Mojo Champ Kit

[SWA]

Mojo Champ Kit Problems

Good Morning MWJB

As promised the volts from ground to V1 pins are as follows:

1 = 0
2 = 0
3 = 2.23
4 = 0.21
5 = 0.21
6 = 0
7 = 0
8 = 2.12
9 = 0.21

Look forward to hearing your views.

Cheers

[MWJB]

Morning SWA,

You have no plate voltage at V1 (pins 1 & 6). Check to see where this disappears - rectifier pin 8, 1st filter cap (far L), 2nd filter cap, 3rd filter cap, junction of 100K plate resistors feeding pins 1 & 6...

Heater voltage at pins 4&5, 9 will be AC. an either be read to ground & double the reading, or more commonly read from pin 9 to pins 4&5.

[SWA]

Mojo Champ Kit Problems

Hi again MWJB

Here we go for the volts en route to V1 pins 1 & 6.......

5Y3 pin 8 =411
1st cap =411
2nd cap =411
3rd cap =353
100K junction=286
pin 1 = 0*
pin 2 = 0*

*continuity to pins 1 & 6, via my 2 red wires, checks OK.

Very grateful for all your thought & input.

[SWA]

Mojo Champ Kit Problems

Whoops - that should read..........

5Y3 pin 8 =411
1st cap =411
2nd cap =353
3rd cap =286
100K junction=286
pin 1 = 0*
pin 2 = 0*

Cheers

[MWJB]

Somehow your 100Ks do not connect to pins 12AX7 1 & 6, or are shorted before they do.

[MWJB]

What do you read at the other ends of the 100K plate resistors, what is the voltage drop accross them?

[SWA]

Mojo Champ Kit Problems

Thanks MWJB we're getting there

I just switched her back on &, miraculously, have volts at pins 1 & 6 - all 286 of them - nothing dropped across the plate resistors!

[MWJB]

That's not good, it means that your 12AX7 is turned off somehow (check continuity to ground at cathode resistors at the top of the board, hot sides connect to pins 3 & 8). You should be seeing around 2/3 of that 286v at pins 1 & 6 (so 190vdc-ish), a little over 1.5vdc at pins 3 & 8.

[SWA]

Mojo Champ Kit Problems

Hi again & thanks for your continuing efforts; it is much appreciated.

Continuity to ground for the V1a & b cathode resistors is good.

AC into the house is down a bit at the moment & the 286 figure has dropped to 283vdc but still right to pins 1 & 6.

I've currently got 2.21 & 2.09vdc at pins 3 & 8 respectively (is 2.3 normally recognized as max?)

Although I'm a junior member here, I've done more than a few valve amp projects over a 40 year period & I've never had anything like this: in actual fact I've never had anything not amplify the signal. Puzzled!

[MWJB]

OK I can see what's up now...those 2x100K look to actually be 100ohms (brn/blk/brn) all your signal is going straight to ground! Should be brn/blk/yel.

[überfuzz]

I can't really see how the wires go to the rectifier tube, but triple check it. If all is well, move on to the filter caps.

What I can tell is.
1. Store each blind tap separately.
2. You're getting voltage slightly off because you've got the red/black tap. It should be the 230 V tap. We're at 400 VAC here in Europe, remember. That makes 230 VAC.

[MWJB]

Uberfuzz wrote: "2. You're getting voltage slightly off because you've got the red/black tap. It should be the 230 V tap. We're at 400 VAC here in Europe, remember. That makes 230 VAC."

We're 240VAC in the UK, we call it "230" just to fit in with mainland EU...but it's really 240 (or significantly over 230VAC on average). Selecting the 230VAC primary tap will push up voltage further. 411vdc at the rectifier, with a Sovtek 5Y3 is normal & nothing to be concerned about.

[überfuzz]

Oh, I didn't know that. Sorry!

We're at 400 VAC (3 phases) in the main land of Europe. Wish makes 400 / cos (30) = 230.9 VAC.

[Bruce / Mission Amps]

Listen to what MWJB just said... you have 100 ohm plate load resistors and they are supposed to be 100,000 ohms.
You have the filament supply balancing resistors installed as the plate load resistors on your 12AX7.

[SWA]

Mojo Champ

Good Morning All

Well, MWJB, after your last advice I went straight out & bought a 'daylight' magnifying lamp for the bench & you (& Bruce - thanks) are absolutely right. Several important lessons were learned from this.......

1. My eyes aren't what they used to be!
2. Don't believe anything written on packaging!
3. Measure twice & cut once (so to speak); I've told staff & my kids this a thousand times. I failed to check with my DMM immediately prior to populating the board - for whatever reason!
4. Practise what you preach!

I'll be attending to the resistors, & taking on-board a few other recommends, over the coming days & then I'll post an update.

My sincere thanks to everyone involved in helping me out.

Cheers & have a good day.