Avo 8, 7 and 40 transformer output voltages - UK Vintage Radio Repair and Restoration Discussion Forum

SeanMcK · 28th Oct 2008, 9:32 pm

When trying to work out where the problem is in the AC side of an Avo 7, 8 or 40 etc is there any point in measuring the AC voltage across the rectifier = the outputs of the transformer?
If so does anyone have the fsd voltage for an Avo 8 Mk3?
Would it be safe for the transformer to try it with the rectifier etc disconnected?
Thanks

SPCh · 28th Oct 2008, 10:36 pm

Not really; it's used as a current transformer, and the voltages you might measure will not be very helpful. If you think there's a transformer probem (e.g shorted turns) you COULD disconnect the output and measure voltages; the turns ratios are well-documented, but only relevant without load. Also, you'll need a good AC voltmeter to give you anything like the truth. I've never encountered a faulty transformer: the problems are invariably either the rectifier(s), the switching, or the feed resistors on the primary side.

The fsd voltage for a Model 8 Mk III is 125mV at the meter, but bear in mind that the movement indicates MEAN current, not RMS voltage, so there's a factor of 1.11 to take into account.

By all means disconnect the rectifier, but bear in mind all of the above. I can help with the sums if you need it.

SeanMcK · 28th Oct 2008, 10:58 pm

Ta.
Could you explain "current transformer"?

SPCh · 28th Oct 2008, 11:32 pm

Not easily: it's more an attitude of mind than a matter of turns ratios.

"Normally", you apply a voltage to the primary and get the transformed voltage out.
You then lose a bit (about a volt) due to the rectifier, and what's left drives a current through the load.

With a current transformer you have to think backwards. The current through the load (the important thing, here) defines the load voltage, and this is increased by the rectifier to demand a slightly higher voltage at the transformer output. This is transformed to the primary, which therefore demands a slightly higher voltage than you might be expecting to present it with (though the current, of course, is turns-ratio accurate for what you want). Then for higher voltage inputs a series resistor provides the appropriate voltage drop from the input terminals. The Ohms Law arithmetic is all quite straightforward, but you have to think current rather than voltage. And in the moving coil meter application, don't forget the form factor coefficient.

28th Oct 2008, 9:32 pm	#1
SeanMcK Retired Dormant Member Join Date: Sep 2008 Location: Londonderry, Northern Ireland, UK. Posts: 58	Avo 8, 7 and 40 transformer output voltages When trying to work out where the problem is in the AC side of an Avo 7, 8 or 40 etc is there any point in measuring the AC voltage across the rectifier = the outputs of the transformer? If so does anyone have the fsd voltage for an Avo 8 Mk3? Would it be safe for the transformer to try it with the rectifier etc disconnected? Thanks

28th Oct 2008, 10:36 pm	#2
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: Avo 8, 7 and 40 transformer output voltages Not really; it's used as a current transformer, and the voltages you might measure will not be very helpful. If you think there's a transformer probem (e.g shorted turns) you COULD disconnect the output and measure voltages; the turns ratios are well-documented, but only relevant without load. Also, you'll need a good AC voltmeter to give you anything like the truth. I've never encountered a faulty transformer: the problems are invariably either the rectifier(s), the switching, or the feed resistors on the primary side. The fsd voltage for a Model 8 Mk III is 125mV at the meter, but bear in mind that the movement indicates MEAN current, not RMS voltage, so there's a factor of 1.11 to take into account. By all means disconnect the rectifier, but bear in mind all of the above. I can help with the sums if you need it.

28th Oct 2008, 10:58 pm	#3
SeanMcK Retired Dormant Member Join Date: Sep 2008 Location: Londonderry, Northern Ireland, UK. Posts: 58	Re: Avo 8, 7 and 40 transformer output voltages Ta. Could you explain "current transformer"?

28th Oct 2008, 11:32 pm	#4
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: Avo 8, 7 and 40 transformer output voltages Not easily: it's more an attitude of mind than a matter of turns ratios. "Normally", you apply a voltage to the primary and get the transformed voltage out. You then lose a bit (about a volt) due to the rectifier, and what's left drives a current through the load. With a current transformer you have to think backwards. The current through the load (the important thing, here) defines the load voltage, and this is increased by the rectifier to demand a slightly higher voltage at the transformer output. This is transformed to the primary, which therefore demands a slightly higher voltage than you might be expecting to present it with (though the current, of course, is turns-ratio accurate for what you want). Then for higher voltage inputs a series resistor provides the appropriate voltage drop from the input terminals. The Ohms Law arithmetic is all quite straightforward, but you have to think current rather than voltage. And in the moving coil meter application, don't forget the form factor coefficient.