2w Sub Miniature Express

[mdd1959]

I'd start by removing the tubes and then seeing if the fuse holds in. You can check your voltages too. If everything seems Kosher, then you can install the tubes one by one to see if any are giving you a problem.

Is there a warranty on the tubes? I'm thinking about building one of these and wonder what the warranty is on the kit.

[UR12]

I'd start by removing the tubes and then seeing if the fuse holds in. You can check your voltages too. If everything seems Kosher, then you can install the tubes one by one to see if any are giving you a problem.

Is there a warranty on the tubes? I'm thinking about building one of these and wonder what the warranty is on the kit.

The parts are warrantied for a period of 90 days. I would also suggest that you remove the tubes and see if the fuse still blows. Are you still running the amps on 100v or are you running them at 120v. If these are still being run on 100v then you may experience problems with the heater winding on the tranny and the tubes. I would pull the tubes and see if you still have a problem with the fuse blowing. If it doesn't then you need to check some voltages . If it does not then put the 2 power tubes and with a speaker attached turn the amp on and see if it stays on. If it still remains on the install the preamp tubes one at a time. If you find a tube that when you plug it in it blows the fuse then stop there.

On another note. I have a new batch of power transformers coming in with a 100v, 120v, 200v, 220v and 240v taps. If it turns out your power transformer has been damaged then I can send you one for proper voltage operation in Japan as soon as they come in.

[MCK]

Hey structo, panrixx, mdd1959, thanks for your kind posts.

Nothing seems burnt. Visually all ok. Starting the troubleshooting. Great link to that troubleshoooting site by the way.

All tubes light up.
Took all tubes out to see if I can isolate this as a tube problem. If it is then I should be able to power up and keep the pilot lit with no fuse blow.
Tried this but still ended up with a fuse blow. Running out of fuses here...
Slight smell emanating from the power supply side I think.

Next step, will test all caps and diodes etc on power supply.

[MCK]

... I would also suggest that you remove the tubes and see if the fuse still blows. Are you still running the amps on 100v or are you running them at 120v. If these are still being run on 100v then you may experience problems with the heater winding on the tranny and the tubes. I would pull the tubes and see if you still have a problem with the fuse blowing. If it doesn't then you need to check some voltages . If it does not then put the 2 power tubes and with a speaker attached turn the amp on and see if it stays on. If it still remains on the install the preamp tubes one at a time. If you find a tube that when you plug it in it blows the fuse then stop there.

On another note. I have a new batch of power transformers coming in with a 100v, 120v, 200v, 220v and 240v taps. If it turns out your power transformer has been damaged then I can send you one for proper voltage operation in Japan as soon as they come in.

Dana, hi. Thanks for your help.

Yup, the fuse still blows with the tubes pulled out. All diodes & caps on the power supply side seems to be ok. Next is to test the voltages I suppose. I can do this since the fuse does not blow right away. Any suggestions on how to test the PT? There is some good ideas on the link panrixx shared http://www.geofex.com/ampdbug/pwrtrans.htm but I don't know how much of this is relevant for the Lil' Devil PT.

I am running the amps on 100V as I thought that was ok despite having slightly lower voltage on my B+. I think its a great idea to cover 100V with a revised PT. Any timeline on these? Sounds like I might want to order 2 even if my current problem turns out to be unrelated.

Regards & Thanks

[MCK]

Ok. Some voltage testing done.

With Power pot at max (ie build manual pg 42 Test 2) I am getting 88V. This used to be 147V when the amp was working.

With Power pot wound down (ie build manual pg 42 Test 2) I am still getting 88V. There is something wrong here.

Not doing anything else now. Please let me know what you think. Is this an indication of PT failure? Should I stop using my other working Lil Devil on 100V?

Thanks for your all help.

[Structo]

Sounds like the VVR is toast.

[mdd1959]

MCK,

Bummer about the problems.

I'd proceed by trying to isolate the AC from the DC side.

I'm not familiar with this amp but perhaps you can disconnect the AC side of the DC supply by removing the fullwave rectifier or the diodes or the transformer wires. If the fuse still blows, It could be the filament winding (I'm assuming the filaments are AC on this amp). If the fuse doesn't blow, then the problem is most likely in the DC section.

