F5m kit

Jump to Latest
#1,722
Studied your photos and all PS wiring looks correct. Just to confirm, with the variac set to 70V, power supply +out to gnd exceeds +26VDC and -out to gnd exceeds -26VDC? And +out to -out exceeds 52VDC? Dumb question: what does the AC output of the variac measure with a meter?
 
  • Like
Reactions: rayma
#1,723
SimpleJim said:
After correcting my mistake (post 1704 w/pics) with J74 in incorrect spot, I'm powering up again via variac and getting very high rail voltage exceeding 26v on both channels before I get 70v on the variac measured off V+ and ground on the PS boards. Antek 4218 Trans, I should be around 25v on the rails I believe. Reading 17.8v from the rectifiers. All pots full CCW. Put new J74/K170's in after earlier mistake. Any ideas?
Click to expand...
Are you sure you are measuring from ground? I don't see how you can have a higher voltage at the load than you have on the output of the rectifiers. What do you measure from V- to V+?
 
#1,726
AVD, I repowered and both rails came up to 24.5v @ line voltage now, V+ to ground on both sides. I can't explain why after several repeated high readings. However moving V+ to V- immediately after was 36.5v and started smoking, 3 amp line fuse blew. I didn't attempt to repower, I've been at it all afternoon and will only do something stupider. I calibrated the Variac previously and marked the correct setting on the dial as it's an Amazon special and was off v's
 
#1,727
Check to make sure your output devices are properly insulated from the heat sinks. A short could connect one of the rails to ground through R6 or R7 (magic smoke). Disconnect PS leads from amp boards and measure resistance to ground from amp board PS + or - while pressing a bit on the boards and output devices.

Another possibility:
Pressure on V+ or - PS connection point on PS board causing a short to ground? Any possibility there is lack of clearance between components or wire leads protruding on underside of PCB and the chassis bottom plate? PS PCB flexing? I see you have at least one wire running under the PS PCB so there must be some clearance. With power to the amp boards disconnected and PS caps discharged, measure resistance from V+ and - pads to circuit and chassis grounds while pressing on things.
 
#1,728
With erratic and confusing readings, fuses blowing up, smoke coming out of ????.... I would step back here and take things one step at a time if this amp was in front of me. I would ask myself questions like "is this power supply functioning properly with no amplifier boards connected to it? "Ok, looks great".... "now, let's hook it to the left channel PCB that I have verified is built properly, that has no shorted components on the board or to the heatsink"... "ok, that works fine... let's do the same with the right channel now..."
 
  • Like
Reactions: SimpleJim
You must log in or register to reply here.
Top Bottom