New build: 2204 Mojotone kit

[Tortuga]

And so, here we get caught up to present moment...

Metro loop needs a ~ 300V supply coming from the PI power node... dropping resistor has to be selected per instructions... check...

So, I swapped out the 2-position terminal strip I'd added for grounding the metro board as well as the 68K grid stop resistor for V1, with a 3-position one. Now, everything has a solid place to live



...and I routed the power under the board and drilled another hole for the lead to join in the fun



This bitch ready to

[Tortuga]

Another (bad) clip... really seeing how rusty I am

Some decent KARRRAANNNGGG here, though. Tones are definitely killer - really gotta get my chops in order .

Kicked on my SD1 (main input) and Carbon Copy Bright (loop) in the clip.

[Tortuga]

Scott (Fretless) asked me for an edge of breakup clip, so here's a crack...



Kicked in the carbon copy and Tube Screamer to see how it handled the gain. Again, y'all - this is really prelim stuff... haven't touched the bias at all yet, and pedals are literally being thrown at it to see what's up.

Noticing quite a bit of noise when the guitar is near the cab... These are single-coil P90-type, so I anticipated that, but it may be something that could be tamed. Wondering if a shield for the bottom of the chassis is warranted?

[greatmutah]

Sounds like a Marshall!

Also it wouldn’t hurt to put shielding on the head cab under the chassis.

[Tortuga]

Thanks Eric - pretty sure I'm gonna get a sheet of something to put in the bottom of that cabinet

Now, for something completely different...

Mains fuse popped - literally 5 minutes before my son arrived to check it out with me

Checked home depot, lowes... nobody has the right one

Have to run to electronics store in the morning. Praying they have the right one

...then I'm gonna need to do some circuit testing to see what's up. Hope this is just an abnormality or at least an easy fix

[Tortuga]

Found fuses today… amp ate two more before I hooked it up to my current limiter. Unfortunately, it’s indicating a short somewhere in the early power section (all tubes pulled and still in standby).

Checked components in the bias supply and everything else I could think of, and am getting concerned that the PT may be blown. Sure hoping not… this has been deflating, to say the least.

Does anyone know how to test a PT?

[GuitarBilly]

This could be a good read

http://www.geofex.com/ampdbug/pwrtrans.htm

[Zozobra]

If it's popping fuses in standby with the tubes pulled then check your heater wiring. Usually when a big Px pops it smells nasty. Like a really metallic tuna sort of smell.

[Tortuga]

This could be a good read

http://www.geofex.com/ampdbug/pwrtrans.htm

Love it - thanks Bill!

Started reading (actually, fell asleep to ) this write-up last night... between that and the one you posted, I should be able to do some diagnosis later this afternoon

https://robrobinette.com/Tube_Guitar_Am ... lows_Fuses

If it's popping fuses in standby with the tubes pulled then check your heater wiring. Usually when a big Px pops it smells nasty. Like a really metallic tuna sort of smell.

Can't tell you how much I appreciate hearing this... yeah, no smell/magic smoke... hoping/praying that it's not that...

But, fixing/re-doing the heater wiring does NOT sound fun

Haven't gotten too far with the troubleshooting stuff yet, but in the meantime, I'm thinking I should be able to disconnect the heater winding from the first tube socket (again, not gonna be fun because it's buried under other wiring/components ) and see if it's still pulling excess current/popping fuses, right?

[Zozobra]

Yeah, just disconnect the heaters and insulate the ends of the wires. If the fuse popping stops then you've at least isolated the fault to something in the heater wiring.

[Tortuga]

Yeah, just disconnect the heaters and insulate the ends of the wires. If the fuse popping stops then you've at least isolated the fault to something in the heater wiring.

Done... no glory

Tried pulling the HT secondaries, same result - current limiter indicating a short in the remaining stuff, which isn't much...

Went ahead and pulled everything, including the transformer, and now I don't know how to check windings. Thought I'd check resistance through the coils, to see if this rings a bell with anyone...

Primary has 3 different voltage inputs:
* Blue wire is common
* Black is 120V - reads 8.1 ohm against blue common
* Orange is 220V - 15.5 ohm
* Yellow is 240V - 16.2 ohm

HT Secondary has two red wires and a red/yellow - according to the diagram, the red/yellow is the center tap
* red-red: 62.5 ohm
* red-red/yellow: 72.2 ohm
* other red-red/yellow: 82.2 ohm
(this seems strange - I would think the outer reds would be the larger resistance, and that each would be nearly equal resistance to the red/yellow center???)

6.3V secondary is giving me .3 ohm, which is concerning - however, it's a step-down, so I wonder if there's enough coil there to offer much resistance?

Called Mojotone - they are having me send all the details, and will see what they can do to help me. Unfortunately, since I'm not the original purchaser, I'm guessing this will not be a normal warranty thing, so I may be on my own.

Wiring diagram I've been using



All my power wiring work undone...



Postmortem

[Tortuga]

Found a YouTube guy that shows how to test PTs in old radios (pretty much same concept as what's in our tube amps)... he suggests using a signal generator to check the transformer secondaries.

So, I hooked up my handheld scope w/ generator, set it for 1V @ 60HZ, and let it fly...

...and, voltage was definitely picked up on the other side of both secondaries. That's the first possible sign of life I've gotten from this thing, and now I'm even more confused. More testing / reading to go, methinks.

