Kevin's Hafler p-500

[mlucitt]

Small update to keep the thread alive. I pulled the board out of the Hafler amplifier and checked for solder run cracks and cold joints, I could find none, the work was top-notch. The PC-19C board is through-hole plated so the components get a good bite if there is any solder on either side. I did see that R32, one of the current dropping resistors had drifted from 47 Ohms to 55 Ohms, a change of 17%, slightly out of tolerance for a 10% carbon resistor. I also measured the diodes and all of them check out the same on my fluke meter except D7, D8, D10, and D16 (all are 1N4148 except D16 is a 1N4003). Those four diodes seem "leaky" to me. In the positive direction (forward biased) the reading in "Diode mode" is .564 (Fluke meter beeps), in the negative direction (reversed biased) the reading is 1.960. All the other "normal" diodes on the board show a similar positive reading but the negative reading is .0L (infinite resistance or open). Just for fun, I tested a new 1N4148 diode out of circuit. It measured exactly like all the other "normal" diodes on the board. I think the forward meter reading on a Fluke in "Diode mode" is the diode voltage drop, is that right?

Am I on to something? D7 and D8 are the input DC offset phase controllers, I would think that if they were bad the DC offset would be horrible (it's just mildly off at .140 mV), but could explain that. D10 bypasses Q10 and Q11, could be an issue for the negative side of the board. And D16 (also on the negative side of the board) drops the current for the front end (I think). Maybe Joe can jump in here. Schematic reattached here.

Thanks,
Mark

 

[Gepetto]

Hi Mark
Tying up things for the storm, will look and respond later on. Yes the forward mode on the Fluke meter is the forward voltage drop that is being measured.

 

[mlucitt]

Joe, it looks like that TS might go over your house!

I replaced D16, the 1N4003 in the rail and it made no change, the reverse bias reading is still off (as stated above) but it must be because of another component on the board. I will be replacing D10 next, and probably D7 and D8 for good measure. It will be interesting to see what they measure out of circuit.

 

[mlucitt]

I replaced D10, again no change to the readings on the board, also the replaced diode tested good out of the circuit. That leaves D7 and D8, the least likely suspects originally. I also discovered that R4, the 47K Ohms supply to the DC offset circuit for the positive side measures 5M Ohm (it's not broken or cracked - it handles millivolts). It's sister, R7 on the negative side measures right at 47K Ohms. These resistors are metal film and should be reliable and steady. Who knows maybe I am closing in on the problem because R4 is adjacent to D7 and D8 is nearby. Plus I like talking to myself here.

 

[laatsch55]

Welcome to my world. Not much to say as I'm not familiare with that amp Mark. i like your troubleshooting process.

 

[mlucitt]

I CANNOT believe I don't have a 47K Ohm resistor in the shop! I probably have every other value in E12... It's OK because I just now sent Mouser another order...

 

[laatsch55]

OH man----no 47K -------that's like blasphemy.

 

[mlucitt]

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?

 

[laatsch55]

And so.......

 

[mlucitt]

I'm waiting for my Mouser order to arrive so I can replace that 47K Ohm resistor and hopefully get that Hafler P-500 back online so it can be shipped back to Kevin.

Ever seen a resistor like that Lee? Magical.

 

[laatsch55]

Not without some sort of outward indication. It's bad enough not knowing what's F&^%$# up, now we have resistors doing the incognito thing!!

 

[NavLinear]

I'm waiting for my Mouser order to arrive so I can replace that 47K Ohm resistor and hopefully get that Hafler P-500 back online so it can be shipped back to Kevin.

Ever seen a resistor like that Lee? Magical.

I've seen through hole resistors that have fractured but they've were installed in hardware that was subjected to major environmentalconditions. Maybe heat was a factor butit did look virgin. Who knows - maybe a manufacturingproblem or something happened during the board fabrication process. I do hope this fixes your problem.

I have been reading your posts but I don't know much about the system. Good luck with this!

 

[kevin]

Thats just so hard to believe that a little DOO-HICKEY like that can cause problems like that.. So So many of those things in there.. lol... You guys on here amaze me !!!!!! I only have three more Haflers Mark !!!!! Lmao..... I wouldn't dare !!!

 

[laatsch55]

there's way more of those do-hickeys than a guy can account for Kev, absolutely scandalous.

 

[kevin]

Yea It just blows my mind that yall can find a problem in all that mess.... Thats trouble shooting at its VERRY VERRY VERRY best !!!!!!!!!!!!!! What a privilage it is to be here on this site !!!!! MEAN THAT !!!

 

[laatsch55]

Well it's a priveledge to have you!!

 

[mlucitt]

Mouser order came this morning - YIPEE! 47K resistor replaced on the board and I'll be putting the board back in after my second cup of coffee. It will be nice to hear that Hafler P-500 rocking both channels.

 

[kevin]

Cool Mark !!!! Can't wait to see what happens !

 

[laatsch55]

Waiting patiently..............

 

[mlucitt]

Those Haflers are no fun to work on, I like the organization of the PL much better.