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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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30th Dec 2016, 6:37 pm | #281 |
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Re: My first valve radio project - Philips 462A
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30th Dec 2016, 6:43 pm | #282 |
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Re: My first valve radio project - Philips 462A
Caught red handed!!!
I have examined the voltages at the 4 connection points on the suspect tag strip (see photo). There is 80V at the point where C29 (C17) used to be connected! Despite this looking like it is connected to nothing at all. See connection point b in the photo. There is even 60V measured at the connection point at the end (a) which has never been used or soldered to. The other positions c and d have capacitors attached to them and both have high voltage on them (240V and80V respectively). So, can we presume that there is leakage along the tag from the voltage sources of c and d and thats affecting the positions at b and a. How is this leaking voltage when the tag is made of plastic? How has this fault arisen I'm wondering. |
30th Dec 2016, 6:49 pm | #283 |
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Re: My first valve radio project - Philips 462A
YES!!! We've done it. I've tested for audio and I get much high max volume and a nice and even increase in volume from 0 - max. SUCCESS!!
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30th Dec 2016, 6:55 pm | #284 |
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Re: My first valve radio project - Philips 462A
It's probably made of Bakelite and could well be conductive by now. Over the years it absorbs moisture very slowly (like the old wax capacitors do). Dust and dirt are also attracted to it and provide an increasingly conductive path. Over a period of time, the leakage increases and actually starts to break down the Bakelite which becomes permanently conductive. The only answer is replacement of the tagstrip.
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30th Dec 2016, 7:06 pm | #285 |
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Re: My first valve radio project - Philips 462A
Ok, you're on the road to being an ace troubleshooter now
I can't say for sure what the tag strip is made of but from the photo you've posted it looks like Paxolin, that's the most common material used, there's three ways the voltage can leak through, we, or at least I do, normally call the leakage "tracking" 1st is by a build of a conductive layer of general crap on the tag strips surface, the crap can get conductive over time. 2nd is an actual breakdown of the insulation in/on the surface layer of the tag strip. 3rd is an actual breakdown of the insulation below the surface level, eg: within the tag strips body. There might be other mechanisms that elude me at the moment, long term moisture or damp ingress can accelerate the above reasons or even be the start of them. In higher voltage circuits such as HT where there is a large potential difference (voltage) between the tags the tracking can leave a visible carbonized mark (a track) between the tags, this often happens if one tag that's being tracked to has a low impedance path to HT -ve and the HT +ve can supply enough current to heat up the track which has now in effect become a resistance do to carbonization, if a lot of current can flow then sometimes you can visually see the heating effect in the form of small sparks making a random path between the tags, some times it tracks like a miniature version of forked lightening or sometimes it tracks in the form of a single track like a miniature version of bolt lightening, however in this instance (your receiver) the HT is tracking to a high impedance part of the circuit and thus the current flow will be small and it's heating effect very tiny. EDIT: Just seen your later post so Lawrence. |
30th Dec 2016, 7:36 pm | #286 |
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Re: My first valve radio project - Philips 462A
Excellent piece of work both of you. That has to have been one of the trickiest faults to track down.
Well done. Mike |
30th Dec 2016, 7:42 pm | #287 |
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Re: My first valve radio project - Philips 462A
My old set is covered in oily, black, dirty, dusty stickiness - I guess most are. The back of the tag strip is very dirty and it smears onto my finger as I rub along it. So, perhaps this is conductive. Are tag strips something that often fail like this? Very difficult to detect.
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30th Dec 2016, 7:44 pm | #288 |
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Re: My first valve radio project - Philips 462A
I would, when convenient replace R6 and R7 (Trader)
Next thing I would check is the health of the output stage, the voltage on the pentodes anode, pentodes screen and the voltage on +ve side of C26 (Trader) followed by the bias voltage on the pentodes control grid. Lawrence. Last edited by ms660; 30th Dec 2016 at 8:06 pm. |
30th Dec 2016, 7:47 pm | #289 |
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Re: My first valve radio project - Philips 462A
Thanks Mike - and thanks for your help on this too - what a marathon journey that was. Very satisfying to have tracked it down to the tracking! Just shows how something very simple can cause such a lot of issues. I hope not all restorations as tough as this one!!
