Half Dead Hafler Dh 500 Help

[frm]

I have a 30 year old DH 500 I built from a kit. The right channel is dead. The right side Mosfets are stone cold. I checked the bias; 11.9 mA, the DC offset; between -80 and -140 mV. The rail voltage is 90 V both sides. The left side works normally, Bias; 350mA, Offset; 33mV. This is an early DH 500 so it has the PC 10 boards (no offset adjustment). The amp has not been modded in any way. It appears that there is a bias issue. Looking at the circuit diagram it looks like maybe Q9 could be a suspect but again, I wouldn't know. The resistors around it check out. But I haven't checked every resistor on the board yet. Unfortunately that is about as far as my electronic ability goes at this point goes.

Right now I am using a $100 DH 225 which sounds a little shrill when I turn it up so I would like to get the old boat anchor back in service. I guess I could tear the PC 19 boards out of the 225 and freshen them up and put 'em in the 500 but I thought I might try fooling around with the 225 when the 500 is back in action. I would appreciate any help that anyone might be able to give me.

Thanks

[frm]

Sorry. I forgot to attach a copy of the Hafler PC 10 board schematic
with my original post. I should also say there are no blown fuses and the protection relay is working, has continuity, and is not dropping out, disconnecting the output jacks. The red light in the power switch is not flashing.

Thanks

[tauro0221]

Hi,
Did you check the voltage before or after the fuse? You have a fuse for each +/- 90 volt feeding the right/left board. Check the voltage at the PC10 +90 pin 3 and PC 10 -90 at pin 10.

[bear]

Ok... Q9 of course must be considered... but also look at the diode string with the zeners, they should not be shorted, and the non-zeners should read like diodes with the diode position of the DVM... you can pull the Q9 and shove it into the hfe check (my $3 Harbor Freight DVM does that) and see if it "works". The driver transistors may be suspected as well.

Was there an "event" that happened when it stopped working?? If so what?

Check the voltage between the rail and the base of each driver transistor - you can check it using any point that is electrically connected, but don't slip and short something out doing this. Then check the voltage between the two bases (Q12, Q13). Compare to the "good channel".

In fact it makes some sense to check all the obvious places like this on the good channel, make a copy of the schematic, write them in... then do the same for the bad channel.
Post it here, probably someone, maybe even me, can tell you where to look next and what to do next...

The good part is that it sounds like something as simple as a bad solder connection on the old PCB... so a good idea is to touch up all the solder connections... make sure the rails are bled down fully, even flip the fuses up so there is no residual to arc to the tip of the iron... definitely do this first. Bad solder joints may be invisible to the naked eye.

_-_-bear

[frm]

Thank you for your suggestions. I checked the voltages at 3 & 10. Both are 90 volts. I can't recall any event that might have caused this, but about a year and a half ago the relay started to kick out after the amp run running for a while, with or without any input signal. Since I was traveling to St. Louis for business on a regular basis, I contacted Musical Concepts (?) and they were going to repair it or talk me into replacement boards. The week before I was going down there, I checked it out again and it was working fine and continued to work until a couple of months ago. I am buying a new DVM this week (I flamed my old one accidentally) and I will start checking as suggested. Thanks again. I will post my results.

[bear]

Touch up all the solder connections on the bottom of the board...

_-_-bear

[tauro0221]

Hi,
Lets check the resistance between pin 8 and 6. I do not know if it is pin 8 or 9. This will check the F4. The output to the speaker goes thru the Fuse and if the fuse open will go thru resistor R36 in parallel with CAP 15 and then goes thru the inductor and the R38. One test you can do is powered the amplifier and put a signal and read the voltage at pin 4 & 9 and compare it with the left channel. This will tell you if the problem is in the output transistors or in the preamp of the amplifier.