Hafler DH500 humm, internal wiring, and other questions

[alex333]

Hello all,
First, let me start by saying I'm somewhat of a newbie in electronics. I can replace parts, soldering is no problem, but I've never taken any electronics classes.

The whole story starts when the I noticed that the power switch on my Hafler DH500 does not light, so I decided to open it up and look around. I dusted things off, cleaned the fuse blocks and other connectors with deoxit, made sure all connections were tight, and closed it up.

Right afterwards, I hear a AC hum on the speakers, unless I disconnect my cable TV from my Mitsubishi TV (which is connected to my Yahama pre via hdmi, which is connected to my Hafler). The amp sounds great, better than before, except for the hum, but only when it finds a ground via the CATV.

Neither the mitsubishi TV, yamaha receiver, nor Hafler have a ground on the AC plug, so I'm thinking the Hafler is finding a ground through the CATV. The cable company came out and put an AC current filter on their line and verified their ground is good. Moving the speakers to the Yam, no hum, so I'm sure it's the Hafler. Any ideas on this?

So, at this point I've started a list of changes to improve the unit, and at the same time try to fix the issue.

Check every connection with a DVM to find a bad or cold solder joint

Install an IEC connector to be able to easily swap power cords

Remove the power switch completely, to reduce number of connectors and points of failure, (+ the unit is on all the time anyways) (??)

Replace the RCA connectors with new ones

Replace the input wiring from the RCAs with shielded Cardas

Replace most of the wiring, since some are frayed plus somewhat thin (??)

Replace the ground bus between the large caps with a thicker copper strap

Remove the output fuses completely (??)

Replace the remaining 4 fuse blocks on the audio path with inline (to get them off the floor)

Misc questions:
What brand / type of audio wire is a good fit for this amp? I've read of undesirable results with some expensive wires. I'm thinking a little thicker can't hurt either, maybe 16 or 14ga.

Where can I find good inline fuse blocks (off the floor) to replace the stock audio fuses, and the copper strap?

I've read replacing the electrolitics is a sure bet with this amp. Exactly, what does that mean? Is there a parts list I can work from to upgrade the sound?

Any info / thoughts are appreciated.

Cheers, Alex

[eds65gto]

Hi:
Send the DH 500 to me and you will never have to worry about it again ! PM me for shipping details.








Seriously see www.musicalconcepts.com for mods and info.
Also see the posts below for the DH 200 / 220 mods.
The DH 200 / 220 is the same a the 500 except it has less output transistors , so lower power out.

DO NOT remove the output fuses ! If you do and one of the output transistors shorts then the B+ , approx. 60 volts , will appear on the speaker leads and you will have a fried speaker voice coil.
Hope this helps
Ed

[alex333]

Changes I've made at this point.
Replaced power switch and fuse
Installed IEC connector
Replaced RCA connectors w Cardas
Replaced the signal input cables w Cardas shielded

Could not find any lose connections, but no more hum, not sure why... sounds great though (was using my Yahama receiver for a while, real amp = so much better)

Next..
Rewire AC cable to the first circuit board with 12ga cable (currently goes from 10ga on power cable to 18ga post switch / fuse)
Replace the ground bus cable between the large caps with a copper bar
Install a bypass cap and resistor on each of the main caps, 6800 ohm 2 watt, and 2.2mfd film
Move output fuses directly off the posts
Rewire most remaining wires, some are frayed, 16 ga?

Misc Qs
What brand / type of wire is a good fit for a rewire? Seems like the Cardas 15.5ga Litz is the top of the line, but $$.. any other options? How about plain old cat5, or even stranded copper wires from HD?

Any info / thoughts are appreciated.

Cheers

[bear]

Running the AC through the stock switch, imho is a terrible idea. They tend to arc and progressively increase in contact resistance (usually).

It's not a bad idea to install a 120vac coil relay with 4 contacts on it for switching the AC.

_-_-bear

[Leolabs]

Get a Hafler DH series power amp manual and recheck the wiring again.

http://www.hafler.com/home/