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Refurbing a Dual 604

Thanks Patrice B,

It's got a dedicated black wire not tied to the RCA cables. It's terminated on the chassis by itself. Once I re-assemble I'll give it a try with the ground cable connected, but I'm ready to try without the ground cable if it is still humming. The only other thing I have to try if this still hums is to remove the strobe lamp to see if that is the cause, but I'm getting ahead of myself.
 
So I buttoned it up and plugged it in and still have hum.... Might be a little less hum, but it's still there. I even flipped the plug at the wall and flipped the RCA's (left to right). No change. I pulled and re-inserted the sled. Nothing worked. I disconnected the dedicated ground wire at the amp. Nothing worked to resolve the hum.

In looking at Patrice B's post I misread it. He was talking about removing the grounds on the RCA cables - not the dedicated ground wire. I'll try that tomorrow and we'll see what happens. I can also try the removal of the strobe from the circuit. One way or the other I'm going to chase this down.

What's left? Well....

- Transport screws (some day)
- Try out the new stylus
- Dust cover (some day)
- Fix the hum (Work on that tomorrow)

Just for fun. Here is a pic of the old RCA's I cut off and replaced. Rather corroded.

IMG_0715.JPG
 
I have been doing this for years. Do not alter the cables.
The strobe does not hum. Not on anything I have fixed. Sometimes a cartridge might pick up a teeny weeny bit of hum from strobe. Moving the arm along its arc would demonstrate this.
 
Thanks NoTransistors,

So tonight I guess I'm doing some additional research. I won't be removing the RCA grounds - at least not permanently. The hum is low level and I can certainly hide it by running music at a moderate level, but if I turn up the volume just a little when there is no music the hum is there and annoying. I don't get any hum on my Dual 506. Here's what I think I can try next.

1. Cleaning the headshell contacts again. I may not have done a good enough job. Also I tried pulling it and re-inserting it last night multiple times, but it didn't seem to matter to the hum. Maybe a good shower with Deoxit?
2. Examine the boards to try to locate the other electrolytics listed in the service manual that I didn't find. (I did see some small disk (ceramic?) caps on the boards. I can check which those are in the parts list in the service manual.
3. Maybe think about the rectifiers. Is it possible that one is leaky - causing the hum? I think I physically saw two. I'll check the parts list and diagrams to see if there are more than that.
4. Check the ground connections. There are 3 on the chassis - two with the RCA cables (for which Patrice B issued a warning) and one dedicated ground. I can certainly inspect and clean the ground connections.
5. Try disconnecting the strobe light. Easy to do and would make me feel better by eliminating that as a culprit - even if it doesn't end up helping.
6. Maybe look into disconnecting the motor in case the hum is being introduced by it.
7. Try it without the sled installed. I forgot to verify this last night and that might tell something.
8. I wonder if the cartridge is live without the TT plugged in? I'll try unplugging the AC supply for the TT and see if the hum goes away. Not sure that will tell me that the problem is inside the TT, but it will be some additional information.
9. So I can certainly disconnect the RCA grounds temporarily under the TT - maybe one at a time for each channel. NoTransistors doesn't think this is a reasonable possibility or even desirable, but if I get desperate it is something I can try.
10. Failing all of the above then I'll have to scratch my head, but maybe the cables or wires to the tone arm might be next.
 
NoTransistors said:
The hum is not from the electronics. Unplug from A.C. and see for yourself.
Click to expand...

Thanks NoTransistors. That just jumped to the top of my list for tonight! Assuming that pans out that would then narrow down the issue to the signal path from the amp to the stylus (including any grounding issues along the way.) It would be very cool to eliminate all the active circuitry that is involved in spinning and blinking things.
 
As NoTransistors said, do not alter the cabling as there is no improvement in hum with dedicated ground wire disconnected. I was just observing that some of the Duals had chassis ground through RCA's and through dedicated ground wire (my 1019 was an example of this). Yours appears to be correctly chassis grounded only through ground wire finally.

Sorry for the confusion!
 
Ok. Some more info.....

Unplugged the TT with the amp on and the hum is still there.

