Loud hum STD4

[andylevine]

Rebuilt the amp for the STD4 project a while ago (2 months or so) but never had enough of the bits together to test well.  FINALLY today after getting mech working properly and woofers back in the carcass, I powered up, heard the loud 'pop' I usually hear, and then hear a loud hum coming out of woofers (no tweeters in cabinet yet). Not that you need it, but heres a link to a video where you can hear it: 

www.youtube.com/watch?v=2PyR8iB_W-g&feature=youtu.be   (turn up your volume)

Powered down, unplugged the woofers and I definitely still hear the hum from somewhere in the chassis.  It's very hard to isolate, but I want to say it's near the crossover (SN1).  I've started unplugging bits one at a time. The amp, no change. The DCC, no change.  I thought it might be the ballast for upper light so I unwired...no change.  The closest I came to any change was when I unplugged the chase lighting power supply....its not where the hum is coming from, but it did seem to affect the hum, diminished it greatly.  I looked to ensure that I hooked up the grounding wire to that and all is in place properly.

Not sure where to look next.  Suggestions welcome

Thanks
Andy

[Ron Rich]

Hi Andy,
Sounds like 120vac hum ? I am unclear from what you wrote--is the amp un-plugged or not, when you hear this ??
Ron Rich

[andylevine]

Hi Ron,

I tired numerous ways, with and without amp plugged in, so ...hard to say. The video was with the amp plugged in, and woofers hooked up. Obviously if I unplug the woofers no sound from them but hum still exists. Just tried again and I think it is coming from amp. The mute/trip relay engages when plugged in, opens again when unplugged. I will say, I still think the lighting power supply also affects the 'size' of the hum.

Isn't 120vac hum usually a bad ground or a short somewhere??

Thanks
Andy

[Ron Rich]

Andy,
Boy, you got me--- All I can envision is un-plugging everything, from all AC outlets--then if still humming, it "hastabe" the light transformer(s) somewhere, as nothing else is ( shooodbe !) ,getting "117vac" ?? Ron Rich

[andylevine]

PLEASE don't say I stumped you Ron Rich.... As Princess Leia once said, "You're my last hope, Obi Wan!!" ;-) So, pretend I didn't say ANYTHING about what I thought I did (because you can never trust the things we rookie say, anyways)...pretend you ONLY saw the video. Where would you go from there?

Taking your advice above I literally unplugged absolutely everything from every socket, every A/C etc, every jack on every component. No DCC, no controller, no black or grey boxes, no light no nothing. The ONLY things left plugged in are the amp, the 5 wires to the speakers (yellow, green red, blue and black) and the plug to the wall. Same big hum. I even went to the extreme of not plugging amp in to the A/C socket in the unit, instead opting for an external extension cord, in case I screwed up the wiring when reinstalling harness. No help. That pretty well isolates it to the amp, right??? If so, then what??

[tonyj]

When I bought my SPS160 it had a nasty hum similar to that and found a shorted output transistor on one channel as well as a burned up resistor on the driver board. Replaced both outputs on that channel, changed the resistors to flameproof type as suggested, recapped and all is well.

[andylevine]

Thanks tonyj, it's got to be something like that. I just pulled the amp, hooked it up to my home turntable and a decent pair of bookshelf speakers. Hum still present. I TOTALLY rebuilt everything in this amp, replaced both output transistors, all transistors on the driver card and all resistors even close to out of spec on all cards. Replaced all electrolytic caps as well. I'm no expert in doing these sort of things though, but was pretty careful and my soldering looks good. However, it would not surprise me at all if I screwed something up. Will take amp down to my workshop later and see if it's anything obvious, but my skills are slim at best. I really don't want to send it out as part of this project is to learn new things.

[andylevine]

Finally getting back to this.... the hum was from.....drumroll please.... Yup, blown right channel output transistor! still don't know why, though. I replaced ALL transistors on driver board and all four of the output transistors when I recapped the amp, so it must be something I did improperly. I was very careful setting the bias after I rebuilt. Also found one of the 100 ohm flameproof resistors toasted on the board.

So today, I replaced both output transistors on the right channel (again), replaced all four transistors on the driver card (again), replaced the flameproof resistor that was toasted and a few others that we're within tolerance but close enough to warrant changing.

I'm going through the amp troubleshooting steps in Tony's SHP Amplifier guide....in the continuity tests, everything is fine, except steps G and H. Yes, I know the manual has an error on those steps... should read 'Collector' as opposed to 'Emitter'.... still... while I have connection between C66(-) and Q16/26 Collector, I have exactly 100 ohms resistance on both. I cant figure out where this is coming from. The only 100ohm resistors even close are the flameproofs, but they shouldn't be part of the path I don't think. The way I see the schematic, is Q16 collector -> Q18 base (via the #12 wire from the connector) -> C66(-)

I'm afraid to plug this in again until I figure out whats going on. Any clues Ron Rich , robnyc or tonyj All suggestions greatly welcomed.

Andy

[andylevine]

So apparently that circuit DOES go through the 100 ohm flameproof. Gonna reset the bias and keep my fingers crossed. Wish me luck.

[tonyj]

Sorry - was away on vacation and just saw a notification.  I'm not sure why the replacements would have blown - you've verified that there isn't a short on the right channel speaker wiring I assume?  Other than that, mine didn't give me any problem after I replaced my outputs, recapped and updated the 4 resistors to flameproof type as recommended.  The outputs I installed were NTE 181, with mica insulators and heat sink compound and I assume you've done the same.  Your amp also has the modification to "hardwire" the problematic connector issue as well?

[andylevine]

tonyj - I did all the replacements (again), checked for shorts and found none, reset the bias and fired it up. Sounds AWESOME. Shocked really at how loud I can turn this up without distortion. I still have crappy needles and am super pleased with sound. Will put new needles in after finishing a few more adjustments. The sound is WAY different than my M100B, as it should be. Love the warmth of my B, this one sounds like high school!!

[tonyj]

Great news - glad to hear it's working!