I have an old Altec-Lansing 9444a pro amp which is my main listening amp. The other day it blew a mains fuse on power up. I opened it up and everything looked ok but it would blow a fuse every time I turned it on, even with the speakers disconnected. I disconnected the cable that attaches the power transformer to the low voltage power supply board and it stopped blowing fuses. I'm pretty sure the problem is not being caused by the low voltage supply because it has its own fuses which are fine. The problem must have been caused by something when low voltage is applied to the main board and protection circuit. Now its not blowing fuses, with everything hooked back up. Any ideas? Schematic is attached.
could be oxide on the connection from the transformer that you cleared by simply disconnecting and re-connecting was adding enough resistance to create a higher inrush current at turn on.
it does warrant also ensuring no intermittant/cracked solders joint on the connection or elsewhere on the PCB that was likely flexed.
next would be main filter condition and age.
it does warrant also ensuring no intermittant/cracked solders joint on the connection or elsewhere on the PCB that was likely flexed.
next would be main filter condition and age.
Interesting. As for the mains filter, I have replaced all of the electrolytic caps so that should be good.
if the transformer secondary is open, a bad high impedance connection would be similar to an open circuit so why not?
maybe i misunderstood what is stated in the introduction of this article https://www.iosrjournals.org/iosr-jeee/Papers/Conf.17017/Volume-2/8. 59-63.pdf?id=7590
maybe i misunderstood what is stated in the introduction of this article https://www.iosrjournals.org/iosr-jeee/Papers/Conf.17017/Volume-2/8. 59-63.pdf?id=7590
I don't know but the OP said "I disconnected the cable that attaches the power transformer to the low voltage power supply board and it stopped blowing fuses."
but now it's no longer blowing fuses....with everything connected
what's your theory scott?
it may well be an intermittent connection
what's your theory scott?
it may well be an intermittent connection
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I had a 10a fast blow in there but its never blown before. I have a 4a in there now cuz I'm out of 10a fuses and no problem.
The only thing that springs to mind is that there was a problem with the connection on one side of the secondary and that was upsetting the flux in the transformer causing a surge at switch on
Usually the last thing you did causes the problem that occurs.
Did you change caps a few days before the main fuse started blowing? Where did you buy the caps? There is a lot of overaged junk on secondary market - ebay, craigslist, gumtree, local parts shops. Buy electrolytic caps from authorized distributors: farnell, mouser, digikey, RS etc.
Usually power amps have slow blo (MDL) fuses. Schematic says 10a slo blo.
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On units this old, mica heat washers under the output transistors can fail, shorting the rail to ground. This unit has to be 40 years old?
Usual practice is to replace them when the transistors fail after 20 years but the washers can go first in home use because low wattage doesn't stress output transistors. Modern silicon washers last longer than mica but mica is still available. I've found selection of silicon washers bad at newark but okay at digikey.
You can gather data by putting individual fuses in holders on the 4 wires coming out of C3, C4, the rails. I'd use 4 amp slo blo in each. If any blows you know what part of the circuit is at fault. DC protection circuit should pull the output relay if one fuse fails, to protect your speaker. If speakers are very expensive, I'd listen on some salvage speakers for a few weeks. I put 6 1/2" drivers out of dead projection TV's in 12" long cardboard boxes with a reflex port in the back for not-to-bad speakers for the TV room or for long term test.
Usual practice is to replace them when the transistors fail after 20 years but the washers can go first in home use because low wattage doesn't stress output transistors. Modern silicon washers last longer than mica but mica is still available. I've found selection of silicon washers bad at newark but okay at digikey.
You can gather data by putting individual fuses in holders on the 4 wires coming out of C3, C4, the rails. I'd use 4 amp slo blo in each. If any blows you know what part of the circuit is at fault. DC protection circuit should pull the output relay if one fuse fails, to protect your speaker. If speakers are very expensive, I'd listen on some salvage speakers for a few weeks. I put 6 1/2" drivers out of dead projection TV's in 12" long cardboard boxes with a reflex port in the back for not-to-bad speakers for the TV room or for long term test.
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This is not a power supply blowing fuses problem.
The low voltage supply (+/-15V) feeds input Op Amp U1, and emitters from level shifting transistors Q1 and Q2.
Current through them drives Darlington Vas transistors Q3/Q5 and Q4/Q6
IF because of any problem excessive current passes through them, both upper (positive) and lower (negative) halves of amplifier will also pass excessive current, both at the same time, shorting positive rail to negative rail and blowing fuses, big time.
Removing +/- 15V stops that, but of course amp is not repaired either.
I think you had an intermittent in that area, it´s momentarily stopped but may reappear.
The low voltage supply (+/-15V) feeds input Op Amp U1, and emitters from level shifting transistors Q1 and Q2.
Current through them drives Darlington Vas transistors Q3/Q5 and Q4/Q6
IF because of any problem excessive current passes through them, both upper (positive) and lower (negative) halves of amplifier will also pass excessive current, both at the same time, shorting positive rail to negative rail and blowing fuses, big time.
Removing +/- 15V stops that, but of course amp is not repaired either.
I think you had an intermittent in that area, it´s momentarily stopped but may reappear.
Another possibility is some residual magnetism in the transformer core. That could cause saturation at switch-on, and the inrush current would be very high. Power cycling several times, applying AC, might have degaussed it enough to clear the “fault”.
I think if that occured, there would have to be 2 faults simultaneously. If U1 came up with an incorrect value or Q1 or Q2 shorted, the DC balance should go way off and the protection relay should light the light and disconnect the speaker. OP hasn't complained about the protection light going on.
I think the "disconnect low voltage supply fuse" causing the fault to clear up is a red herring. I think perhaps different humidity or different temperature of before & after that may have more to do with fault clearance.
This unit has a fan, which pumps a 40 year old unit full of dust. That stuff can carbon track, which causes faults that come & go as the humidity rises & falls. Rails are unspecified but 100 v rail caps indicates that likely there is +-80v floating around in there to short out.
I'd say a thorough cleaning, inspection for carbon tracks, and perhaps replacement of the mica washers is in order. Certainly doesn't cost much when done by an amateur. Just hours on the bench with alcohol a tooth brush a paper towel and a bright light. My tube amps have certainly needed such treatment after 50 years to clear up flakeys. The high wattage Peavey amp came from the factory with flakeys and the cleaning happened on the way to replacing all the burned PCB tracks and poking around to find the good looking bad solder joint that caused "don't use channel A" label and the DC faults to happen in the first place.
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