Check resistors R3, 15 meg and R4, 2.2 meg. One of these may be open. It's possible your AVC line goes negative and doesn't have a path to bleed this voltage off?
Norm
:I have an Arvin Model 544 with a unique AVC problem...tube complement is 12SA7,SK7,SQ7, 50L6, and 35Z5. The 12SQ7 serves as the detector, with the two rectifiers separated from each other. One acts as the signal detector and passes it on to the first AF and in addition feeds the AVC bus as you normally see. The other rectifier in the 12SQ7 ties directly to the AVC bus. The set only picks up the strongest local station and I have noticed the AVC bus voltage is -6V (as measured with a DVM) and it doesn't change much when tuned off station. If I load the AVC bus with a Simpson (to read the voltage) the set is very sensitive and picks up many stations. All tubes have been swapped and all DC voltages measure in spec. The set also has a floating ground. Why are the detector rectifiers connected this way, why doesn't the AVC voltage change when off station, and why does the darn thing play great when I load the AVC bus. I assume the root cause is the AVC staying too negative but I can't figure out why?? Any ideas out there?
Hi Joe
:
: Check resistors R3, 15 meg and R4, 2.2 meg. One of these may be open. It's possible your AVC line goes negative and doesn't have a path to bleed this voltage off?
:
:Norm
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:
:
::I have an Arvin Model 544 with a unique AVC problem...tube complement is 12SA7,SK7,SQ7, 50L6, and 35Z5. The 12SQ7 serves as the detector, with the two rectifiers separated from each other. One acts as the signal detector and passes it on to the first AF and in addition feeds the AVC bus as you normally see. The other rectifier in the 12SQ7 ties directly to the AVC bus. The set only picks up the strongest local station and I have noticed the AVC bus voltage is -6V (as measured with a DVM) and it doesn't change much when tuned off station. If I load the AVC bus with a Simpson (to read the voltage) the set is very sensitive and picks up many stations. All tubes have been swapped and all DC voltages measure in spec. The set also has a floating ground. Why are the detector rectifiers connected this way, why doesn't the AVC voltage change when off station, and why does the darn thing play great when I load the AVC bus. I assume the root cause is the AVC staying too negative but I can't figure out why?? Any ideas out there?
The AVC discharge path is through R4 and your volume control. Since you didn't mention a volume control problem figured it was ok. Be sure your volume control hasn't gone way up in value.
Remove R3, 22 meg. This will keep AVC for resting at a negative voltage.
The diode directly on the AVC line would keep it from going positive. The tube has two detector diodes. Guess they felt had to use both. Maybe someone has a better idea?
Look at Philco 40-215. In that radio one diode is detector and other AVC.
Norm
::Norm,
:R3 and R4 are okay...If I put a 1meg resistor in parallel with the avc cap, the avc bus voltage drops from -6v to -1.3v and regulates with the weaker stations. The sensitivity is outstanding...so I guess I will add the resistor...there are some notes on the schematic for this set that there have been some mods to the original design...maybe the resistor I am adding somehow fixes part of the past problem. By the way, do you know why the 12SQ7 diode sections are wired separately like they are rather than the typical parallel connection I normally see? The direct connection from the one to the avc cap makes no sense to me. As always, thanks for your help..
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:Hi Joe
::
:: Check resistors R3, 15 meg and R4, 2.2 meg. One of these may be open. It's possible your AVC line goes negative and doesn't have a path to bleed this voltage off?
::
::Norm
::
::
::
:::I have an Arvin Model 544 with a unique AVC problem...tube complement is 12SA7,SK7,SQ7, 50L6, and 35Z5. The 12SQ7 serves as the detector, with the two rectifiers separated from each other. One acts as the signal detector and passes it on to the first AF and in addition feeds the AVC bus as you normally see. The other rectifier in the 12SQ7 ties directly to the AVC bus. The set only picks up the strongest local station and I have noticed the AVC bus voltage is -6V (as measured with a DVM) and it doesn't change much when tuned off station. If I load the AVC bus with a Simpson (to read the voltage) the set is very sensitive and picks up many stations. All tubes have been swapped and all DC voltages measure in spec. The set also has a floating ground. Why are the detector rectifiers connected this way, why doesn't the AVC voltage change when off station, and why does the darn thing play great when I load the AVC bus. I assume the root cause is the AVC staying too negative but I can't figure out why?? Any ideas out there?
It may be R4 open? R3 actually feeds a negative voltage from your oscillator to AVC line.
Norm
:I have an Arvin Model 544 with a unique AVC problem...tube complement is 12SA7,SK7,SQ7, 50L6, and 35Z5. The 12SQ7 serves as the detector, with the two rectifiers separated from each other. One acts as the signal detector and passes it on to the first AF and in addition feeds the AVC bus as you normally see. The other rectifier in the 12SQ7 ties directly to the AVC bus. The set only picks up the strongest local station and I have noticed the AVC bus voltage is -6V (as measured with a DVM) and it doesn't change much when tuned off station. If I load the AVC bus with a Simpson (to read the voltage) the set is very sensitive and picks up many stations. All tubes have been swapped and all DC voltages measure in spec. The set also has a floating ground. Why are the detector rectifiers connected this way, why doesn't the AVC voltage change when off station, and why does the darn thing play great when I load the AVC bus. I assume the root cause is the AVC staying too negative but I can't figure out why?? Any ideas out there?