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Old 27th Dec 2016, 11:10 pm   #141
indigo.girl
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Default Re: My first valve radio project - Philips 462A

Quote:
Originally Posted by G8KBG Tony View Post
No. The second picture in post #139 is not the end of C28 I'm referring to.

Reconnect the (off) white wire you cut from the other end.
I see - you mean the yellow wire - this connected to the other end of the C28 which was also then connected to chassis. I've managed to shatter C28 while I was trying to remove the clamp holding it down... so I'll just reconnect the yellow wire from the transformer to the chassis....
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Old 27th Dec 2016, 11:13 pm   #142
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Default Re: My first valve radio project - Philips 462A

No! The centre tap is NOT connected to chassis.

You will see where it should go if you look at the circuit diagram.
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Old 27th Dec 2016, 11:21 pm   #143
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Default Re: My first valve radio project - Philips 462A

If in doubt there is now an easy way to identify the center tap tag on the transformer, you've already proved that the whole of the winding (end to end) has continuity (391 ohms) so do a resistance check from either pin 4 or pin 6 of the rectifier to all of those tags shown in the 2nd pic in post#138 one of those tags should show a resistance reading of approx 195 ohms to either one of those valve pins, that tag is the center tap, the center tap needs to be connected to the chassis...via.... the bias resistor network, R19 & R20, when that's confirmed the resistance reading between the center tap tag and chassis should be approx 101 ohms.

This post might cross.

Edit: Edited since first posting...read again, my apologies.

Lawrence.

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Old 27th Dec 2016, 11:35 pm   #144
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by g8kbg tony View Post
no! The centre tap is not connected to chassis.

You will see where it should go if you look at the circuit diagram.
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Old 27th Dec 2016, 11:39 pm   #145
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Default Re: My first valve radio project - Philips 462A

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Quote:
Originally Posted by g8kbg tony View Post
no! The centre tap is not connected to chassis.

You will see where it should go if you look at the circuit diagram.
Attachment 135005
Re your thumbnail diagram...Yes and yes.

Lawrence.
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Old 27th Dec 2016, 11:44 pm   #146
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Default Re: My first valve radio project - Philips 462A

This may help. It's a bit bigger.

The centre tap goes to C1, C2, C3, C15 and R2.
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Old 27th Dec 2016, 11:52 pm   #147
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Default Re: My first valve radio project - Philips 462A

Hi
It would help if we stuck to one circuit diagram, I thought it was suggested early on that the Trader sheet was used.
Nicola has both Trader and Manufacturers
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Old 27th Dec 2016, 11:54 pm   #148
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Default Re: My first valve radio project - Philips 462A

OK then. Which one does the OP want to use?
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Old 27th Dec 2016, 11:59 pm   #149
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Default Re: My first valve radio project - Philips 462A

YES WE'VE DONE IT!!

I've connect the centre tag from the transformer to the negative tail of the C27 smoothing cap. I now get 285VAC from the AZ31 anode to chassis as expected and a whopping 666V HT from pin 2 (cathode).

This is turn gives me 265V from anode to chassis at each of the other valves.

I get crackles from the speaker as I switch from SW - MW - LW and best of all when I inject a signal from an MP3 across the gram input sockets I get amplifier sound out of the speaker! I have to turn volume up high to hear it but in principle its up and running!

Congratulations to G8KBG Tony for spotting the snipped wire!! Eagle eyes
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Old 28th Dec 2016, 12:19 am   #150
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Default Re: My first valve radio project - Philips 462A

Good result, the 666 volts on the rectifiers cathode, are you sure, in theory the maximum voltage there would be approx 403 volts DC if there was 285 volts AC on rectifiers the anodes, that voltage of 403 volts would only be there until the other valves started to warm up, at which point it would fall to a lower value.

The 265 volts DC on the other anodes is higher than quoted in the Trader sheet but not destructively so but I would say it needs to be lower, it could be the audio output valve not drawing enough current, what's the DC voltage between C26 +ve and chassis with the valves warmed up? and again confirm the voltage on the anode of the audio output valve (pin 2 of the EBL21)

EDIT: If there really is 666 volts DC on the rectifiers cathode connection then it's panic stations and switch off.

