Original pog repair

[slackshampoo]

I got an old POG unit that started acting funny on me. I went to turn it on and just got faint sputtery sounds. I double checked I was using the right power supply, an electro-harmonix 18V 500mA one spot. Sure enough it was the right power supply but it looks like it went faulty. I checked and it was delivering 27V! Excellent
I hooked the pedal up to my zuma at 18V 500mA and I'm getting the same sputtering sound. Im assuming something was damaged from the high voltage. It's all smd so I'm not sure where to start debugging here.

Anybody have a schematic or possibly can point me in the right direction? I'm out of my depth so any help would be much appreciated. Hopefully one of these big chips isn't toast.

[bluebunny]

Is this a real post or are you just trying out your posting fu?  This board is for the latter.  Let us know if you want this moved somewhere that'll get some traffic.

[idy]

Very common for those power supplies to read very high when there is no load. You would need to check the voltage when it is plugged in to the unit (or something else, a dummy load like a 10k resistor.)

[duck_arse]

moved, cause I'm interested.

is there a failure mechanism in one-spots that allows the output voltage to shoot up to 27V? aren't they regulated? isn't output regulation inherent to switch-mode-ing?

[PRR]

> isn't output regulation inherent to switch-mode-ing?

Not really. By the time you get a switcher working and robustly, regulation is nearly trivial, but not inherent.

The 1 SPOT® by Truetone IS regulated, but it is not clear this is truly a 1 SPOT (electro-harmonix ?).

[Rob Strand]

The 1 SPOT® by Truetone IS regulated, but it is not clear this is truly a 1 SPOT (electro-harmonix ?).

I'm confused about the Truetone and electro-harmonix as well.

If you look inside the  Truetone you can see it has a transformer as well as claiming to be switch-mode and
clearly having switchmode devices in there.   There also looks like a bank of linear regulators.

https://truetone.com/cs7/

It's not so easy to derive the exact topology.  There's many options.  The main uncertainty in this case is how the 18V is derived.
Just a few. 
-  mains in ->  transformer to low voltage ->  switch mode for isolation only and only on 9V  -> linear regulators on 9V
   unregulated output from transformer for 18V that would explain 27V with no load.

-  mains in ->  transformer to low voltage ->  switch mode for isolation only on 9V and 18V -> linear regulators on 9V,
    regulator on 18V (can't see one).

-  mains in ->  transformer to low voltage ->  switch mode for isolation only on 9V and 18V -> linear regulators on 9V
   maybe no regulator on 18V

Not really. By the time you get a switcher working and robustly, regulation is nearly trivial, but not inherent.

So my argument is the 18V is ambiguous.  Is the truetone 18V output supposed to be regulated? and it derived from a mains source or a switchmode.

More the issue what is the correct PSU for the POG.   Is it supposed to be 18V regulated and will it handle 18V unregulated.  And if it supposed to be 18V regulated did the 27V blow it up.

I guess all of the above is leading to how the unit blew-up.   If the 27V was the cause then perhaps that's a place to start debugging the pedal.    It would reduce the amount of debugging required.  In order to take that direction we need to answer the main question.  What input voltage can the POG handle?

According to this page a schematic is available on "the other forum",
https://falseelectronics.blogspot.com/2017/08/electro-harmonix-pog-big-box.html




It looks like it should handle 27V in.   Nonetheless worth checking the 12V rail is good,  R37 not fried,
R39 not fried, getting about 20V across D4,  getting 3.3V at output of switcher (output of L1 inductor
and 10uF SMD cap C39 - sorry got chopped), 3.3V at U6, 3.3V at U23.

[PRR]

...inside the  Truetone you can see it has a transformer as well as claiming to be switch-mode and clearly having switchmode devices in there.   There also looks like a bank of linear regulators. ............... ..............

That's a deluxe model. It has an AC output for pedal who swing both ways.

If we really wanna talk 1-SPOT the designer/support-staff is lurking among us.

However "1 spot" can also be like "aspirin"... it has become a generic name for "ANY" little many-way supply box. The reference to "E-H" suggests this may be something else.

[Rob Strand]

That's a deluxe model. It has an AC output for pedal who swing both ways.

