Dobro amp 6107

[angelodp]

Will do, thanks Martin.

[angelodp]

My attempt at installing an IEC with a proper fuse holder was a fail? I could not figure out what the issue was. I had 120v incoming but no indicator light, no heater voltage, no DC?? I tore it out and went back to the original setup and it's fine/works. I am still running an external ground. With the two prong plug, I get a pronounced buzz/hum.... reverse the plug and dead quiet. Back when I figure this out.

BTW what is the story with the 3.2M dual gang on/off switch volume pot?

Best A

[maxkracht]

No idea why your modern power cord doesn't work, but the extra hum when reversing polarity is because you have a "death cap". It gets noisy when the hot instead of neutral has a reference to chassis ground through that cap. Best to remove when you have the 3 prong power cord or replace with a class Y safety cap.

[Stevem]

I have had many a new IEC jack with its built in fuse holder on new gear that get in for repairs that does not pass voltage and the reason is that the clips that grip the fuse itself are spread too wide.

Just go in there with a thin screwdriver and bend each one of the 4 metal tangs closer together.

[angelodp]

What I believe to be the closest I can find to this amp (this schematic does not have the 3.2M dual gang vol pots), does not show a death cap. In the pic at the front is that the death cap under the two R's with leads twisted together? I did not use an IEC with internal fuse holder.

The way this amp came in its switching and fusing the neutral, which is quiet. Trying to determine the "Death Cap"

Thanks

[Stevem]

There is no so called death cap because the amp has No polarity switch on the primary side of the power trans.

[maxkracht]

There is no so called death cap because the amp has No polarity switch on the primary side of the power trans.

Perhaps we disagree on terms, but plenty of amps have what I would consider a "death cap" without a ground switch. Look at the original schematic posted... It is hard for me to see in the pictures, but the silvery looking component connected to the power switch looks like it could be a cap connected from primary to ground.

[Stevem]

Sorry yes, but the LA278 schematic here does not show a death cap.

I guess it was redrawn with out it or something?

[angelodp]

Here is a layout of the amp as it was when it arrived.

I tried to implement the IEC and moved the Hot to the fuse & switch. I also tried lifting the .017uf (Death Cap?). The amp had AC but no DC and no heater voltage. I am getting in a new fuse holder, but it seems to be ok.

As of now the Amp works fine, but the switch/fuse is on neutral not hot. I have an external ground connected to the chassis.

[maxkracht]

Didn't realize the dual pot was paralleled. I'm guessing they just had a bunch of those or it was cheaper than a 1-1.5M single pot. You could replace that with a more readily available switched 1M pot if necessary. You can also take the switch part off of regular alpha pots and add a different value wafer, In case you meant 3M total resistance with two of those in parallel.

Mains input polarity should not have anything to do with how anything operates on the secondary of the transformer, just a safety thing. I'm guessing you had some sort of connection problem. Is the lamp in series with the filament for some reason? If you are getting AC on the secondary, mains wiring shouldn't be your problem in terms of DC not working.

[angelodp]

This is quite puzzling. I have now gone back to the IEC with fuse inline, per the original. The only thing I have basically done is swap the Hot and Neutral and introduced a chassis ground and it does not play. The panel light comes on, but the tubes are not conducting.

[sluckey]

Disconnect the green wire from chassis. Now check resistance from the black wire to chassis. Repeat for the white wire to chassis. Both wires should read infinite resistance to chassis. If not, find out why.

[angelodp]

Hi Steve, I reverted back to the old setup. Will this test work in the original setup? I am running an external ground to the chassis. So I disconnect that ground and check as you said.

[angelodp]

Yes both wires are infinite Resistance.

[maxkracht]

I believe he was trying to confirm that there are no connections from either side of the primary to ground. If there are no connections to chassis, it shouldn't matter which hot/neutral connects to which primary wire. There must be something else connected to the primary if flipping the wires causes problems.