Full thread - http://www.speakerplans.com/Forum/topic23310.html
First Post - Topic: Carver PM 1.5 Power Factor
Posted: 26 January 2009 at 12:09am
Just had an interesting experience on the bench. Been working on a pair of old Carver PM1.5's for a long standing forum member. The one on the bench at the moment had been attacked by someone rather ham-fisted with a soldering iron and started working once the solder bridges and unnecessary wire links had been removed. Have had to replace the low voltage supplies smoothing caps as they'd gone dry and stopped actually storing charge. That was a highly entertaining job for starters because of their location and the fact that whoever built them had used a whole roll of solder on the board! I could go on for ages about how strange a design they are: Lighting dimmer for PSU regulation, no less than three sets of supply rails, 10 output devices per channel but only two connected to the output!
Anyway, I started running the amp up into a 4R load as a kind of final check before screwing the case on and deeming it finished but after only a few seconds of running at full power (slightly clipping) the amp cut out. Very quietly, no bangs or anything. Turned out the mains fuse had blown (6.3A slow blow). So I replaced it with the one from the other amp, same value. Started the testing again and it blew the fuse again. Now at this point I've run out of 6.3A 35mm fuses so it's now got a 20A one temporarily but have downrated the plug fuse to 5A for testing and I decide perhaps measuring the input current would be a good idea.
Very quickly the reason for the fuse blowing was obvious, the amp has an appalling power factor. At low powers it's about 0.3, rising to 0.5 at high power. To give you an idea, with the amp delivering 300WRMS from one channel into a 4Ohm load, it's drawing over 5A or 1300VA from the mains!!! Just before it took out the 5A fuse in the plug I clocked it at over 3000VA and this is still only with one channel running! Maximum output power is about 800WRMS/4R for short bursts (100 ms or so)