Lawrence.

Last edited by ms660; 28th Dec 2016 at 12:26 am.
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Old 28th Dec 2016, 12:38 am   #151
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Default Re: My first valve radio project - Philips 462A

Looks like the TWO [duell] Tonys posts 134 and 136 have made a break through
But that is a high voltage!

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Old 28th Dec 2016, 12:49 am   #152
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Default Re: My first valve radio project - Philips 462A

Sorry to alarm you there Lawrence - no its 280V HT from pin2 on AZ31 (not the 666!).

The EBL21 anode voltage still seems a bit high (measured again as 265 V DC rather than the spec value of 245V). However, the anode voltage on the two ECH21's is 240V DC spot on with spec data.

Will remeasure it all again tomorrow and see where we stand - but so far after fiddling around with tone control and volume knobs I can get a very listenable audio out the speakers from my MP3 signal injector. What a day and what a thread!! I've learnt loads so I don't mind its taken all day. Thanks to EVERYONE who has commented - a real team effort

But for now that's me OVER AND OUT!! .....until tomorrow
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Old 28th Dec 2016, 12:52 am   #153
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by dave walsh View Post
Looks like the TWO [duell] Tonys posts 134 and 136 have made a break through
But that is a high voltage!

Dave W
yes, Tony Duell actually spotted what was wrong in theory and then Tony number 2 spotted the snipped wire. Excellent team work
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Old 28th Dec 2016, 12:52 am   #154
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Default Re: My first valve radio project - Philips 462A

The lack of 666 volts is a relief...phew.

So far so good.

Hit the pillow time for me.

Lawrence.
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Old 28th Dec 2016, 1:26 am   #155
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Default Re: My first valve radio project - Philips 462A

Comgratulations! Sorry, been away with other things, you know how it gets ..... Anyway, well done on getting it to this state! You have established that there is a path for audio, so there can't be much more wrong with it now .....

Indications earlier seemed to point towards a faulty power transformer, which is why I didn't go into more detail about the rectifier "bodge"; but it it had looked as though the AZ31 were faulty and the power transformer sound, I would have suggested getting two 1N4007 diodes and two 220Ω, 3W resistors; soldering a resistor in series with the anode lead of each diode; joining the two cathodes together, and this junction to C26 (Trader) / C1 (Manufacturers') positive; and the other ends of the two resistors to pins 4 and 6 of the AZ31 socket. This arrangement uses silicon diodes to replace the diode valves within the AZ31. The series resistors are there to make up for the fact that a silicon diode is a better conductor, when the current is in its preferred direction, than a diode valve.

But that's probably not something you will need to bother with now.
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Old 28th Dec 2016, 7:57 am   #156
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Default Re: My first valve radio project - Philips 462A

The fact that you are using a digital multimeter means that your Dc voltages will be higher than the diagram as the meter puts little load on the circuit unlike an analogue meter which would have been used for the manuals circuit readings.
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Old 28th Dec 2016, 8:00 am   #157
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Default Re: My first valve radio project - Philips 462A

Glad to see this morning that you are almost there, and have a decent HT and working LT.

Mike
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Old 28th Dec 2016, 9:28 am   #158
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Default Re: My first valve radio project - Philips 462A

I would suggest that there are lessons for us all here:-

1. Specify which service sheet is being used at the very beginning and stick to it. This would have prevented the needless changing of a 330pF capacitor in the mistaken belief that it was the audio coupling capacitor.

Personally I use the manufacturers service sheet if there is one, but I appreciate that Trader sheets seem to be more widely available. Also those offering assistance may not have the same service sheet as is being used by the OP.

2. Make sure that the OP has reliable test equipment and is using it correctly. In the case of a test meter this means using the correct sockets for the test leads, selecting the appropriate function such as AC or DC Volts and selecting a range suitable for the expected result. If necessary switch down the ranges until a sensible reading is obtained.

Had the voltage readings been correct, we wouldn't have suspected a mains transformer fault.

