Hi, I have a 2year old Miniwatt N3 that had been working quite well but just stopped working. I thought it was the fuse but the light on the switch would not work with out the fuse working. I removed the fuse and checked it and seems like it did not blow. Any one have thoughts on what may be wrong? Thanks.
- vkengeri
- vkengeri
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Is main transformer common for both channels? Are tubes light?
1. You can measure with voltmeter /in AC mode of voltmeter/ is there AC on taps of transformer - high and 6.3 v/...Is there DC +U /in DC mode/ after bridge rectifier. Maybe something is wrong in PSU?
Please, give us results. Next measuring advice - after first.
1. You can measure with voltmeter /in AC mode of voltmeter/ is there AC on taps of transformer - high and 6.3 v/...Is there DC +U /in DC mode/ after bridge rectifier. Maybe something is wrong in PSU?
Please, give us results. Next measuring advice - after first.
I can't see power transformer. Maybe +U DC and 6.3 V AC are formed direct from AC net? IMO, it's dangerous and don't open amplifier, if it is connected to the wall. Maybe something is wrong with 6.3 former and/or power diode rectifier.
IMO, You can ask some person, who understand electronic and SS rectifiers.
IMO, You can ask some person, who understand electronic and SS rectifiers.
miniwatt N3 | ???????????? - ?????
There is inside picture.....it seems something like HF impulse PS, maybe problem is here....with IC on Allum. cooler plate.
There is inside picture.....it seems something like HF impulse PS, maybe problem is here....with IC on Allum. cooler plate.
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Tubes not lighting up means there's no heater power supply. Now the problem is whether the plates are not getting power as well. Looking at the internal pic, I think plates are supplied directly from a rectified mains while heaters gets a stepdown transformer? So.. first thing i would do is to unplug all the tubes and check if there's supply for the plates. If plates are powered, then it could be the step down transformer for the heater.
Hi, I have the same problem with this amp. Died while playing music. Power switch lights up, but not the tubes. Fuse is fine.
The heater source is DC, and it seems the filaments are connected in series. All the filaments on mine are OK, but there is no DC coming from the source. B+, however, is fine.
Here's a picture of the inside. The lower left corner is part of the heater supply (two blue capacitors, etc).
The heater source is DC, and it seems the filaments are connected in series. All the filaments on mine are OK, but there is no DC coming from the source. B+, however, is fine.
Here's a picture of the inside. The lower left corner is part of the heater supply (two blue capacitors, etc).
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So the 12V is out?
This 12V comes from a switch mode supply?
High voltage comes from the same switch mode supply?
First check the board for dry solder joints (as an indication: the tin is not shiny but dull gray) and resolder any suspicious spots.
If fine and you are confident with high voltage, locate pins 4 and 5 of the outer tubes (EL 84). These carry the heater voltage and should give a reading of 6,3 volt DC.
Check back.
This 12V comes from a switch mode supply?
High voltage comes from the same switch mode supply?
First check the board for dry solder joints (as an indication: the tin is not shiny but dull gray) and resolder any suspicious spots.
If fine and you are confident with high voltage, locate pins 4 and 5 of the outer tubes (EL 84). These carry the heater voltage and should give a reading of 6,3 volt DC.
Check back.
All the solder joints in this amp are dull grey, because it uses lead-free solder.
You need to check out the primary side filter capacitor. The biggest one, and the first one in the chain directly after the bridge rectifier. That was the problem on mine. It was causing the B+ to be too low, and the heater voltage to be virtually non-existent (just a few mVs). I had to unsolder it completely to notice that it was actually faulty. It had leaked a little bit on the bottom, where the leg comes out. Checking it further, it couldn't actually store any energy.
Symptoms? Check B+ with the amp ON. It should normally be about 300V, but your value will likely be lower. Then power off. If the voltage disappears immediately (rather than trickling away gradually), that cap is dead.
A replacement cap can be somewhat difficult to find. You need a 105 degree rated cap, preferrably (because it is H O T inside). The original value is 270µF/400V, but I used 100µF/400V which worked just fine. Physical size is very critical. The original cap's dimensions are 25 mm diameter, 40 mm length - and you can not in any way go any higher than this. Not wider, not taller. It will absolutely not fit.
You need to check out the primary side filter capacitor. The biggest one, and the first one in the chain directly after the bridge rectifier. That was the problem on mine. It was causing the B+ to be too low, and the heater voltage to be virtually non-existent (just a few mVs). I had to unsolder it completely to notice that it was actually faulty. It had leaked a little bit on the bottom, where the leg comes out. Checking it further, it couldn't actually store any energy.
Symptoms? Check B+ with the amp ON. It should normally be about 300V, but your value will likely be lower. Then power off. If the voltage disappears immediately (rather than trickling away gradually), that cap is dead.
