Need repair info on Hafler DH200

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I have very low volume out of both channels. It sounds pretty clean but mostly upper frequencies. Source is directly from cd player(2) so should be plenty loud. All fuses test good. Power supply is +/-60VDC. Looked at bias both sides were at ~450ma,both heatsinks warm to touch (Adj. bias to 275ma). These are old amps from friend and noticed this one has 1/4 phone jack as input connectors???
I have worked on many SS amps and Tube also. I have electronics degree and experience. I have the schematic.
Looks like all original caps. I did come across an older thread talking about "220 with tin can sound". That sounds very close to my problem. Did pull a couple of the output transistors and measured with Fluke DMM. Measured results would at one time look like shorted and remeasure and would look ok??? I'm assuming that if it biases though the outputs are ok?
Thanks for any help provided
RG
 
most of these amps due to quality of parts and age might have parts inside that drifted from origfinal values ... most of the times these are capacitors not especially the big ones but small, signal and feedback related

make a close inpection there is also a chance that values of resistors have drifted if the machine is exposed to extreme heat or moisture

Finally its been some models that had also wiring and soldering issues which will mean that absense of ground in certain places might cause a million complications

kind regards sakis
 

UncleArt

Member
2010-03-03 12:05 am
I have a TA1600 that I've relegated to the junk pile for the exact same reason...and because I have no desire to replace surface-mount devices.

The TA1600 used early SMD electrolytics on the PC board; when these dried up, it was as though someone put a big pad and a 1 K high-pass filter on the inputs of this thing.

If the DH200 is at all similar, you will likely find a few dried out small signal caps in the input/driver stages.
 

djk

R.I.P
2001-02-04 4:23 am
USA
"Look for bad grounds especially around the input area."

Both channels behave the same, and they are on different boards.

Considering how the inputs have been changed to 1/4", he may or may not have an R45. That would mainly cause a ground loop problem, depending on what was done.

"I believe there may be a bridge/mono switch"

Not on a stock DH200, the DH202 bridging kit was hardwired. A DH220 bridging kit (DH222) did have a switch mounted inside the unit.
 
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DC offset

I have very low volume out of both channels. It sounds pretty clean but mostly upper frequencies. Source is directly from cd player(2) so should be plenty loud. All fuses test good. Power supply is +/-60VDC. Looked at bias both sides were at ~450ma,both heatsinks warm to touch (Adj. bias to 275ma). These are old amps from friend and noticed this one has 1/4 phone jack as input connectors???
I have worked on many SS amps and Tube also. I have electronics degree and experience. I have the schematic.
Looks like all original caps. I did come across an older thread talking about "220 with tin can sound". That sounds very close to my problem. Did pull a couple of the output transistors and measured with Fluke DMM. Measured results would at one time look like shorted and remeasure and would look ok??? I'm assuming that if it biases though the outputs are ok?
Thanks for any help provided
RG
Hi, what were your DC offset readings.
 
I have a TA1600 that I've relegated to the junk pile for the exact same reason...and because I have no desire to replace surface-mount devices.

The TA1600 used early SMD electrolytics on the PC board; when these dried up, it was as though someone put a big pad and a 1 K high-pass filter on the inputs of this thing.

If the DH200 is at all similar, you will likely find a few dried out small signal caps in the input/driver stages.

SM capacitors are some of the easiest things to change and the replacements are easy to get and inexpensive. Panasonic FK and FP series are likely far better than the originals and available at DigiKey. I prefer changing SM vs through hole as it's quicker and easier. Good soldering iron is a MUST though. A Metcal STTC-126 tip is excellent for parts replacement, particularly SM.

 
Just to let everybody know latest status. Saw the original Dh-200 repair thread and read about the input blocking caps going bad (C1 i think). Pulled mine and were only measuring in the pico range. Replaced and all is working correctly now. First hafler I've heard and I am quite surprised at it's sound. Better than I expected.
I still have another one that doesn't bias up at all in the left channel. BTW what is the "perfect" bias point for this amp??
RG
 
Just to let everybody know latest status. Saw the original Dh-200 repair thread and read about the input blocking caps going bad (C1 i think). Pulled mine and were only measuring in the pico range. Replaced and all is working correctly now. First hafler I've heard and I am quite surprised at it's sound. Better than I expected.
I still have another one that doesn't bias up at all in the left channel. BTW what is the "perfect" bias point for this amp??
RG

Hi, I’ve looked around on the net for a exact factory Bias setting for the DH-200, couldn’t find one.
What I read from others, it’s around the 240ma mark, running mine between 240ma-- 250ma at idle, anything above this turns amp into a heater.
Can’t hear any audible different running higher Bias settings.
For a amp that’s over 30 years old would have thought it’s time for a recap, you have already found one bad cap.
 
Just an update I have the first amp up and running. Playing in my system for about a week and no problems. Low offset also.
The second DH-200 has bigger problems. I have replaced all the electrolytics on the driver boards but they still measure the same. Left channel powers up but the bias reads high 270-280ma with P1 adjusted for minimum all the way one direction. The other channel is dead cold bias showing around 5ma. Trying to adjust P1 makes no difference. Same readings before the cap changes. Both channels show huge DC offset of 1.3 and 1.4 VDC at the output. There were no speaker fuses in place to start with but have jumpered fuseholder and still the same readings. I'm starting to suspect some grounding issues. PS caps were replaced by someone to 18000uf per side. Tested all transistors with fluke on diode test in circuit and they all look good. Looked at bias resistors in the dead channel in series with bias pot and all are right value.
Thinking about swapping good driver board from other amp to decide whether it is in the drive or output stage. Little hesitant to tear down a well operating amp though.
I have checked for cold solder and retraced all traces with iron.
Any ideas????
RG
 
The DH-220 manual has a table of voltage values expected at various points. The schematics of the DH-200 and DH-220 are similar enough that you can interpolate this DH-220 table for your DH-200. Checking voltages at various points should point you to some possible problem areas.

Failure in the first 4 transistors (input dual diff amp) can lead to big problems in DC offset at output. These transistors must be closely matched on hFe.
 
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