3. Only change one component at a time and make tests after doing so. It is likely that if just the audio coupling capacitor had been changed and then the set was powered up via a test lamp then, once the faulty mains switch had been sorted, HT would have been present. The smoothing caps could then have been changed, preferably one at a time, rechecking the HT after each change. This would have immediately have showed up the missing connection to the centre tap of the HT winding.

4. If test results are not as expected, the do the test again using different test points. We had assumed that there was a connection from the HT winding centre tap to the chassis, albeit via bias resistors in this case. A voltage test from the centre tap to HT positive would have shown that this was not the case, as would a resistance check from the centre tap to the chassis.

I realise that I am saying this with the benefit of hindsight. I also intend no criticism of the OP.
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Old 28th Dec 2016, 11:45 am   #159
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Default Re: My first valve radio project - Philips 462A

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I would suggest that there are lessons for us all here:-

1. Specify which service sheet is being used at the very beginning and stick to it. This would have prevented the needless changing of a 330pF capacitor in the mistaken belief that it was the audio coupling capacitor.

Personally I use the manufacturers service sheet if there is one, but I appreciate that Trader sheets seem to be more widely available. Also those offering assistance may not have the same service sheet as is being used by the OP.

2. Make sure that the OP has reliable test equipment and is using it correctly. In the case of a test meter this means using the correct sockets for the test leads, selecting the appropriate function such as AC or DC Volts and selecting a range suitable for the expected result. If necessary switch down the ranges until a sensible reading is obtained.

Had the voltage readings been correct, we wouldn't have suspected a mains transformer fault.

3. Only change one component at a time and make tests after doing so. It is likely that if just the audio coupling capacitor had been changed and then the set was powered up via a test lamp then, once the faulty mains switch had been sorted, HT would have been present. The smoothing caps could then have been changed, preferably one at a time, rechecking the HT after each change. This would have immediately have showed up the missing connection to the centre tap of the HT winding.

4. If test results are not as expected, the do the test again using different test points. We had assumed that there was a connection from the HT winding centre tap to the chassis, albeit via bias resistors in this case. A voltage test from the centre tap to HT positive would have shown that this was not the case, as would a resistance check from the centre tap to the chassis.

I realise that I am saying this with the benefit of hindsight. I also intend no criticism of the OP.
Thanks Graham. This is a fair summary of the lessons learnt on the journey. As a first timer my own ignorance certainly played a part - especially with misidentifying and removing the 330pF cap instead of the audio output valve! At that point I had very little understanding of the circuit and was just 'following orders' However, these silly mistakes are almost necessary to begin with as its the mistakes that show up gaps in knowledge that then get filled. When I first read through the traders sheet it made virtually no sense whatsoever - but now after this journey of discovery I feel I have a good working grasp of this particular set.

Having a faulty multimeter is just bad luck and I'm not sure how anyone can guard against that one! I must be having bad luck with kit because the wire came out the back of the probe from my new meter last night too! Think that might be the reason for the erroneous and devilish 666 V HT I scared everyone with last night....

Changing things one at a time is definitely good advice and good practice in any scientific method. But I also read lots of advice to 'remove all the duff capacitors before you even switch on' to protect your circuit. Its a tricky one to judge. Could a rule of thumb be to always remove the audio output first - then switch on with lamp limiter - then remove the obvious suspects one by one testing each time?

....and mixing up a traders sheet with a manufacturers sheet is something you only do once

Everyone's patience, diligence and encouragement on this forum is gold dust to anyone starting out on this hobby. Well done everyone for making this a get place to hang out
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Old 28th Dec 2016, 12:09 pm   #160
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Default Re: My first valve radio project - Philips 462A

Quote:
Changing things one at a time is definitely good advice and good practice in any scientific method. But I also read lots of advice to 'remove all the duff capacitors before you even switch on' to protect your circuit. Its a tricky one to judge. Could a rule of thumb be to always remove the audio output first - then switch on with lamp limiter - then remove the obvious suspects one by one testing each time?
Opinions on that vary. I don't even change the audio coupling capacitor before applying power. The first thing I do after power application though is to check there's no positive voltage on the grid of the output valve. Generally there is, so the coupling capacitor has to be changed.

I do attempt to reform the smoothers before applying power though and change these if necessary.
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