Home Forums Electronics Restoration Dial Lamp and 35W4 circuit on Electrohome Roland 6T8 AA6 tube radio…

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  • #18402
    Doug Parker
    CVRS Member

    I have an Electrohome Roland 6T8 AA6 radio that has a 33 ohm resistor that is connected between the 35W4’s anode and the dial lamp filament tap at pin 6.
    I have gotten 2 schematics for the 6T8 radio. One schematic has the R15 – 33 ohm resistor in the circuit and the other schematic does not.
    My chassis was untouched and it came with the resistor in question installed from the factory. I would assume that this was a later revision.
    As far as saving the 35W4 for eventual filament failure during radio power on which is the better of the two circuits to run with?

    #18405
    Ralph Spracklin
    Forum Participant

    Hello Doug

    While I am by no means yet an expert on these things, I will attempt to explain it.

    This 35W4 Tube was designed in the mind-set that it could be used in a circuit using a Dial Lamp or without a Dial Lamp. That is why the the filament on this tube is Tapped. Because your Radio uses the Dial Lamp, a a Shunt Resistor is needed. Obiviously the Design Engineer, did not include it in the one schematic as it was not needed because that schematic was for a model that did not have a Dial Light. Therefore your Radio, because it has a pilot light and the resistor, it will be OK, with no added failure dangers to the 35W4 tube itself. The resistor acts as a fail-safe (not certain if that is the correct word to use). should the Dial lamp go out. Let’s see what others on this site think of my explanation. See data provided on Franks Radio Data Site for the 35W4 tube. And by the way this Dial Lamp should be a #40 or a #47. And the 33 ohm R15 resistor according to the schematic is correct.

    Ralph

    #18410
    John Greenland
    CVRS Member

    Hi Doug,

    Attached is the recomended way to use a 35W4 as taken from the latest RCA tube manual.

    I beleive that the resistor in question from the plate of the 35W4 is to limit the current though the 35W4.
    You will notice on the attached sheet that the caps in the chart are in the order of 30 UF or so, not the 250 that is currently in use in this radio.
    This size cap would take a higher initial current.

    Also, according to the diagram in the attached sheet a shunt resistor is across the panel lamp and not in series with it.

    I would keep the resistor in place.
    Regards,
    John G. VO! CAT

    #18413
    Ralph Spracklin
    Forum Participant

    John. I am now wondering if the Shunt Resistor on Dougs radio is in series or is it shunted across (in parrallel) wth the dial lamp, which to me seems to be the logical place for it. Maybe Doug will advise as to its actuall location.

    Ralph

    #18428
    Doug Parker
    CVRS Member

    John and Ralph. Thank you for your input. The R15 – 33 ohm 1/2 watt resistor is factory installed in my chassis from pin 5 (plate-anode of the 35W4) to the pin 6 and one side of the lamp just as the schematic shows. Obvious there was two different schematics for the same radio. The untouched chassis had the two (50 ufd – 150 volt) filter capacitor sections. I would agree that the R15 is used to limit the current flow through the 35W5 as it is in series with the anode to the rest of the circuit. I measured the voltage drop across R 15 when there was no signal applied and the voltage drop across the resistor was almost 1 volt. That works out to 3 milliamps approximately. The voltage at pin 7 of the 35W4 was 94 volts DC and the line voltage was measured at 120 VAC. I swapped in another 35W4 I had and the voltages were the same. I will install the chassis back into the radio as everything was working properly. Again, Thank you.

    #32505
    Rui
    Forum Participant

    Greetings to all,
    I just stumbled into this conversation and, since I recently acquired a Electrohome 6T-8 and am in the process of restoring it, I would appreciate some help.

    Does anyone know anything about the printed circuit used in this radio? It appears to be a PC 151A, but I can hardly read it. I have no indication of the values for the capacitors – are those values in mF? nF? The schematic provided by Doug Parker (thank you, Doug, your schematic was a life saver) makes no reference to the printed circuit. I managed to check the resistors mas got readings that make no sense when checking the capacitors.

    I would prefer not to replace the printed circuit with individual components, if I can avoid it, but would like to receive suggestions from someone knowledgeable about these things. Of course, after that I may have more questions, and I am hopeful someone will kindly answer them.

    So long,

    Rui

    #32506
    John Greenland
    CVRS Member

    There is a very clear schematic for this radio at Pacific Tv.

    http://pacifictv.ca/schematics/electrohome6t7.pdf

    The values for the hybrid cct are there . If you are not sure about the value then why not do a comparison with a similar 5 tube radio that uses individual components?
    Most components are going to be in a similar range of values.

    TTFN

    John G.

    #32507
    Rui
    Forum Participant

    Thank you, John.

    Yes, that is the schematic I have now, and it is clear enough for my needs, unlike the one I was using before. Unfortunately, I do not have a similar radio handy, to compare the values with; and I am confused with the range of values: from 0.005 to 220? What, mFd? I hate to have to guess when working on these radios, so easy to make a mistake.

    Regardless, thank you for the suggestion – worthwhile looking for another radio and comparing. Any more suggestions, from you or anyone else… I would love to have them.

    So long,

    Rui

    #32508
    John Greenland
    CVRS Member

    Hi again,

    I was not thinking of you having another radio but comparing schematics.

    This one attached ( partial ) seems to have better identified the actual component values in the hybrid circuit.

    It is an Electrohome 5C23. No rf amp like the 6T7 or 8 . But if you look at the position of the components inside the hybrid circuit, you can probably answer your questions about values

    TTFN

    John G.

    #32510
    Rui
    Forum Participant

    Thank you, John,

    Yes, I will do that.

    Right now I have managed to get the radio to work; and it works just fine. The values are not that pressing anymore, but I am still curious.

    So long,

    Rui

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