Hafler DH101 preamp repair and mods?

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anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Wildcat,
Yes, I see that post was a few years old. I responded to a new post message.

I agree with what you have said. I to have seen incredible destruction in products. These days it's usually a poor Marantz or McIntosh product I see messed up. It can take days to restore a unit to it's proper configuration. I'm working on a McIntosh right now where the previous idiot tech rewired and added his own circuitry. It went into him with a meter reading problem only, and that tech managed to wreck the output transformer (they never go), then did this creative work. I hate having to charge a customer to restore things like this, but I can't "eat it" either.

I have made changes to products from time to time, but they are carefully considered changes that always make a measurable (and positive) difference, in addition to being audible. You have to tread very carefully if making any change to a circuit.

I also agree with the CJ gear. Good stuff, very basic design but it works and sounds good unless you're a fan of Audio Research. They also make good equipment, but they represent a different type of sound.

-Chris
 
stenks

stenks

Member
Joined 2011
A

Aaron M

Member
Joined 2018
My Hafler DH 101 has big noise just like between FM stations from left channel, I re-capped all electrolytic and tantalum capacitors, but the noisy is still there, and it is not changed by adjusting any knobs and buttons except when the Tone button is pushed on the noise getting smaller.

I have limited skill and knowledge, I appreciate that you can give me some guidance to trace the cause of problem.

Thanks in advance.
 
L

llwhtt

Member
Joined 2008
Since you say none of the controls make any difference and it's only the left channel that rules out the everything before the volume control and it should also eliminate the power supply. This leaves us with more than likely a noisy transistor in the line stage. There are six transistors in the line stage Q5-Q10. Do you have the service manual? HiFi Engine has it if you don't. With the manual you will be able to locate which transistors are in the line stage. Q5 and Q6 should be matched to each other, Q7 to Q8 to each other and Q9 to Q10 to each other. Matching with a DMM that has hfe test might be good enough. Replace one pair at a time until the noise disappears, hopefully.

Craig
 
A

Aaron M

Member
Joined 2018
Since I don't have spare transistors with me, I exchanged Transistors of both channels starting from Q5, after I exchanged Q8 and the noise shifted to Right channel, so the problem transistor must be Q8. The model of Q7 and Q8 are NP MPS 6523, tomorrow I will go to electrics store try to find this model. Do I only need to replace Q7 and Q8 of left channel or replace for both channels? Thanks.

Aaron
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Aaron,
Which area are you in? You may be further ahead to order from Digikey after you find an equivalent.

If some happy soul suggests and ECG / NTE or other replacement brand, keep walking. Those parts are often not the greatest.

To cross reference the part, first look up the original (MPS6523) Turns out that the exact part data sheet is available at Digikey, but the part is discontinued there. It is a low noise part, but in a pinch a 2N3906 would work. It's just a PNP signal transistor with medium - high gain. You might be able to use a BC560 (better) and that is a low noise transistor. It would make sense to replace both channels with this part. Look carefully at the data sheets as the pinout is different (rotate the part).

-Chris
 
anatech

anatech

Administrator
Joined 2004
Paid Member
Hi Phil,
That's an option as long as they aren't pulls or remarks.

Personally, I'd be installing the BC560. In fact, I would replace all the MPS6523 in there with BC560 and if there are any MPS6521, I would replace those with BC550. Better, lower noise parts. They are cheap and Aaron is going to have to work to make a minimum order amount up. The BC550/560 will be new manufacture with today's improved processes as well.

-Chris
 
A

Aaron M

Member
Joined 2018
Hello Chris,

I am in Vancouver. I checked Lee electronics store's website in Vancouver there is no MPS6523 for sale, but the other two model 2N3906 and BC560 you proposed are available in this store, 4 pcs or 5 pcs bagged. I will go there buying a bag of BC560(4pcs).

Thank you so much for your professional and patiently instruction.

Best Regards

Aaron
 
A

Aaron M

Member
Joined 2018
Hi Chris, Phil,

I replaced the original Q7 and Q8 for both channels with BC560 today, the noise of the left channel disappeared, however, I found another problem: sound from left channel is very small, when I push the "Tone" button down (switch it on), left sound is getting louder but still smaller than right and is not rich with less bass, two channels are not balanced,

I am wandering whether it is caused by the fault capacitors in tone control circuit?

Thanks

Aaron
 
A

Aaron M

Member
Joined 2018
llwhtt said:
New transistors installed correctly with regards to pins and in the correct location, NPN/PNP? The tone circuit didn't have a problem before did it?

Craig
Click to expand...

Hi Craig,

Yes, installed new transistors according to the data sheet and in correct location, before did this I hadn't check and notice it due to loud noise from L channel. I swapped C7&C8 of both channels, the issue didn't remove.

I remember when I re-capped several days ago, the C26 on PC-1 was bursting because of bad quality, the fuse was burned too. Will this have impact to the issue?

Thanks
 
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