alternator whine in PG MS2250

[marko]

I'm 99% sure one of my beloved MS2250 amps is faulty and inducing alternator whine into the system. checks i have done so far is run amp with muting plugs and also isolated speakers and used an external speaker and the whine remains

power supply seams ok and the neighboring amps seams fine and +/- comes from the same source and cables ran together.

how do i start trouble shooting an amp that has this issue? plays superb apart from this. only work it's had done since my ownership is new caps (did all 3 amps at same time using same caps/technique).

only thing i haven't tried is swapping amps round to see if the problem moves with the amp but I'm almost sure it wouldn't..

HELP!

[Perry Babin]

Did you have ALL of the speaker wires disconnected from the amp except the short wires for the test speaker?

How bad is the whine (barely audible or loud)?

Does the level change with the gain control?

Are the 'muting' plugs RCA plugs with the center conductor and shields shorted together?

Is there any DC on the RCA shields on the amp?

What's the resistance between the RCA shields on the amp and the case of the amp (no RCAs plugged in, no speaker wires connected to the speaker terminals)?

What's the resistance between the RCA shields on the amp and the ground terminal of the amp (no RCAs plugged in, no speaker wires connected to the speaker terminals)?

It shouldn't be a problem if the amp is screwed down to the body of the vehicle but if it is, you may want to isolate it to see if that makes a difference.

1moreamp is the resident PG guy so he may have some experience with this sort of problem.

[marko]

will go through that thorough list very soon but for now everything is back in and to be fair the whine isn't as bad as it's ever been so not ruling out anything just yet! alternator whine is the single most frustrating thing in the whole of the car audio world for me, just a PITA to eliminate

How bad is the whine (barely audible or loud)?

was really bad last week to the point i turned the system off, now i doubt i would notice it except when stationary and music turned down.

Does the level change with the gain control?

no difference, always have gain on minimum with these amps.

Are the 'muting' plugs RCA plugs with the center conductor and shields shorted together?


yes, think you call them "shunts"?

Is there any DC on the RCA shields on the amp?


not checked yet..

What's the resistance between the RCA shields on the amp and the case of the amp (no RCAs plugged in, no speaker wires connected to the speaker terminals)?

not checked yet..

What's the resistance between the RCA shields on the amp and the ground terminal of the amp (no RCAs plugged in, no speaker wires connected to the speaker terminals)?

not checked yet..

It shouldn't be a problem if the amp is screwed down to the body of the vehicle but if it is, you may want to isolate it to see if that makes a difference.

definitely isolated (on wooden battens).

[Perry Babin]

If the whine varies in level, you'd probably need to measure the resistance and voltage when the whine is bad.

Yes, RCA shunts.

If the noise varies gradually, you should notice whether it's worse when warmer/cooler, when the air is more or less humid, etc. When the noise level is related to ambient conditions, it could be due to a poor quality connection or due to something like leaked electrolyte on the board.

[marko]

noise remains the same at any level.

all amps had a recent recap so doubt any are leaking.

i will go over the earth connections and main earthing point for system next weekend just to be sure. what resistance is acceptable when measuring from amp earth to chassis? or indeed earth terminal to chassis? i did rub the paint down prior to bolting the earth clamp down but you never know..

Mark.

[Perry Babin]

The resistance from the amplifier's ground terminal (B-) to the body of the vehicle should be too low for most common meters to read. If the amp can produce full power, this ground isn't causing the whine.

The resistance from the RCA shield to the ground terminal on the amp should be at least 1k ohms.

If there was leakage before you replaced the caps and electrolyte from the old caps saturated the board, that electrolyte could be causing problems now. When electrolyte saturates the board, the saturated area of the board must be cut away completely. Most people don't want to do this because it leaves holes in the board but it's the only viable option if you want the amp to be reliable, long term. If you replaced the caps before any of them leaked, this isn't a concern.

