Bottlehead Forum
Bottlehead Kits => Kaiju Stereo 300B amp => Topic started by: Jamus on June 02, 2016, 07:02:32 AM
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I'm close to finishing my Kaiju build but I'm having a problem at the driver testing stage.
IA and OA on the D socket board were spot on spec. However the voltage at 46U measures 130VDC and the trim potentiometer does not work at changing the voltage at all. For what it's worth the A side worked no problem.
I'm wondering if the fact that turning the screw on the trim potentiometer doesn't change voltage does that indicate that the potentiometer is faulty?
Thanks,
Jamus
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I'm wondering if the fact that turning the screw on the trim potentiometer doesn't change voltage does that indicate that the potentiometer is faulty?
This is very, very unlikely.
There's a set of pads that says Kreg on the A side, what voltage do you get here? Are you 100% sure that you have TL431's where they should be an PN2907's where they should be? Are the PN2907's facing the right way?
What voltages do you get at OA and IB?
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You could measure across the outer pins of the pot and see if resistance changes as you turn the screw.
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This is very, very unlikely.
There's a set of pads that says Kreg on the A side, what voltage do you get here? Are you 100% sure that you have TL431's where they should be an PN2907's where they should be? Are the PN2907's facing the right way?
What voltages do you get at OA and IB?
Kreg is 3.6VDC on A side, 2.4 on D side
OA and IB are both 300VDC on A, 299 on D
Doubled checked all transistors are in the correct position.
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You could measure across the outer pins of the pot and see if resistance changes as you turn the screw.
Just checked and the resistance does change after turning the screw, but not the voltage
*edited for clarity
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Excellent data, very helpful. R3 may not be soldered well.
The pot is configured as a variable resistor, and makes a voltage divider together with R3. The divided voltage is compared with the internal 2.5v reference. If the R3 leg is not connected then the adjusted voltage is stuck at 2.5v, which would give the observed 130 volts.
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I touched up R3 and all the other terminals for that matter but no changes in voltage at 46u.
Then I screwed something up big time. I was remeasuring IA one OA and the multimeter lead must have touched both terminals. Loud pop, bright flash, now I get no reading at OA. How bad did I eff this up? :'(
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You probably took out the 2N2907 and the MJE5731. Do the LEDS look exploded?
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LEDs look okay. So I'll replace the PN2907s and MJEs. Anything else to replace based on the initial problem?
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When you get the transistors replaced, I would recommend posting some photos of the offending board, both top and bottom sides if at all possible.
You may also have a jumper in the TL431/trim pot part of the circuit that isn't where it should be.
-PB
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Getting back to the original issue. I had exactly the same thing happen today with a Paramount, which has the same circuit. I had built a replacement board for the 5670 and the tube was stuck at 130V no matter how the trimmer was set. Closer inspection showed I had completely missed installing the 4.99K resistor that is in series with the trimmer. Installed it and it worked perfectly.
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I replaced the MEJs and PN2907s but I still get 0 VDC at OA and the B side LEDs don't light up. Here are several photos of the board. I tried to get it from different angles but let me know if you need better pictures. Note the damage between IA and OA :'(
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Well, I was hoping someone more knowledgeable might wade into these waters, but since it's been quiet I'll take a stab.
First of all, your symptoms seem to be 0V at OA, but the LEDs are lighting up on this side. Is that correct?
The cause that makes the most sense is a dodgy solder joint. From your pictures, it looks like some of your joints weren't heated long enough for the solder to properly flow. For instance, look at the rightmost hole of bA in IMG_3161. The wire can be seen coming up through the hole with solder on it, but the pad looks untouched by solder. This is just one example, this issue is seen on several terminals.
Solder needs to get to its liquid state and stay there long enough that to seep onto whatever the metal surfaces, which need to be hot enough to allow that. The pins of the A side 5731 are hard to make out in the photos (the Bside is clearly blobby (the other sign of not enough heat)), but they would be highly suspect for your symptoms.
The other cause that could be an issue is a fried Zener. Test your diodes by measuring the resistance both ways across each diode. One way should read very high (in the high KΩ or MΩ), and the other way should read over limit.
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Yes, check the zener diode string. If it is shorted the voltage at OA will be pulled down.
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Yes, check the zener diode string. If it is shorted the voltage at OA will be pulled down.
