Hafler DH-120 Repair Help Bridging Problem

[Dick West]

I got a used DH-120 from a guy who said that althought the amp did not work it had never been bridged. Well . . . I find the left channel works but the right channel is dead and has a huge DC offset. Apparently the amp's mono/stereo switch was flipped to MONO while the inputs and speakers were still connected in normal stereo mode. Also the output speaker protection fuses were burned out on both channels.

The point on the right channel PCB where the left channel signal is injected to bridge the amp has a burn mark. This insertion point is eyelet B and is on the back side of the input diff amp part of the circuit.

Can anyone help me begin to sleuth where the parts have been damaged. First off I would suspect Q2 is damaged. What other parts of the right channel may have been damaged from using an amp switched to mono mode while inputs/outputs still connected in stereo mode?

Thanks......

[Dick West]

Here is the schematic for the DH-120

[PRR]

Maybe I'm missing some Hafler trick in that fuzzy schematic, but I don't see how running the amp "mono" with source and loads connected "stereo" will do any harm at all. You will get the left input, in both speakers, out of phase. Big deal.

Does the manual caution against doing this?

It is also hard to understand how point B could burn. It is the feedback node. Current from the same and opposite outputs is limited by the 22K; that won't burn even if you feed 120V into the speaker output terminals (many other things may burn, not point B).

Is the upper supply rail resistor, 47 ohms?, intact? If not, there may have been some sneak path down the right side of the input pairs. Even so, it is hard to see where the current would go TO.

Kid pushing pennies through the ventilation slots?

Was this a kit? (I forget if Hafler sold kits after he left Dyna.)

[Dick West]

Hi PRR,

Thanks for reading my message. The Hafler manual specifically warns against making connections to the black terminals or to the right input jack when switched to the MONO mode. The warning is in big bold letters!!

Also, the manual stipulates that when switched to MONO the AMBIENCE/NORMAL switch must be set to NORMAL.

Obviously inadvertant switching to MONO when connected for STEREO is a BIG NO NO.

FWIW, the output fuses of both channels were blown but they were only the 2 amp variety. All other fuses are OK and of proper rating. The amp is very clean, all solder joints were inspected, all other voltages are normal.

Also, I checked the output MOSFETs with my trusty ol' Hafler MOSFET checker. All are OK.

The point B on the right channel PCB has a scorch mark so it points to the damaged area. Also, the 22K resistor in series with the left channel output that is injected at point B on the right channel is intact. Careful inspection shows no obviously burned parts.

Check the pdf file for the manual at Hafler.com Technical Archives for a better presentation of the schematic.

Any other hints or hypotheses are most welcome . . .

Dick

[Zen Mod]

no hints ,but question

I'm curious-anobody here with listening experience-how DH120 sounds comparing to bigger DH cousins (200 and 220) ?

[Dick West]

If you closely inspect the schematics of the DH-120, P-125, Pro-1200, SE-120 and Hafler 9130 you will see they are all almost identical. The Hafler 9130 uses a few better quality film caps so has a slightly more refined sound.

I have been using a custom built DH-120 with much larger transformer and power supply and separate diode and cap banks for each channel. This little amp really rocks -- all things considered.

It is the best 60 watt amp I have ever heard. It is a little weak on the low end but its mids and highs are, I think, superior to those of the DH-220.

Hope that helps. Now, I need some help in repairing this right channel an a very clean and otherwise pristine DH-120.

[Zen Mod]

Dick- english is not my native lng ;maybe is this main reason-but I can not make connection between your big experience with Hafler amps and your original (first ) question...
anyway- from my experience in repairing amps (in last 15-20 years I repaired at least 200 amps) easiest way is just to desolder EVERY transistor and diode - and check it - at least with plain diode test , and then return it on PCB; off course-best way to do this is ONE BY ONE ,just to ensure that you will not mix them in inproper way (hehe-bene there-done that ) .

if ,after that ,your amp is still nonfunctional ,it's time for checking resistors and caps

frankly- I have good explanation for just 30% of broken amps- why and where are broken ( but I assume that short circuit between owner's ears is most offten case ).

sorry if I underestimate your knowledge in any way , but I must say my experince in that field.....

[anatech]

Hi Dick,
I haven't had a chance to look at the schematic yet, but ... Why was the question of bridging raised? Did you expect to be told the truth? Could someone else have access to this amp and done something unintelligent?

I have seen outputs get hooked up to inputs on the same amp, and another amp's outputs get connected to an amplifier's inputs. The latter in an effort to increase power output. Yeah, I know, it doesn't make any sense.

I would suggest you test the transistors, replace the diff pair with a matched set. Check any other components that you may suspect. I was almost never told the truth on a warranty job, or "Dad's" stereo. Approach this as you would any other unknown repair job.

-Chris

[Dick West]

ANATECH,

You are correct about trust. I am having a little trouble in this and another situation when people tell me one thing only to find that something else was the reality.

My question was really meant to encourage someone with more experience than mine to suggest possible parts that get damaged when bridging goes awry. I agree, the input diff pair must be checked and I do have a matched pair to use as replacements.

But, I wondered if there are any other particular adjacent parts highly likely to be damaged in this type of event. The speaker fuses although blown were only 2 amps in value so parts damage should not be extensive. I don't have a good soldering station or solder sucker so Choky's suggestion to remove everything is not practical.

I will start with the input diff pair and go from there and will greatly appreciate any other suggestions you might have after reviewing the schematic.

Thanks . . .

[Zen Mod]

Originally posted by Dick West
I don't have a good soldering station or solder sucker so Choky's suggestion to remove everything is not practical.



. . .

resolder transistor with heating all three legs;
then heat one eyelet and just stick tip of graphite pencil in hole;
move solder tip away , wait few secs ,then move pencil;
eyelet (hole) is clear