Plate Voltage on Detector high

5/14/2008 1:46:10 PMRon

I am working on a Zenith 4G903, http://www.nostalgiaair.org/Resources/845/M0024845.htm When I Opened it up I saw that R8 coming off the selenium rectifer was charred and crumbled when I touched it. I replaced all of the caps and discovered the selenium rectifier was bad as well. I replaced it with a diode and the radio worked, but only picked up strong stations. I checked the voltages on the tube pins from those that are listed on the schematic. The only one that was high was on pin 5 of the Is5 it has +34 volts instead of the listed 19. I replaced R8 and it shows +100v coming out of it which is what the schematic reads. I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value. What am I missing? Is 34v acceptable voltage for the plate of 1s5?

Thanks,
Ron

5/14/2008 5:20:59 PMDoug Criner

Ron, are you measuring voltages to B-, the "common return"?

When you replace a selenium diode with a silicon diode, as you did, all the DC voltages normally increase because of higher forward drop of a selenium diode - and it's usually necessary to add a dropping resistor. It's strange that just one voltage went up.

You could touch up the IF xfmr coils and see if sensitivity improves. It looks like those IF xfmrs have turnable slugs in the cores.
Doug

:I am working on a Zenith 4G903, http://www.nostalgiaair.org/Resources/845/M0024845.htm When I Opened it up I saw that R8 coming off the selenium rectifer was charred and crumbled when I touched it. I replaced all of the caps and discovered the selenium rectifier was bad as well. I replaced it with a diode and the radio worked, but only picked up strong stations. I checked the voltages on the tube pins from those that are listed on the schematic. The only one that was high was on pin 5 of the Is5 it has +34 volts instead of the listed 19. I replaced R8 and it shows +100v coming out of it which is what the schematic reads. I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value. What am I missing? Is 34v acceptable voltage for the plate of 1s5?
:
:Thanks,
:Ron

5/14/2008 5:48:45 PMRon

I was measuring from the negative side of the electrolytics, should I measure from someplace else. I had read that I might need to put a resistor in line with the diode, but when I checked the voltage it measured 100 volts like the schematic lists.

:Ron, are you measuring voltages to B-, the "common return"?
:
:When you replace a selenium diode with a silicon diode, as you did, all the DC voltages normally increase because of higher forward drop of a selenium diode - and it's usually necessary to add a dropping resistor. It's strange that just one voltage went up.
:
:You could touch up the IF xfmr coils and see if sensitivity improves. It looks like those IF xfmrs have turnable slugs in the cores.
:Doug
:
::I am working on a Zenith 4G903, http://www.nostalgiaair.org/Resources/845/M0024845.htm When I Opened it up I saw that R8 coming off the selenium rectifer was charred and crumbled when I touched it. I replaced all of the caps and discovered the selenium rectifier was bad as well. I replaced it with a diode and the radio worked, but only picked up strong stations. I checked the voltages on the tube pins from those that are listed on the schematic. The only one that was high was on pin 5 of the Is5 it has +34 volts instead of the listed 19. I replaced R8 and it shows +100v coming out of it which is what the schematic reads. I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value. What am I missing? Is 34v acceptable voltage for the plate of 1s5?
::
::Thanks,
::Ron

5/14/2008 7:05:55 PMEdd

I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value.

ALSO check the 1U5’s 1st grid biasing resistor carefully…..R11, and its companion over at R14 is also tied into that loop.

73's de Edd

5/15/2008 12:29:14 AMRon(62266:62260)

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: I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value.
:
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:
:
:
:ALSO check the 1U5’s 1st grid biasing resistor carefully…..R11, and its companion over at R14 is also tied into that loop.
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:73's de Edd
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:Both check fine, am I worry about something I should not? It just seems that 15 volts high is beyond the tolerance level, but I am pretty new at this, so should I just do an alignment and see how it performs? Thanks for the help. Ron

5/15/2008 12:58:43 AMNorm Leal(62267:62266)

