Express build 1st startup, B+ voltages too high?

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sluckey
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Re: Express build 1st startup, B+ voltages too high?

Post by sluckey »

Is -9V after D11 an expected measurement?
No. You should see approx. -40 to -50 at the anode of D11. How much resistance do you measure from the negative side of C21 and C22 to ground?
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xtian
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Re: Express build 1st startup, B+ voltages too high?

Post by xtian »

Simplify until you find the error. Suggest you disconnect the B- output of the bias board (white wire marked "bias"). Measure for DC at the junction of D11 and R3--dial bias adjustment pot while measuring.

Are all those components on the yellow board soldered in properly? (Maybe on the underside of the board?)
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shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Sluckey wins the contest, C22 has 0 ohm resistance across the terminals. So I will replace it. Any particular cap I should choose, or just any generic 10uf 100V electrolytic cap?
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Does this one seem ok? 450V, but should be able to fit it onto the board.

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xtian
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Re: Express build 1st startup, B+ voltages too high?

Post by xtian »

Yes, that will work, but is certainly over spec'd in terms of voltage.
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RJ Guitars
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Re: Express build 1st startup, B+ voltages too high?

Post by RJ Guitars »

I'm late to the party but looks like there is still plenty of fun left...

I've used this bias board on a few different builds although I don't have any pictures of it in a more traditional Wreck build. I did use it in an amp development platform that I built as a test platform for comparing output transformers... and it is an Express circuit. So if you can see past the layout differences maybe my component choices will help you sort out the issues and get things dialed in.
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shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Replaced the bias caps today, but still I am getting 0 bias voltage, and only -9 VDC after D11. I then replaced R29 and D11, but nothing changed. The AC voltage before D11 is just under 600V.

I am truly at a loss about what do try next, hopefully someone can point me in the right direction.


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Re: Express build 1st startup, B+ voltages too high?

Post by pompeiisneaks »

I'm seeing what look like potentially not well soldered junctions in multiple places on the bias board, I may be seeing things so my apologies if they're not what I think, but I've taken your picture and have added some red arrows to areas that look to not have a solid solder connection, if this is the case, they could be causing problems making the connections go through, thus why you're seeing 0V. I would go over all of those connections and reflow some fresh solder in them and see.
bias.jpeg
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shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

The bias board was originally soldered from below. Since the picture you annotated was taken, I have re-soldered all connections from above. I'll put up a new picture tomorrow.


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sluckey
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Re: Express build 1st startup, B+ voltages too high?

Post by sluckey »

I don't see a ground connection to the positive end of the bias filter caps. Should be connected to chassis at the same place the HT center tap connects to chassis.
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romberg
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Re: Express build 1st startup, B+ voltages too high?

Post by romberg »

shipwreck wrote: Mon Jun 12, 2017 4:43 pm
Those are R23 and R24, located on the power supply board instead of on the can caps. Drain resistors, I think?
Those resistors look really, really wrong to me. I have no idea what schematic you are using. But from looking at what is visible from the topside of that board, it looks like you have either 100k or 50k connecting the *CENTER* of one leg of your rectifier to ground. At least I'm assuming one of those resistor ends hits the chassis through that trace that bumps into the mounting bolt. The other resistor goes to a trace that connects to something the picture does not show. If they are suppose to be either a voltage divider or bleed resistors they won't work for that purpose connected to the center of the rectifier.

These resistors are not doing anything positive for your main power supply. But I don't think they are what the problem is with your bias supply. As others have suggested, I'd disconnect that white wire. With no tubes installed the (disconnected) white wire should show no continuity to ground. If something under the main board (where the white wire connects) is shorting to ground that would cause low bias voltages. It is hard to tell from the pictures as I'm assuming there is a trace on the main board from where the white wire connects and the bias feed resistors.
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M Fowler
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Re: Express build 1st startup, B+ voltages too high?

Post by M Fowler »

Those bias caps are grounded by the top left screw, I would make sure that standoff is grounding well.

The two 100k resistors are the drain resistors.
shipwreck
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Re: Express build 1st startup, B+ voltages too high?

Post by shipwreck »

Yes, M Fowler is right, and I did a continuity test on all the standoff screws. But anyway, this weekend I will disassemble the whole bias board and check all connections once more.
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Express build 1st startup, B+ voltages too high?

Post by shipwreck »

IMG_2295.JPG
Ok, redid the whole bias board today, see attachment. I checked for continuity to ground, and was surprised to get continuty beep from the yellow high voltage wires feeding the bias board. Is this expected, or does it indicate a fault in the PT? Haven't tried to power it up yet.

I also switched the location of the blue wire from C15.
IMG_2295.JPG
Ok, redid the whole bias board today, see attachment. I checked for continuity to ground, and was surprised to get continuty beep from the yellow high voltage wires feeding the bias board. Is this expected, or does it indicate a fault in the PT? Haven't tried to power it up yet.

I also switched the location of the blue wire from C15.
IMG_2261.JPG
IMG_2261.JPG
IMG_2295.JPG
Ok, redid the whole bias board today, see attachment. I checked for continuity to ground, and was surprised to get continuty beep from the yellow high voltage wires feeding the bias board. Is this expected, or does it indicate a fault in the PT? Haven't tried to power it up yet.

I also switched the location of the blue wire from C15.
IMG_2261.JPG
IMG_2261.JPG
IMG_2261.JPG
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shipwreck
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Express build 1st startup, B+ voltages too high?

Post by shipwreck »

IMG_2261.JPG
Here is the PT datasheet, I got the wrong one in the previous post. So the 300V secondaries show continuity to ground, is that as it should be??
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