DH-500 bias problem

labjr · 19th February 2011, 01:49 PM

A friend brought over a DH-500 with a dead channel. He had both, capacitor and semiconductor rebuild kits for the PC-10 driver board.

After installing all the components and finally getting the driver board back in it won't bias correctly. When P1 is turned fully counter clockwise it stays around 450-460ma. I can't get it any lower and it runs hotter than the other channel.

I'm pretty sure I installed every component correctly. Does this kind of thing happen when you rebuild the driver boards? Is there a mod to bring the bias control back within the proper range?

BTW these amps definitely aren't very easy to work on.

Mooly · 19th February 2011, 02:00 PM

The D500 manual I have shows P2 as the bias pot. P1 is to zero the DC offset.

labjr · 19th February 2011, 02:05 PM

Quote:

Originally Posted by Mooly

The D500 manual I have shows P2 as the bias pot. P1 is to zero the DC offset.

The PC-10 version of the driver board has only one pot for bias. Thus there is no P2.

Mooly · 19th February 2011, 02:08 PM

Would have to see a circuit in that case... I was going off this,

labjr · 19th February 2011, 02:32 PM

The circuit is very similar. I'm thinking there's nothing wrong with the circuit and the new parts have just moved the operating point a little.

I'm hoping someone has seen this before and can comment on tweaking the circuit a little.

Mooly · 19th February 2011, 04:57 PM

The vital thing is the voltage across Q9 (C to E) as that voltage determine how heavily the drivers and outputs conduct. So you need to be sure that it is actually working as intended and that the voltage across Q9 varies as you turn the pot. If it doesn't you have a fault and the voltage may by being limited only be the zeners D8 and 9.
The pot should be able to pull the C-E voltage right down to near zero I would have thought.

If the circuit is OK then reducing the value of R28 or increasing the value of R29 will allow the bias to be reduced.

labjr · 19th February 2011, 05:23 PM

The pot will only let me bring the CE voltage to about 2.6v. It will go up when turned clockwise but I didn't want to go too far and destroy something. Q9 checks good. P1 measures 0 when turned down. Maybe there still is a fault in the circuit. I'll have to poke around some more.

Mooly · 19th February 2011, 06:16 PM

Q9 and its two resistors and pot operate "in isolation".
Just remove R29 and the C-E voltage should fall to near zero whether there is another fault in the amp or not (giving zero bias in the outputs) as Q9 saturates. A basic diode check test of Q9 is not always sufficient... if there is an element of doubt and you not 100% sure it's OK then you have to swap it.

Are you sure the two resistors are the correct values.

labjr · 20th February 2011, 02:52 PM

You were right. Q9 was bad. Didn't check bad though. And the parts were all new. What's the chances of that happening?

Thanks so much for you help. I would have continued to think the circuit needed to be changed.

Mooly · 20th February 2011, 06:21 PM

Your welcome and good to know you have fixed it. Very unusual to get duff new parts but it happens I guess.

19th February 2011, 01:49 PM	#1
labjr diyAudio Member Join Date: Apr 2003 Location: MA	DH-500 bias problem A friend brought over a DH-500 with a dead channel. He had both, capacitor and semiconductor rebuild kits for the PC-10 driver board. After installing all the components and finally getting the driver board back in it won't bias correctly. When P1 is turned fully counter clockwise it stays around 450-460ma. I can't get it any lower and it runs hotter than the other channel. I'm pretty sure I installed every component correctly. Does this kind of thing happen when you rebuild the driver boards? Is there a mod to bring the bias control back within the proper range? BTW these amps definitely aren't very easy to work on.

19th February 2011, 02:00 PM	#2
Mooly diyAudio Member Join Date: Sep 2007	The D500 manual I have shows P2 as the bias pot. P1 is to zero the DC offset. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

19th February 2011, 04:57 PM	#6
Mooly diyAudio Member Join Date: Sep 2007	The vital thing is the voltage across Q9 (C to E) as that voltage determine how heavily the drivers and outputs conduct. So you need to be sure that it is actually working as intended and that the voltage across Q9 varies as you turn the pot. If it doesn't you have a fault and the voltage may by being limited only be the zeners D8 and 9. The pot should be able to pull the C-E voltage right down to near zero I would have thought. If the circuit is OK then reducing the value of R28 or increasing the value of R29 will allow the bias to be reduced. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

19th February 2011, 06:16 PM	#8
Mooly diyAudio Member Join Date: Sep 2007	Q9 and its two resistors and pot operate "in isolation". Just remove R29 and the C-E voltage should fall to near zero whether there is another fault in the amp or not (giving zero bias in the outputs) as Q9 saturates. A basic diode check test of Q9 is not always sufficient... if there is an element of doubt and you not 100% sure it's OK then you have to swap it. Are you sure the two resistors are the correct values. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*

20th February 2011, 06:21 PM	#10
Mooly diyAudio Member Join Date: Sep 2007	Your welcome and good to know you have fixed it. Very unusual to get duff new parts but it happens I guess. __________________ ------------------------------------------------------- *A simulation free zone.* *Design it, build it, test it.*


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19th February 2011, 05:23 PM	#7
labjr diyAudio Member Join Date: Apr 2003 Location: MA	The pot will only let me bring the CE voltage to about 2.6v. It will go up when turned clockwise but I didn't want to go too far and destroy something. Q9 checks good. P1 measures 0 when turned down. Maybe there still is a fault in the circuit. I'll have to poke around some more.

20th February 2011, 02:52 PM	#9
labjr diyAudio Member Join Date: Apr 2003 Location: MA	You were right. Q9 was bad. Didn't check bad though. And the parts were all new. What's the chances of that happening? Thanks so much for you help. I would have continued to think the circuit needed to be changed.

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