20-05-2024, 08:09 PM
(This post was last modified: 20-05-2024, 08:10 PM by Mike Watterson.)
Tolerances when testing a valve in the equipment
Especially any Pentode.
Because of how they were made the resistors more than 100 k Ohms tend to go high and less than 1000 Ohms go low. Often the output valve screen grid (g2) connects to an HT or the output transformer, so not an issue. Many resistors were 20% and grid leaks (g1 to 0V) can be 40K to 5 M Ohm on the output valve and it won't make much difference. The cathode resistor should though be within 10% and nearest 5% part is fine for replacement, such as 220 Ohms for a part marked 200 on the schematic. Screen grid to HT resistors going high will reduce gain, but fine if within 30% of original value.
AGC capacitors are not critical as long as working.
Electrolytics can be +90% and -40% and it won't matter as long as they are neither leaky (oxide broken down or high ESR (Effective Series Resistance), usually due to drying out. Same voltage or higher for replacements and even twice the value is fine except on two places. The rectifier valve has a maximum direct connected capacitance on data sheet (peak current) and the replacement FM discriminator or similar should be nearest next highest value, thus 4.7uF for a 4uF part. It is usually -ve to the diodes (semiconductor or 2 in an EABC80) and +ve to chassis.
Nearest +20% is fine for replacing any paper caps. I usually replace the "tone correction" with a 1KV ceramic of nearest value. Others are replaced by suitable 250V or 400V metpoly/polyester caps. I may use PCB types with tied on and soldered extension wires and sleeves rather than the round yellow ones. Many paper caps in battery sets can be replaced by 50V or 100V ceramic parts. I might snip out most of the "live" end of a paper cap and leave it in place.
If the g2 voltage is near HT with no valve and drops alarmingly and cathode voltage is madly high, did you really replace cap feeding g1? If the g2 drops a lot but cathode voltage is OK, suspect an HT line resistor gone high, or the g2 to HT resistor if there is one.
Don't rely on an old tester bridge from 1960s or earlier, but a DMM or a modern component tester (discharge caps twice before testing). Unplug mains before testing or changing parts.
If you have no leakage tester reading socket voltages with HT live and no valves fitted will reveal leaky caps. A DMM even on 20 M Ohm range can't see capacitor leakage. You need uA meter and 100s of volts, or the safe flashgun based tester.
Especially any Pentode.
Because of how they were made the resistors more than 100 k Ohms tend to go high and less than 1000 Ohms go low. Often the output valve screen grid (g2) connects to an HT or the output transformer, so not an issue. Many resistors were 20% and grid leaks (g1 to 0V) can be 40K to 5 M Ohm on the output valve and it won't make much difference. The cathode resistor should though be within 10% and nearest 5% part is fine for replacement, such as 220 Ohms for a part marked 200 on the schematic. Screen grid to HT resistors going high will reduce gain, but fine if within 30% of original value.
AGC capacitors are not critical as long as working.
Electrolytics can be +90% and -40% and it won't matter as long as they are neither leaky (oxide broken down or high ESR (Effective Series Resistance), usually due to drying out. Same voltage or higher for replacements and even twice the value is fine except on two places. The rectifier valve has a maximum direct connected capacitance on data sheet (peak current) and the replacement FM discriminator or similar should be nearest next highest value, thus 4.7uF for a 4uF part. It is usually -ve to the diodes (semiconductor or 2 in an EABC80) and +ve to chassis.
Nearest +20% is fine for replacing any paper caps. I usually replace the "tone correction" with a 1KV ceramic of nearest value. Others are replaced by suitable 250V or 400V metpoly/polyester caps. I may use PCB types with tied on and soldered extension wires and sleeves rather than the round yellow ones. Many paper caps in battery sets can be replaced by 50V or 100V ceramic parts. I might snip out most of the "live" end of a paper cap and leave it in place.
If the g2 voltage is near HT with no valve and drops alarmingly and cathode voltage is madly high, did you really replace cap feeding g1? If the g2 drops a lot but cathode voltage is OK, suspect an HT line resistor gone high, or the g2 to HT resistor if there is one.
Don't rely on an old tester bridge from 1960s or earlier, but a DMM or a modern component tester (discharge caps twice before testing). Unplug mains before testing or changing parts.
If you have no leakage tester reading socket voltages with HT live and no valves fitted will reveal leaky caps. A DMM even on 20 M Ohm range can't see capacitor leakage. You need uA meter and 100s of volts, or the safe flashgun based tester.