Hi
Plate and screen voltage on a 1S5 or 1U5 (both being talked about but only one in a radio) will read high using a modern meter.
Voltage is being supplied through large value resistors. Older meters will load down voltage so they read lower. Modern Digital meters don't load as much so voltage will be higher.
This is the 1st audio amplifier in a radio. As long as audio doesn't distort voltage is ok.
Norm
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:: I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value.
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::
::
::
::ALSO check the 1U5’s 1st grid biasing resistor carefully…..R11, and its companion over at R14 is also tied into that loop.
::
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::
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::73's de Edd
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::
::
::
::
::Both check fine, am I worry about something I should not? It just seems that 15 volts high is beyond the tolerance level, but I am pretty new at this, so should I just do an alignment and see how it performs? Thanks for the help. Ron

5/15/2008 6:19:15 PMDoug Criner(62282:62267)

Norm, I thought about the meter impedance - but the schematic says all voltages are measured with a VTVM. Hmmm.
Doug

:Hi
:
: Plate and screen voltage on a 1S5 or 1U5 (both being talked about but only one in a radio) will read high using a modern meter.
:
:
:Norm
:

5/15/2008 11:31:44 PMNorm Leal(62286:62282)

Doug
In that case your 1S5 tube emission may be a little low? It should still operate ok.
Norm
:Norm, I thought about the meter impedance - but the schematic says all voltages are measured with a VTVM. Hmmm.
:Doug
:
:
::Hi
::
:: Plate and screen voltage on a 1S5 or 1U5 (both being talked about but only one in a radio) will read high using a modern meter.
::
::
::Norm
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:

5/16/2008 9:10:32 AMRadiodoc(62296:62282)

Doug,
The older VTVM that was specified as to use to measure probably had a 1 meg input considerably lower than the 10 meg plus of the VOMs of today.
Radiodoc
***************

:Norm, I thought about the meter impedance - but the schematic says all voltages are measured with a VTVM. Hmmm.
:Doug
:
:
::Hi
::
:: Plate and screen voltage on a 1S5 or 1U5 (both being talked about but only one in a radio) will read high using a modern meter.
::
::
::Norm
::
:

5/15/2008 9:17:03 AMRadiodoc(62273:62266)

Ron,
I don't think the higher plate voltage on the tube is anything to worry about. RCA tube data doesn't really specify a specific voltage on the plate, rather it is specified the plate "supply" voltage be 90 volts thru a 1 meg resistor to the plate and the screen "supply" voltage be 90 volts supplied thru a 3 meg resistor to the screen. As Norm stated, if the audio sounds good and is not distorted, then everything is probably fine.
Radiodoc
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::
::
::
::
::
::
::
::
:: I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value.
::
::
::
::
::
::ALSO check the 1U5’s 1st grid biasing resistor carefully…..R11, and its companion over at R14 is also tied into that loop.
::
::
::
::
::73's de Edd
::
::
::
::
::
::Both check fine, am I worry about something I should not? It just seems that 15 volts high is beyond the tolerance level, but I am pretty new at this, so should I just do an alignment and see how it performs? Thanks for the help. Ron

5/15/2008 9:53:20 AMRon(62276:62273)

Thanks guys, I won't worry about the voltage, do an alignment and enjoy the set!
Ron
:Ron,
:
:I don't think the higher plate voltage on the tube is anything to worry about. RCA tube data doesn't really specify a specific voltage on the plate, rather it is specified the plate "supply" voltage be 90 volts thru a 1 meg resistor to the plate and the screen "supply" voltage be 90 volts supplied thru a 3 meg resistor to the screen. As Norm stated, if the audio sounds good and is not distorted, then everything is probably fine.
:
:Radiodoc
:****************
:
:
:::
:::
:::
:::
:::
:::
:::
:::
::: I have checked R7,R9,R10,R12, and R13 and they all measure very close to the stated value.
:::
:::
:::
:::
:::
:::ALSO check the 1U5’s 1st grid biasing resistor carefully…..R11, and its companion over at R14 is also tied into that loop.
:::
:::
:::
:::
:::73's de Edd
:::
:::
:::
:::
:::
:::Both check fine, am I worry about something I should not? It just seems that 15 volts high is beyond the tolerance level, but I am pretty new at this, so should I just do an alignment and see how it performs? Thanks for the help. Ron