UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post

Croozer · 21st Sep 2019, 2:16 pm

Just a thought - has the set been connected to the mains lately? If not, you may find that the capacitances will come back up if you apply a low-ish (ca. 20V) DC voltage across each section - charging and discharging each section in turn a couple of times. Remember also the values on the can are nominal only and the set will probly operate on wide tollerances, (16 mFd was common for resevoir capacitances) as long as they aren't actually leaking and you could avoid a replacement.

The point Laurence makes about the 100mFd being positive-to-chassis ie "upside down' is interesting. If you check the diagram you will see that there are none of the typical cathode resistors to raise valve cathodes above chassis (and grid) potential. Rather its the cathodes that are at deck voltage and a -ve feed is provided to make the grids negative - hence the 'upside down' condenser - a bit like an old-fashioned 'grid bias' circuit in a battery set.

21st Sep 2019, 2:16 pm	#32
Croozer Pentode Join Date: Jul 2016 Location: Edinburgh, Scotland, UK. Posts: 110	Re: Philips BX281U20 valve radio Just a thought - has the set been connected to the mains lately? If not, you may find that the capacitances will come back up if you apply a low-ish (ca. 20V) DC voltage across each section - charging and discharging each section in turn a couple of times. Remember also the values on the can are nominal only and the set will probly operate on wide tollerances, (16 mFd was common for resevoir capacitances) as long as they aren't actually leaking and you could avoid a replacement. The point Laurence makes about the 100mFd being positive-to-chassis ie "upside down' is interesting. If you check the diagram you will see that there are none of the typical cathode resistors to raise valve cathodes above chassis (and grid) potential. Rather its the cathodes that are at deck voltage and a -ve feed is provided to make the grids negative - hence the 'upside down' condenser - a bit like an old-fashioned 'grid bias' circuit in a battery set.