My Hafler DH-200 is oscillating on one channel. ( Edit: Solved, finally)

zebulon1

zebulon1

Working on my own stuff. Finally
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I had mentioned the Hafler in another thread, which got me thinking. I thought I would post my discovery.
I haven't finished troubleshooting it, my real work got in the way but I did have time to hook it up to the Signal generator, Fluke - set to AC, 8 ohm load and scope.
The right channel works well up to 19.4 volts AC with no clipping. After that it blows the speaker fuse.
The left channel is oscillating right off the bat.
Visual inspection:
Inspection of the left channel reviled two 100 ohm resistors R27 and R33 toasted, melted but still reading 99.9 ohms.
Tested all the transistors on my trusty DCA75 Pro and they all check good. Pre's, drivers and outputs.
Started checking the diodes but couldn't finish due to work. So far they check fine.

Note: I did a visual check of the boards but missed the burned resistors. Of course I didn't see them until I had the board loose. They may of been all right before I ran the amp for the checks.

The previous owner said it sounded distorted on the left channel.

Hafler DH-200 Service Manual
 
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:lurk: Comparing 200 (pc-6) and 220 (pc-19) driver boards. The only differences are that they added a dc offset adjust circuit, they changed the current sources to resistors in the input stage, they added a 120 pF cap from collector to base of Q 13, and they changed the base stop resistors (R40 and R41 (on the PC6 board) from 220 to 470 ohms. The last 2 were probably to improve stability, as the mosfet outputs tend to oscillate if gate resistors are too low. Maybe it will help to add the cap and change the resistors. YMMV
 
Got back to this Hafler after it had been in the way through a few rebuilds on other units.
I replaced the all the electrolytic caps including the bi-polar's and replaced the burned resistors.
Checked all the transistors, diodes and remaining resistors for serviceability.
The fuse holder clips were crusty and cleaned.
The culprit I think was the poor solder connections on the bottom of the board. It looked like someone had desoldered the board but was careless in the resoldering.
After re-doing all the solder connections, it fired up and now a beautiful scope trace.
Now I can do all the little details to make it sweet.

DSC04723.JPG
 
Excellent.

Other than the MC upgrades which aren't really considered all the little upgrades that make it sweet, can we get that list you are doing to this amp? I grabbed a long list built from the DIY board discussion and am about halfway through that list. Amps work fine and eventually I'll go further but I'd like to see what your upgrades are.
 
Well I lied. It wasn't fixed.
I had some nasty offset volts on both channels. Turns out a few of the carbon comp resistors were way out but the R36 2w resistors were open on both boards.
Now it works.
Tons of power through those Mosfet outputs. Wow.
Little package - large sound.

Edit: It was still acting up.
 
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I never had it fixed.
This is embarrassing because it's a fairly simple amp. Use's several known circuits and should be a easy fix at my skill level. Non-the-less.
Anyway:
I wanted to repair this amp instead of going for the mods and kits.
The sad thing is, I had chased my tail the whole repair and am I'm sure I didn't cause any of the chain reaction issues along the way (Maybe a broken wire here and there ;)). It would of been easier to start fresh building a new one.
Some E-Caps were marginal.
Poor solder connections
Loose fuse holders
Corrosion
Miss wired jacks.
Broken wire strands at the amp boards
A bad Q9 transistor. Replaced it with a NPN, 2N5551A.
Several resistors were way out of specs. R35, R26, R27, R32, R33.

Here, It's been playing for an hour and sounding really nice on the bench Klipsch book shelf's .

DSC05011.JPG
 
That amp does look a lot easier to work on once is it is spatchcocked. Easy to get to everything with the amp boards removed from the heat sinks, too.

Don't know what you did to fix it but apparently it needed a small handful of parts thrown at it to work right.
 
Mostly I needed to get kicked down a few notches. She certainly got her licks in.
I'm reading the current thread on "Am I the only one?", thinking about contributing. Not sure where to start :D
 
zebulon1 said:
Mostly I needed to get kicked down a few notches. She certainly got her licks in.
I'm reading the current thread on "Am I the only one?", thinking about contributing. Not sure where to start :D
Click to expand...
I imagine we've all been humbled by something we thought was a simple device/repair... I know I have.

Difference is, most of us don't talk about it! :D

Hey, you got it fixed. That's what counts.
 
One reason I mentioned this effort is to finish the thread. After searching for info on this and the DH-220, most spoke of performing the mods. Less had tangible tech issues.
I posted this thread in hopes of help with something I'd never seen but it turns out it was several issues that I should of handled.
In the end It still isn't going to help others as I didn't document the work very well.
A few notes:
If the Bias is high (Around .350mV at it lowest setting). Q9 has failed. Replacement 2N5551 works well.
Blowing Speaker fuses? The twin fuse holders for the +-B tend to get loose causing intermittent fuse contact. Re-squeeze the rivets and secure the clips.
Recap it with fresh quality caps. Note: Some are Bi-polar.
Check all the resistors for proper value. Several were way out.
A DC Balance problem might be a failure of R39 for the feedback. This is not that uncommon.
Replace the input RCA jacks. Very poor, older design.
If it was a kit built. Re-secure all the soldered connections. Go so far, as to desolder and strip new wire ends for good contact.
Repaste the outputs. MOSFET's run hot.
 
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