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tubelab SSE popping fuses with no tubes installed

johnzm

Member
2010-07-14 8:43 pm
Hey guys,

I have had a good 6-8 months of no issues on this amp, but now another one has cropped up.

it is currently popping the fuse right at start up, with no tubes installed.

any advice to start with, on troubleshooting?

674230d1523363743-popping-fuses-startup-img_20180115_143124-jpg
 
At first, make a current limited power supply, inserting a series lamp (Filamentary, not led nor fluorescent), making an extension cord, or with two crocodiles, and replacing the fuse with the lamp. So you will stop blowing fuses unnecessarily. The lamp must be more or less twice the power expected to consumed by the DUT. Without tubes, it must not light or at most, a brief flash when charging the 'lytics of the rectifier sector, but post this flash, lamp must be almost dark.

Second, measure with a VOM some important points: rectifiers, 'lytic caps, wires, switches, etc.

Third, disconnect (Unsoldering) some points in away to individualize the part of the set that is causing the fault.

Take care with high voltage points in the circuit.
 
The usual cause of start up fuse shorting is the diode rectifiers, but they do not look to be installed in your amplifier.

The power supply filter capacitors are across B+ to ground. I would check those filter capacitors to make sure one or both have not shorted or become leaky.
 
popping the fuse right at start up, with no tubes installed.

As stated, it looks like the solid state rectifiers are not installed, If so remove them and try again.

This happens without the rectifier tube?

If so it has to be in the power transformer or its wiring. We can use the process of elimination to narrow things down.

As mentioned the light bulb tester can make life easier and save a lot of fuses.

Assuming the fuse blows or the bulb glows even without a rectifier tube, disconnect one secondary at a time from the board and test again. If all secondaries have been disconnected from the board and the fuse still pops, the board is not the problem since it is no longer connected to the transformer. There isn't much left except the transformer and its wiring. Look carefully at all the wiring, especially where the wires come out of the transformer and go through the chassis. Any worn spot needs to be fixed.
 

johnzm

Member
2010-07-14 8:43 pm
no rectifier tube.

i finally got a couple tools and pulled all of the all the secondaries and it still popped. i checked the wiring through the chassis and it looks solid as well.

should i get an ohm reading between the 6.3 and ground? i see about 9.25k.
between the 375/375 im getting about 48.8
and the 5 and 6.3 are showing very low resistance

so im assuming at the moment its either a switch problem (tested but i have a second one to verify with)
a wire hitting the chassis somewhere
or a bad power transformer, which is only a couple years old.

does anyone have any ideas on further testing the power transformer?
 
At first, make a current limited power supply, inserting a series lamp (Filamentary, not led nor fluorescent), making an extension cord, or with two crocodiles, and replacing the fuse with the lamp. So you will stop blowing fuses unnecessarily. The lamp must be more or less twice the power expected to consumed by the DUT. Without tubes, it must not light or at most, a brief flash when charging the 'lytics of the rectifier sector, but post this flash, lamp must be almost dark.

This is the way it’s done. Give it a try. Your problem will become apparent.

Did you tape off your secondaries when you disconnected them? Is you power switch wired in series with the PT primaries?
 

johnzm

Member
2010-07-14 8:43 pm
ok,

i put the light bulb in and it lights dim when i power it up.
i am able to confirm that the transformer is putting the proper amount of power out of all of the lines.

can anyone confirm that the next step would be pulling and replacing the power caps?
 
I'm currently traveling and will be away from my lab until Dec 31, so I can't try a transformer and a light bulb to see if dim is normal. What wattage bulb did you use?

should i get an ohm reading between the 6.3 and ground? i see about 9.25k.

If you are measuring this on the transformer alone, and not the board, no this isn't right. The board does have a 10K resistor from the 6.3 volt input to ground, so this is normal for a transformer connected to the board.

i finally got a couple tools and pulled all of the all the secondaries and it still popped.

The transformer, switch and fuseholder blew the fuse without any connection to the board? The same setup makes the bulb glow dim while putting out all the proper voltages? If so, I would double check that the fuses are indeed the correct fuse for the SSE. I use a 2 amp slow blow fuse in the USA, and recommend a 1 amp slow blow for users in 230 / 240 volt countries.

If the transformer alone makes the bulb glow dim, try reconnecting the secondaries back to the board one at a time, with no tubes installed, and rechecking the glow. Start with the 6.3 volt, then the 5 volt, then the HV winding. None of these should change the bulb glow.

If all pass, then use a 60 or 75 watt bulb and install the rectifier tube. On power up the bulb should light up at about half brightness, get slightly dimmer within a second or two, then begin to get brighter as the B+ voltage comes up. Once the caps have charged the bulb should grow dimmer, back to near it's initial brightness, since there should only be about 12 watts of power being drawn by the board without tubes.
 

johnzm

Member
2010-07-14 8:43 pm
so i decided to tempt fate, and just plugged it right in, with all tubes plugged in. it has no sound out the speakers.

all the heater voltages look good, the transformer voltage looks good, but im not getting any DC out of the rectifier.

additionally when testing based off this thread
https://www.diyaudio.com/forums/tubelab/158719-simple-se-checkout-dummies.html#post2051548

ground to R4 is getting low/no voltage at all.

based on this, i ordered another rectifier tube.
 

johnzm

Member
2010-07-14 8:43 pm
got the new rectifier in.

i left all the fuse stuff bypassed, and no light bulb, and i powered it up.
i managed to check a few things and saw 502 volts out of the rectifier. i let it play music for awhile (cant tell if it was any good as i am using a pair of speakers i've never heard before)
and powered it off. put it back through the fuse and first time i powered up it popped the fuse.
so one thing seems to be resolved but i still do have something amiss in there.