Phoenix Gold M100 distorted sound.

[Anders]

Hi! Got some issues with a M100.
When I got this M100 the clip led was on all the time, but it seems to work ok. Then I read about this cap leakage and did a recap on the six 2200uF just to be safe. Every thing worked out fine when I tested it in the car for a few minutes with a sub, but suddenly it died and the ovr. led flashes. I replaced the Q2 transistor (ZTX749) and it went alive again. The clip led is on as before. I have it on the bench now and it sounds harsh distorded in some way, not the warm natural sound I recall it have back in the nineties when I bought a new one.
I plugged in my cheap oscilloscoop and got this strange wave with power supply and even with 12v battery. It´s getting better after a while, but not perfekt.
Tested two other kenwood amps and the wave is perfect from start.

This is what I got for the moment.
Idle after 30min no signal. 3,8A
With very low 100Hz signal after 30min 3,2A
DC offset Left 0.033v - Right 0.014v
Clip Led always on.

Have someone any idea whats wrong?

Sorry for the bad English!

[Silverhorse]

Hello Anders

idle current should be no more than 2A.
did the amp had repairs before?

[Anders]

Hello Anders

idle current should be no more than 2A.
did the amp had repairs before?

Hello!
I bought it from a fiend of mine who bought it new in the nineties, so no repair before the recap and the transistor.
The idle current rises very slowly from 1,4A at start, 3,2A after 10min with no input. Yesterday 3,8A after 30min.
Are the yellow pots in there for the bias or DC-Offset on the M100? Haven't touched them.
This high drawn Idle current issue and the clip led was from the beginning when I tested the sub, but I didn´t test it with oscilloscoop before it died.

[Silverhorse]

That's a very clean M100 you have there!

If the 6 blue psu caps are still the original ones then replace them first before you do something else! They catch fire if you don't.

The yellow pots are for bias. Take a picture of them or mark the position with something. Than turn them slowly counter clockwise and see what happens.
If the idle current is going down you are on the right track. Than listen if the distortion is less.

[Eric D]

Do both channels sound bad (right and left), or just one of them?
Try to play them on a full range speaker to hear the most detail.

[Anders]

That's a very clean M100 you have there!

If the 6 blue psu caps are still the original ones then replace them first before you do something else! They catch fire if you don't.

The yellow pots are for bias. Take a picture of them or mark the position with something. Than turn them slowly counter clockwise and see what happens.
If the idle current is going down you are on the right track. Than listen if the distortion is less.

I have already replaced the six blue caps in both my M100s, the old ones where leaking.
Is there anything to messure before and when I turn the pots, exept current draw? It´s nice to have some value to go back to if it dosen´t solve the problem.

[Anders]

Do both channels sound bad (right and left), or just one of them?
Try to play them on a full range speaker to hear the most detail.

It´s both channels, most in the midrange and up. Not much, but it´s there and is not pleasant to listen to in the long run.
Is there something I can messure in the amp to have some clue whats going on?
Have someone got a Schematic for this amp?

[Mr. Wild]

I'm not sure about this but I remember hearing that if these amp have been clipped on a regular basis, the clipping light circuit gets damaged and starts causing problems.

[Silverhorse]

you can measure voltage across the output emitter resistors. Make sure you have a microvolt meter.

[Anders]

Solved the clip led problem. Transistor Q122 A1038 was kaputt.
Thanks for your help!

[Silverhorse]

you find the problem, good for you!
is this transistor part of the clipping circuit?

[Anders]

you find the problem, good for you!
is this transistor part of the clipping circuit?

Don´t know for sure, but think so. Only have my poor mans schematic
The sound is better after the bias adjustment.