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 Post subject: Line in question
PostPosted: Sep Sun 26, 2021 3:59 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
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Location: Bristol, Rhode Island
Hi all, this question got buried in another thread so I thought I would re-post if that’s OK. Thank you!

I have a Firestone 3071 that I've recapped. The Riders schematic includes a schematic to modify/wire in a phono input. I would like to add this.

However it does not look like this phono circuit fits the actual schematic. Specifically I would love some advice on how to apply it to my radio.

First, at the left side of the DPDT switch, the node of R43,C15 and R45 is disconnected from 6U7 screen grid in phono mode. If switched back to radio position, that node is reconnected to the grid. Note that there is nothing else on that node.

But if you look at the original schematic at right, that node of R43,C15 and R45 goes to 6U7 grid, but there's also a leg going off to the left just above R43. That goes to the 1st audeio 6L7 grid only.

With this lead unaccounted for on the phono schematic, I don't know where to make the break.

And there appears to be an error with respect to the right side of the switch. Please correct me if I'm wrong?

In radio mode, phono schematic shows high side of volume pot to left side of R34.
In regular schematic, high side of volume pot goes to secondary of the IF and and R44 in to the eye tube grid.

So I have a few uncertainties about how to wire this phono circuit into the schematic. Can anyone help? Thanks!


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 Post subject: Re: Line in question
PostPosted: Sep Sun 26, 2021 4:43 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8119
Location: Near Brandon, Iowa
The DPDT does two things:

(1) When in Phono mode, one pole of the switch removes screen grid voltage from the 6U7 tube. This "kills" the IF amp tube and is done to prevent audio signal leakthrough from the radio to the audio amp stage. In Radio mode, the screen grid is connected and the tube will operate normally.

Strictly speaking, this switching is not needed provided you are willing to retune the radio off-station when you want to use the phono input (and tune the station back in when you are done with the phono mode). In other words, a SPDT switch would do if you are willing to put up with a bit of inconvenience. (If the set is going to be seldom used in the Radio mode, it might be better to not implement this portion of the switch).

(2) When in Radio mode, the other pole of the switch selects the Demodulator circuit signal source. When in Phono mode, the arm of the switch is disconnected from the demodulator and connected to the Phono input jack "hot" contact.

The arm of the new DPDT switch must go to the "top end" of the volume control pot, which is the end lug opposite the grounded lug. The wire from the demodulator circuit which originally went to the VC top end lug is first removed from the pot and relocated to the pole of the new DPDT switch that will give continuity to the switch arm from the demodulator circuit when the switch is in Radio mode.

Really, that's all you need to know to make the circuit work. The two drawings have different pickoff nodes for the demodulated signal but either will work. You'd have to examine your radio to know exactly how your set is wired... but you can implement the switch as described above without needing to know this info.


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 Post subject: Re: Line in question
PostPosted: Sep Sun 26, 2021 5:10 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 3542
Location: Bristol, Rhode Island
Thanks. I was mainly confused about the missing node wire shown in the left side of the switch. But I have no problem switching to SW mode or some quiet place in the dial, instead.

One other question the phono schematic brings up: Should I also remove the eye tube lead from the high side of the volume pot, or just the lead going to the IF secondary?

Thanks,

Ted


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 Post subject: Re: Line in question
PostPosted: Sep Sun 26, 2021 5:49 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 8119
Location: Near Brandon, Iowa
Diverted wrote:


One other question the phono schematic brings up: Should I also remove the eye tube lead from the high side of the volume pot, or just the lead going to the IF secondary?


You need not do anything with the magic eye tube connections. However, if you plan to use the set as an actual radio receiver only infrequently, you might want to switch the eye tube heater off when the unit is in Phono mode. Eye tubes are expensive and are getting more so; shutting off its heater would conserve the tube. No other wiring changes are needed.

Going this route would (once again) require the use of DPDT switch; but instead of switching the IF amp tube's screen grid bias, that part of the switch would be wired in series with the eye tube heater such that the connection to it is broken when the mode switch is placed in Phono mode.


Last edited by lorenz200w on Sep Sun 26, 2021 7:21 pm, edited 1 time in total.

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 Post subject: Re: Line in question
PostPosted: Sep Sun 26, 2021 5:54 pm 
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Joined: Jan Thu 24, 2013 11:50 pm
Posts: 3542
Location: Bristol, Rhode Island
Thanks Lorens. Great idea, I appreciate that. SInce I have mostly DPDT switches, it's a convenient and easy mod to do.

Ted


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 Post subject: Re: Line in question
PostPosted: Sep Sun 26, 2021 8:23 pm 
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Location: Ashhurst, New Zealand
IMHO who ever drew up the schematic for the phono input unintentionally drew the connection from the top of the switch 51B onto the AGC line. He/she should have extended that line to the secondary of the IF transformer at the capacitor #4 as it was in the unmodified schematic.

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