Electrical pop/thump from Dual 1019 when cycling

I have a Dual 1019 that Doctor Dual fully restored a few years ago. It works like a champ except it has in the past month or so developed a problem that has me frustrated. When cycling, everytime the mute switch engages or disengages, there is an audible pop or thump through the speakers. It's especially loud when the motor switches off after the arm returns to rest. It makes the noises whether or not the cartridge sled is in place.

My first suspicion was a bad snubber cap, so I replaced it with a .01µf 1000V cap, but that made no difference at all. I next burnished the mute switch sleds with some superfine sandpaper folded in half and shot some contact cleaner on there... no dice.

I'm at a loss... any other ideas about what to check?

 

Sometimes this works....reverse the polarity of the power plug if possible.

 

It was suggested to me on Vinyl Engine that moving the power plug to a different out than the amp could also help. Right now it's plugged into the same Monster Power Center.

 

I wish I had a solution for you. With my power amp a 'pop' would be disaster for my speakers.

 

PhxJohn was telling you to pull out the plug and put it back in the opposite way. "Change its polarity." Did you try that?

 

Yes, and it makes no difference.

The former Assistant Service Manager at Noresco -N.Q.I.-Dual of Canada, Klaus Adlhoch, gave me a very detailed response on Vinyl Engine, which I have pasted below. I can't say that I fully understand what needs to be done - this is a bit over my head - but it appears that there is some stray EMF causing the problem. This weekend I will try to explore some of the areas he directed me towards:


Hi,
Cleaning the muting switch contacts is a good idea but this does not influence back EMF noise/pop, generated by the power switch. Let's separate the two sources of noise.
The power switch for the 1019 and units of that era used this switch slide:



As can be seen in the picture, the "fingers" need to be re set as outlined in order to give proper pressure to the main switch lever (item 128).
Keeping the contacts clean also reduces the back-EMF spark. As for the cap, it is a good idea to uses a combination RC unit that has a 100 ohm resistor in series with the 10 nF capacitor. This should also be an X2 rated safety item. The combined resistor/capacitor will reduce the pitting on the switch points. The cleaner to points, the less noise ensues during switching.
It is also possible to reduce the noise by using a different power socket/outlet for the changer as opposed to the amp. The pop noise (a spike created in the power line) is further distanced from the amp by this method. Keep in mind, the preamp is very sensitive, so any noise in the power line can work its way into the preamp stage.
Now the muting switch. Cleaning that is a very good idea which also reduces pops in the audio line. This muting switch has a priority function, which is not sound related. The muting switch fingers have, as a first job, to place the cam in a null or home position. In order to accomplish that, the fingers that touch the cam must also be re-stressed. The secondary mute finger are then adjusted to open and close (short) the sound path, as the cam is either rotating (sound mute) or at the home position (sound open).
It is also important to clean the power contacting tips on this rail as well as the contacting tips in the power box:



Unfortunately, there will probably never be a total silent switching, because of the amount of power that is involved and the back EMF it creates. Back EMF can be seen in light, power switch or brushes in a motor. It is simply the resistance to change and follows the laws of inertia. These are simple ways to reduce this unwanted by-product.
Regards,
k

 

What is it that you don't understand?

My understanding of this is:
First, make sure the power switch makes proper contact. Re-bend the leaf contacts, so they make good contact.
Second, the audio mute switch needs to be re-adjusted. There are fingers that touch the cam, that need to be re-stressed to make proper function.

Best regards

 

Klaus's advice was crucial to me getting Dual 1249 running smoothly about a year ago.

 

I do not know enough about it to suggest... But what entered my mind was that maybe a worn capacitor, or resistor may be a part of the problem. Keep in mind when that Dual was made the AC voltage as a norm was not quite as high as today... All I keep thinking now is DeoxIT.

Interesting memory... Years ago I drove to see a friend who had just begun working for Dual as a repairman. I went to Dual to see him to possibly find a job. We had both been line repairmen for BSR before. Dual took quality control very seriously. I could not tell you what year that was. But, I remember while on my drive home I heard on my car radio for the very first time the just released, "Money" by Pink Floyd. Those were odd days, yet in an exciting way.

 

Danke!

I believe that the mute switch is properly tensioned. I have visually inspected it, and it does indeed do its job with no buzzing or channel drop-outs.

I will definitely take a close look at the slide switch, which could well be the issue.

The only capacitor in there is the snubber, which was the first thing I replaced.

 

Another thing just occurred to me... maybe there is something going on with the power cable, which is detachable? The power box junction has four contact blades, not unlike the headshell sled - maybe there is a loose connection or oxidation there? Hmmm....

 

Please do post the fix should you find it. Mine does the same thing.

 

Absolutely! Keep yer fingers crossed.

 

THIS!

As if you didn't know?

 

Stay tuned...

 

Assuming your model has the little junction box for the power cable, be careful opening it up. On my 1249 the connections basically fell apart when I opened it up. I had to solder some of the wires together. Not sure why Dual didn't use a simpler terminal strip for these connections.

 

Good knowledge, thanks!

 

After two months of head-scratching and poking around, I've ID'd what the source of the problem is: the metal contact "fingers" on the start/stop switch and/or the blades on the power plate were pitted. I burnished the surfaces with 200 grit sandpaper followed by contact cleaner on a Q-Tip, and the popping noise has been reduced to the point where it's no longer a big deal.

I'd like to be able to get it to the point where it's totally eliminated, but I'm not sure that's achievable.