21st Dec 2009, 1:27 am | #141 | |
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Re: Newb Stumped on a Derwent
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I'm not convinced that the 5000R exists as an actual component, as I can't see it.
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21st Dec 2009, 1:28 am | #142 |
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Re: Newb Stumped on a Derwent
no tiredness is getting the better of me I disconnected got better access and took the reading 23v. Re soldered back on with better access and reading is 23v. Sorry i'm a i think bed should be the order of the errr night.
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21st Dec 2009, 1:30 am | #143 |
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Re: Newb Stumped on a Derwent
If i touch one probe (the common) to pin 5 it springs to life!!!!!!!!!!!!! red (dc probe does nowt)
lots of volume quality not great though, th emore you turn the vol up it distorts, tone works well This must mean something |
21st Dec 2009, 1:38 am | #144 |
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Re: Newb Stumped on a Derwent
Capacitor in circuit 6V measured.
Capacitor disconnected 23V measured. Capacitor reconnected 23V measured. That doesn't add up. Either 6V was wrong or you disturbed something when disconnecting and reconnecting the capacitor. Given that you have 23V on the cathode I'd expect to see 23V between pins 4 and 5, so we're getting somewhere.
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21st Dec 2009, 1:41 am | #145 | |
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Re: Newb Stumped on a Derwent
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21st Dec 2009, 2:03 am | #146 |
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Re: Newb Stumped on a Derwent
OK bed beckons and what a long day...........
Thanks to all for your input and sticking with the newb it must be frustrating for you, but I sneakily think it brings out the remote sherlock in you. Got me hooked, I started this morning around 10:30 and now look at the time 01:00 Yeeeek. Lucky for me the wife is ok with it, means she can watch dross tv. Can't wait to see how this fault hunt turns out. Snooooooooze |
21st Dec 2009, 8:39 am | #147 | |
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Re: Newb Stumped on a Derwent
I think the penny has dropped at last!
The "wire link" between the chassis/earth tag on the 75 and pin 4 (control grid) of the 42 wasn't a "wire link" at all. In fact it was a 510K resistor inside a length of sleeving. This was the grid leak resistor for the 42. When the "wire link" was cut at the chassis/earth end it removed the grid leak resistor from the 42. The grid leak resistor was replaced with another by shifting one end of another 510K resistor, spuriously connected across the 500R cathode bias resistor, from pin 5 to pin 4 of the 42. This means that there were only two things wrong with the wiring as found:- 1. There was a spurious 510K resistor connected across the 500R cathode bias resistor. This would have had little or no effect on circuit operation as it would have been swamped by the 500R. 2. The 50uF cathode bias bypass capacitor was connected the wrong way round. So we are back to square one. THE VALVE VOLTMETER COMMON PROBE. Quote:
When you connect the common probe to pin 5 you are providing a low resistance path from pin 5 to earth/chassis restoring circuit operation. You may be able to prove this by unplugging the valve voltmeter from the mains and touching the common probe on to pin 5. If this theory is proved I would suggest that the way forward is to lower the value of the cathode bias resistor either by replacing it with say a 330R or by shunting it with say another 500R.
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21st Dec 2009, 10:40 am | #148 |
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Re: Newb Stumped on a Derwent
Well deduced Graham. I wonder if this might have more to do with earthing the 75 cathode. It would be interesting to see how the last IF is wired to the detector as it might give a clue as to whether it is meant to have the cathodes connected together.
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21st Dec 2009, 11:04 am | #149 |
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Re: Newb Stumped on a Derwent
Ok head still fuzzy read your post looks like you have it even if i only partially understand. Trouble is so much has gone on its "wood for the trees now"
I have attached a diagram of how it looks now with a couple of queries and if someone can suggest what I do next. Last edited by oldticktock; 21st Dec 2009 at 11:28 am. |
21st Dec 2009, 11:29 am | #150 |
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Re: Newb Stumped on a Derwent
I'm asuming that your drawing is a true representation of the wiring as it exists at present.
The 50uF capacitor is the wrong way round. It should be connected negative end to chassis and positive end to pin 5 of the 75. Do not replace the dotted 510K. It isn't needed. Green, Yellow, Brown indicates a 540 ohm resistor. --------------------------------------- Try the common probe to pin 5 test with the valve voltmeter disconnected from the mains supply ie mains plug out. Is performance still improved? If not this indicates that 500/540 ohms is too high a value. It's possible that the value is changing when current flows through the resistor. Not unknown with carbon resistors. I'll call the 500/540 the cathode resistor. Try reducing it in value to 330R and take new resistance and voltage readings. You should be aiming for about 16V DC from cathode to earth ie "across" the cathode resistor.
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21st Dec 2009, 11:35 am | #151 | |
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Re: Newb Stumped on a Derwent
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I will change the 50uf around again I thought when someone suggested turning it, it seemed weird to have + to earth I will leave out the 510k where one used to be and will replace the 540 with a 330 I will try the probe test Cheers Station X |
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21st Dec 2009, 11:38 am | #152 | |
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Re: Newb Stumped on a Derwent
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I expect this to be working by tea time today!
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21st Dec 2009, 11:44 am | #153 |
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Re: Newb Stumped on a Derwent
ok before doing anything else I tried the probe test
Valve Volt Meter disconnected from mains, probe on pin 5 of 42.... nothing Valve Volt Meter connected to and switched on, probe on pin 5 of 42.... station heard albeit distorted lots of vol etc Now I will rotate the 50uf and install the 330 |
21st Dec 2009, 11:50 am | #154 | ||
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Re: Newb Stumped on a Derwent
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Yes when I typed that I thought you would think that and edited the post. I'm getting there slowly but surely. Have to say this is fun. By tea time would be great, I would so love to have this working for xmas. I've got myself a little am transmitter AM250 and want to send this radio some good OTR broadcasts sit down in front of the fire with a whisky and a cigar and soak up the atmosphere. |
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21st Dec 2009, 12:04 pm | #155 |
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Re: Newb Stumped on a Derwent
Ok changes made, radio fires up and can now be heard.
Volume was loud but distorted now volume has dropped off (vol at max) but still can be heard but very very distorted. |
21st Dec 2009, 12:06 pm | #156 |
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Re: Newb Stumped on a Derwent
Voltages? Resistances ?
Try disconnecting one end of the 50uF.
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21st Dec 2009, 12:14 pm | #157 |
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Re: Newb Stumped on a Derwent
There are a number of Philco sets with the same lineup. In all cases the 42 cathode is earthed and grid bias is taken from the energising coil which is in the -ve HT path.
Has this been hacked about due to a speaker change? |
21st Dec 2009, 12:15 pm | #158 |
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Re: Newb Stumped on a Derwent
Ok Will do.
I have to nip out while I can, will be back later to post update. Back in about 1 hour (she who must be obeyed). Steps in the right direction now..... I see the summit..... head down and push on. |
21st Dec 2009, 12:18 pm | #159 |
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Re: Newb Stumped on a Derwent
PJL.
Quite possible I'd think. That O/P TX held in with one screw doesn't look quite right. We have to work with what we have though.
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21st Dec 2009, 12:19 pm | #160 | |
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Re: Newb Stumped on a Derwent
Quote:
pin 5 does goto earth see my lastest diagram speaker and all associated looks original |
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