looking for help faultfinding 2ch audio amp

THE_RB

Joined Feb 11, 2008
5,438
Ok, it sounds like they are setting the crossover using the bias current, it's a fairly common method in biasing old class AB amps.

I assume that 10mV is measured by measuring the voltage across R22? Is it a 0.22ohm resistor (based on it's colour codes)? I noticed before you said something about it being measured at 0.4 ohm, which was likely just your meter probe resistances adding to the resistor value.

If you are using Ohms law to calc the current based on R22 voltage you need to use I = V/0.22 as that is the resistor's actual value. If the manual says you need 30mA that would be; V = IR (V = 0.030*0.22) = 0.0066v which is very hard to measure correctly, something does not sound right there.

Does the manual have an adjustment procedure (order) to adjust the two trimpots? They look to affect each other which may be the reason you are topping out on one of the trimpots.

If you can provide more info on exactly what the manual says, and exactly how you are measuring it that will help a lot!
 

Thread Starter

snh

Joined Oct 13, 2012
11
Yes measuring 10mV across R22 and yes it is (supposed to be) a 0.22ohm resistor.

Measured 0.4ohm out of circuit with a Fluke 87 DMM.

I was following step 1 in the procedure listed in the attachment of my original post here (page1). Couldn't do step 2 (thermal compensation check) as I don't have a sig gen or scope.

Don't laugh but I was thinking of getting one of these http://www.seeedstudio.com/depot/dso-nano-v2-p-681:1595af6435015c77a7149e92a551338e.html
to do some quick and dirty (cheap) audio signal tracing.

What do you think?
Thanks RB
 

THE_RB

Joined Feb 11, 2008
5,438
Re the little nano scope yeah that is probably good enough for hobby audio work and would be a heck of a lot better than no scope! :) There have been a few threads lately about scope choices, with people suggesting a few options so you might want to check out those threads.

Ok the adjustment procedure says to adjust vr3 to set the dc output voltage then vr1 to set the current. It does say to repeat until correct, so like many amps they probably affect each other.

Vr1 and tr2 make a constant current adjustable regulator and its current is the only current source to drive the entire output stage, both hi and lo sides.

It seems very strange that you can turn Vr1 up full and not get more than 10mV on R21!

Can you say if both amp sides are giving the same 10mV result, and both pairs of trimpots in roughly the same position? Are you sure your multimeter is correctly measuring the 10mV (and is on DC 200mV range!!).

Also it would be nice to know some resistor values, particularly R27 in the output stage and also R19 and R23.

Volts and resistance of R9 will show how much current is being supplied out of the current regulator.

Also can you check the voltage across the triple diode pack in the current regulator, and the current going through it which will be roughly; volts R7 / resistance R7.

If the two amp sides are biased very differently you could compare readings around the triple diode packs. We can't say without knowing the current but your diagram shows 1.4v across the diode pack, which I would have expected to be more like 2.1v. Maybe there's a shorted diode?

The good thing about stereo amps is that you usually have a good channel to compare voltage readings etc to the bad channel, which makes life easier. :)
 

Thread Starter

snh

Joined Oct 13, 2012
11
Firstly, thanks again for your help with this - it's really great to be able to ask a question online and get a response such as yours.

I have the amp back together and setup in the bedroom and in service :) i.e. playing music so can't take any measurements now. Also I am going away for a couple of weeks but will get back onto it and post my results when I return.

But FYI:
R25/R26 0.22ohm
R21/R22 0.22ohm
R27/R32 47ohm
R9/R10 1k
R17/R18 1k
R19/R20 47ohm
R23/R24 330ohm
R28/R29 6.8k
R33/R34 100ohm
R35/R36 10k
R39/R40 220k
R37/R38 220k
VR3/VR4 20k
R7/R8 39k
R30/R31 22k
R13/R16 10ohm
R14/R15 1k
VR1/VR2 50ohm
R11/R12 270ohm
 

THE_RB

Joined Feb 11, 2008
5,438
I can understand if it's all back together and sounding ok and you would prefer to let it go at that. :) Somestimes it's good just to get jobs "done".

Anyway since you provided resistor values I'll comment on that. With r9 at 1k your red schematic numbers show 3.7v and 1k so the current regulator is supplying 3.7mA to drive the output stages.

R23 is at 1.1v and is 330 ohms so that's "using up" 3.3mA. So there's only 0.4 mA into BOTH the output stages (into the bases of the darlington drivers tr6 and tr8). That sounds low to me, like the output stage is underbiased for sure.

Also you are showing 0.5v across r27 and r19, which are 47 ohms, so that's 10.6mA through the darlington drivers tr6 and tr8.

You said you got 10mV on the output resistors at 22 ohms which is 45mA, so that indicates 10mA through the drivers and 35mA through the power trannies. That shows the output stage is all working and all 4 transistors are working ok, but it looks far from properly set up if the voltage on the output resistors is supposed to be 30mV and you are getting 10mV!

If you do end up taking the lid off and doing some measurements I have a feeling either the current regulator is not producing enough current (maybe a bad diode pack?) or that the power trannies you replaced are the wrong type and don't have enough gain.

Or (here's a long shot) maybe it's supposed to be 30mA quiescent current through the output stage which is a reasonable enough number BUT the manual has a typo saying 30mV instead of 30mA in the setup instructions?

I mistakenly thought before you were talking about 30mA as the quiescent current, as it is the type of number I would expect. If the manual is right about 30mV and those resistors are properly 0.22 ohms, that would spec a quiescent current of 136mA which just seems wrong to me for that size amp!

Maybe one of the audo guys who design tranny amps could speak up? My background is from repairing audio amps and I have to admit it was mainly the 80's when I did a lot of discrete audio amp repairs so my memory is a bit rusty. Personally I'm thinking 30mA is a lot closer to a good value than 136mA!
 
Last edited:
Top