Crown microtech problem

[Bigdrums]

Hi everyone: I have a microtech 600 with one side not working. Finally chased the big problem down to the power supply or right after. Channel 1 measures +15v on the red lead off the filter caps and -15v on the blue lead coming off the filter caps at a reduced setting on the variac. However channel 2 measures near nothing on the red lead and -27v on the blue lead. I checked resistance to ground from each of the 4 heatsinks. Ch 1 pos to ground is is infinity, neg to ground is 0 ohms. Ch 2 pos to ground is 6.2 ohms and neg to ground is 2.8 k ohms. Not sure whats going on there except I thought the heatsinks were insulated. I'm sure the 6.2 ohms to ground is probably at the heart of the problem, except that when I lift the blue & red from ch 2, neither one measures any voltage to ground, only across the 2 at 27v. Any suggestions on how to fix this? crown_micro-tech-600_sm.pdf you will find the schematics way down at the bottom of the pdf. Thanks

[x-pro]

Check breakdown of Q412 and Q409 Q410 Q411. Resistance of 6.2 Ohm is R421 + R417 + broken K-E channel Q412

[Bigdrums]

Would that account for the negative voltage where positive should be?

[x-pro]

Would that account for the negative voltage where positive should be?

Where did you write about this earlier?
Do you understand the principle of operation of one channel from the picture on page 10 of the manual?

[x-pro]

If in channel 2, between +Vcc and GND 6.2 ohms, then the upper transistor on the LOW SIDE is either broken or fully turned on by some level from some element.
It is better to make all measurements without load, without signal, in the stereo mode of the amplifier.

[Bigdrums]

I appreciate your input. I might be wrong but am concerned in that lifting the red & blue leads coming from the main filter caps on both channels, meter ground tied to speaker ground, I measure only
a very small positive voltage (tenths of a volt) from red lead and very small negative voltage from blue lead on ch 1. Same thing on ch 2 except both red and blue leads have a small negative voltage.
I've got to this point because of the -59 v or so measured previously on pin 5 or 6 of j500 on ch 2. Ch 1 measures +27 v if I remember correctly. So there was no +/- on main board and output board ch 2 like there is on ch 1. Tring to back track to the source of the problem.

[Bigdrums]

You are correct. I do not understand this circuitry at all. I would have to have someone walk me through the entire repair. Think it best I just stop with this particular repair as the design is so different from anything I've seen before

[x-pro]

Can you see the difference?
The first circuit is a standard amplifier with bi-polar power supply.
The second one is yours. The power supplies are unipolar.

[x-pro]

Therefore, measurements relative to the speaker's ground pin will not be correct. This point is not directly connected to the power supply. It is virtual, i.e. it appears during amplifier dynamics.

[x-pro]

Look at the movement of current in a closed circuit. This occurs when a positive half-wave of a sinusoidal signal is applied to the input of an amplifier.

[Bigdrums]

Yes, I agree speaker neg is not the same as common ground, as I found a short while ago. I do know how filter caps in a typical configuration will get rid of the left over ripple by
dumping it to ground. But on this amp the caps are not tied to ground, so how do they filter the ripple with no place to dump it?
If I turn my variac to 50%, leads from the caps lifted, I see a few hundredths of a volt, measured to chassis ground, proper polarity at this point. Why don't I see VCC?

[x-pro]

In a unipolar power supply, the filter (capacitor) does not have to be connected to ground. For example, one transformer winding, no midpoint, one diode, capacitor. In this case there would be no filtering? Or should the minus of the capacitor be connected to ground?

[x-pro]

I do know how filter caps in a typical configuration will get rid of the left over ripple by
dumping it to ground.

They do not discharge to ground, but short the variable component of ripple through themselves to the source (in this case to the transformer winding).

You need to imagine that a capacitor is a stable power battery with a constant voltage.

[Bigdrums]

I guess I don't see what the caps are there for except to store the reserve voltage. A 2 lead output on the trans going to the bridge rectifier. Rectifier output goes to each lead on the cap,
then on as +/- rail voltages. I don't get it.
The +/- rails have to be clean or ripple free, but this is a new concept to me

Another strange thing to me is using my DMM, I check across the leads on the power trans. I get .042 voltage drop on the npn side all trans even swapping leads. I get .012 voltage drop on the pnp side. This thing actually works like this?

[x-pro]

Look at the diagrams and descriptions of the operation of a bridge amplifier with a single-pole power supply.