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30th Dec 2016, 7:49 pm | #290 | |
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Re: My first valve radio project - Philips 462A
Quote:
Lawrence. |
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30th Dec 2016, 8:42 pm | #291 | |
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Re: My first valve radio project - Philips 462A
Quote:
Va (pin2) = 258 VDC Vg1 (pin3 - control) = -3.35 VDC Vg2 (pin4 - screen) = 263 VDC Vh = 6.3 VAC V4 AZ31: Va (@pin4 and @pin6) = 280 VAC Vh (across pins 2 and 8) = 4 VAC @C26 = 285VDC Last edited by indigo.girl; 30th Dec 2016 at 8:48 pm. |
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30th Dec 2016, 9:08 pm | #292 |
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Re: My first valve radio project - Philips 462A
So, all looking good now . Just tidying up work to do. I have mended the on/off switch ganged to volume control and an in the process of wiring in the bluetooth adaptor. I will also order some replacements for R6 and R7.
There are a couple of black capacitors left. C39 (C24 traders) across the output transformer - 1nF C36 (C10 traders) AVC line decoupler - 47 nF. Worth replacing? And what about the C15 (C28 traders) RF bypass cap that I snipped out at the start? What role does this play and is it important if I'm not using Radio frequency for this set? (C3 and C4 traders are band pass couplers so not in use so no need to replace). I'm also wondering what people do to insulate their solder joints. I've covered the legs of the caps or resistors I've added with plastic sleeving but the ends where they have been soldered are still exposed. Is there something I can use to cover them (tape seems to just fall off after a while). Last edited by indigo.girl; 30th Dec 2016 at 9:24 pm. |
30th Dec 2016, 9:30 pm | #293 |
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Re: My first valve radio project - Philips 462A
Well done for the perseverance Nicola, the next set just has to be easier.
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30th Dec 2016, 9:47 pm | #294 | |
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Re: My first valve radio project - Philips 462A
Quote:
Subtracting the valves anode voltage from the anodes HT feed voltage gives a voltage difference of 27 volts, that voltage is dropped across the anodes load (the output transformer in this case) we know from the manual(s) that the transformers primary that supplies the anode has a resistance is 800 ohms, a quick ohms law gives an anode current draw of 33mA. Next is to obtain the valves data sheet: http://www.r-type.org/pdfs/ebl21.pdf Scrolling down to page 11 shows the valves characteristic IaVg curve plotted at two different anode voltages, Ia = anode current, the vertical scale on the right. Vg = control grid bias voltage, the bottom horizontal scale. We know that the anode current through the valve being checked is 33mA, note that point on the Ia scale on the right then project that horizontally until it hits the curve, but which one? There are two curves shown (the big ones) the first one intersected is for an anode and screen supply of 250 volts, the second one is for an anode and screen voltage of 300 volts, the anode voltage on the valve being checked is 258 volts so the best match curve for that is the 250 volt curve, however our screen voltage is not the same as the anode voltage but the difference isn't great so the 250 volt curve will do, so from the point of the horizontal intersection already mentioned project down vertically to the grid bias voltage on the bottom horizontal scale, that should equate to a grid bias voltage of approx. 6 volts -ve. If you know scroll down to page 13 the first part gives operating figures such as voltage, current, grid bias etc, for the valve when used as a pentode output valve, in the right hand column you'll see the various data for the valve when the anode and screen grid are operating with voltages at 250 volts respectively, that's the column we're interested in as those two voltages are the ones we referred to in the graph on page 11, the first down the column is the cathode resistor, no need to bother with that as there's no cathode resistor being used in your receiver, next down is the grid bias voltage of 6 volts -ve, next down is the anode current of 36mA, you can now see that those figures are close enough to tally up with the figures that were determined from the valve that was measured in the receiver. The only fly in the ointment is that the grid bias voltage given above is higher than the grid bias voltage actually measured on the valve, that can be explained by the fact that the voltage on the grid is supplied via a high impedance source and the test meters input impedance is pulling (loading) the real figure down. Here's a suggested read for learning about valve operation, in my own opinion it's one of the best available as a download on the web. Inside The Vacuum Tube by John F Rider: http://www.tubebooks.org/Books/rider_inside.pdf It starts off with the very basics and is relatively easy to follow and hopefully understand, learn from the beginning up to and including the triode valve at least. Quiet time reading, long term it will hopefully prove useful Hope all this is some use. Lawrence. Last edited by ms660; 30th Dec 2016 at 9:52 pm. |
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30th Dec 2016, 10:10 pm | #295 |
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Re: My first valve radio project - Philips 462A
Heatshrink Sleeving, available in various colours and diameters from the usual suppliers (CPC, Maplin, RS, etc.) is useful for covering solder joints, etc., Nicola. It shrinks to 50% of it's original diameter when heated. Other types of sleeving are also available.
