MUSICAL FIDELITY A2 Quiescent current / Bias problem Right Channel

[sviru]

Hi Guys.

Unfortunetly my Musical Fidelity A2 plays different on one channel.

Sound from right channel is a litte bit dull, less vivid. When I listen to music the whole soundstage goes to the left side of the room. It makes me crazy.

I started to look for a reason why... I have checked preamp - ok, capacitors - seems to be ok.

Today I checked Quiescent current.

Left channel - 4 transistors x 300mA. That is ok - proper value for this amp.

Right Channel - 2 transistors x 300mA and 1 x 200mA and 1 x 360mA.

I was measuring on the emitter resistors 0.22Ohm.

The wrong value is on the pair MLP120P.

Why is it happening? What can be wrong in the circuit? Is it a bad trim pot? TR108? What can be a possible cause?


Thanks for help. It is a great little amp - I need to repair it.

[Jan Dupont]

Since the lower pair (BUZ900P) shows the right quiesent current, I would suspect the upper MLP120Ps.
However your measurements doesn't add up......
200+360mA on the upper side, and 300+300mA for the lower side?
Have you measured any DC on the ourput ?

[sviru]

Yes. On one resistor there is drop 96mv - and on the other one about 53mv.

I have checked DC at the output - there is about -48mv on the left channel and about -15mv at the "wrong" right channel.

Values after warm up - on the other resistors drop 80mv - 360mA.

What are my options?

Damaged TRANSISTORS? Old capacitors? Old trimpot? I'm confused

[Highspeed]

Hi Sviru

I have repaired some MF A1 and A2

The main problems were the electrolytics caps because these things runs hot....

Try at first C103 but as these amps get olds, maybe all electro. caps need to be replaced.

Good Luck!

[sviru]

hi Highspeed. Can you tell me if You ever had that kind a problem?

I'm concerned about MLP120P - is it possible that one of them is bad?

One is using quiescent current at level of 400mA and the second uses only 200mA. How this is possible? Is there any way that transistor is leaky? If it takes so much current then there must be something wrong.

I have just switched 3300uF caps from one channel to another - nothing has changed.

Replaced also c103 - no change.

The worst thing is that MLP120P is not avaiable anymore. I can't find any replacement for that transistor.

Any ideas how to check MLP120P transistors?

[darkfenriz]

Check voltage drops on the resistors around short circuit protection. Diodes polarity as well, hope it was never 'repaired' according to the schematic you've attached.

Pozdrawiam

[RocketScientist]

It's not that uncommon for paralleled output devices to not equally share current. The only solution is to either match them (which usually means you need a larger number of devices to choose from) or to use bigger emitter/source/drain resistors which presents other trade offs.

Some manufactures install matched sets of output devices for just this reason. Generally, some current mismatch is to be expected, and it's not likely *anything* that's audible--especially at low to moderate listening levels. The imbalance has probably been there since your amp was new. It's especially common with MOSFETs as their Vgs threshold typically varies more than the beta of bipolar output transistors

The imbalance is more an issue when pushing the amp hard at full power into low impedance loads where you might be approaching the SOA (safe operating area) of the transistors. In that case it can mean the one getting more current might fail.

First, I'd be sure something is really wrong with your amp. If you swap both the input and output cables (i.e. keeping the soundstage the same but switching which channel drives which speaker) does your lopsided soundstage reverse? If not, you need to look elsewhere (like your speakers, source, room or hearing).

If the problem does follow one channel of the amp, I'd look for a bad cap as has already been suggested.

[sviru]

I have just checked drop on resistors R24 and R28 - Tadaaam! Different on bad right channel.

64mv and 27mv RCH
64mv and 64mv LCH

Thanks for the tip - now what can I do with this?

Check the resistors? TR108? Diode?

[RocketScientist]

Originally posted by sviru
I have just checked drop on resistors R24 and R28 - Tadaaam! Different on bad right channel.

64mv and 27mv RCH
64mv and 64mv LCH

Thanks for the tip - now what can I do with this?

Check the resistors? TR108? Diode?

See my post above, I think you're looking in the wrong place. First verify one channel really does have a genuine problem (if you haven't already). If so, I would expect something like a capacitor degraded from heat. It's rare for MOSFET's to "degrade" in any meaningful way. The same is even more true of resistors.

Ideally, it would be best to have someone with an Audio Precision (or something like one) test out the amp to verify it's operation. That way, if you find a tangible problem, it's easier to find it, fix it, and verify the fix. Otherwise you're just shooting in the dark with the sort of symptoms you have described.

[sviru]

Unfortunetly there is no one to help me in my town.

The symptoms I have described - RCH plays different - no clarity, strings and guitar not so vivid, etc - even my girlfriend can hear this

That is why I wanted to check transistors. And I had o good feeling - one transistor works on a low quiescent current - the other one to high. The rest is ok.

On the short circuit drop is lower at the transistor with the lower quiescent current.

I'am not an engineer but I suspect that those clues can help solve this problem. Otherwise I will have to get rid of this amp - I don't want that


Ps. I have switched input / Output cables - even speakers. No change. Still the same thing. I'll try to replace all the caps from one channel to the other one.