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Old 26th February 2004, 04:00 AM   #1
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Default Hafler DH 500 problem

One day the music just died. After a little troubleshooting I find the relay in the Hafler amp doesn't fire. The manual says it could be the relay or the circuit board. The PC Board looks fine. Is there a way to check the relay or is it auutomatically the problem? Thanks in advance!
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Old 26th February 2004, 09:18 AM   #2
djk is offline djk
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Many things to check.

Does the pilot light come on(power switch)?
(no light, no power on primary side: switch, cord, fuse, fuse holder, transformer))

Is it steady or does it blink?
(blink? bad thermostat)

Are the internal fuses OK? 93V at both channels?
(15A line fuse, bad rectifier)

How much DC is on the output of either channel?
(trip point is less than 2V)

Could be 48V relay coil, could be drive transistor, could be the C in the RC time delay.

Check all in the order suggested.
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Old 26th February 2004, 06:14 PM   #3
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Sorry I thought you could see the light on solid from there ;-}

5 Internal fuses check fine, 2 external as well, + _ 93Volts fine on both sides

How much DC is on the output of either channel?
(trip point is less than 2V)
??
0V unless I measured this wrong. Just on low scale DC between - and + output posts? If so 0V.

Assuming 0V is correct, how do I check the relay coil or is it just replace the parts you mentioned untill it works? I understand some electronics but never learned to troubleshoot to component level. I can replace the parts easy enough once located though. I truly appreciate your help. I miss my music and the boom box with 3" speakers with bass boost is making me cranky. At least that's my excuse.
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Old 27th February 2004, 01:57 AM   #4
djk is offline djk
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the DC must be measured inside at the circuit card. The binding posts are after the relay.
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Old 28th February 2004, 03:14 AM   #5
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Default Take it to a good tech, please

I don't mean to be offensive, but from the description of your electronic abilities you may not have the background to safely work on this amp, or have much chance of repairing it. I know that sounds pretty snooty, but this is a big amp with hihg voltages and repair parts are expensive and hard to find if you do torch it. I have the schematics and manuals for this amp, and know it pretty well, having repaired a number of them, so I know what I am saying here.

The real risk is that you will fry the output mosfets, which are very difficult to find and very expensive. There are 12, and they cost $15-20 each if you can find them, but those will probably not be matched and this circuit was designed for matched triples of output mosfets. So I'd suggest hands off unless you are willing to accept the fairly high risk that you will blow the outputs.

Having said that, it could be a very simple problem of adjusting the DC balance, for which there is a pot. Observe the output voltage on both channels. If one is more than a few millivolts from zero, possibly that channel just needs a tweek. Go gently, if you don't see improvement stop and turn off the amp and leave it for an experienced repair person.
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Old 28th February 2004, 04:27 PM   #6
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My previous post was wrong. There is no DC offset adjust on this amp. The pot I mentioned adjusts bias current and is not relevant to this problem.

In my opinion, it's still not a good idea to tackle troubleshooting this amp without the schematic, some tools and basic knowledge of electronics. Without more information than you have given, it is hard to guess where the problem lies.
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Old 28th February 2004, 08:18 PM   #7
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Quote:
Originally posted by slowhands
My previous post was wrong. There is no DC offset adjust on this amp. The pot I mentioned adjusts bias current and is not relevant to this problem.
My DH-500 has two pots per channel - P1 is the offset adjustment, P2 is the bias adjustment. They are really cheap open pots. Cleaning them may help, better yet replace them.

Check the DC offset of the signal at the output FETS where 6 leads come together (there are no source resistors). If it is more than a few volts, something is seriously wrong, otherwise you might be able to tweak the offset back into limits.

BTW, idle bias should be 450 mA/channel. Put your ammeter in place of one of the rail fuses. I cannot hear any difference in the sound quality until I drop below 150 mA. Although I didn't listen full range for long. I use the amp to power my subwoofer.

Edit: I can scan the schematic if you need it.
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Old 28th February 2004, 09:51 PM   #8
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There may be several versions of the PC-10 driver board for this amp, because my example and documentation only show one pot. My schematic and parts list, for SN 7117876 dated 1981, and and the physical amp have only one pot, P1, for setting bias current.

I did check the Hafler P-505 schematic, and the PC19c driver boards do have two pots, an input offset adjustment and bias set pot. Possibly the PC19C is a later version, it looks to be interchangeable with the PC10 connections. this amp is essentially identical in ratings, but has balanced XLR inputs ("Professional", no less).

I'm not sure this is much help to the fellow with the dead DH-500, but it kind of explains why I recalled the input offset pot on this series amp. It's there on some of the series, but not all.
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Old 24th September 2012, 06:30 PM   #9
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I'm going to make a thread all about Hafler's DH amps. The DH-500 used a PC-10 card. It was nearly the same as the DH-200's PC-6 card. The DH-220, P225, P230, later DH-500s, and others used a PC-19 card. The PC-19 went through three revisions, A, B, and the last was C. Not sure what the differences are, but the PC-19C has two pots. One for DC Offset, and one for the bias. PC-10 and PC-6 have ONE pot.. bias only.

Yeah I know this thread is dead, but thats my $0.02
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Old 24th September 2012, 10:29 PM   #10
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What, and leave out my DH-120? I did a lot to it including changing to Exicon's after some too successful stress testing, and changing the compensation from VAS loading to traditional Miller. Big improvements in sound.
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