solder a short across the LW coil or the switch contact. Use as little signal as possible so AGC is at max gain (0V, goes negative. You can monitor DC volts on AGC line or AC volts at volume control. The MW loop coil is only "wrong" if no position changes the gain. THE LOOP NEVER AFFECTS TUNING, only gain. The number of turns sets gain at 600 kHz. It's unlikely to be wrong.
7) Remove the short on LW coil. If the signal level drops on 1400 kHz (215 m), then the switch isn't Zero Ohms on MW, clean it.
Only do Part B, LW, if the LO tuning on MW is correct at 600 kHz and 1400 kHz
1) The three switches are open on LW. T6 sets the LF end, about 170 kHz (1750m). The T4 sets the HF end, typically 252 kHz is a good choice unless it's the rarer set that tunes to over 300 kHz. These interact. So repeat. Do not worry about LW coil or T5, those only affect gain, not tuning. Do not adjust T1, T2 or T3, though they all affect LW as they are to set up MW. The T6 and T4 may not set band limits correctly if Part A isn't properly done.
2) Adjust T5 at about 252 kHz on weak signal generator or RTE1. If the LW loop is wrong it only affects the gain, never tuning. If T5 has no effect on gain, re-check MW and T1 at about 1400 kHz. Only consider the LW loop aerial being wrong if all the TUNING is correct, but reception is totally rubbish.
I do get the main UK MW stations here on the 1939 Lissen from dusk and the R4 LW all day. Tuning accuracy is reasonable and the trimmers on the main tuning cap look "normal". See where Limerick City is on a map. I'm 10km further west. The 1945-1946 Model A also correctly tunes but is a lot quieter.