Ah that's what's going on.   So presumably no mains transformer in the lower models.

However "1 spot" can also be like "aspirin"... it has become a generic name for "ANY" little many-way supply box. The reference to "E-H" suggests this may be something else.

I've see people call them "Isolate Power Supplies" but that's not accurate either because not all have isolate outputs.   The generic name, and nothing to do with guitars, is a DC Power Distribution Box.   If the outputs are isolate then you add the word "Isolated" - even then not clear if the isolation is between outputs or from input to output.

[slackshampoo]

Thanks for moving my post to the correct spot. I don't contribute here very often so sorry for not posting in the right location. I hadn't considered the no-load voltage discrepancy. I just tested it and it is lover than 27V with the load but it's still high at 22V at the J & G terminals on the pcb. It's not a true branded 1-spot. I was using that term loosely (technically incorrect I suppose), the whole aspirin Kleenex thing. Here's a picture to clarify. Thanks for the engagement.

[slackshampoo]

[Rob Strand]

Thanks for moving my post to the correct spot. I don't contribute here very often so sorry for not posting in the right location. I hadn't considered the no-load voltage discrepancy. I just tested it and it is lover than 27V with the load but it's still high at 22V at the J & G terminals on the pcb. It's not a true branded 1-spot. I was using that term loosely (technically incorrect I suppose), the whole aspirin Kleenex thing. Here's a picture to clarify. Thanks for the engagemen

That's just an unregulated DC power supply.    27V unloaded and 22V with a light load is very typical.   It might drop to 16V at full load on the label.  (The output has ripple so the dips of the ripple could be below the average DC measured on the multimeter.)

My understanding is EHX pedals use that type of adaptor.    The EHX adaptors were unregulated.  If you can find a manual for the POG it might spell out a recommended adaptor.

In short (at this point) I don't think the PSU is the cause of the problem.    It's probably an unrelated fault.   Could be a faultly cap issue.   It might be worth doing a web search for your exact same problem.

Edit:
look here, second post,
https://www.harmonycentral.com/forums/topic/1627645-broken-ehx-pog/

you aren't the only one to see this.

[slackshampoo]

It looks like it should handle 27V in.   Nonetheless worth checking the 12V rail is good,  R37 not fried,
R39 not fried, getting about 20V across D4,  getting 3.3V at output of switcher (output of L1 inductor
and 10uF SMD cap C39 - sorry got chopped), 3.3V at U6, 3.3V at U23.

That's just an unregulated DC power supply.    27V unloaded and 22V with a light load is very typical.   It might drop to 16V at full load on the label.  (The output has ripple so the dips of the ripple could be below the average DC measured on the multimeter.)

My understanding is EHX pedals use that type of adaptor.    The EHX adaptors were unregulated.  If you can find a manual for the POG it might spell out a recommended adaptor.

In short (at this point) I don't think the PSU is the cause of the problem.    It's probably an unrelated fault.   Could be a faultly cap issue.   It might be worth doing a web search for your exact same problem.

Edit:
look here, second post,
https://www.harmonycentral.com/forums/topic/1627645-broken-ehx-pog/

you aren't the only one to see this.

Thanks for these suggestions, I'll definitely be checking this out shortly and share what I find. I've reached out to EHX as well so well see where it goes.

[slackshampoo]

Tested everything suggested:
R37 reads 2.3M so that's good
R39 & D4 are omitted because there's no U5
12V rail on U4 is good
3.3V off U6 & U23 looks good as well

I'll check the other IC's later when I get a chance.

[Rob Strand]

R37 reads 2.3M so that's good

2.3 ohm, yes? should be a low value.   I expect so otherwise everything else would fall in a heap.

(and Yes, I read somewhere some parts might not be loaded.)

Skimming over a few threads it seems like there's a few cases where the pots have gone faulty.
Slider pots are notorious for faults.   You could try measuring the pot resistance by setting all pots to the mid setting
then measuring from wiper to the max'd terminal and from wiper to the min'd terminal.
(If you haven't done this before:  power must be off, and you need to interpret the in-circuit measurements
by looking at the schematic and accounting for the effect of the surrounding circuit.)