my new Hafler DH110 project

[kuene]

Hi all. I picked up a local DH-110 on the cheap, as it presents a slight low hum. I figured I'd start digging through solder points and caps - maybe get one of the kits out there. I don't really know what I'm doing, but I've fixed a bunch of gear when it comes to just tinkering with bad solder points and caps - so, why not. A couple things I've noticed on my inspection of the unit so far.

1) It's incredibly clean inside, hardly a spec of dust to be found.

2) when listening to amp through headphones, powered on, no inputs connected, you hear the hum only (or nearly only) when phono (1 or 2) is selected.

3) when listening to amp via power amp hookup, hum is detectable equally no matter which input selected

4) every pic/diagram of this pre I've seen shows a single transformer mounted diagonally at back of case. Mine has two wired in, mounted flat on bottom of case.

I'll attach some pics. To me, this looks like it could have been a kit build, but I have nothing to compare it to. It's not messy, but some solder points look a little heavy. Anybody have any clue the reason for a PO to put in two transformers? Do you all suggest I start with power supply section caps, as well as input jack solder?

Thanks!
-Andrew

 

[kuene]

Here some pics inside the unit - please note the two transformers... (why?)

 

[sregor]

The original transformer is probably 48 volt center tapped - Someone used two 24 V CT wired in series to make the same secondary voltages.

 

[kuene]

I see. Perhaps that’s a place I should start, anybody know a part number/values or link for proper transformer? I have the manual but it doesn’t seem to list the details for the originally supplied transformer.

 

[Semanon]

I see. Perhaps that’s a place I should start, anybody know a part number/values or link for proper transformer? I have the manual but it doesn’t seem to list the details for the originally supplied transformer.

There’s nothing inherent different between the two transformer vs. one transformer. Still, if the hum is in both channels, the power supply is the right place to start.

You might also short (or make some shorting plugs for) the unused phono inputs these can create noise when left open.

 

[kuene]

Hello all,
Some diagnostics updates. If I don't have anything in the phono inputs - I can reduce the hum by touching anything metal in the chassis. I did get some shorting plugs and when I have them in the phono inputs the hum is reduced... but not gone. The hum is still present - but lower volume and mostly in the left channel, and it's very very low volume in the right channel. This is all about hum in the line outputs from the preamp and as heard via my hafler dh-200.

Below notes are all with both phono inputs shorted:

- pretty sure the hum is 120hz

- Via headphones there really is no discernible hum in either channel.

- to make sure its not my power amp, when I swap the preamps outputs to power the amp, the hum and quite channels will swap accordingly. Power amp seems clean.

- The hum is consistent no matter which input or tape monitor selection is selected.

- It is also unchanged by any of the tone or balance controls, or the filter, mono, etc. switches.

- The hum doesn't change much at all when the volume knob is increased up to around noon, but it will decrease a little when I turn it up to around 1 o'clock ("9" on the face plate), but then gets louder again after going higher than that. So when I have a source playing, the hum isn't very audible, and gets less audible the more i turn up the volume given that the source signal doest get louder.

I don't know if these troubleshooting notes mean anything in so far as where I should look next, but thanks for reading and for any advice.

_Andrew

 

[BodybyFisher]

The original transformer is probably 48 volt center tapped - Someone used two 24 V CT wired in series to make the same secondary voltages.

It's very interesting that someone used 2 24V transformers. I found this thread because I am recapping and improving the DH-110 I built in 1984 and am considering changing to a toroidal transformer and you answered my question, OEM was a 48 volt center tapped model.

I haven't followed this thread to see what the hum was the OP was getting, mine is quiet, but one of the phono RCA jacks needs resoldering, it hums intermittently.

Thanks

 

[BodybyFisher]

Hello all,
Some diagnostics updates. If I don't have anything in the phono inputs - I can reduce the hum by touching anything metal in the chassis. I did get some shorting plugs and when I have them in the phono inputs the hum is reduced... but not gone. The hum is still present - but lower volume and mostly in the left channel, and it's very very low volume in the right channel. This is all about hum in the line outputs from the preamp and as heard via my hafler dh-200.

Below notes are all with both phono inputs shorted:

- pretty sure the hum is 120hz

- Via headphones there really is no discernible hum in either channel.

- to make sure its not my power amp, when I swap the preamps outputs to power the amp, the hum and quite channels will swap accordingly. Power amp seems clean.

- The hum is consistent no matter which input or tape monitor selection is selected.

- It is also unchanged by any of the tone or balance controls, or the filter, mono, etc. switches.

- The hum doesn't change much at all when the volume knob is increased up to around noon, but it will decrease a little when I turn it up to around 1 o'clock ("9" on the face plate), but then gets louder again after going higher than that. So when I have a source playing, the hum isn't very audible, and gets less audible the more i turn up the volume given that the source signal doest get louder.

I don't know if these troubleshooting notes mean anything in so far as where I should look next, but thanks for reading and for any advice.

_Andrew

What was the outcome of your hum?

I have the original manual and they address hum. Have you seen that?

I don't have the manual at hand but one of the recommendations was to flip the plug in the wall.

I'll pull the manual tomorrow and post a photo of the paragraph on Hum.

 

[milanomike]

In case you are unaware, on these DH110's the RCA jacks are problematic (at least on units 40 years old). The jacks were kind of cheap to begin with. If I understand correctly, the hum is on phono only? I would look carefully at the jack solder joints on the board for cracks, even if you don't see any, I'd consider reflowing/touching up the solder joints. This worked great on my unit to alleviate hum and dropped phono signal for a while anyhow.

 

[BodybyFisher]

In case you are unaware, on these DH110's the RCA jacks are problematic (at least on units 40 years old). The jacks were kind of cheap to begin with. If I understand correctly, the hum is on phono only? I would look carefully at the jack solder joints on the board for cracks, even if you don't see any, I'd consider reflowing/touching up the solder joints. This worked great on my unit to alleviate hum and dropped phono signal for a while anyhow.

I have planned to do that thank you. I built my unit in 1984 and yes the RCA jacks were cheap. I spent last night looking for better quality replacements.

As I mentioned elsewhere here my phono plugs are very tight gold Plated plugs and the no doubt have stressed the RCA jacks and if I can't replace them I'll reflow the solder joints. I plan to remove that board anyway to clean up the ribbon cable pins.

Thanks