25-07-2020, 10:08 AM
(This post was last modified: 25-07-2020, 10:13 AM by Mike Watterson.)
Conclusions:
The DK96 oscillation failing under certain conditions like touching or extending an aerial seems to be related to condition. The original didn't work well on OC2 and not at all on OC1. A replacement DK96 didn't exhibit the aerial issue, in fact MW & LW receptiion was louder with a whip or touching the wire to the whip.
A beginner mistake not checking IFT coli continuity. That would have saved a lot of time.
Be suspicious of any component that's obviously resoldered, and area where any leads seem inexplicably snapped (or cut?). Be very suspicious of any part with far too long / original length leads and where one end goes to a place with lots of tags. One end of 33K HT feed resistor was to wrong tag, would only work well on MW.
Alignment
IF is first. Keep reducing the signal so AGC is at max gain. AGC action will make it hard to see correct peaks. IF was stamped on the IFT bases, but it can be anything from 128 kHz to 1700 kHz. Mixing up 455, 465, 470 etc will usually make tracking poorer. Below 455 KHz and above 480 kHz is very rare on domestic sets between 1937 and today. Always leave FM till after AM.
The first thing is to make sure tuning capacitor isn't faulty and you know where the mechanical alignment of the pointer is. Usually the lowest frequency, or a special mark at that end of the scale. Many tuning caps increase slightly in capacitance as fully opened due to rotor edges coming close to stator. One solution on LW/MW, if in doubt, is to put tuning to an empty part of the band on LW (or MW, only one) with rotor a bit past half closed and tune the LO coil for maximum noise with the two main trimmers about 1/4 turn open as the LW & MW RF tuning in the lower frequency range is little affected by the trimmers and RF tuning is usually set by coil position on the ferrite rod. Then move the scale pointer to that frequency!
Iteratively repeat adjusting RF & LO trimers at HF end (about 260 KHz or 1400 kHz) and recheck lower end peak RF, no lower than 600kHz, or the LW scale equivalent, but only on MW. Or if there are now LW trimmers and LW is more important, then LW only. Doing this resulted in loud R4 LW and louder RTE LW. Then the pointer did seem to line up to a logical point on the scale fully closed. It's MUCH easier with alignment instructions. Never align RF & LO coils and trimmers at the ends of the scale. See actual MW alignment instructions and around 1400 kHz to 1500 kHz and about 600 kHz are typical.
If there are bands created by switching fixed parts with no trimmers or separate coils then don't align them. Check switched in capacitors if madly out.
Other thoughts
Use a shunt regulator (two suitable rectifiers for 1.3V to 1.5V) on the LT PSU so any inadvertent short doesn't blow the filaments. On this set the B7G power connector had corroded pins, sockets and wires. I had to cut back all four wires to reliably resolder them. Connect LT first and disconnect last. Check voltage on LT- to HT- bias resistor from time to time (it's 180 Ohms to 1000 Ohms depending on O/P valve type and HT. Series filament sets don't need it as the O/P valve f+ is usually at LT+ and the DAF96/DAF91/1U5 etc is usually f- to LT- = 0V. On series sets the DAF91/1U5/DAF96 etc should be 1.3V to 1.35V, also that's a reasonable range for external LT on a parallel set. Similarly, the 6.3V valves are absolutely fine at 6.0V AC, in fact an accurate 6.0V is better than a poor 6.3V that's really 7V. The actual range is about 5.75 to 7V. Life about halves with every 10% increase from 1.35V or 6V.
Testing Frequency Changers
Even an AVO VCM163 is poor for Heptodes, Octodes and Nonodes as well as Triode-Pentodes and Triode-Hexodes. You can only do a basic emission test, the gain tests are only of significance for the Triodes or treating g1 to g2 as as triode on the single valve types. You need either a known good valve or a known good radio.
The DK96 oscillation failing under certain conditions like touching or extending an aerial seems to be related to condition. The original didn't work well on OC2 and not at all on OC1. A replacement DK96 didn't exhibit the aerial issue, in fact MW & LW receptiion was louder with a whip or touching the wire to the whip.
A beginner mistake not checking IFT coli continuity. That would have saved a lot of time.
Be suspicious of any component that's obviously resoldered, and area where any leads seem inexplicably snapped (or cut?). Be very suspicious of any part with far too long / original length leads and where one end goes to a place with lots of tags. One end of 33K HT feed resistor was to wrong tag, would only work well on MW.
Alignment
IF is first. Keep reducing the signal so AGC is at max gain. AGC action will make it hard to see correct peaks. IF was stamped on the IFT bases, but it can be anything from 128 kHz to 1700 kHz. Mixing up 455, 465, 470 etc will usually make tracking poorer. Below 455 KHz and above 480 kHz is very rare on domestic sets between 1937 and today. Always leave FM till after AM.
The first thing is to make sure tuning capacitor isn't faulty and you know where the mechanical alignment of the pointer is. Usually the lowest frequency, or a special mark at that end of the scale. Many tuning caps increase slightly in capacitance as fully opened due to rotor edges coming close to stator. One solution on LW/MW, if in doubt, is to put tuning to an empty part of the band on LW (or MW, only one) with rotor a bit past half closed and tune the LO coil for maximum noise with the two main trimmers about 1/4 turn open as the LW & MW RF tuning in the lower frequency range is little affected by the trimmers and RF tuning is usually set by coil position on the ferrite rod. Then move the scale pointer to that frequency!
Iteratively repeat adjusting RF & LO trimers at HF end (about 260 KHz or 1400 kHz) and recheck lower end peak RF, no lower than 600kHz, or the LW scale equivalent, but only on MW. Or if there are now LW trimmers and LW is more important, then LW only. Doing this resulted in loud R4 LW and louder RTE LW. Then the pointer did seem to line up to a logical point on the scale fully closed. It's MUCH easier with alignment instructions. Never align RF & LO coils and trimmers at the ends of the scale. See actual MW alignment instructions and around 1400 kHz to 1500 kHz and about 600 kHz are typical.
If there are bands created by switching fixed parts with no trimmers or separate coils then don't align them. Check switched in capacitors if madly out.
Other thoughts
Use a shunt regulator (two suitable rectifiers for 1.3V to 1.5V) on the LT PSU so any inadvertent short doesn't blow the filaments. On this set the B7G power connector had corroded pins, sockets and wires. I had to cut back all four wires to reliably resolder them. Connect LT first and disconnect last. Check voltage on LT- to HT- bias resistor from time to time (it's 180 Ohms to 1000 Ohms depending on O/P valve type and HT. Series filament sets don't need it as the O/P valve f+ is usually at LT+ and the DAF96/DAF91/1U5 etc is usually f- to LT- = 0V. On series sets the DAF91/1U5/DAF96 etc should be 1.3V to 1.35V, also that's a reasonable range for external LT on a parallel set. Similarly, the 6.3V valves are absolutely fine at 6.0V AC, in fact an accurate 6.0V is better than a poor 6.3V that's really 7V. The actual range is about 5.75 to 7V. Life about halves with every 10% increase from 1.35V or 6V.
Testing Frequency Changers
Even an AVO VCM163 is poor for Heptodes, Octodes and Nonodes as well as Triode-Pentodes and Triode-Hexodes. You can only do a basic emission test, the gain tests are only of significance for the Triodes or treating g1 to g2 as as triode on the single valve types. You need either a known good valve or a known good radio.
