20th Dec 2009, 2:16 pm | #41 | |
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Re: Newb Stumped on a Derwent
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220k resistor is wired as follows, one pin from speaker to chassis loom has a wire rail attached, then the 220k is next on the rail, from that the 220k is connected to a 0.1 uf yellow cap which links into a wire that connects to pin4 in one direction, and chassis in the other. 500ohms is the answer for pin 5 Last edited by oldticktock; 20th Dec 2009 at 2:24 pm. |
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20th Dec 2009, 2:24 pm | #42 |
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Re: Newb Stumped on a Derwent
Given 30V DC on the cathode and a 500 ohm cathode resistor that gives a combined anode and screen current of 60mA. I would expect to see about 40mA.
However more bias should give less current, so something doesn't add up. Can you locate the cathode resistor and check it's colour coding?
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20th Dec 2009, 2:28 pm | #43 |
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Re: Newb Stumped on a Derwent
Yes quite right the thick wire could be HT are we then saying that the 220k is anode load to the 75 ,
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20th Dec 2009, 2:30 pm | #44 | |
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Re: Newb Stumped on a Derwent
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gahhhhh! my heads turning into spaghetti wiring |
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20th Dec 2009, 2:31 pm | #45 | |
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Re: Newb Stumped on a Derwent
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20th Dec 2009, 2:31 pm | #46 |
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Re: Newb Stumped on a Derwent
Measure it with the set off. Let's see what it is.
Cheers, Steve P.
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20th Dec 2009, 2:41 pm | #47 |
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Re: Newb Stumped on a Derwent
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20th Dec 2009, 2:44 pm | #48 |
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Re: Newb Stumped on a Derwent
So we have HT rail----220K resistor-----------0.1uF capacitor-----------chassis.
Is there anything conected to the junction of the resistor and the capacitor?
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20th Dec 2009, 2:53 pm | #49 |
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Re: Newb Stumped on a Derwent
yes it spits down to the 75 looks like the first small pin after heater
just to be clear, the chassis end point appears to be a tag which is on the base of valve 75's holder looks like the chassis rivet point with a leg on it, the neg end of the smoothers are on this as well. one pin of (4 pin loom)...... (ht rail).....(220K).....split here off to 75.(0.1uf).... hits centre of new wire one side to pin 4 grid 1 other to chassis (tag on rivet of 75 holder |
20th Dec 2009, 2:56 pm | #50 |
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Re: Newb Stumped on a Derwent
Yes, but is there anything connected to the junction of the resistor and capacitor? If not I cannot see what useful purpose these components serve.
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20th Dec 2009, 3:03 pm | #51 |
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Re: Newb Stumped on a Derwent
this might be easier than my numpty ramblings,sorry if i'm frustrating you guys you've all be more than helpful.
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20th Dec 2009, 3:16 pm | #52 |
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Re: Newb Stumped on a Derwent
That looke OK apart from the link between the two valves - Pin 4 on the 42 to the unmarked pin on the 75. That won't work - check again here. If it's right, disconnect this link from the 75. Also, the wires to that capacitor to the HT and the valve. Should be joined surely.
The cathode volts are high - check that resistor - what value is it? Higher than it's marked at a guess. Cheers, Steve P.
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20th Dec 2009, 3:18 pm | #53 |
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Re: Newb Stumped on a Derwent
more detail
the link from 42 to 75 is a chassis tag on the 75 valve holder |
20th Dec 2009, 3:22 pm | #54 |
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Re: Newb Stumped on a Derwent
The capacitor goes to pin 4 of the 42 valve. Take the other link from this pin off. Then try it.
Cheers, Steve P.
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20th Dec 2009, 3:22 pm | #55 |
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Re: Newb Stumped on a Derwent
where is the grid leak for the 42, the grid seems to be connected directly to chassis
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20th Dec 2009, 3:39 pm | #56 |
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Re: Newb Stumped on a Derwent
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20th Dec 2009, 3:48 pm | #57 |
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Re: Newb Stumped on a Derwent
Cut the wire which goes to pin 4 of the 42 and connect a high value resistor between pin 4 and the wire.
Anyone got the pin out for the 75?
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20th Dec 2009, 3:54 pm | #58 |
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Re: Newb Stumped on a Derwent
yes, the 510k you have going to pin 5 should go to pin 4, the link from pin 4 to chassis should be cut, also the 50 uf seems to be connected the wrong way round, the + should go to pin 5, - to chassis
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20th Dec 2009, 3:55 pm | #59 |
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Re: Newb Stumped on a Derwent
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20th Dec 2009, 4:02 pm | #60 |
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Re: Newb Stumped on a Derwent
The cathode bias resisitor is a 1% type and seems to be green-brown-black-brown which is 5K. Which is just as well as I*I*R says it should be a 2W part and I would guess this is a 0.6W.
The wire wrapped around the 330 resistor looks like a dry joint. |