Unfortunately you are correct, all Zeners on that side are 0 ohms. I really wish I could take back that momentary lapse in hand eye coordination that kicked off this shitshow!
I was just telling my wife how I would be sad when this build was done because I really enjoy building these kits. I guess the gods were listening and gave me some bonus rebuilding time!
So I'll order 5 new Zeners and touch up that board (thank you Josh for the thorough look at the photos too by the way). I'll report back when that is done.
Thanks!
Jamus
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OK, glad you found the problem. That string is not too terrible to replace. Once it is back together the behavior should be that upon startup OA immediately goes to about 350V. That's what the zener strings regulates the voltage to. Then as the 5670 heater warms up the shunt regulator kicks in a pulls it down a bit to about 300V. That way the zener string drops out and thus has no negative influence on the sound. PJ is pretty tricky sometimes.
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You are also more than welcome to test the amp without the zeners in place. They are a preventative measure that protects your amp from 5670 tube failure.
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I replaced the Zeners today. Interestingly one of them was split in two right through the body. After replacing those the voltage is good everywhere except for 46U.
I usually use a pencil tip style solder iron tip which doesn't transfer heat as well but is good in small spaces like the PC board. I switched to a broader tip and reflowed all the joints on the board to eliminate any blobs. Still, I get the same 130V at 46U.
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***SEE LATER POST*** I'm leaving this as posted but lined out. The reference to pin 2 should be pin 8 of the D socket.
What is the DC resistance from the 5670 pin 2 to ground? This is a test of the R3 issue mentioned (not very clearly) earlier by myself, PB, and Doc B. It should be between 5000 and 15000 ohms, and it should change as you adjust the trim pot.
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From terminal D2 to ground I get out of limit reading on my meter across all ohm settings.
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***SEE LATER POST*** I'm leaving this as posted but lined out. The reference to pin 2 should be pin 8 of the D socket.
OK, there's the problem. D2 is wired to kreg on the A side, which goes to the trimmer R4, then to the 4990 ohm resistor R3 and finally to the -reg on the board, and then is wired to ground. Somewhere along this path a connection must not be made. I still suspect R3 to be the problem.
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Paul,
Tonight I pulled off all the parts that make up this chain on the A side of the board and reinstalled them all. I felt really good about the connections. Reattached the board and rechecked voltages. Despite all that I still get the same 130V at 46U. What else can I try?
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***SEE LATER POST*** I'm leaving this as posted but lined out. The reference to pin 2 should be pin 8 of the D socket.
Sorry, I was a little short on explanation there.
"D2 is wired to kreg on the A side," - so attach one lead of your ohmmeter to D2, and probe the resistance from there to kreg - should be near zero, if not then the wire between them is faulty or badly soldered or connected to the wrong place....
" which goes to the trimmer R4," Hard to get at the trimmer leads, so move on to:
" then to the 4990 ohm resistor R3" Leaving the first lead attached to D2 and probe resistor R3 (4990 ohms) - both leads. One of them should show the trimmer setting (between zero and 10K depending on the trimmer setting). The other should be close to 4990 ohms greater than the first resistor lead. Too complicated to do here, if this is the problem post results and we can dive onto it.
" and finally to the -reg on the board," so move the probe to -reg (keeping the reference at D2). Should be the larger of the above two measurements. If not, the solder at one end of R3 is bad.
" and then is wired to ground" Move the probe to a nearby ground terminal. If it's not the same, then the wire from -reg to ground is at fault, or badly soldered.
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Thanks Paul. Your initial instructions were probably 100% clear for the majority of builders on here but I'm pretty green at this. I'll remeasure those resistances today and will report back.
Thanks,
Jamus
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"D2 is wired to kreg on the A side"
Just want to be sure you meant to say B side here?
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Ok, here's what I measured...
D2 to Kreg on B side = 0.5 ohms
D2 to -reg = 423k ohms
D2 to ground = 423k ohms
D2 to R3 on A side (both ends of resistor) = out of limits
i also measured from both ends of R3 to ground and got 427k ohms and 433k ohms respectively
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***SEE LATER POST*** I'm leaving this as posted but lined out. The reference to pin 2 should be pin 8 of the D socket.
You are right about the B side - I went by the circuit diagram, which seems to be different from the construction manual. Applies to the D2 to R3 - should be R3 on the B side. My apologies for the confusion.
