Zenith Model 808

6/25/2009 9:43:00 PMSteve

I recapped and replace all the resistors including the candohm. The radio plays good picking up many stations around the dial and on all three bands. I have the radio running on 112vac from my variac per the setup data. Voltage reading were taken to chassis ground. The 42 power output tube has -15 volts on pin 4 (G1). The setup data says it should be .6 volts. The 6A7 tube has 241 volts on pin 4 (G2). The setup data says it should be 70 volts. All the other voltages are within reason. Do these two readings indicate that I still have a problem with this radio?
Thanks
Steve

6/26/2009 1:12:12 AMZ-

:I recapped and replace all the resistors including the candohm. The radio plays good picking up many stations around the dial and on all three bands. I have the radio running on 112vac from my variac per the setup data. Voltage reading were taken to chassis ground. The 42 power output tube has -15 volts on pin 4 (G1). The setup data says it should be .6 volts. The 6A7 tube has 241 volts on pin 4 (G2). The setup data says it should be 70 volts. All the other voltages are within reason. Do these two readings indicate that I still have a problem with this radio?
:Thanks
:Steve
:

If the radio plays well, maybe you shouldn't be too concerned about those readings as data sheets specify those were done using a 1Kohms/V voltmeter! Using a DMM will definitely show higher voltages.

Without looking at tubes specs, that -15V bias sounds about right for a 42.

Syl

6/26/2009 6:06:20 AMEdd

6/26/2009 8:45:41 PMSteve

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Sir Steve. . . .

I also concur on the -15 VDC being of the correct biasing vicinity for the AF output tube, so that is signifying that the
250 ohm leg that is held above ground on the Candohm, is of the proper value.

While we're on the Candohm aspect. . . can we assume that your sets first resistor value off the main B+ is a 11K ohm unit followed by a 5700 ohm and then the 85 ohm unit ?

Those constitute the voltage divider elements , the most interest of your situation being the 11K-57K junction which feeds the other tubes screens. WITH the one exception, being on your 6A7, wherein ITS screen grid is acting as an "ersatz"plate for the local oscillator circuitry, and ITS B+ supply is from the MAIN B+, coming in thru the R9 and R10. . . .19 and 11K feed resistors.

Therefore, I would EXPECT a healthy level of B+ there, more so than the specified 70 VDC , additionally they spec'd the other screens at that SAME 70 VDC level, even though they are using different supply derivation points.

An interesting experiment might be to use your digital metering in reading all of the sets screen voltages again, but this time shunt your digital metering unit with a 100K resistor to assimilate the loading aspect of their specified 'tousand ohm per volt metering
and it being in its 100VDC range. Then, see what the voltage readings variances are.

73's de Edd

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Thanks Ed & Syl for your help.
I did shunt my Fluke 87 with a 100K resistor. Pin 4 of the 42 tube know reads -1.9vdc. Pin 4 of the 6A7 tube reads 166vdc. I think 70vdc for the 6A7 that is stated on the data sheet must be wrong. The alignment went fine and the radio plays great.
Thanks
Steve

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:Sir Steve. . . .
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:I also concur on the -15 VDC being of the correct biasing vicinity for the AF output tube, so that is signifying that the
:250 ohm leg that is held above ground on the Candohm, is of the proper value.
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:While we're on the Candohm aspect. . . can we assume that your sets first resistor value off the main B+ is a 11K ohm unit followed by a 5700 ohm and then the 85 ohm unit ?
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:Those constitute the voltage divider elements , the most interest of your situation being the 11K-57K junction which feeds the other tubes screens. WITH the one exception, being on your 6A7, wherein ITS screen grid is acting as an "ersatz"plate for the local oscillator circuitry, and ITS B+ supply is from the MAIN B+, coming in thru the R9 and R10. . . .19 and 11K feed resistors.
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:Therefore, I would EXPECT a healthy level of B+ there, more so than the specified 70 VDC , additionally they spec'd the other screens at that SAME 70 VDC level, even though they are using different supply derivation points.
:
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:An interesting experiment might be to use your digital metering in reading all of the sets screen voltages again, but this time shunt your digital metering unit with a 100K resistor to assimilate the loading aspect of their specified 'tousand ohm per volt metering
:and it being in its 100VDC range. Then, see what the voltage readings variances are.
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:73's de Edd
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