Dumbleator build, help with strange voltages

[norburybrook]

Hi Guys,

Well after successfully finishing my 50w #102 build I thought a Dumbleator was in order.

So, I've got it all built up but I've got strange voltages from the power section.

Instead of seeing B+ voltages of 300v or so I'm getting readings in the region of -69vDC and -57 vDC with -68 v DC going to the main board!!

Anyone hazard a guess why I'm getting negative voltages?

the transformer is giving me; 130v AC and 3-3 AC for the heaters, my mains voltage here is a healthy 240 v AC.

one day I'll make something and it will just work!!

cheers

Marcus

[MKB]

Be sure and double/triple check the wiring of the diodes and the input of the power supply in general. If the diodes are backwards that will give you a negative voltage, and if the reverse negative voltage is applied to the positive terminals of the power supply caps, they are possibly damaged or destroyed as well.

The wiring side of your PS board is not shown, so its only possible to guess the connections.

Recently I mistakenly connected a 47uF 350V electrolytic backwards in a test (over 30 years of electronics experience does not immunize you from such things) to a 300V supply, and the cap got very warm quickly. I just threw it away as it is probably destroyed.

[norburybrook]

the diodes and cap polarity were the first thing I checked, and they're the same as the layout.

hmm actually I noticed my caps got very warm as they melted the hot glue I'd used to secure them.

I used the layout Tony posted i.e. the original Dumbleator layout for my wiring.

I'll make a trace showing what goes where as I think that will help.

cheers

Marcus

[67plexi]

In the layout that is posted the first two radial filter capacitors are backwards ground going to B+.


Steve

[norburybrook]

thanks Steve, I was just looking at the schematic and came to the same conclusion.

Hopefully I wont have fried these caps. It's not that their expensive but buying 2 caps is impossible form Mouser or Tubetown without hideous postage.

thanks

Marcus

[MKB]

As 67plexi noted, this layout has the polarity backwards of the two 47uF caps in the voltage doubler, as does norburybrook's build. If they got hot enough to melt glue, they are indeed dead and should be replaced, I wouldn't trust them. Sorry to see this, it's frustrating. But it is good they didn't blow up on you; those big caps with all that energy can make quite a bang and mess.

[norburybrook]

I've just taken them out and tested them they both still read 47uF so I put them back in the correct polarity and I'm seeing better voltages now but still -ve??????????

Is this because they're knackered?

I don't have any replacements at the moment.


Marcus

[Charlie Wilson]

Marcus, those caps see the highest B+ in your Dumbleator. As MKB states if they got that hot, don't trust them. If they short worse things can happen then having to replace the caps. I would consider them semi-Knackered at this point.
CW

[norburybrook]

thanks Charlie, could that explain why I'm still seeing negative voltages though?

Marcus


p.s. I've ordered some more.

[Charlie Wilson]

Marcus, were are you seeing the negative voltages? Also, replace the diodes.
CW

[67plexi]

Marcus, Your standby lead is grounded out also defeating the voltage doubler. Brown lead on the layout.


Steve.

[norburybrook]

Steve,

Its confusing looking at the pic with my lines drawn on I see. The standby switch /brown wire is actually black ,as is the ground wire so they do indeed look confusing, it's not grounded though it goes to the standby switch.

Charlie, I'm seeing negative voltages at the B+ points. The diodes are still measuring a 0.6v drop across them in the correct direction , I presume that means they're still working.


Marcus

[Charlie Wilson]

Marcus, you could disconnect the B+ line from the power board that goes to your other board and rule out maybe a short on the preamps tube board that is pulling your voltages down. My concern is that your power transformer probably isn't to happy right now (is it getting hot?). You do have an appropriate value fuse installed? The thing is, we know that your main filters took a hit. Maybe you could remove the first dropping resistor and see what the voltages do at the top of the first filters. If is still negative there than it is those caps. The prudent thing to do would be while you are waiting for the new caps, double and triple check your wiring with the unit not powered. My .o2.
CW

[norburybrook]

Thanks guy's I'll wait for my new Caps and then report back.

I thought this was going to be an easy build compared to the #102

I'm surprised no one else had mentioned the layout was wrong on Tony's Dumbleator sticky.


Marcus

[norburybrook]

OK, this is really embarrassing ...


the reason I had negative voltages was................




I HAD THE LEADS FROM MY MULTI METER THE WRONG WAY ROUND



I'll get my coat....................


Marcus