Warm Audio - TB12 Tone Beast ORANGE - No Signal

[NoisyIndividual]

I've pulled many a leaking cap that my meter insisted was fine.
Also, bad caps might read ok in circuit or out of circuit, but put proper voltage across them and they poop.

 

[p0p0p0000]

So Far my stupid self has concluded not conclusively that, I have two failed diodes that caused the resistor to over current and burn.. but what caused the diodes to fail?? Should i be visually inspecting for a short near those diodes? Should i pull the diodes and retest? Could this be a grounding issue.. Getting somewhere slowly.. I like to move it move it... Do you suggest i just replace all caps and then test??

 

[Khron]

I have two failed diodes that caused the resistor to over current and burn.

And which diodes might that be?

 

[p0p0p0000]

D9 and D10, (they might be connected together??) measurements of all diodes in previous post.. and measurements for the regs?? .. ill just repost them here..

2nd measurement is with probes reversed..
D8 - 0.610V, OL
D10 - 0.549V, 0.864V
D9 - 0.552V, 0.929V
D7 - 0.613V, OL
D11 - 0.607V, OL
D12 - 0.609V OL
D17 -(connected to r8) 0.729V OL

U1 : 0.931V , 0.557V
U2: 0.55V , 0.86V

These were all taken in circuit with the 4 big caps removed as in pic in previous post.

 

[NoisyIndividual]

The only thing I got from those measurements is that D17 might be a zener.

 

[Khron]

Could D9 and D10 be the anti-parallel diodes (between their outputs & inputs), to protect the regulators from "backfeed" (when the input voltage drops below the output)?

 

[p0p0p0000]

Not sure whatsoever, I have however just ordered a metric **** ton of capacitors and a few resistors off mouser, so i guess we will see how that goes. Also i need to thank you Khron, you unknowingly influenced and helped me save a motu 828mk2 that someone was giving away ... Now you understand why i was asking such weird questions about those diodes...

 

[mjrippe]

Also the cap measurements, what do you think?? 920uF on the worst is nearly 10% out of spec..

Ten percent for an electrolytic is actually pretty decent. Many are rated +/- 20% (or worse!).

 

[Khron]

Ten percent for an electrolytic is actually pretty decent. Many are rated +/- 20% (or worse!).

Standard tolerance for electrolytics is indeed +/-20%, but

Yeah, well, capacitance isn't always the only (or even best) indicator of a capacitor's health, or longevity.

 

[p0p0p0000]

A quick update.. I have found that C27 a 470uF 25V cap was dead short, pulling it out confirmed this. Also i am thinking that the 2 diodes i thought were dead, are probably ok and giving 2 voltage drop readings because they are still in circuit and connected in parallel to other components and or each other (i can confirm by pulling them out i guess and this is something i learnt from a youtube video).. I also am thinking i may have found the root cause (unless is was just the failed cap), i believe in my excitement and haste of getting my first decent mic pre i forgot about plugging unplugging condenser mics and phantom power conventions and might have pulled the mic out and inserted whilst phantom power was engaged (mic is fine)... My other main suspect is still the AC adapter though, despite numerous people and videos indicating that if the voltage is the same , increased amperage shouldn't matter as the device will only pull what it needs...

 

[p0p0p0000]

OK... the plot thickens? i purchased an ESR meter and all the caps i have pulled out seem to measure good... I have been probing the board in continuity and diode modes with dmm to try and understand more which is slowly working. I was probing the DC input jack and have realised something. The AC adaptor i bought has what seems to be a much longer barrel than the one thats supposed to be supplied with the unit (comparing from photo sent by warm).. I have noticed the input jack is a 3 connector design (the longer barrel connects to extra side connector, shorter doesn't)and wonder whether this could in fact be the source of the problem... in the pic left dc tip is adaptor i bought left is incorrect dC supply provided but looks same length as correct one by looking at photos.. edit :.... it turns out the longer jack doesn't go all the way in so this is highly unlikely a factor..

 

[p0p0p0000]

Hrmm... decided to do maybe what i should have done first... put all the caps back in and simply replaced the burnt resistor... IT POWERS UP...
it WORKS... kind of... well the resistor hasn't burnt out yet, and appears to be not getting hot... Nearly all functions seem to work... except.... when the LPF is engaged there is what sounds like low level mains hum... and when the clean capacitor switch is engaged there is massive noise like when you touch a guitar cable... needless to say i disengaged both of these functions immediately, will be contacting a repairer shortly... This unit has some ghosts and ghouls in it.... still considering replacing all caps...