Driver testing failure

[Doc B.]

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.

[Paul Birkeland]

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.

[Jamus]

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.

[Paul Joppa]

***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.

[Jamus]

From terminal D2 to ground I get out of limit reading on my meter across all ohm settings.

[Paul Joppa]

***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.

[Jamus]

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?

[Paul Joppa]

***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.

[Jamus]

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

[Jamus]

"D2 is wired to kreg on the A side"
Just want to be sure you meant to say B side here?

[Jamus]

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

[Paul Joppa]

***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.)

[Jamus]

Just checked across both R3s. I get 5k ohms A side, 2.5k ohms B side as expected.

[Jamus]

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?

[Paul Joppa]

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.