A replacement cap can be somewhat difficult to find. You need a 105 degree rated cap, preferrably (because it is H O T inside). The original value is 270µF/400V, but I used 100µF/400V which worked just fine. Physical size is very critical. The original cap's dimensions are 25 mm diameter, 40 mm length - and you can not in any way go any higher than this. Not wider, not taller. It will absolutely not fit.
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Well add me to the list of those who own mini watts that die while playing music. Was working just fine and when I stepped out, came back and though power light still on and fuse fine, tubes aren't lit and they're cold….
I've had it for less than two weeks and will shell out to return to China….
I've had it for less than two weeks and will shell out to return to China….
Blown capacitor
My Miniwatt also suddenly gave up. Fuse is ok but no tube heating.
When I opened it up, one of the capacitors in the front section (signal filter cap) had been blown to bits. Sticky insulation everywhere...
This is probably not the cause of the failure but the output voltage must have been quite significant for the capacitor to blow like that.
Could this possibly be a clue to the design flaws of the Miniwatt?
My Miniwatt also suddenly gave up. Fuse is ok but no tube heating.
When I opened it up, one of the capacitors in the front section (signal filter cap) had been blown to bits. Sticky insulation everywhere...
This is probably not the cause of the failure but the output voltage must have been quite significant for the capacitor to blow like that.
Could this possibly be a clue to the design flaws of the Miniwatt?
I've summerized the modifications I made to my N3 (also called Appj 0901A) in this post APPJ EL84 SE mini Tube Amp, PA0901A. I recently also exchanged the speaker terminals with better ones. I haven't seen any glaring design flaws, it's simply a cheap assembly and component selection that is common on many consumer electronics devices.
It does have a cheap SMPS with minimal protection, and the filter capacitors are smaller than usual (probably also due to the chassis constraints), so they have higher ESR. The fix is quite simple and cheap, just change the capacitors with quality low-ESR ones and maybe add a varistor at the input. The power supply controller/driver on my amplifier is a popular TOP250YN, it is cheap to exchange if a shorted capacitor damaged it. I exchanged some dodgy capacitors on mine as preventive measure and it works well.
It does have a cheap SMPS with minimal protection, and the filter capacitors are smaller than usual (probably also due to the chassis constraints), so they have higher ESR. The fix is quite simple and cheap, just change the capacitors with quality low-ESR ones and maybe add a varistor at the input. The power supply controller/driver on my amplifier is a popular TOP250YN, it is cheap to exchange if a shorted capacitor damaged it. I exchanged some dodgy capacitors on mine as preventive measure and it works well.
According to diagram I found on the net, the input gain valve has its heaters wired in series for 12.6 volts, while the two EL84s are wired in series. Consequently, a check for heater voltage is best done by measuring 4&5 on the input valve. Pin 5 is GND and pin 9 is left floating.
The heater voltage is somehow also tied to the cathode CCS.
The SMPS use the same transformer for 12.6 volt and B+, with the on-led being tapped from B+. If there is no glow in any of the valves and the on led is not working, I would suspect the issue to be on the primary side of the SNMPS transformer.
A good place to start would be to check voltage across capacitor E03 as proposed by Welcome in his posting above. It would not be the first time I come across capasitors with developing a DC resistance as failure mode when subjected to high ripple currents.
The only thing between bridge rectifier and E03 is component TH1 (probably a termistor or VDR) that I assume is there as a precaution against more extreme outcomes in case of faulty components in the switching circuitry.
Messuring DC straight on bridge rectifier (pins 2 and 4 on my diagram) could also be useful.
Finally, be carefull when working on SMPS as the primary "chopper" side of the transformer is working on rectified mains. If one is available I would recommend an external isolation transformer while amp is open. Maybe also consult "power supply" section of this forum if in doubt for safety measures while dealing with Switch Mode Powers.
Lars
The heater voltage is somehow also tied to the cathode CCS.
The SMPS use the same transformer for 12.6 volt and B+, with the on-led being tapped from B+. If there is no glow in any of the valves and the on led is not working, I would suspect the issue to be on the primary side of the SNMPS transformer.
A good place to start would be to check voltage across capacitor E03 as proposed by Welcome in his posting above. It would not be the first time I come across capasitors with developing a DC resistance as failure mode when subjected to high ripple currents.
The only thing between bridge rectifier and E03 is component TH1 (probably a termistor or VDR) that I assume is there as a precaution against more extreme outcomes in case of faulty components in the switching circuitry.
Messuring DC straight on bridge rectifier (pins 2 and 4 on my diagram) could also be useful.
Finally, be carefull when working on SMPS as the primary "chopper" side of the transformer is working on rectified mains. If one is available I would recommend an external isolation transformer while amp is open. Maybe also consult "power supply" section of this forum if in doubt for safety measures while dealing with Switch Mode Powers.
Lars