30th Dec 2016, 12:18 am | #241 |
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Re: My first valve radio project - Philips 462A
Great tip Les I'll try that in the morning and see what R6 really is
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30th Dec 2016, 12:19 am | #242 |
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Re: My first valve radio project - Philips 462A
A trick you could try to measure R7 even with a meter that can only read up to 2MΩ would be to put a resistor less than 2MΩ in parallel with it, and measure the resistance of the combination (which obviously will be less than 2MΩ). When resistors are connected in parallel, it is the inverses of the resistances that add up; i.e. 1 / R = 1 / R1 + 1 / R2, where R1 and R2 are the individual resistances and R is the combined resistance. If you substitute your known resistance for R1 and the combined parallel resistance for R, you can rearrange the equation to work out the actual value of R7.
For instance, if R7 really is 2.2MΩ, and you connect 1 MΩ in parallel with it, the combined resistance will be 1 / (1 / 1000000 + 1 / 2200000) = 687500 Ω, but a standard digital multimeter with a "1999" display will only be able to show this as 687 kΩ or 688 kΩ. (To work this out using a calculator, enter: Code:
1 [exp] 6 [1/x] + 2.2 [exp] 6 [1/x] = [1/x] EDIT: Post crossed with Les. Great minds think alike ..... Also, I'm with Mike (crackle), this is proper edge-of-the-seat stuff!
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30th Dec 2016, 12:37 am | #243 | |
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Re: My first valve radio project - Philips 462A
Quote:
I have taken my 1Mohm variable resistor that I was using half open for R7 and am now using it fully open to replace R6. i.e. R6 is now 1M ohm (lower than its spec of 2.2M but in the ball park). To get something close to R7 of 470K ohms I have connected together 6 x 47Kohm resistors in series (my entire stock) making a total R7 of 282Kohms. So with V2 IN I have measured the Voltage across all the stages from C25 to control grid pin 4 of V2 like before. Interestingly my multi-resistor set up has allowed me to track the bias voltage changing from neg to positive as I move from C25 to pin4 at lots of positions along R7. The voltage at the C25 end of R7 starts negative and then increases at each step along R7 until it becomes +ve finishing at +2.3V (where is meets R6). The diagram attached explains it better... Bottom line is though that even with a known albeit lower resistance at R6 we are still not getting that much neg voltage at pin 4 Anyhow - that's me done too for tonight - I'm off to bed to dream of resistors no doubt Mike and Julie - glad you are enjoying it - me too I think I should start taking bets for what the eventual fault will be once we find it! Last edited by indigo.girl; 30th Dec 2016 at 12:44 am. |
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30th Dec 2016, 12:54 am | #244 |
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Re: My first valve radio project - Philips 462A
Is that -0.2V on pin 4?
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30th Dec 2016, 9:38 am | #245 |
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Re: My first valve radio project - Philips 462A
I've been staring at the drawing in post #243 for ages and to me it makes no sense at all. It doesn't even fit the scenario of some kind of contact between HT positive and the control grid.
All I can suggest is getting rid of the pot substituting a 2.2M resistor and the series string of resistors substituting a 470k resistor. Fit a couple of resistors of the correct value. It doesn't take much current through a 1M resistor to cause a substantial volt drop across it, so stray pick up may be the problem.
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30th Dec 2016, 10:12 am | #246 |
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Re: My first valve radio project - Philips 462A
Could it be crumbling insulation on the wiring? My set had lots of this.
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30th Dec 2016, 10:20 am | #247 |
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Re: My first valve radio project - Philips 462A
If the voltages in the picture in post#243 were taken with V2 fitted then the +ve voltage is being introduced at the junction of R6 and R7 so far as I can make out, the reason that the +ve voltage is lower on the grid is because the grid and cathode are acting as a conducting diode, the grid being the anode of that diode, that diodes internal resistance will be comparatively low. So far it all points to a leakage somewhere unless we're missing something, the most common cause for that is valve holder leakage or tag strip leakage, the voltages shown in the picture seem to suggest that it isn't valve holder leakage.
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30th Dec 2016, 10:22 am | #248 | |
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Re: My first valve radio project - Philips 462A
Quote:
I wonder what the input impedance of Nicola's meter is?
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30th Dec 2016, 10:28 am | #249 |
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Re: My first valve radio project - Philips 462A
I guess there must be a tag at the junction or R6, R7 and C17. Isolating it might provide a clue.
In fact R6 and R7 are physically some distance from C17, so perhaps there's a wire with insulation problems between these points?
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30th Dec 2016, 10:35 am | #250 |
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Re: My first valve radio project - Philips 462A
A picture of that junction area might give a clue?
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30th Dec 2016, 10:48 am | #251 |
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Re: My first valve radio project - Philips 462A
Another observation if I've got it right:
In the picture of the manufacturers chassis layout posted way on back, R6 (trader) connects to R7 (trader) on what looks like a tag strip, the tag next to it has the local oscillators anode voltage on it....leakage possibly? Lawrence. |
30th Dec 2016, 10:54 am | #252 |
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Re: My first valve radio project - Philips 462A
What post number is that Lawrence?
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30th Dec 2016, 10:59 am | #253 |
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Re: My first valve radio project - Philips 462A
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30th Dec 2016, 11:14 am | #254 |
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Re: My first valve radio project - Philips 462A
Got it.
In manufacturers speak we're talking R10, R11 and C29. Plenty of tags and screened wires involved.
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30th Dec 2016, 11:21 am | #255 |
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Re: My first valve radio project - Philips 462A
Yes, that's the one I was on about, all worth checking to eliminate/narrow down.
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30th Dec 2016, 11:29 am | #256 |
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Re: My first valve radio project - Philips 462A
1st refs are manufacturers:
The tag above C29 (C17 Trader) carries the main HT feed. The tag below C29 (C17 Trader) carries the mixers screen voltage. The tag above R10/R11 (R6/R7 Trader) junction carries the local oscillator's anode voltage. All possible suspects, shouldn't be too difficult to isolate. Lawrence. |
30th Dec 2016, 11:44 am | #257 |
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Re: My first valve radio project - Philips 462A
Hi Nicola.
At the moment you don't know if you have a fault or are simply suffering from lack of gain Check C25 if not already by bridging it to see if volume becomes louder. Adding a preamp would enable the volume control to be turned down more to prove it. why not use the IF amp valve as a preamp. Leave the screen grid as is, disconnect the anode and put a resistor between anode and HT line 100k or 220K or whatever you have Disconnect cathode and put a resistor between it and Chassis 1 or 2.2K should do. use a cap 0.01 or so from anode to the new gram input you have created. The grid should have 500K or 1 meg or and feed your signal into that. Feel free to improvise with whatever components you have to hand. I am sure someone will draw it out for me. Good luck Peter. |
30th Dec 2016, 11:51 am | #258 |
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Re: My first valve radio project - Philips 462A
According to the voltages posted thus far, there is a fault.
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30th Dec 2016, 12:11 pm | #259 |
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Re: My first valve radio project - Philips 462A
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30th Dec 2016, 12:19 pm | #260 | |
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Re: My first valve radio project - Philips 462A
Quote:
Do these shielded wires ever cause trouble? |
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