Crown D150A V2

[guitardad]

Ok, this is an oldie. Initial complaint was distorted sound, very low output, and IOC lights on

Upon inspection, I found bad/open R2 (2.2k) and an open circuit board connection under the Jack panel that buts up to the main PCB.

So I fix that, change the Electrolytics on the PCB that feed the OP Amp PS. They were ok both with ESR test and Cap Value test, but I changed them anyway.

Fire it up and all seems well with no load. Good signal all the way to the posts. NO offset.

Connect a speaker load, and it starts lighting up my bulb limiter and humming away. No sound. Same issue L or R
Same thing even when input signal is disconnected. As long as a load is connected, something is amiss.

Checked every last transistor. They seem ok. Diodes too.

I can't recall ever having a power amp do this loaded but not unloaded.

IOC lights DO NOT come on.

Any Ideas?

[guitardad]

Schem in image

PDF manual was too big 32MB, but is at Electrotanya

CROWN D-150A II POWER AMPLIFIER SM Service Manual free download, schematics, eeprom, repair info for electronics

[Jazz P Bass]

I do not run power amps on a limiter.

It is a very usefull tool for checking basic voltages at idle.

But it can be abused.

[g1]

Agree with Jazz, ditch the limiter. It may be interfering with proper operation under load. And considering the amp was not blowing any fuses or showing any excess current fault, it shouldn't be in any more danger than when the user was running it that way.

[Enzo]

Agree^^^ and agree^^^^

But if an amp acts that way regardless, the first thing to do is find out what it is doing. Look at the output, is it going DC offset? Is it doing something else? One very important thing to remember with these Crowns is they are DC coupled throughout. Yes, if you put some DC in, it will drive DC on the output. How's that for low end response?

[Jazz P Bass]

I would imagine 'welder' is not too far off of a term.

[guitardad]

Yeah it didn't make sense that I could see perfect signal at the Spk Posts on a scope unloaded, and then loaded have it act up like this. So it may be working fine and the bulb limiter is making me chase my tail. lol

I know this unit has a protection circuit and I had seen a video with a previous Gen model D150A running on a bulb limiter, but that was a 200W bulb and I think mine is only 100w.

Friend of mine borrowed my variac so this was all I had to limit myself to any further damage. Didn't consider the bulb limiter to add its own issues.

I'll try without and see what sparks fly if any.

Thanks!!

[Leo_Gnardo]

I would imagine 'welder' is not too far off of a term.

I s'pose you could do some minor tack welding with a D150, but the old 2000W Delta Omega Crown would be more like it.

Yes having a 100W lamp on the AC line would play hob with the '150 delivering power. I use a 250W "sun lamp" bulb as the final test on the lite bulb limiter before committing to variac or full line testing for suspect amps.

D150's a very nice amp. One of my favorite recording outfits "Le Mobile" used a pair for their large monitor amps, in bridge mode. Who knows maybe they still do.

[km6xz]

I have never been a fan of bulb limiters except when a kitchen table hobbyist wants to do a quick check as to whether their experiment is shorted since running any circuit at 1/2 or less of the design voltage will impact the operation in ways that might be confusing to those very same experimenters who use bulb limiters. In an emergency they serve a purpose but there is no use on a bench with normal test instruments.
A thought experiment of "what would the circuit be expected to do if the supply is cut in 1/2, which regulators would work and which would not, which biases would be moved in which direction", and many more questions would need to be asked. Running something up on a variac while monitoring current and output tells a lot of information as to whether the next step of adding a load is safe. From no load and stable, the next step on a device in unknown condition would be adding a load that is safe for the drivers alone.
Maybe 100 ohms. If there output section is open, the drivers can still be seeing the load. If it is stable at 100 ohms and the bias on the outputs looks normal, switch to a 8 or 16 ohm load.
That all goes out the window when running 1/2 input voltage that varies greatly with circuit current draw do to the cold/hot resistance of tungsten varying so much. When hot, that 100 watt bulb has a resistance of about 144 ohms but when cold, about 9 ohms. It takes a tungsten filament about 0.1 seconds to go from 9 ohms to 144 ohms so imaging what your circuit is seeing if you were very quick with the variac and varied the line voltage over a 15 to 1 ratio in cycles from cold to hot to warm to hot many times a second. It is not always easy to predict the outcome.
I always instructed my techs to not make any measurement until they could predict what the results should be. If it is not as predicted, there are two possible reasons, 1, the tech does not understand the circuit well enough(happens to us all), or 2, there is an important clue that moves him closer to a diagnosis.
Random testing without a strong sense of what it should be at a particular test point is wasted time and just adds to confusion. Introducing such a variable as a varying supply voltage over a 15:1 ratio makes all the measurements suspect.
To prevent techs from wasting time and getting confused I banned bulb limiters from my shop. There were 23 benches all with quality current and voltage metered variacs and all the other test instruments that could reduce confusion and I did not want any that created more confusion.