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Old 28th Dec 2016, 11:45 am   #159
indigo.girl
Retired Dormant Member
 
Join Date: Apr 2016
Location: Loughborough, Leicestershire, UK.
Posts: 754
Default Re: My first valve radio project - Philips 462A

Quote:
Originally Posted by Station X View Post
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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