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 Post subject: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 12:48 am 
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Joined: Jul Sat 24, 2010 11:17 pm
Posts: 45
Location: Washington State
I am working on an RCA T7-5 that does not appear to have anything ever done to the wiring or any components replaced. I do not know if the radio was working. I did not plug it in with so many bare rubber coated wires with some obvious shorts. My question is how it could have worked with the wires on pins 3 &4 swapped from what the schematic shows. This diode make the bias voltage for the 6K7 tubes. I do not know exactly how the 6H6 is constructed internally but connected this way the Plate is being used as the cathode and vice versa. How could this have been working?


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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 2:28 am 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 336
6H6, pin 3 is anode and pin 4 is cathode. Could you tell us where these two pins are connected in your radio?

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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 2:35 am 
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Joined: Jan Mon 16, 2012 4:15 pm
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Location: Near Brandon, Iowa
6H6 is a double diode. One diode is the audio signal demodulator, the other one creates the negative voltage used by the AVC bus. These are independent functions in this radio (often, a single diode is used to perform both functions).

It doesn't matter if pins 3 and 4 (anode and cathode of the audio signal demodulator) are swapped, since the signal (audio-modulated RF) is symmetrical about zero volts... results are the same. Try it and see.

It would be problematic if the cathode and plate of the AVC diode were swapped... but the audio demod just doesn't care.


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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 3:46 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4786
Location: Lexington, KY USA
Perhaps there were different versions of this model, but the Rider schematic on NA shows one diode doing all the detecting for both audio and AVC.

http://www.nostalgiaair.org/pagesbymode ... 015833.pdf

The schematic lacks pin numbers for the tubes, but these are shown on the wiring diagram. It's too bad the draftsman left this work for the service tech, but it's not unusual.

Attachment:
Screenshot from 2021-06-08 22-18-20.png
Screenshot from 2021-06-08 22-18-20.png [ 208.44 KiB | Viewed 691 times ]

Diode 1, on pins 5 and 8 of the 6H6 is the detector. Both the audio signal and the AVC are sourced from the IF can terminal that has the 2.2Meg resistor and 0.01uF capacitor connected to it.

The cathode of diode 1 is connected to chassis ground. If pins 5 and 8 are reversed, the audio should be OK, but the AVC will not work at all.

Note that the schematic fails to show that there are a couple of resistors and a fixed capacitor inside the IF can.

Diode 2 of the 6H6 is supposed to act as a DC clamp for the AVC line. The cathode on pin 4 is shown connected to a negative bias voltage (-3.1V), while the plate on pin 3 connects to the AVC line. If these two pins are reversed, the AVC will barely work, since the clamp will then severely limit how negative the AVC line can go.

Ted


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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 7:25 am 
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Joined: Jul Sat 24, 2010 11:17 pm
Posts: 45
Location: Washington State
Thanks for the input. When I get the rewiring done I will try swapping the Diode 2 wires as Lorenz suggested. I have now attached my schematic as I should have done originally. I think it is much clearer as to understanding the situation.


Attachments:
RCA T7-5 Wiring Schematic 1 Excerpt.jpg
RCA T7-5 Wiring Schematic 1 Excerpt.jpg [ 313.01 KiB | Viewed 678 times ]
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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 1:09 pm 
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Location: Victoria, Australia
The whole circuit does show that set to be "back biased" via R17 & 18. That will provide a fixed bias voltage for the grid of the Pentagrid via D2, as well as AGC voltage. It is built for metal tubes.

Marc


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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 4:02 pm 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 336
D1 diode performs the detection of the audio signal and at the same time provides the necessary AGC negative voltage for the grids of the RF stages in order to limit the level of strong RF signals. D2 and R17 provide the necessary negative bias at the grids of the RF stages since the cathodes are directly grounded. The role of D2 is to prohibit the AGC voltage to be grounded via the R17 and hence no AGC action would take place.

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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 5:56 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 4786
Location: Lexington, KY USA
Attachment:
RCA T7-5 Wiring Schematic 1 Excerpt.jpg
RCA T7-5 Wiring Schematic 1 Excerpt.jpg [ 152.16 KiB | Viewed 625 times ]

This version of the schematic is nice and clear, and clearly has Diode 2 of the 6H6 hooked up backwards.

Some long ago draftsman came back after lunch and got it mixed up.

I wonder where this re-drawn version originated. It's the sort of mistake frequently found in SAMS redrawn schematics.

I'm pretty sure the NA Rider print is a photo reproduction of the prints issued by RCA. The Rider print correctly shows the plate of the diode connecting to the AVC line, on both the schematic and the wiring diagrams.

The bottom line for this is that the radio must be wired to match the Rider print in order to work correctly.

Ted


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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Wed 09, 2021 6:32 pm 
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Joined: Mar Thu 21, 2019 3:53 pm
Posts: 336
I agree with you Ted. The anode of D2 should be connected to the AGC line. The original schematic seems to be correct.
http://oldtech.net/RCA/T7-5/35P390.gif

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 Post subject: Re: RCA T7-5 Detector
PostPosted: Jun Thu 10, 2021 12:34 am 
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Joined: Feb Sun 01, 2009 2:56 pm
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Location: Victoria, Australia
Salient point; If that is the factory wiring leave it alone, or if its cactus, rewire it as it was done in the factory: It worked when it left the place.

Marc


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