Bad Voltage readings on V2 and V1 - Mojotone Champ 5F1 build

guytrongt100

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Hi All,
I need some help on this Champ - everything is built to the Mojotone diagram. The voltage readings are below (with all tubes installed) and seem pretty disturbing and the amp makes a horrible loud buzzing sound. Sad part is I built a 5e3 and a Blackface Princeton Reverb with zero issues, this was supposed to be a walk in the park. I'm not an electronics guy so troubleshooting is not something I have any knowledge of. I have checked all the solder joints and they appear good, wires are tight, and I didn't find any wires touching each other anywhere on the board. Tested all resistors and the values are good.

B+1 = 353 vdc
B+2 = 323 vdc
B+3 = 290 vdc

6V6:
Pin 1 = 240 vdc
Pin 2&7 = 6.5 vdc
Pin 3 = 350 vdc
Pin 4 = 323 vdc
Pin 4 = 105 vdc
Pin 1 & 8 are jumpered

12ax7:
Pin 1 = 174 vdc
Pin 3 = 1.6 vdc
Pin 4&5 are Jumpered and Pins 5&9 = 6.5 vdc
Pin 6 = 265 vdc
Pin 7 = 268 vdc
Pin 8 = 221 vdc

Any help will be very much appreciated!!
 

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milocj

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Are you using an old stock tube? Im pretty sure that pin 1 isn't connected in most 6V6s but that in some rare instances they do have an internal connection and will be extremely noisy. I had an issue with one in a dual 6V6 amp and I remember it being linked to pin 1 and 8 but I can't remember the situation.
 

guytrongt100

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Check out the pictures and the pdf diagram that are attached. They are brand new JJ tubes that came with the amp kit and they will stay there until I get this issue corrected and then burn it in for 72 hrs. Then the NOS tubes will go in.
 

King Fan

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Nice-looking build, very tidy for a Champ. I'm not looking into your voltage table yet, and anyhow smart folks can do a better job than me.

To try to resolve one question, 6V6 pins 1 and 8, you're right, that's how Mojo shows it, but it isn't at all necessary and might not be a good idea. But, I doubt it's causing your other voltage problems. Why? Cuz pin 1 has no connection on the 6V6GT (glass tube); it needed grounding when they had metal shells. OTOH for simplicity I'd leave pin 1 bare and just run a wire from pin 8 to the cathode resistor/cap.

From your file:

1648933688845.jpeg
 
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guytrongt100

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Thanks, I can follow directions (most of the time :)) and solder, but that's about the extent of it. Yea, mojo didn't do that on the 5e3, not sure why they did it on this one. I can make that change when we figure out what the major malfunction is. Thanks!
 

Junior Little

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Not an expert, not even by a long shot, but please tell us you're using a light bulb current limiter?
You definitely don't want 200+ VDC on the wiper of your volume pot. It seems to me you should check your continuity and/or resistances because something isn't wired correctly. Amp off and caps drained before doing so, of course.
 

guytrongt100

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Sorry to say, not using a light bulb limiter. I do have snuffer stick that I drain the caps with. I am electronically challenged, so you will need to tell me how to check the continuances. I did check all the resistors and they gave correct readings.
 

Junior Little

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Please do not plug the amp in again until at least until the wiring checks out. Or, much better: build a light bulb limiter. They're easy to build and it can save your amp and possibly your life.
Put your meter in continuity mode and trace through the layout you have in your first post. Check each one off as you go through and verify. For example, does pin 7 of V1 go to the wiper of the volume pot? Does pin 8 of V1 go to the junction of the negative feedback resistor and cathode bias resistor? Do the remaining connections, especially the power tube pin 5.
Drained caps and unplugged!
And build a current limiter! :cool:
 

guytrongt100

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Thanks!!
I tested all the resistors and they all read good and are in the correct locations. When you have 2 resistors going to one hole the reading should be half the value of the resistor?
I tested the continuity on all the lines that run on the underside of the board and they all read 0 on the continuity setting. In my untrained brain, how do I know the coupling caps are doing their job? I told you I know zero about electronics!!
 

zook

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Hi All,
I need some help on this Champ - everything is built to the Mojotone diagram. The voltage readings are below (with all tubes installed) and seem pretty disturbing and the amp makes a horrible loud buzzing sound. Sad part is I built a 5e3 and a Blackface Princeton Reverb with zero issues, this was supposed to be a walk in the park. I'm not an electronics guy so troubleshooting is not something I have any knowledge of. I have checked all the solder joints and they appear good, wires are tight, and I didn't find any wires touching each other anywhere on the board. Tested all resistors and the values are good.