[marko]

looking at the serial number on this pic tells me it's this amp that displayed the most whine, the other had very little with the speaker next to my ear! this one was more noticeable but again had to have the speaker next to my ear to notice.

if the board is saturated then it's staying that way! there is no way I'm gonna cut a rare board up, never knew this could happen! if this is the route of the problem then i may more the amp on to midbass or sub where it's not such an issue..

[1moreamp]

Morning guys.
I checked what info I have, and this amp should have only optically coupled feedback to the 12 volts side of the amp. In fact I see no direct connection of any kind between the audio side and the 12 volt side of the amp except thru opto-couplers. <2500 volt breakdown isolation.
So the 1K ohm grounding resistor common to many amps is not present in this amp. And if you have a connection between the two I would be highly suspect of one of the power toroids being shorted somehow as this is the only circuit pathway possible in this amp, if the amp has been reassembled properly. Pray the toroid is not shorted as PG will not sell them and you must rewind it by hand. Worry not I have tech spec if this is the case.

You said the amp was recently re-capped ? then I am also suspect of bad board install in the case, so Perry's question about resistance and case contact are very high on my probability list.
These amps can be a pain to reassemble properly < over a hour typically being careful not to cause any case to board shorts >. I actually use some thin fiberglass circuit board sheeting inserts on all of mine that i work on instead of the plastic insert PG used. I feel the fiberglass board is a tougher insulator for the bottom of the board.
Also if when it was re-capped the thru hole solders for the caps were not made well then there is a problem here also of the caps not being in circuit correctly to filter power supply noise adequately.

This is a few of the noise related things i have seen since early 1992, and this is why I am extra careful on reassembly of any of this series of amp because any unusual case contact that may make a connection between the audio side and the 12 volt side will give a ground loop in the MS series.

You can try to lift the grounding by removing the hold hold screws on the amp PC Board while leaving all the power transistors clamped. This board connects to the case thru the mounting screws that hold the board down to the case. this should float the case and if your noise goes away I am suspect that you must take it all apart and find where it was contacting the case on the audio side of things. You can also run this amp out of its heat sink for a few moments only depending on its output bias settings. I suggest you mark the bias setting with a marker and then turn them all the way down before you do such out of sink testing. Bias has absolutely nothing to do with the noise issue your having, but heatsink contact on the audio side does.
I have also seen +12 volts on the case/heatsink cause such issues, here again this is a reassembly problem where +12 gets shorted to the case body somehow, but this usually cause power supply failure issues due to ground being brought to a +12 volt level for the power supply circuitry. this can cause blown traces to appear like magic, and then the amp will try to run while not being properly grounded on it power supply side. Seen this only once or twice over the years, rare but possible.

On the older 275's and the like they had a grounding switch that allowed you to contact the RCA shield to 12 volt ground. I never found any use for this as it always caused noise issues in my installs, so I always left it floating and not grounded at all. They even had this same switch on the ZPA series 1 and the later removed on the series 2, and installed shunts floating the RCAs away from ground all together.

So I would be looking hard for any contact of the audio side to the 12 volt ground most likely the case/heatsink after reassembly from the re-cap. The PC board hold down screws test is how I start this procedure.

Please let me know what you find out in your ground isolation testing, as this is most likely what is causing you noise problems, but if its not then I am very intrigued as to where else such a issue could be caused. Nice to see you around Marko, hope all is well out your way...Cheers...C

[Perry Babin]

I see no signs of saturation on the board in the photo.

When a board becomes saturated with electrolyte and there are traces on the top and the bottom of the board (as there are for most large filter caps on double-sided boards). current flows from one trace, through the electrolyte-saturated board, to the other trace. This produces heat (power dissipation in the conductive board) which results in the board burning whenever voltage is applied to that area of the board. This makes the amp unusable unless the saturated part of the board is completely removed.

[marko]

just had a quick look (will have a proper play this weekend), with rca's connected i'm getting 50ohms between the earth connection and chassis of all amps. and with them disconnected getting about 2m ohms, not sure what that says and will do more tests tomorrow..