Metropoulos Forum

Been a while since I posted anything (new profile name cuz I forgot the old one!) ...anyway I have a Metro JMP 50 that I recently replaced all the caps with mullard mustards. Just a note - before performing the cap replacement the amp worked wonderfully... But now when I turn the amp completley on (standby swith off) there is a gigantic hum that increases in volume (and intensity) and then blows the HT fuse. I've checked, double checked and triple checked all the wiring (even under the board thinking I loosened a wire) and all appears to be fine. I am completely stumped and really bummed out Especially because we have 5 gigs booked for April and I was REALLY needing to use this great amp with me Aracom PRX that will be arriving next week (yes please shoot me now!)...
I've built a couple of Metro kits in the past and I've never experienced this specific symptom. Any ideas?? Could it be the OT or PT?? As you can tell I'm reaching here... Thanks All!!

Remove the power tubes and turn the amp on play mode if the fuse does not blow then your power tubes are bad if it does blow double check the grounding especially for the filters if that's fine then you might have a leaky new cap you replaced, try replacing the first two one at a time towards the end of your trouble shooting. Also when the PI tube goes bad it will induce Hum. Let me know what happens.

God Luck

Art

Hey Art - I removed the power tubes and no hum - fuse did not blow. Put power tubes back in and it hums and the volume of the hum gets louder and louder...to the point where I turn it off..don't want to blow my last HT fuse. I also replaced the caps bac to sozo's...still get the violent HUM...now I did notice that the white wire from the board to the SS rectifier was severed...it must have inadvertantly been cut when dressing up the wires (ugh) ..so I re-soldered it on - but still the HUM - thi si not a small HUM - it's huge. Could powering up sans the white wire to the SS rectifier have caused irreversable harm to the PT or someting else?

...also swapped out power tubes and get the same HUM that gets louder and louder and louder...getting desperate here..anyone have any suggestions??

so all you did was install the mustard caps? go back through exactly what you did and check for a wire that maybe fell out underneath from soldering if you did replace them with the board installed. if you removed the board recheck all the connections. i don't suppose you have had enough time to check the bias or even get voltages. something you did along the way is causing it if it was fine before you replaced them.

Mark

Hi Mark,
Here's all that I did; replaced caps on board with mustards, replace POT's with PEC's, repaired the AC cord (insulation was spitting on the outside of the grommet), and redressed the cables (that's when I must have snipped the white wire going from board to rectifier). Could that have done some serious damge??
I went back and replaced all the suspect caps with the original sozo's...still get the HUM - I unsecured the board from the chassis and checked all the wiring underneath - looks real good. Checked the AC cord soldering - looks good. POT's / wiring are all good as well. So I started checking voltatges - basically running thorugh the step by step process that George provides via the build instructions - in going back through the process I noticed that I'm only getting 305 vdc at the (+) terminal of the bridge rectifier - should be around 350 vdc?? ...this is with all tubes removed. I'm wondering if I blew something up when I discovered the white wire going from the board to the rectifier was disconnected...(i must have accidentaly cut it when snipping a zip tie) ...also need to stress that this isn't just a nice little annoying, steady kind hum...it's a HUIM from HELL - and only gets bigger and louder and more intense as the seconds tick by...so much so I finally have to turn the standby switch back on for fear of blowing up my home.

Mike

Tossing up some pics...Hoping maybe someone sees something I don't...

Another....

Pots... (1 of 3)

Pots and board (2 of 3)

Pots (3 of 3)

Last one...

Did you remove the primary wires of the OT and rewired them?

No sir haven't touched them...could my symptoms be indicitive of a sick OT? At this point (and as an act of desperation) I was thinkning I should replace my rectifier, OT and PT...probably overkill but I am at my witts end...

I'm not an expert (especially in the power section) but here are some thoughts...

Since you replaced the pots before the hum, you may have a loose ground somewhere.

The white wire from the rectifier to the board supplies the bias voltage so if you turned the amp fully on with it disconnected you probably damaged your output tubes and maybe the screen resistors. Maybe the screen resistors need to be replaced? (Just a guess)

Has the HT fuse blown with new tubes AND the white rectifier wire reconnected?

Here's what I would try:
- Remove output tubes. Turn amp on but leave on standby and check AC (335V per instructions) and DC (450V) at the rectifier. Check heater AC voltage on output tube sockets. If the AC is okay the PT is probably fine. If the DC is low (below 400VDC), the rectifier diodes may be damaged.

If above checks okay and you have a load box/attenuator:
- Install output tubes and set the bias trim pot fully clockwise and power the amp fully on and see if the HT fuse holds. If the fuse still blows, wait for the experts to chime in. (Sorry, just sharing some things you may have overlooked in hopes that it may help.)