Hafler DH300 Monoblock forever?

Thank you. So, it sounds like if it has the PC7 board in there, it replaced what was there originally for that channel, and I'll need to find another board to revert it back to the original condition it was in as a DH-200, correct?

There is no PC7 board, just the regulat PC6 boards like my DH-200 has. It looks like it's wired the same also, the only difference I see is the input jacks are like those you'd find to plug full size headset type plug into, not phono jacks, so the also don't have the 2.2 ohm resistor attached to them like my phono jack inputs on my DH200. I can say this much, the chassis and faceplate for rack mount, both have silk screened DH-300 on them so it may have started life as a true DH-300 and maybe someone has already reversed it and removed to PC7 board....? I'll have to get a plug like these and get soem signal into it and see what it does.

Yes, both inputs are going right up to pins 1&2 on the PC6 boards. If I convert back to regular phono jacks on the inputs I imagine there must have been some good reason for the 2.2 ohm resistor on the right input jack of my similar DH-200, between it's (-) and ground...? There isn't one on the left channel, nor was there supposed to be at least looking at my original wiring diagram. Thanks for all your help!

Im going to show some of technical ignorance here, but are you referring to the big caps, 10,000 uF that are connected via the bridge rectifier? I think I can pick up a set from QA-Con , I think that's the supplier on eBay for about 50 bucks. They also sell full kits of all the smaller caps as well. What's the story I hear about "conditioning" amps that haven't been used for a long time with a variac or other type set-up? Thanks again! Off to radio shack to get some phono-kack bulkhead mounts to take out these headset jack type input jacks.

Wow, almost sorry I asked, now I can't plead ingnorance. Will have to see if I can locate someone with one to lend me. I have a plain vanilla DVM I use so I don't think that it will fill the requirement for "True RMS" readings.

On another note, I went and got some adapters to convert the 1/4" mono input jacks to allow them to accept my phone(RCA) jacks from my preamp. The newly received DH300 works but it has considerable hum through both channels (at zero volume) as soon as I plug the input leads from the preamp into the back of the Hafler. Both channels do work, so it's set up for stereo, although I have not done anything with the 2.2K ohm resistors I picked up, knowing my standard DH200 has that across the ground leg and chassis ground on the right channel. Could that have anything to do with the hum, or is it that I have some floating ground or something with these 1/4 mono plug inputs that I think are isolated from chassis ground.

thx, I screwed up and am pretty sure I grabbed some 2.2K ohm resistors at Rat Shack instead of true 2.2ohm, will need to correct that, and report back to you.

So far I tried the new DH300 and tried reversing the power plu polarity and that helped considerably, but there's still hum as soon as I plug the input cables from the pre-amp into the Hafler. I have not yet tried to ground the Hafler chassis with a jumper or anything back to the pre-amp. I'm still using the 1/4 plugs with adapters that allow me to connect my RCA phono plugs into the DH300 and the 1/4" jacks appear to have plastic mounts isolating them from the chassis. Any thoughts?

I tried the exercise and grounded one (-) of one input channel. It didn't make any audible diff. Here's what I found tonight. The hum will emit from the speakers once either one is conncted to the amp WITHOUT any pre-amp input plugged in. Then when I plug the right hand channel input in, it gets somewhat worse as soon as I touch the jack tip at all as it enters the jack, even before the positive tip seats down to it's clip, and when I plug the left channel input jack in it get even worse. With no input cables plugged in, and no speakers connected, I show .3 volts DC at the right channel posts and .5 volts DC on the left channel posts. I took a look at wire routing inside and both pin #1 feeds are going to the positive of the input jacks from the PC6 boards. It does look like perhaps the wire routing might be able to be improved, it's wired electrically the same as my DH200, but they ran the #6 outputs wires up to the bridge between the large caps and then back down to the output posts, more like a star config. My DH200 runs them (per recommened Hafler diagram) from the PC6 board to the positive output post, and then there's another jumper that runs from there up to the solid wire bridge between the caps. Woudl you think any of this routing difference could be creating some induced hum on the output channels?

OK, thx again, will go that route and let you know how I make out!