Ashdown MAG 250 - blowing fuses

[audiopete]

So this thread is to continue with the MAG250 I have here that I mentioned in this thread - http://music-electronics-forum.com/t28046/#post368670 - schematic attached.


What I've found is a base voltage of 0.881V on TR16/TR17 and base voltage of 0.123V on TR3/TR11

TR13 b - 1.435V
TR8 b - 0.685V

Are these supposed to be close or could this imbalance set up oscillation in the power amp? I also observed approx 17KHZ oscillation on the output as mentioned by jpfamps in the previous thread.

With the output devices out of the amp it runs fine. The output devices test negative for shorts.

[g1]

Is the amp oscillating when you are measuring those voltages? If so, you may not get accurate DC readings.
With the different base voltages at the output transistors (rather than roughly equal voltages of opposite polarity), I would expect DC on the output, is that the case?

[Jazz P Bass]

Yeah, not enough negative voltage are presenting thenselves.
Case in point: TR8. The base should have some kind of a negative voltage.
If C9 was grounded, then that would turn TR12 full on.
TR6 & TR7 would be a good place to provide a few BCE voltages.

[audiopete]

Is the amp oscillating when you are measuring those voltages? If so, you may not get accurate DC readings.
With the different base voltages at the output transistors (rather than roughly equal voltages of opposite polarity), I would expect DC on the output, is that the case?

No the output devices were removed before I took those voltages. The oscillation showed up before I removed them on a dummy load and current limiter. Yes to DC on the output.

[audiopete]

Yeah, not enough negative voltage are presenting thenselves.
Case in point: TR8. The base should have some kind of a negative voltage.
If C9 was grounded, then that would turn TR12 full on.
TR6 & TR7 would be a good place to provide a few BCE voltages.

I'll get back to you with those shortly.

[Jazz P Bass]

If you are going to 'run' the amp without output devices, at least connect a 1K/ 1watt resistor across the output transistor B/E pads. (one on the NPN side. One on the PNP side)
Just for testing the drivers, mind you.
Do not expect power out.

[audiopete]

So oops my mistake on a couple of those last readings. Don't know why I disregarded those negative signs on my meter -

TR11/TR3 base (negative)-0.123V
TR8 base (negative) -0.685V

So the negative voltages are there but still mismatched.

TR6 - B -60.6V
C -60.2V
E -61.1V

TR7 - B -60.1V
C -0.685V
E -60.7V

Does this mean TR7 is not turning on?

[audiopete]

If you are going to 'run' the amp without output devices, at least connect a 1K/ 1watt resistor across the output transistor B/E pads. (one on the NPN side. One on the PNP side)
Just for testing the drivers, mind you.
Do not expect power out.

Thanks. Will this affect the readings I just took?

Solid state power amps is a subject where I need to learn more.

I should have mentioned all electrolytic caps have been replaced. There was an issue of the negative leg of C12 arcing on the chassis because they were too long and not trimmed so I changed all the Ecaps on the power amp pcb.

[Jazz P Bass]

TR12 & TR14 voltages: BCE

And either install one set of outputs or one set of 1K resistors to make the circuit complete.

[audiopete]

OK thanks JPB, I am going to put the output transistors back in and do these readings for you again in a little while. I have to go snow plowing now, as I live in the land of snow squalls.

I want to look at the oscillation again on the output and do a freq counter and AC voltage reading on it out of curiosity.

[audiopete]

I took all the measurements again with output transistors installed, 8 ohm load installed and unit in fail mode. If I pull the load, the amp comes out of fail. Sometimes when I power up with the load again, it comes up functioning normally, but then it will go to fail seemingly randomly.

TR11 B +12.41V C -15.03V E +11.78V TR17 B +12.70V C +13.33V E +12.02V

TR3 B +12.41V C -15.01V E +11.78V TR16 B +12.71V C +13.34V E - couldn't get the probe in to test safely but I assume it's close to TR17 emitter

TR8 B +12.07V C -15.02V E +12.39V TR13 B +13.26V C +13.39V E +12.74V

TR7 B -14.46V C +12.07V E -14.98V TR12 B +12.69V C +13.24V E +13.25V

TR6 E -15.01V B -14.99 C -14.48 TR14 C +13.35V B +13.24V E +12.68V

TR15 E +11.95V B +12.37V C +13.23V

[Jazz P Bass]

It would help to have voltages when it is 'coming up normally'.
These voltages are obviously the failed state.
Case in point: TR11 Base: +12.41V Emitter: (eek) +11.78V

TR8 is not turning on.(or is it TR7?)
Something is wrong that TR8 base is not a negative voltage.

I have to look through my files.
There was a post awhile back revolving around a crappy/ underrated relay.

[audiopete]

Yah all hail the bulb limiter. OK i'll pull the load and take the voltages again tomorrow for you in working state. Could the arcing that was happening at the 4700uF cap on V- to chassis ground have sent spikes down the line and semi fried a transistor or two?
It would seem that the relay could be a repeatable fail point that I would not expect from a transistor. Does the relay mute the audio path with the 10uF cap to ground? What about a bad C4 leaking or possibly causing an oscillation? My guess is C4 is used to snub relay switch noise.

[audiopete]

Sorry I had a water freezing emergency today with one of my tenant's homes so I was not able to get this info for you. I will get it up asap. Thanks again for your help Mr. Jazz P Bass.

[audiopete]

Hmmmm why can't I go back and edit my posts?

Here's the latest test results with amp running properly - no load.

TR11 B -0.465V C -61.9V E +0.008V TR17 B +0.470V C +62.2V E +0.007V

TR3 B -0.465V C -61.9V E +0.008V TR16 B +0.470V C +62.2V E - +0.007V

TR8 B -1.01V C -61.8V E -0.464V TR13 B +1.016V C +62.2V E +61.7V

TR7 B -61V C -1.01V E -61.8V TR12 B +61V C +1.02V E +61.7V

TR6 E -62.1V B -61.9V C -61.3V TR14 C +62.3V B +61.6V E +61.2V

TR15 E -1.01V B -0.311V C +1.025V