Kevin's Hafler p-500

[laatsch55]

Ah....the sweet "Thump" of success!!

 

[kevin]

Yep.... I just gotta figure out why it wouldn't do that with the Hafler on it.. I know that Hafler is twice the the amp that ole plate amp is...

 

[laatsch55]

OH!! That was the plate amp?? Stiill no=-go on the Hafler??

 

[kevin]

Nope... I was kinda waiting for suggestions..

 

[speakerman1]

Nope... I was kinda waiting for suggestions..

Could it be broke?

 

[mlucitt]

It was working when it left here, in fact it went into the box still slightly warm. Did the box look OK upon receipt?

 

[laatsch55]

Nothing wrong with shipping slightly warm amps, that wqay the packing and equipment get cozy.

 

[grumpyoldman]

I finally found a problem, the magic 47K resistor. From all appearances completely normal. However, it is open. I've never seen this in a metal film resistor, even the carbon resistors crack first before they open. Now, I may replace this defective part and something else will show up, but no doubt this was contributing to the problem. Who knew? My explanation while looking at the schematic goes like this: R4 is fed +85V from the diode train of D1-D3. R4 drops almost all the voltage and sets up about +1.8V to the 1K Ohm DC offset null potentiometer P1. The null DC voltage is wiped off the pot and sent to R3, another 47K Ohm resistor to combine with the audio input to turn on Q3 or Q5 depending on the polarity of the input waveform. The other function of R4 is to allow current flow to the base of Q1. When Q1 conducts it sends +1V to the emitter bias resistors R15 and R16. These 47 Ohm resistors reduce the emitter bias voltage on the differential pair of Q5 and Q6 to +.6V each allowing them to conduct the negative going input signal present at the base of Q5.


Thoughts?

I think you're my hero.

Thanks to you I've brought my baby back to life. The right channel died 15 years ago and I've never found the problem, unitll last night when i found this little thread. I pulled it out from the depths and armed with my trusty meter, tracked down R4. Dead as can be. Stopped by rat shack tonight and dropped $1.49 on a 5 pack of 47k resistors. Gingerly pulled the old one (little fresh solder helps that old stuff flow, i always forget) and dropped in my $0.30 miracle. For the first time since Saddam was in power the mosfets on the right side got warm. I gleefully plugged in my testoscillator (heh) and set up a crappy old speaker.

The feels. Oh the feels. It's not often I feel giddy.
Clean sweeps up and down both channels.
Tomorrow it reunites with the DQ-10s. Can't wait.

Thanks so much.

 

[laatsch55]

I shall say you're welcome for Dr. Lucitt in absentia. Mark hasn't been around for quite awhile.

And welcome to.Phoenix Grumpyoldman!!

 

[Northwinds]

Why did Mark split the forum??

 

[laatsch55]

His wife was having medical issues. They camr out ok, but decided he didn't have time for audio projects...

 

[kevin]

I'll be damn !!! Mark worked hard on fixing that Hafler for me !! He would not give up !!! And now he has fixed another !!! I'm glad I saw this !! The P-500 is a monster !!! I'm not even using mine ... But I see it sitting on the shelf everyday and Know its ready to go !!!! Thanks to Mark !!!