hafler dh220

[larryg]

what is the best way to test the output mosfets? this is on the channel that is dead (hum/distortion). i have only an digitial volt/ohm meter. as i've read in other threads, this amp is built like a tank! also,with the mosfet, which pin is what, ie: gate-source-drain ?(t03 pkg) and what should i see when testing? any help will be much appreciated.

[slowhands]

As I recall the Hitachi lateral mosfets in TO-3 packages were pinned gate (left), drain (right) , source (case), looking from the underside, with the two pins closer to the top. Due to the physical constuction of lateral mosfets, this pin orientation is different than that of vertical mosfet TO-3 packages (GSD), so be careful of that.

The quick and dirty test I do using a DVM is to set the meter in diode mode and apply voltage between the gate and source using just the meter leads. For instance, put the positive lead on the gate and the negative lead on the source to turn on an N-channel device, and reverse them to turn it off. Usually these fail with a short from gate to drain or source, which will read as a very low resistance in either direction. If you get a solid infinite resistance when off and a falling resistance when on, the device is probably good. There is an intrinsic diode between source and drain which should read open in one direction and a diode drop in the other. Reverse the polarity for a P-channel device. Again, this is a crude go/no-go test to sort good from bad, not a functional test.

These devices were matched in sets of 2 per polarity; the number hand stamped on top should match for both. I believe they were matched for Vgs(on) and transconductance and this is required for proper operation, since there are no source resistors in this design to encourage current sharing. Hafler used a dedicated tester, the Hafler DH-910 MOSFET test fixture, or equivalent. In the unlikely event you have a curve tracer, it will suffice also.

[JTT]

I know this doesn't address your question, but do you know that cct schematics and other detailed info for pretty much all of of the Hafler gear is available for download from their website. If you aint got detailed info, this might help.

JohnT

[larryg]

thanks slowhands,and jtt! this info is very much appreceiated!

[fab]

Hi

Have you fixed your amp already?
Are your output mosfets good or bad?

In case your mosfet are good, and you still need some help, I own three dh-200 amp (very similar to dh-220) and I repaired them several times (because I have modified them a lot and not because they are not reliable). The DH-200 is a very good design to start with - it has been designed by Erno Borbely (http://www.borbelyaudio.com/) for Hafler. And with a few modifications, it can sound even better!

[Variac]

I have a 220 I'm going to use for my woofers. I'm interested in upgrades. Also is there a way you could tell me which are the feedback resistors. I hear that replacing them with high quality ones makes a difference. I've got the owner's/builders manual so if you know the numbers.......

[larryg]

Hi varac! I'm not sure which resistors are the feed back ones. I'm
just a novice at this point. Bet if the schematic were posted, there
will be many who could point them out.

[Variac]

I thought that fab might be able to help, and also mention some good mods.

[fab]

Yes I can help but I am new on the forum and I am afraid to be doing "threadjacking"! I want to respect Larryg original thread subject on how to repair his amp (in fact, is it repaired already?). So, if Larryg says it is OK, I can give some valuable upgrades ( not subjective ones but those that everybody can hear!) on this thread OR you can start a brand new thread on DH-200/220 sound improvements and I will gladly participate. My own amps designs are based on the DH-200/220 circuit topology.

[fab]

The audio feedback resistors are:

R26 = 3k3
R28 = 150

The DC feedback resistor is R25 = 33K

The high frequency (above audio) feedback resistor is R27 = 1K

I have not tried to change them from carbon type to other better material in my DH-200 amps. So, if you do, just tell us the result.