If it still blows, you could try lifiting the filament winding wires off the board and see what happens on the filament section. You have to get it to a point where one section or the other is not popping the fuse. Because the fuse hangs in a bit before blowing, it might be a component breaking down. Try isolating to find what section is causing the problem; either the filament or the DC supply, etc.

I'd also do a visual inspection to see if there's any bridging of the traces or something shorting although that usually cause an immediate blowing of the fuse.

Best of luck!

Edit: I see that you've Trouble Shooted a bit. Sounds like the VVR could be the issue. Perhaps Dana will chime in.

[MCK]

Structo, mdd1959, thanks for your input. Though its sad to have to take it apart and go through this I am happy its happening now while my knowledge of this build is still fresh. I am sure I will learn a lot as a result of this problem. So yes, there is a silver lining to all clouds. And oh, this is yet another reason why I have 2 Lil' Devils... The other one is still working though I might need to ground it if Dana decides operating on 100V is not advisable with the current PT.

If VVR is busted then I guess it will be a smaller fix than the whole PT etc so lets hope its just that. Also if thats the case then I should be able to disconnect the whole power supply PCB and test it on its own and see that there is no fuse blowout and also see the proper voltages across the board. Will wait a little to see if Dana has any other guidance (re VVR etc) and then proceed to this testing to complete the troubleshooting data dump.

[UR12]

Ok. Some voltage testing done.

With Power pot at max (ie build manual pg 42 Test 2) I am getting 88V. This used to be 147V when the amp was working.

With Power pot wound down (ie build manual pg 42 Test 2) I am still getting 88V. There is something wrong here.

Not doing anything else now. Please let me know what you think. Is this an indication of PT failure? Should I stop using my other working Lil Devil on 100V?

Thanks for your all help.

A couple of more checks. If you are measureing like step 2 in the manual then you are measuring the voltage BEFORE the VVR and this should stay the same no matter the position of the power control I will need to change the manual in that regard. Try removing the B+ wire going from the power supply board to the main board. Just unsolder the end going to the main board marked B+. Then with the wire lifted measure the voltage on the end of the wire you removed and see what voltage you have. If it goes back to normal ~147 then I would guess the tranny is OK, if it is not normal then you will need to do a few more checks on the power supply board to see what is going on. Also measure your heater voltage at the 2 green wires going onto the main board. These normally measure 6.3VAC. I am going to make a guess that you will be down around 5vac with 100v input to the tranny and that is not good for the tubes. The tubes can run fine on 147VDC but they need a full 6vac on the heaters.

[MCK]

Dana, thanks for that. From memory, the heaters were testing 5.8V before and 147V was with the B+ connected to the main board. Will disconnect & test & report in few minutes.

PS. Running out of fuses here but getting better at timing when it'll pop and turn it off...

[MCK]

Ok. B+ lifted I read 97V across ground & B+. Heater reads 5.4VAC which is lower than before... This is starting to feel more & more like a PT failure. Thoughts? Time to ground #1?

[UR12]

Ok. B+ lifted I read 97V across ground & B+. Heater reads 5.4VAC which is lower than before... This is starting to feel more & more like a PT failure. Thoughts? Time to ground #1?

You have to remember that you are reading the pulsating DC voltage comming off the HV diodes without any filter caps connected. When the filter caps are connected on the board (via the wire you just lifted) that voltage should go up with out the tubes in there and your reading with it connected to the board was 88. Try putting your meter on AC volts and measure the voltage at the 2 red wires on the Power supply board labled HV AC in.

[MCK]

Try putting your meter on AC volts and measure the voltage at the 2 red wires on the Power supply board labled HV AC in.

With B+ Still disconnected from the main board, red connected to HV in on power supply board reads 216.2VAC

That sounds about right doesn't it?

[UR12]

Yes that sounds about right. Maybe you will get lucky and the mosfet will be bad. Can you remove it / unsolder it from the board temporarily? Are you still blowin fuses with the red wire disconnected from the main Board?

[MCK]

Sure. You might also recall I have a spare MOSFET... I am always happier with doubles & spares... removing it now. Would you like me to install the spare rightaway?