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Old 29th February 2012, 01:34 PM   #1
db5owat is offline db5owat  United States
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Default Hafler DH 101 last ditch effort

First, I am (was) a kit builder. I therefore have limited skill and knowledge. When I am guided I do well with repairs and can solder fairly well.

I have a Hafler DH 101 preamp I built that and later modified with a kit from Musical Concepts. The modified preamp sounded great! At some point I rec'd a Tandberg amp/preamp/tuner (3000 series) from a friend and the DH 101 was largely relegated to a shelf. I would bring it out from time to time and recently repaired the plastic strip holding the RCA jacks.

Yesterday I decided to hook it up for my son to use and discovered something is wrong. It plays but the sound is distorted. At first I thought it might be blown speakers but I have determined it is the preamp. It sounds very much like blown speakers or a partially blown inline power amp fuse. The distortion seems spread throughout the audio spectrum and is exacerbated as the volume is increased. It sounds "overdriven".

I'm stuck as I have no idea what might cause this and do not have the technical skill to trace the problem.

Is there anything I can do to repair this myself or is it time to sell to someone with the skill to repair. Any obvious possibilities come to mind? It is such a great sounding little preamp and it has sentimental value to me, I'd love to be able to repair it but can't put too much money into it. It would probably be a steal for someone on Ebay that could repair it.

I appreciate your guidance.

Last edited by db5owat; 3rd March 2012 at 12:52 PM. Reason: remove typos
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Old 29th February 2012, 02:25 PM   #2
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Hi,

Do you have a voltmeter? If so, the first place to start would be the power supply. With your voltmeter negative lead on pin 10 of each amplifier board in turn (labelled PC-4), measure the voltage on pin 12 (+18V) and then pin 11 (-18V). I suspect it's a supply problem if the distortion is present on both channels.
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Old 29th February 2012, 03:36 PM   #3
db5owat is offline db5owat  United States
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I don't have a voltmeter but I can borrow one. Assuming it is the power supply (as determined by testing for the voltages you described) what would the next step be? I recognize this is a bit of a silly question, but at that point does one replace the entire supply or is there a likely component(s) needing replacement?

I have seen this Ebay kit to upgrade the DH 101 power supply, but my concern is that my preamp has already been upgraded by Musical concepts and I don't know how this kit might integrate with an already modified one. I wish I was a little more proficient in this arena.
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Old 29th February 2012, 03:46 PM   #4
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Quote:
Originally Posted by db5owat View Post
I don't have a voltmeter but I can borrow one. Assuming it is the power supply (as determined by testing for the voltages you described) what would the next step be? I recognize this is a bit of a silly question, but at that point does one replace the entire supply or is there a likely component(s) needing replacement?

I have seen this Ebay kit to upgrade the DH 101 power supply, but my concern is that my preamp has already been upgraded by Musical concepts and I don't know how this kit might integrate with an already modified one. I wish I was a little more proficient in this arena.
If there is a fault in the supply, the next step would be to determine which components have failed and replace them as necessary. From the age of the kit, ~1978 according to the manual, it would be a very good idea to replace all of the electrolytic capacitors as the originals would most likely have dried out by now. A few measurements with a voltmeter should help pinpoint the problem.
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Old 29th February 2012, 03:54 PM   #5
db5owat is offline db5owat  United States
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Quote:
Originally Posted by currentflow View Post
If there is a fault in the supply, the next step would be to determine which components have failed and replace them as necessary. From the age of the kit, ~1978 according to the manual, it would be a very good idea to replace all of the electrolytic capacitors as the originals would most likely have dried out by now. A few measurements with a voltmeter should help pinpoint the problem.
Would using a kit like this be good place to start if one assumes age dictates wholesale replacing older parts?
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Old 29th February 2012, 04:02 PM   #6
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Would using a kit like this be good place to start if one assumes age dictates wholesale replacing older parts?
I would take some measurements before doing anything else. There is no guarantee that this kit would fix the problem, although it might be useful to have for spares, if you were so inclined. Only when you have located the fault will you know a) whether it is worth repairing and b) what it might cost.
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Old 29th February 2012, 04:11 PM   #7
db5owat is offline db5owat  United States
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I would take some measurements before doing anything else. There is no guarantee that this kit would fix the problem, although it might be useful to have for spares, if you were so inclined. Only when you have located the fault will you know a) whether it is worth repairing and b) what it might cost.
Yes, I'm sure you're correct on that. I shouldn't look for a "quick fix" until I know what's broken . Frankly, I just don't know what to test other than the 2 areas you described...... Step one is securing a voltmeter.

Here is an important question, I assume all measurements with the voltmeter are made with the preamp unplugged from power? I would be quite hesitant to be tinkering in a powered up unit.
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Old 29th February 2012, 05:30 PM   #8
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Originally Posted by db5owat View Post
Yes, I'm sure you're correct on that. I shouldn't look for a "quick fix" until I know what's broken . Frankly, I just don't know what to test other than the 2 areas you described...... Step one is securing a voltmeter.

Here is an important question, I assume all measurements with the voltmeter are made with the preamp unplugged from power? I would be quite hesitant to be tinkering in a powered up unit.
As you will be taking voltage measurements, power must be applied so you will need to exercise caution, taking care to protect yourself and not to accidentally short out anything in the process with the meter probes. If there are exposed mains terminals within the pre-amp, position the case such that you don't have to lean or reach across these areas while it is powered up as you take your measurements. If you are uncertain, would you be able to post a photo of the inside from above?
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Old 29th February 2012, 05:35 PM   #9
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This pre-amplifier had a ground-loop problem -- easy to remedy. There's been a post on the unit that addresses the problem: DH-101 with a noisy phono stage
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Old 1st March 2012, 05:46 PM   #10
db5owat is offline db5owat  United States
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Originally Posted by currentflow View Post
As you will be taking voltage measurements, power must be applied so you will need to exercise caution, taking care to protect yourself and not to accidentally short out anything in the process with the meter probes. If there are exposed mains terminals within the pre-amp, position the case such that you don't have to lean or reach across these areas while it is powered up as you take your measurements. If you are uncertain, would you be able to post a photo of the inside from above?
Here are some pics:

* They are pretty small. I couldn't seem to upload any of the larger .jpg files even though they were within the size limitations.
Attached Images
File Type: png DH 101.1.png (168.6 KB, 190 views)
File Type: png DH 101.2.png (176.2 KB, 183 views)
File Type: png DH 101.3.png (179.9 KB, 182 views)
File Type: png DH 101.4.png (143.6 KB, 180 views)
File Type: png DH 101.5.png (170.6 KB, 176 views)

Last edited by db5owat; 1st March 2012 at 05:49 PM. Reason: Add info
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