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PPX1200 humming
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scott_fury View Drop Down
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    Posted: 18 May 2020 at 8:57am
Hi,

I have a Citronic PPX 1200 that has a permanent hum from both output channels, even with both volume controls down. I haven't used the amp for a while, but pretty sure this is a recent development. I have used the same signal source/speaker setup on a couple of other amps with no hum. It also happens without any input connected.

Is there a way to diagnose what it is from the frequency of the sound? Can anyone guide me as to where to begin?

Many thanks!
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Try switching the ground lift switch on the back and see if that makes any difference to start with.
could also be capacitors, these amps are getting on a bit now, so it could probably do with a service.
I have nearly 30 ppx amps they are very reliable.
100% Earth Moving Bass
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scott_fury View Drop Down
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Originally posted by GAZ. GAZ. wrote:

Try switching the ground lift switch on the back and see if that makes any difference to start with.
could also be capacitors, these amps are getting on a bit now, so it could probably do with a service.
I have nearly 30 ppx amps they are very reliable.

Haha! Yes the switch had been nudged possibly. It was sort of in the middle. Bit embarrassing but at least it’s a simple fix!

I’ve seen a few pics of your PPX collection. Had a few issues think you helped save a channel on my 1600 a year or so ago with some pics you posted for me. Which does lead me onto another problem I have....

One of the LED clip lights has stopped working. I guess I check the LED first, but is there any other cause for this to fail? Both channels work fine BTW.
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Could be the transistors for the clip lights. On a couple of my amps, the problem is the other way round causing clip lights to be constantly lit. Doesn’t affect operation and as such I have not repaired this yet!
I’ll take a picture of the transistors that you need to check when I’m next at the lockup.

Could also be dry joints on the led

Edited by GAZ. - 18 May 2020 at 5:42pm
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Will be those silly little ZTX 213 transistors,is a whole bunch of them around the clip light.
Be seeing you.
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Originally posted by GAZ. GAZ. wrote:

Could be the transistors for the clip lights. On a couple of my amps, the problem is the other way round causing clip lights to be constantly lit. Doesn’t affect operation and as such I have not repaired this yet!
I’ll take a picture of the transistors that you need to check when I’m next at the lockup.

Could also be dry joints on the led

I’m pretty hot on limiting everything before it gets to the amp so it’s not been a problem, but this lockdown has meant I’ve got through a lot of my to do list, so whilst I’m in the PPX zone i may as well tick all jobs off :) . Will open up the 1600 and have a test.
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Originally posted by jbl_man jbl_man wrote:

Will be those silly little ZTX 213 transistors,is a whole bunch of them around the clip light.

Thanks for that jbl_man. Think you helped me out last time too. Remember seeing pics of some lovely new raised up resistors inside one of your PPX’s. 

If that’s the culprit, are ZTX213 transistors available, or is it worth looking for better alternatives? It’s bipolar pnp transistor?
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M4trix View Drop Down
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Originally posted by scott_fury scott_fury wrote:

Hi,

I have a Citronic PPX 1200 that has a permanent hum from both output channels, even with both volume controls down. I haven't used the amp for a while, but pretty sure this is a recent development. I have used the same signal source/speaker setup on a couple of other amps with no hum. It also happens without any input connected.

Is there a way to diagnose what it is from the frequency of the sound? Can anyone guide me as to where to begin?

Many thanks!

https://www.youtube.com/watch?v=3Mjc5JJcu5I

LOL

Edit: Btw, according to the PPX1200 schematic (PPX1600 is same but different resistor values) , it could be anything around the clip LED. Those check markings
on the schematic are not mine. It's on the original schematic. Looks like the clip LED issue is a common problem. LOL

MPSA93 = MPSA92 (still active)
MPSA43 = MPSA42 (still active)
ZTX213 (dead as a Dodo bird) = 2N3906 or 2N5401 (same pin order)




Edited by M4trix - 19 May 2020 at 2:43am
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scott_fury View Drop Down
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Originally posted by M4trix M4trix wrote:

MPSA93 = MPSA92 (still active)
MPSA43 = MPSA42 (still active)
ZTX213 (dead as a Dodo bird) = 2N3906 or 2N5401 (same pin order)


Thanks M4trix! That’s very helpful. Was a bit concerned how’d I be able to check a duff transistor for which pins were which.

I’m not too familiar with testing transistors but assume I would need to test continuity between the base and collector/emittor pins (this confirms junctions are good and checking both ways confirms if it’s pnp or npn). If the junctions are in tact I then test for forward voltage drop across the junctions, the higher reading is the emittor, I think?



Edited by scott_fury - 19 May 2020 at 8:15am
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odc04r View Drop Down
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Yes, for a PNP look for the usual back to back diode drops between EB and EC with base more negative than emitter. Should be high resistance from EC.

Or buy yourelf one of the these - https://www.ebay.co.uk/itm/HOT-LCR-TC1-Transistor-Tester-ESR-Capacitance-Meter-Electronic-Component-NPN-PNP/353043023189?

I think they are brilliant for checking small components of all kinds


Edited by odc04r - 19 May 2020 at 8:41am
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kedwardsleisure View Drop Down
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I've had plenty where the led just snaps off.
Another good one is where someone has taken the front panel off to clean it or whatever and when they've put it back they've bent the LED out of the way so it doesn't shine through.
And they do just fail.

ZTX super e-line transistors are very good, developed by uk-company Ferranti no doubt for some cloak-and-dagger application. They just don't get on well with amplifiers!
Kevin

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scott_fury View Drop Down
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Originally posted by odc04r odc04r wrote:

Yes, for a PNP look for the usual back to back diode drops between EB and EC with base more negative than emitter. Should be high resistance from EC.

Thanks for this. That’s a big help.

Did consider getting a tester but thought I should learn the hard way first, as it helps me understand how they all work. You’re right though, a worthy bit of kit for my electronics toolbox. 

Also, I assume that they won’t test a component on a board, and I’m hoping I can test these transistors without having to pull out the board and unsoldering them if they’re still good?
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