If I am reading it correctly, it seems the 4.99K resistor R3 is actually nearly 100 times too large. You can probably measure the resistor directly. (The other R3 on the A side should be 2.49K ohms - might as well check them both while you are at it.)
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Just checked across both R3s. I get 5k ohms A side, 2.5k ohms B side as expected.
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Ok, here's what I measured...
D2 to Kreg on B side = 0.5 ohms
D2 to -reg = 423k ohms
D2 to ground = 423k ohms
D2 to R3 on A side (both ends of resistor) = out of limits
i also measured from both ends of R3 to ground and got 427k ohms and 433k ohms respectively
why do I get resistance readings from R3 to ground but not R3 to D2? Does that help with isolating the issue? Since the difference across R3 to ground is about 5k ohms (433-427) does that mean that R3 is placed correctly?
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My apologies again; I finally figured out that the A and D sockets are wired differently. (I was not present when the manual was written, and I don't have a Kaiju in my lab.) The circuit diagram shows 5670 pin numbers for the A socket, not the D socket. The relevant socket terminal is D8 (not D2). I feel really bad about this.
Here is my earlier post, edited to say what I intended to say:
"D8 is wired to kreg on the A side," - so attach one lead of your ohmmeter to D8, and probe the resistance from there to kreg (A) - should be near zero, if not then the wire between them is faulty or badly soldered or connected to the wrong place....
" which goes to the trimmer R4," Hard to get at the trimmer leads, so move on to:
" then to the 4990 ohm resistor R3" Leaving the first lead attached to D8 and probe resistor R3 (4990 ohms) on the A side - both leads. One of them should show the trimmer setting (between zero and 10K depending on the trimmer setting). The other should be close to 4990 ohms greater than the first resistor lead. Too complicated to do here, if this is the problem post results and we can dive onto it.
" and finally to the -reg on the board," so move the probe to -reg (A) (keeping the reference at D8). Should be the larger of the above two measurements. If not, the solder at one end of R3 (A) is bad.
" and then is wired to ground" Move the probe to a nearby ground terminal. If it's not the same, then the wire from -reg to ground is at fault, or badly soldered.
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Paul, no worries on the mixup I'm just grateful for the help!
Kreg A = 0
R3 = 250 and 5250 ohms
-reg = 8800 ohms and slowly rising
Ground and -reg are the same
I resoldered both ends of R3 multiple times, and get the same reading at -reg. Could then be an issue with the board/terminal preventing the connection? I must have touched up R3 a dozen times since it was first suggested as the likely source of the problem.
Thanks again for helping out.
Jamus
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Those look better - presumably the trimpot is set to250 ohms, very close to one end of its range. The only anomaly is that one end of R3 should be connected to -reg.
On the back side of the PC board you can see the trace that connects the end of R3 closest to the 431 chip to one terminal of the Cc capacitor and from there to -reg. From your measurements, it seems clear that something is wrong with that connection. The total length of that trace is barely 1/2 inch - I'd say we have narrowed it down pretty close ... :^)
If you can't find it from that, can you post a photo of the back side of the board? Maybe getting more eyes on it will help.
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Okay, I think I see the problem. It really helped knowing where to look so thanks Paul for helping narrow it down within 1/2 inch!
It appears to me that the trace between R3 and Cc is not complete (bottom of the image). If you zoom in on this picture you can see a small notch in the trace. The metal around the Cc terminal is also incomplete.
Am I seeing that right?
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Yup looks like it is open right there. Easiest solution is to make up a wire jumper that reconnects the disconnect.
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Just so I'm clear, jumper from R3 to Cc?
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R3 hole is too small to fit a second wire through. Any other options?
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You can carefully scrape the green solder mask from the trace and tack solder the end of the jumper to it. The soldering also needs to be done with some care so the trace doesn't lift off the board.
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I installed the jumper tonight and sure enough that did the trick! Now getting the same resistance at R3 and -reg, powered up and voltages are in spec, trip pot is trimming, all is good in the world.
This was a long strange detour and I couldn't have navigated it without the help of PJ, Josh, PB, and Doc B. Many thanks for sticking this one out!
Best,
Jamus
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Awesome! We will be sending out a replacement for the defective board tomorrow.