Hafler DH-500 short duration cutouts? - diyAudio
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Old 25th September 2006, 10:30 PM   #1
Bogie is offline Bogie  United States
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Default Hafler DH-500 short duration cutouts?

This morning, I hooked the thing up to maggie MMGs...

After about an hour, it cut out for about a second. Did it again a couple of times over the next hour. Wasn't really playing that loud (Alison Kraus live SACD and Steely Dan Two Against CD).

Fan was running, amp doesn't really seem that warmish... Nothing like the fins on the amps I've gotten from DickW...

This amp ran my sides in The Bunker without a hiccup... Have it on in the bedroom now with some grateful dead and 8 ohm speaks, haven't noticed any problems (other than lifting it to the top of the gear stack on top of a tall dresser...). Fan temp feels about the same, and Terrapin Station is almost over...

Any ideas?
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Old 26th September 2006, 12:23 AM   #2
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Bogie,

Are you comfortable running this amp with the lid removed? If so, you could try using something like a plastic toothrush handle to tap various of the little parts on the circuit cards. You may have a bad solder joint or a component with that goes "open" as its temperature changes. A judicious tap on a part can sometimes reproduce the problem and so help pin point where the fault may be.

This is for a one channel cutout.

Do both channels go off and on at the same time? If so then you should monitor the status of the speaker protection relay. Maybe it is acting up and temporarily removing the output signal from the speaker jacks.

Does the sound go off and on slowly or abruptly? If slowly it could be the PS as the energy stored in the large filter caps is used and is not replaced continuously. If so, you could have a loose connection there. Or, perhaps the diode bridge has become intermittant in operation.

But, be careful what you touch with your fingers and keep one hand in your pocket so you won't get a lethal shock from the PS caps.

Describe its symptoms in more detail.....
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Old 26th September 2006, 03:00 AM   #3
Bogie is offline Bogie  United States
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Both channels.

It shuts down abruptly, for maybe a second (sometimes a little longer, sometimes a little shorter. Comes back just as fast, with same volume.

When close, I can hear something making noise inside the amp while this occurs... (that took some patience) Sounds kinda like the startup relay circuit or whatever it is that keeps the thing from slammin' the speaks?
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Old 26th September 2006, 03:47 AM   #4
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The symptoms certainly seem to point to the relay or its control circuit. As I recall its circuit provides a time delay at startup and also monitors DC offset to protect the speakers.

A picture of the circuit cards would help. Three different circuit cards were used on this amp over the years. If you have the later cards the circuit has a small potentiometer that can null out DC offset. If so, you could measure DC offset and correct it back to close to 0 mV. Can you show us a picture if its interior?

If the relay is bad you may have a problem finding a replacement. If something like a capacitor in the relay circuit has died from old age its replacement would be a simple thing. You got some good suggestions over at the PE board.
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Old 26th September 2006, 04:07 AM   #5
Bogie is offline Bogie  United States
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Okayfine - I'll fire some pix tomorrow evening.

Okay - just moved it from the master bedroom dresser (owie... top of stack) to the housepest/general crap room... Cracked the case. Will try to find a camera, since it wasn't where it was supposed to be...

Spiders seem to have liked it... Several little webs. Doesn't look like it was ever wired for the rear-mount fuses. Serial number is 7239088 on the little white sticker on the bottom. Didn't see any "assembled in a factory" type of indicator...

Are the pots you are talking about little donut-lookin' dealies that stand out on their sides from the two main boards? They look like someone splooged some sorta red stuff on 'em where the screwdriver should go - assume it's the audio equivalent of locktite? I don't see anything else that remotely resembles one of the knobby turny things... Not like those P230s...

So I just hook a multimeter up to where, and see what it reads?

Will probably do new caps too - what's a good source that doesn't want a fortune? 20,000 f, 100 volt?


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Old 26th September 2006, 04:30 AM   #6
jaycee is offline jaycee  United Kingdom
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My personal guess is that the relay is fine, but the DC protection circuit is tripping. If you really weren't playing it too loud (pushing the amp into clipping would trip the detector) then it probably means the capacitors in the DC protect circuit need to be replaced.
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Old 26th September 2006, 11:21 AM   #7
djk is offline djk
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Wave a hair dryer on the front end cards, one at a time. My guess is the DC off-set is drifting. Could be the quad comp diff pairs, or the current sources.
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Old 26th September 2006, 12:13 PM   #8
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The Magnaplanar's are quite the current-holics. Maybe one of the output MOSFET's are open? .....causing a premature clip to the top or bottom half of the wave, causing a DC offset with dynamic signals?

Try hooking up only one channel at a time and try to reproduce the problem. This could help determine if this is a particular channel at fault.
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Old 26th September 2006, 01:26 PM   #9
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Bogie,

The PCBs in the DH-500 and those in the P-230 may look identical.

The P-230 you have uses version PC-19C of the main circuit card. It has two pots. One to null DC offset, the other to set Bias or idling current. Now that you have the lid off you should study the DH-500 manual available as a PDF file at the tech archive section of WWW.HAFLER.COM. Can you see if your PCBs are also the PC-19C version?

On page 19 of the manual you can see a picture of the PCB and on it are two triangles that depict the location of the 2 pots. One is P1, the other is P2. P1 is the one used to null DC offset.

You can measure DC offset by putting a meter set to measure DC millivolts across the speaker output jacks. Remove any input, remove the speakers, turn on amp and see what meter tells you about DC offset. I also put a 10 ohm resistor across the speaker outputs when I do the measuring. I use a 10 watt sand block resistor.

The earliest DH-500 amps used the PCB from the DH-200 amp which had no way to null DC offset, except by making sure the first 4 input transistors, the differential amp section, were closely matched on current gain (hFe). On these PCBs the input diff amp transistors sometimes drifted out of spec over the years and the amp began to put out way too much DC offset. A symptom of this was to blow the speaker protection fuses and sometimes idiots did not fix the problem but jumpered out the speaker protection fuses. Go figure.

Anyway, show us the PCBs you have, study the manual, and we may be able to provide more help.
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