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|18th November 2012, 05:02 AM||#1|
Join Date: Mar 2007
Location: Canandaigua, NY USA
Notes- Line6 HD147 guitar amp power section
There isn’t much about servicing this on-line, so here’s what I learned while fixing one recently.
This is probably the most complicated amplifier circuit I’ve ever worked on. Each paralleled output MOSFET (N-ch) is driven by its own op-amp. That means the opamps have to float up and down with the signal. They used a lot of discrete circuitry to make this happen. It’s mostly surface mount so if an output device fails, it can take all sorts of tiny components with it, right back into the level shifting circuitry. Start by checking the 5.1 ohms gate resistors, the protection zeners, the op-amps and work back as needed. Check everything! Large through-hole parts will absorb transients and abuse without a burp, but not their SMT counterparts.
Pay special attention to the 0.22 ohm 2W current balancing resistors. Normally I just look for opens, but the value is very important for this circuit to operate correctly. After everything appeared to be working, I had slightly early and non-symmetrical clipping on one channel. It turned out that one of the 0.22 ohm resistors had been stressed and rose to about 0.5 ohms. That small change is enough to cause noticeable problems, so measure more carefully than might otherwise be the case. It took me way longer to find this than it should have!
Powering the amp is a sure route to more blown parts until everything is fixed. I did find that one can tie the dual supply rails together (+ to + and – to -), then power it with a current limited bench supply of no more than ±15 VDC. That lets you make measurements and confirm that no large currents are flowing.
Use a good DMM with a diode check function to probe all the transistors and diodes. If the zeners are good in the forward direction, they’re almost certainly good in reverse.
The impedance switch on the rear panel adjusts the supply voltage along with signal gain. The schematics don’t give supply voltages but for 4 ohms expect about ± 46 and ±29, for 8 ohms ± 62 and ± 40, for 16 ohms ± 83 and ± 55. My guess is that having the switch set for 16 ohms and driving a 4 ohm load would be risky.
Finally, note that the output is fed back into the signal path at some unexpected locations, so a waveform problem almost anywhere will appear everywhere.
Though intimidating at first, logical thought and dogged perseverance will get you through this like any other amp- just start at the outputs and work your way back with the DVM. There are two identical amp sections; hopefully only one will be bad so you can compare readings with the good one.
I may be barking up the wrong tree, but at least I'm barking!
Last edited by Conrad Hoffman; 18th November 2012 at 06:16 PM.
|28th November 2012, 03:19 PM||#2|
Join Date: Mar 2009
Location: Buenos Aires - Argentina
I tried to help somebody at some other Forum with a similar dead head (preamp and effects working beautifully) and finally he gave up.
Not that we were going anywhere !!
The amp became a nice"POD_in_a_cool_cabinet" driving a used cheap and LOUD Peavey rackmount power amp, so it was not wasted.
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