Removed the sled and the hum changed, but is still there. (Not sure this is a valid check as that leaves the cables open ended at the head.)

Note that all 3 cables are grounded per the service manual. I.e. Both channels and the dedicated ground.

I'm going to swap in my Dual 506 and and Dual 1226 compare the hum again.
 
Well now here's a weird problem. While I was typing the previous post I heard the speed start varying and I went over to watch the strobe and darned if the platter just didn't slow to a stop while I was watching.

I turned it off and then back on again and it started up. The strobe doesn't seem to be quite rock solid. I see the marks on the platter under the strobe light shifting back and forth just slightly. Either I've got a motor going out or there is an electronic problem with the motor speed controller. Stayed lit the whole time this happened until I shut it off by putting the arm back on the rest. I'll be watching this TT closely now.
 
So while watching it I'm looking a little more at the hum issue. When I wave my hand near the cartridge the hum changes. Kind of like a microphone with a bad ground. I'm going to check the wiring on the cartridge body. Although it looks all original there is something going on.
 
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Swapped in my 1226. No hum at all. Time to look at the audio path including grounding on the 604.
 
Tim D said:
So while watching it I'm looking a little more at the hum issue. When I wave my hand near the cartridge the hum changes. Kind of like a microphone with a bad ground. I'm going to check the wiring on the cartridge body. Although it looks all original there is something going on.
Click to expand...

I was going to say 'boat anchor', but 'Theremin' also seems to fit.
 
Found the cause of the hum. It's gone now. :banana:

It was the sled. This sled has spring loaded push rods in it that make the connection with the headshell contacts. The one for the right side ground (green wire) was showing open. I had to trace it all the way back from the RCA's - point by point until I got to the cart. Eventually zeroed in on the one connection at the sled. Some Deoxit and some exercise of the spring loaded pushrod got it to make contact. So that push rod obviously doesn't have a soldered connection to the green wire to the cart. There was apparently some corrosion inside the sled for that push rod preventing it from making contact.

Not only is the hum gone but the whole turntable sounds better. I didn't know what I was missing and thought the sound was generally alright. Man was I wrong.

Here's what is left. Not much.

- Transport/shipping screws (some day. Maybe I'll post a WTB on BT for them.)
- Try out the new stylus (I'll give it a try tonight on one of my better LP's)
- Dust cover (some day)
- Watch for that weird behavior that saw it lose speed. (There's a ghost in the machine somewhere. Maybe the motor is on the way out. I see a few on the big auction site for decent prices.)
 
You did deoxit the speed pots, did you? Probably one on the outside and some fine tuning ones somewhere on the PCB. Service manual is available on VinylEngine.
 
I would not bet on the motor being the culprit. A long time ago, I took a look at one of these turntables. I think that there were four wires to the motor. A pair to the brushes and a pair to a speed monitoring loop. This is a low-end design without the fancy 'slot-less, overlapping-poles, brush-less, etc, etc, etc', design. Concentrate on the electronics, or just find a busted 701 and see if you can get that going. Would be worth the effort. I learned how to do just that. It is cool to watch it finally spin.
 
Well, ever since that one episode it's been behaving. That was freaky. What's more freaky is that it isn't doing it anymore. I'm going to drive it for a while as my daily driver to keep my eye on it.

I'll keep my eye open for the shipping screws and dust cover while I'm driving it. Now I've gotta find some worthy vinyl to try out the new stylus. Hmmm.
 
Your desire to get this table into proper condition is awesome.Iam glad I have not had any serious issues with mine.I have a Goldring Eroica lx mc cart on mine and it sounds pretty good.
 
Grrrrh. It happened again. The speed went a little funny then it's like the motor quit and it just spun down to a stop. Pushing it with my finger wouldn't start it. I shut it off a few times with the tonearm rest and it finally started back up and is now running normally again. This is really strange. It's like a protection circuit for the motor kicks in and disconnects the power to just the motor. The strobe is still lit when this happens. If it was a heat thing you'd think that it wouldn't start up so easily and that it would overheat sooner and stop again - it doesn't do that.

I'm with NoTransistors. Boat anchor. Other than that it is running fine.
 
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