B+1 = 353 vdc
B+2 = 323 vdc
B+3 = 290 vdc

6V6:
Pin 1 = 240 vdc
Pin 2&7 = 6.5 vdc
Pin 3 = 350 vdc
Pin 4 = 323 vdc
Pin 4 = 105 vdc
Pin 1 & 8 are jumpered

12ax7:
Pin 1 = 174 vdc
Pin 3 = 1.6 vdc
Pin 4&5 are Jumpered and Pins 5&9 = 6.5 vdc
Pin 6 = 265 vdc
Pin 7 = 268 vdc
Pin 8 = 221 vdc

Any help will be very much appreciated!!
Pin 7 should have practically 0 Volts. Should be measures in Millivolts. Pin 2 the same. You may have crossed pins 6 and 7 look carefully,
 

dan40

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I don't see the jumper between the 25uf/500r cathode resistor and the 8uf filter cap that is shown on the layout. Did you jumper this under the board? The horrible noise may be caused by positive feedback which can be fixed by swapping the red and blue wire coming from the OT. You can test for this easily but we need to get the voltages right first.
 

guytrongt100

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Dan,
Sorry I'm not following you on that? All the dashed lines are under the board and they all showed continuity. I'm not sure about which jumper you're referring to.
 

King Fan

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A picture may be worth 1,000 forehead slaps... if it helps, lemme insert a jpg of that pdf for us all to look at inline.

1648950113205.png


And here's your pic rotated to the same orientation; I've drawn a red line for the jumper on the board I think he's talking about.

IMG_5355 copy 2.jpg

Is *that* jumper under the board? What about its friend that does the same thing between the left and center filter cap negative poles?
 

dan40

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KF's drawing is correct. Just wanted to make sure you had that small jumper in place under the board. I don't see the jumper between the main cap and screen cap either but I'm assuming that it's there under the board also.

6V6:
Pin 1 = 240 vdc
Pin 2&7 = 6.5 vdc
Pin 3 = 350 vdc
Pin 4 = 323 vdc
Pin 4 = 105 vdc
Pin 1 & 8 are jumpered


On your 6v6 voltages, you listed pin 4 twice. I'm assuming that the second pin 4 was suppose to be pin 5's measurement. Are you actually seeing 105v on pin 5? Should be 0 on that pin. Pins 1 and 8 should be about 20vdc. Check that socket carefully for a wiring error.
 

guytrongt100

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You guys are absolutely correct, I feel pretty dumb that I missed those and just really never saw them while looking the diagram the whole time! Now the fun begins, trying to get the jumpers in without pulling the board. Don't think I'm gonna be able to get at the one under the pilot light.

Thanks so much guys, I really appreciate the help!! A fresh set of trained eyes always helps!
 

King Fan

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Credit to @dan40 for the eyeball expertise.

Yeah, adding those jumpers esp. under the lamp looks (or smells) like burning plastic and cloth to me. It'd be tedious, but I might take down and remove the lamp (those 100Rs would need to be lifted) or the volume pot or both.

Hmm, do you have enough slack to lay the volume pot wiring right down on the control panel? Or maybe the yellow wire could swing to the left for the right jumper and to the right for the left one? But the white wire(s) might be trickier.

1649000666756.png


If you do take down the volume pot wiring, you'd have the option to also revise the AC wiring to switch the hot (black) household wire instead of the neutral (white). Not a *huge* safety difference, but a difference. I suspect you did follow Mojo, but unfortunately their approach is more 'vintage' but slightly less safe (leaves more of the amp 'hot' when the switch is off). Here, I drew you a picture. But... optional. Many vintage amps did end up with switched neutral.

1649001917679.png
 

guytrongt100

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Yes Dan40, thank you very much!

King, thank you also, I need to learn to edit jpeg files like that, makes things very easy and idiot proof! I just saw your response just now, I was making the repairs, testing, and playing through it. I had to do exactly what you said. Only change I had to make was wrapping the leads of the 25uf to the 500r leads so that I could fit the jumper in the hole.

Below are the new numbers:

Don't know how much it matters but the line voltage in the house is 124v today, it's usually around 120v?

B+1 is a little high and so is pin 1 on 6V6. What is a good value on Pin 3 on 6V6?
All others look good to me.

B+ = 407 vdc
B+1 = 341 vdc
B+2 = 286 vdc

6V6:
Pin 1 = 22 vdc
Pin 2&7 = 6.3 vdc
Pin 3 = 394 vdc
Pin 4 = 341 vdc
Pin 5 = .086 vdc
Pin 1 & 8 are jumpered

12ax7:
Pin 1 = 168 vdc
Pin 3 = 1.6 vdc
Pin 4&5 are Jumpered and Pins 5&9 = 6.3 vdc
Pin 6 = 168 vdc
Pin 7 = 0 vdc
Pin 8 = 1.5 vdc

Again, thanks to all that helped, I really appreciate it!!
 
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