26-06-2011, 09:18 AM
Just a point about voltages on the T105:
Loooking at the ERT Service Chart it states that the readings listed were taken using an AVO model 7, (whereas most service sheets state that an AVO 8 was used). Although either meter will read that actual voltage present when the meter is applied to the circuit, those voltages will be lower than they would be with the AVO not applied to the circuit, due to the loading effect of the meter. The loading will be less pronounced at higher voltages, but will be much more pronounced when using an AVO7. An AVO 8 has an input impedance of 20,000 Ohms per volt, but the Avo 7 in only 1,000 Ohms per volt. Even the cheapest and tackiest modern digital have an input impedence of 20 MegOhms, so won't load the circuit to any extent. Hence, the readings will be much higher, and a more accurate representation of what the voltages are before the operaration of the circuit is affected by applying say an AVO 7 or 8.
To give an example of how this would affect the voltage measurements on the Little Maestro T105, consider the potential divider comprised by R14 (47k) and R15 (1 MegOhm). If we were to apply an AVO 7 on the 100V range across R15, this would have the same effect as applying a 100k resistor in parallel with that 1 MegOhm resistor. From Ohms Law we can calculate that the resistance at that point would fall to 90.909K - a tenth of what it would be if a modern cheapo digital voltmeter was place across the 1 MegOhm resistor, and a tenth of what it would be before the AVO 7 was plonked across it. The effect of this would be to stop the circuit from working.
That's why on the ERT Service Chart for the 'T105' it states: 'Readings were taken on an AVO model 7. The voltages below are the only ones that can be taken with the receiver operating without upsetting its performance'.
It then lists Voltage readings at just four test points on the circuit. For example, V1 pin 1 85V; V1 pin 8 75V.
I've no idea how much higher the voltages at those points would be if taken with a modern digital multimeter with a 20 MegOhm input impedance as compared to an AVO 7 at only 1,000 OPV, or even an AVO8 at 20,000 OPV. Not perhaps as high as those observed by Joe, but definately higher.
I can uderstand that many restorers like to use AVOs when referring to service sheets which had readings taken by the same model of instrument, and I can understand why many hold them in high regard and have a great affection for them as robust instruments, but certainly on solid state gear, for the reasons stated, they're of little practical use. I've mentioned before that if we consider a potential divider consisting of two 100k resistors with 12 Volts applied across it, we know from Ohms Law that the true Voltage at the junction would be 6 Volts, and that's what you'd see with a digital voltmeter (20MegOhm input impedance). If howwever, we stuck an Avo 8 (20,000 OPV) across the junction to ground, it would read just 4.8V - an accurate reading of the voltage at that point, because of the loading of the AVO8 on the circuit reducing the voltage. With an AVO7 (1,000 OPV) it would be even worse - it would measure just 1 Volt and would doubtless stop the circuit from working.
I'm not 'dissing' AVOs, and as I said, they do have their uses when taking measurements and comparing them with circuit data on which the measurements werre originally taken with the same model of instrument. However, beyond that, especially where solid state gear and low voltages are concerned, they're best left on the shelf as fond reminders of how life used to be - not as a serious piece of test gear for today's purposes.
Sorry to have drifted off topic to some extent, but I felt that I had to go into some detail to justify why I made the iinitial assertion about the voltages on the Service Chart having been taken with an AVO 7, and the implications of that, be it on the T1105 or any other set if the same instrument isn't used when checking voltatges.
David
Loooking at the ERT Service Chart it states that the readings listed were taken using an AVO model 7, (whereas most service sheets state that an AVO 8 was used). Although either meter will read that actual voltage present when the meter is applied to the circuit, those voltages will be lower than they would be with the AVO not applied to the circuit, due to the loading effect of the meter. The loading will be less pronounced at higher voltages, but will be much more pronounced when using an AVO7. An AVO 8 has an input impedance of 20,000 Ohms per volt, but the Avo 7 in only 1,000 Ohms per volt. Even the cheapest and tackiest modern digital have an input impedence of 20 MegOhms, so won't load the circuit to any extent. Hence, the readings will be much higher, and a more accurate representation of what the voltages are before the operaration of the circuit is affected by applying say an AVO 7 or 8.
To give an example of how this would affect the voltage measurements on the Little Maestro T105, consider the potential divider comprised by R14 (47k) and R15 (1 MegOhm). If we were to apply an AVO 7 on the 100V range across R15, this would have the same effect as applying a 100k resistor in parallel with that 1 MegOhm resistor. From Ohms Law we can calculate that the resistance at that point would fall to 90.909K - a tenth of what it would be if a modern cheapo digital voltmeter was place across the 1 MegOhm resistor, and a tenth of what it would be before the AVO 7 was plonked across it. The effect of this would be to stop the circuit from working.
That's why on the ERT Service Chart for the 'T105' it states: 'Readings were taken on an AVO model 7. The voltages below are the only ones that can be taken with the receiver operating without upsetting its performance'.
It then lists Voltage readings at just four test points on the circuit. For example, V1 pin 1 85V; V1 pin 8 75V.
I've no idea how much higher the voltages at those points would be if taken with a modern digital multimeter with a 20 MegOhm input impedance as compared to an AVO 7 at only 1,000 OPV, or even an AVO8 at 20,000 OPV. Not perhaps as high as those observed by Joe, but definately higher.
I can uderstand that many restorers like to use AVOs when referring to service sheets which had readings taken by the same model of instrument, and I can understand why many hold them in high regard and have a great affection for them as robust instruments, but certainly on solid state gear, for the reasons stated, they're of little practical use. I've mentioned before that if we consider a potential divider consisting of two 100k resistors with 12 Volts applied across it, we know from Ohms Law that the true Voltage at the junction would be 6 Volts, and that's what you'd see with a digital voltmeter (20MegOhm input impedance). If howwever, we stuck an Avo 8 (20,000 OPV) across the junction to ground, it would read just 4.8V - an accurate reading of the voltage at that point, because of the loading of the AVO8 on the circuit reducing the voltage. With an AVO7 (1,000 OPV) it would be even worse - it would measure just 1 Volt and would doubtless stop the circuit from working.
I'm not 'dissing' AVOs, and as I said, they do have their uses when taking measurements and comparing them with circuit data on which the measurements werre originally taken with the same model of instrument. However, beyond that, especially where solid state gear and low voltages are concerned, they're best left on the shelf as fond reminders of how life used to be - not as a serious piece of test gear for today's purposes.
Sorry to have drifted off topic to some extent, but I felt that I had to go into some detail to justify why I made the iinitial assertion about the voltages on the Service Chart having been taken with an AVO 7, and the implications of that, be it on the T1105 or any other set if the same instrument isn't used when checking voltatges.
David