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31st Dec 2016, 2:03 am | #296 |
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Re: My first valve radio project - Philips 462A
Congratulations! This hasn't been an easy one at all, by any stretch of the imagination, so you definitely deserve a or several. [voice="radio announcer"]Coming up after the news: When Insulators Don't, a new play for radio by ......[/voice] It's been a baptism of fire, but hopefully you've learned something from the experience.
Even though there isn't much left on those bands nowadays, you might want to think about getting a simple medium / long wave set up and running next, just so you can say you have done it. (You can build a short-range "pantry transmitter" which will allow you to feed any modern signal source to old sets.) FM receivers are rather more complex beasts, and it will help to have done an AM set first before you tackle one.
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31st Dec 2016, 10:47 am | #297 |
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Re: My first valve radio project - Philips 462A
"There are a couple of black capacitors left.
C39 (C24 traders) across the output transformer - 1nF C36 (C10 traders) AVC line decoupler - 47 nF. Worth replacing?" I would say that C24 (Traders) is worth replacing, it needs to be rated at 600 volts or higher, the reason for that is in the event that the volume is set high and for whatever reason the loudspeaker becomes disconnected or goes open circuit then high voltages will be developed across the output transformers primary that can sometimes cause flash over and damage, C24's function is for tone correction but it also helps to supress any high voltages developed across the transformers primary should the above loudspeaker disconnection situation occur, the value shown in the schematic is commonly available. C10 (Traders) is the AVC/AGC line decoupler, if it was to go leaky or short circuit it wouldn't affect the operation of the audio stage, so no need to bother unless you intend to use the radio section of the receiver. "And what about the C15 (C28 traders) RF bypass cap that I snipped out at the start? What role does this play and is it important if I'm not using Radio frequency for this set?" If you don't intend to use the radio function then I would say that C28 (Trader) can be left disconnected. It's function is to supress RF interference that might be on the mains supply that would otherwise interfere with a radio signal being received. "(C3 and C4 traders are band pass couplers so not in use so no need to replace)." Correct. Lawrence. |
31st Dec 2016, 12:12 pm | #298 | |
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Re: My first valve radio project - Philips 462A
Quote:
Interesting what you say about trying to get a MW/LW set up and running next. I am tempted and feel a little guilty about not doing it for this one after being told off by Heterodyne I think my issue was I don't really know what to do with a MW/LW set once I'm finished... what do people do with their MW/LM sets these days? I do like the idea of trying it anyhow just as a learning experience so I'm not totally against the idea next time I get hold of an old radio. |
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31st Dec 2016, 12:16 pm | #299 | |
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Re: My first valve radio project - Philips 462A
Quote:
Interesting what you say about the voltage across the transformer being high if the loudspeakers are disconnected. So, as a general rule should speaker NEVER be disconnected while playing - does this apply to modern sets and radiograms too?[/QUOTE] |
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31st Dec 2016, 12:41 pm | #300 |
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Re: My first valve radio project - Philips 462A
"Interesting what you say about the voltage across the transformer being high if the loudspeakers are disconnected. So, as a general rule should speaker NEVER be disconnected while playing - does this apply to modern sets and radiograms too?"
Yes, never run the volume up with no load (loudspeaker) on the output transformers secondary, that applies to valve sets, running a transistor amp with no load is generally no problem unless it uses an output transformer with high HT (rare) A loudspeaker disconnect at low volume in a valve set is normally no problem. As an exercise it might be worth while to try and get the radio section functioning, it's more or less been established that the mixer/oscillator valve is probably ok by the ECH21 valve swop earlier on. There's then the option of using a "pantry" transmitter to transmit speech/music from whatever source to the receiver. You've replaced most of the suspect capacitors in the receiver so there shouldn't be too much work to do to get the radio section working. What you could do if you wanted to is to measure the voltages on V1's anode, screen grid, control grids etc for both the heptode and triode sections and the same for the heptode section of V2, that will give an idea as to what's what in the radio section and won't cost anything to do, if you do decide to do the voltage checks do them with the receiver switched to medium wave with no antenna connected. Lawrence. |