Bottlehead Forum
Bottlehead Kits => Eros Phono => Topic started by: armaloney on February 06, 2019, 04:18:58 PM
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I have mounted the shunt regulator board and am at the step of measuring the DC voltages on each of the MJE5721A tabs (p. 55 of manual). The manual states that the voltages should read between 210VDC-230VDC. On the B side of the board, I measure 215.8 V, so that is fine. On the A side of the board, I measure 232.8 V.
Should I be concerned with this voltage, or is that fine?
If it is something worth investigating, what other measurements would be valuable?
Other remarks:
I've visually inspected the circuit and nothing is immediately jumping out at me. (Some of my solder joints could have been better I suppose.) All four LED are glowing, as is the 12AU7.
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I would stop and get this sorted out. What voltages do you see at the Kreg pads?
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On the A side of the board: 1.92V
On the B side of the board: 2.93V
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That is a bit of a problem. If it's not too much trouble, could we review some photos of that area of the build?
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I've attached a few here. Any views you want to see in particular?
Tried to get underneath as best as I could, but would need to disconnect some wires to really get it. (I actually broke the wire from the +275V pad on the power supply trying to get underneath.)
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I would check to be sure that you have the 431 regulators in the correct spots and the 4250 transistors in the correct spots. Also check the red/white jumpers leaving the PC board to be sure they go to the correct terminals, as well as the R1 resistors on the PC board to be sure they have the correct values. The 1.92V on Kreg is indicative of an issue, so you'll want to get that sorted before proceeding.
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Those all look to be in order.
1. 431 regulators are in the right correct spots.
2. 2907 transistors (I think that is what you meant.) are in the correct spots.
3. The R1 resistors are correct value.
4. Jumpers are to the correct terminals.
Any other recommendations? Could it be the tube?
(I'm now measuring 230.6 on A and 215.9 on B the MJE5721A tabs. I still have around 1.9 and 2.9 for kreg on A and B, respectively)
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Are both halves of the 12AU7 glowing?
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Yes. Both sides are glowing.
However, I'm pretty sure I just created a new issue. While taking some measurements, I shorted two terminals together. (I'm thinking it was D5 and D6, so I put too much voltage to the heater.) My A side reading is now about 185V off the tab and 135 mV at kreg.
If the tube wasn't damaged before, it probably is now, right?
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The tube is nearly certainly just fine, but the transistors on that board can be sensitive.
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Ok. I guess I’ll check voltages on the A side of the board and see which transistors may have been damaged.
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With the amp powered off and the voltage bled down, you can check DC resistance between pairs of legs on each of the transistors. A bad transistor will show a short between two legs.
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Yep. Looks like the transistors on the A side of the board were damaged. I’lll take the board off and get some replacements for those to put in.
I’ll post an update after that’s done.
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So I finally got around to swapping out the transistors on the A side of the board.
I plugged the unit in and the RC resistor actually ignited. I'm not sure why that would have happened, but figure my best bet might be starting from scratch with a new shunt regulator board and taking extra care with it. I had some trouble removing the old components from the board, so fear I may have really made a mess of the board.
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Yes, I would recommend doing that or sending it in for repair.
RC doesn't really have any DC across it unless CC is shorted.
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Update:
I ordered a replacement shunt regulator and assembled that. Upon testing, I had the same result as on the original build (about 230V on A tab, kregA at 1.9V, 216V on B tab, kregB at 2.9V).
This led me to believe that the issue was not in the shunt regulator, as it seemed unlikely to have the same issue twice. I swapped out the 12AU7 for another that I have been using in my Crack. Upon doing so, I measured roughly 216 V on both sides of the board, and kreg around 3V on each side. Seems the tube sent with the Eros may have been the culprit all along.
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I would suggest reviewing the wired connection between pins 4 and 5 on the 12AU7 socket. When I've run into problems like you're describing, it will sometimes simply be that this particular connection is not well made, so only half of the 12AU7 is heating (and only half of the regulator is working).
When installing a new tube, you may just jostle a loose connection enough to make the amp work, so it is certainly worth it to go back and have a look.
-PB