Hafler 200 bias

[rudy129]

I would appreciate it if someone could help with my problem. My son was cranking his Hafler amp and blew the internal fuses. I replaced all the blown fuses including the ones that protect the speakers. Both channels play but the right is distorted. I checked the bias and it is only 20ma. When the amp is first turned on the reading is 240ma then quickly drops to 20ma. I checked Q9 in the bias circuit and it's ok. Any ideas what could be causing this problem?

[CharleyW]

So what's the bias reading on the left channel?

[rudy129]

240 ma.

[Dick West]

As you troubleshoot this channel are you monitoring it for DC offset?

You could swap the MOSFETs from one channel to the other to rule out a problem with them.

What color are your circuit cards?. The earlier version has phenolic cards that could not withstand a very high temperature in the solder bath upon construction. Heating and cooling over time may have caused tiny breaks in continuity where some components are soldered. Try reflowing solder on all joints on the back of the circuit card, especially the pre-driver and driver transistors.

After all of this you may have to begin checking and/or replacing pre-driver and driver transistors.

And, I assume you have tried to rotate P1 while monitoring bias setting. Did you replace Q9 or just pull and check it? It will not control the bias circuit if its hFe is too low.

[rudy129]

Hi Dick, thanks for replying! The circuit cards are light brown in color. I pulled Q9 and it checked OK. I'll try swapping the mosfets next. I do get audio from this channel though, it's slightly distorted due to the low bias.

[Dick West]

Let us know how you are doing on your problem.

Did swapping MOSFETs from one channel to the other make a difference? Did you re-flow solder to all the connections on the back side of the circuit card?

I know from experience that those phenolic cards became a problem over the years and re-soldering everything solved a very similar problem for me with an old DH-200 amp.

[rudy129]

I swapped mosfets and still had the problem. I found Q12 & 13 to be bad. I checked Q7,8,10 & 11 and they seem OK. I'll replace 12 & 13 and see what I have. Thanks...Rudy.

[Dick West]

I have been told that those driver transistors are a common cause of failure.

What value of fuses are in the fuse blocks on the back panel? If a 2 or 3 amp fuse is placed there they will often blow before the amp is damaged.

How do you check these transistors? I assume you remove them? How do you unsolder these little guys? Do you have a solder sucker? My soldering skills and equipment are not that good but maybe I can improve.

Good luck.

Dick

[still4given]

I am in the process of trying to fix my P230's. The filter caps are bad. I'm thinking that they may not have gotten that way on their own. Where can I find new MOSFETs if I need them?

Blessings, Terry

[djk]

"The filter caps are bad. I'm thinking that they may not have gotten that way on their own."

Strictly a function of heat vs time.

You can:

Buy 105* caps instead of 85* caps

Or:

Get bigger caps

(in general, ripple current ratings follow the physical SIZE of the can, the more surface area, the higher the rated current)