JTM45 voltage problems

[axe-victim]

I just finished a JTM45 using George's instructions and began measuring voltages (using a Fluke 112) and installing tubes. My measured voltages are high in some cases and non-existent in one case. Here's what I have:

V1 235v --- 1.4v 3.25v 3.25v 393v --- 1.36 3.25v
V2 231v --- 1.5v 3.25v 3.25v 399v 231v 0v 3.25v
v3 255v --- 32v 3.25v 3.25v 429v --- 32v 3.25v
V4
V5
V6 --- 101v --- 350v ---- 350v --- 100v
5.2v 0.36v

What really gets me is 0 volts on V2 pin 8! I checked all the connections using the continuity setting on the meter and check from the component's lead to the pin/ground/where ever. Also, the 100v and 0.36v on the rectifier tube is a concern.

Some background. The chassis is from Metro. The trannies are from Marstran. The board was built by Brian Haberman. Most of the components were purchased at Valvestorm.

Other things I did to find the problems: I double checked all the board connections using the continuity setting on my meter. I checked from the component leads to where ever they were connected. I confident all is correct. I also swapped another tube for V2 and a working GZ34 with the same result.

I'm not too concerned with the high voltages as the Marstran web site indicated that a 120v line would increase these, but they seem a bit inconsistent with the ones from the instructions (pin six on V1-3 is listed as 180v, 280v, and 195v. where mine is 393v, 399v and 429v).

What am I missing?

[neikeel]

Check your ground connection of the cathode follower (100k) and that your tone stack cap is good and the yellow wire between the tie pint of the cap, slope resistor and the 100k is sound (ie not dropped out or a dry joint).

Any pics?

If that valve is not drawing the correct current V1 will be higher than expected.

Did all your voltages check out with just the GZ34 before you put the valves in

[axe-victim]

I have continuity between the 100K lead and ground, so that should mean the solder joint is good (well, sufficient, maybe). Same goes for the other side of the 100K, it has continuity between the 100k lead and pin 8. I think I hit every connection on the board that way, or at least tried too.

I didn't record the voltages with only the rectifier, but I'm confident they were reasonably close or I would have stopped then. Should I go back to that step and record them?

My theory is old and rusty, but here goes. If the 100k resistor was either open or not grounded, wouldn't I still read a voltage as the meter would complete the circuit? I wouldn't read a voltage across the resistor lead to lead.

I'll get some pictures posted soon.

Maybe I should take a day or two away and come back fresh. But that will be REALLY hard to do.
Art

[axe-victim]

image(5).jpeg (80.1 KiB) Viewed 3559 times

Let me know if you want any close-ups.

I pulled the 12ax7s and checked the voltages as per instructions.
352v on standby switch
-51v on pin 5 of V4 & V5

[neikeel]

That pic looks ok - a bit distant but nothing glaring.

The two last voltages sound correct too.

In my experience this sort of issue is a dry joint (one that looks ok but is of high impedence) and simply ringing out with a meter will not necessarily detect the problem.

Same goes for the joints on the tune sockets as well.

I presume you checked that all the components were good with your meter before installing? There have been some duff silver mica caps recently - although they usually manifest themselves with odd noises.

What is the voltage (dc to ground) at turret where the 56k slope resistor, tone stack cap and 100k resistor come together?

[axe-victim]

Well, I have my work cut out for me. I'll check the solder joints in question and then some.

I didn't check all the components; got in a hurry to get done, I guess. I'll pop that silver mica and the tone caps and check them thoroughly.

The voltage at the point in question is zero.

I've got some close-up pics if you tell me what section (s) you want to see.

Thanks for all the help so far. I really appreciate it.
Art

[Reeltarded]

Little more light and a little closer on that end, with v2 in the middle of the pic?

My eyes are worse than any picture you might take!

[axe-victim]

image(8).jpeg (62.46 KiB) Viewed 3523 times

image(7).jpeg (65.09 KiB) Viewed 3523 times

image(6).jpeg (74.34 KiB) Viewed 3523 times

Okay. If I were to bring this amp to any of you to make it right, what would you do? I'm inclined to disassemble the majority of it and re-solder after testing all the components. I do have the time (other than my day job, evening job, side job, upkeep of the house,kids, etc...). Oh, and I still have a lot of solder left!

Peace,
Art

[toner]

V2 pin 5 has no heater wire connected, thus no cathode voltage.

[Reeltarded]

Hey!

[axe-victim]

Holy cow!!!! You caught that one! I went back to the instructions and sure enough, wire pin 4 to pin 5.

Insert red face here.

This is what happens when one doesn't keep in mind exactly what is being connected and why. I certainly will be more aware of the purpose in the future.

I've got some distraction this afternoon and can't get to this until Thursday. I'll post the results after that.

Thanks again for all the help so far!

[neikeel]

............. can't get to this until Thursday. I'll post the results after that!

Good call by Toner, you can see now why clear, well lit pictures help so much!

[Reeltarded]

I thought it might be right there. My eyes are terrible, but the shadows seemed a little thin. lol

[neikeel]

I could not tell from the first pics, sure a whole wire missing but not a solder coloured link across a pre-socket.
Pretty clear on the close ups - Toner was onto it quickly.

[axe-victim]

Jumpered the preamp tubes and did a quick voltage check and everything looks good so far. I'll install the power tubes tomorrow and finish the checks. I'll post the results.