[Tortuga]

...and, thinking about it some more, I decided to get some actual readings. I'm using 1.2V as the input (1/100 my wall voltage), to keep any math (somewhat) simple.

HT Secondaries
* red to red (outer pair) - 171.8mv
* red to red/yellow (outer to center tap) - 96.3mv
* red/yellow to red (center tap to other outer) - 75.1mv
...not sure this is what I should be seeing, as this is supposed to be a step-up - shouldn't it be a multiplier of the 1.2V input?

Heater secondary
* org to blk - 6.3mv
* org to brn (center tap) - 3.0mv
* brn to blk - 3.3mv


Thinking I may build an isolation bar for all the secondary wiring (to make sure everything is safely isolated and accessible for measurement) and connect my variac to bring this thing up and see what it reads at 10 volts, etc

[IAmTheKurgan]

Sounds like a Marshall!

Also it wouldn’t hurt to put shielding on the head cab under the chassis.

He should build a Faraday cage around it


I'M KIDDING, GRIMES!!!


EDIT: just saw the issues that you had with fuses and possibly PT. Fuck.... that sucks man. Hopefully its nothing too serious that can't be corrected topically.

[ajaxlepinski]

Guess #1
Maybe, you have a bad on/off switch? The switch could be burning the fuse by briefly creating a short when flipping it on?

Guess #2
The schematic indicates that the fuse should go between the on/off switch and the voltage selector.
However, you penciled in the fuse's location as between the mains wall plug and the on/off switch.
Electricity can do weird things... it's possible that the on/off switch isn't designed to have the fuse on that side of the switch.
Maybe, the switch is creating a brief short that the fuse wouldn't feel if it were between the voltage selector and the switch?

[Tortuga]

Sounds like a Marshall!

Also it wouldn’t hurt to put shielding on the head cab under the chassis.

He should build a Faraday cage around it


I'M KIDDING, GRIMES!!!


EDIT: just saw the issues that you had with fuses and possibly PT. Fuck.... that sucks man. Hopefully its nothing too serious that can't be corrected topically.

More updates below... pretty sure the PT is defective...

Guess #1
Maybe, you have a bad on/off switch? The switch could be burning the fuse by briefly creating a short when flipping it on?

Guess #2
The schematic indicates that the fuse should go between the on/off switch and the voltage selector.
However, you penciled in the fuse's location as between the mains wall plug and the on/off switch.
Electricity can do weird things... it's possible that the on/off switch isn't designed to have the fuse on that side of the switch.
Maybe, the switch is creating a brief short that the fuse wouldn't feel if it were between the voltage selector and the switch?

Good guesses, but I'm pretty sure it's not in the switch... I did some further testing after pulling the entire thing from the chassis and pretty much confirmed that the primary winding is the culprit. I'm still inspecting components & parts in that early part of the ciruit (power switch, fuse, heater wiring, bias), but I think everything in those areas were done right.

As for the fuse location (before or after the switch), I contacted Mojotone to ask them about it last week. It seems that the wiring diagrams all put the fuse *before* the switch (as God and Marshall originally intended), and they told me they appreciated me pointing it out, and would be amending the schematics to include that correction. The fuse and holder are there to protect the switch and most of the amp components - not the other way around

[Tortuga]

So, as stated, I did some more testing with the transformer out of the chassis, and have pretty much confirmed that the PT is bad.

I did reach out to Mojotone yesterday, and was greeted with some good news in my email this morning

[Marc G]

great of mojotone to sort you out like that even though you didn't buy the kit from them directly. Hope it's just the OT and the amp is up and roaring soon...

[IAmTheKurgan]

Wow that's pretty awesome of them!! I'm glad they took care of you.

[ajaxlepinski]

As for the fuse location (before or after the switch), I contacted Mojotone to ask them about it last week. It seems that the wiring diagrams all put the fuse *before* the switch (as God and Marshall originally intended), and they told me they appreciated me pointing it out, and would be amending the schematics to include that correction. The fuse and holder are there to protect the switch and most of the amp components - not the other way around

"as God and Marshall originally intended".... True that!!!

Glad Mojotone is doing the right thing!!!

[Tortuga]

Transformer has been sent... all indications are good that they'll take care of me

Really stoked about this - can't say enough good about their customer service. I'm gonna be a customer for life






So... a radical idea is hatching...

...gonna get the JMP-sized chassis and faceplates (chassis will ship with the replacement PT)

...rebuild the 2204 into the smaller chassis that I really wanted anyway

...either build or procure the JMP cab I really want

...sell the JCM cab and chassis/faceplates for whatever beer $$$ I can get

This way, I can incorporate all the improvements / changes I discovered during/after the first build and wish I'd have been able to do, as well as get a second build under my mitts, which I think is a really good thing.





...change my mind???

[IAmTheKurgan]

Transformer has been sent... all indications are good that they'll take care of me

Really stoked about this - can't say enough good about their customer service. I'm gonna be a customer for life






So... a radical idea is hatching...

...gonna get the JMP-sized chassis and faceplates (chassis will ship with the replacement PT)

...rebuild the 2204 into the smaller chassis that I really wanted anyway

...either build or procure the JMP cab I really want

...sell the JCM cab and chassis/faceplates for whatever beer $$$ I can get

This way, I can incorporate all the improvements / changes I discovered during/after the first build and wish I'd have been able to do, as well as get a second build under my mitts, which I think is a really good thing.





...change my mind???

It's certainly a crazy idea, but it just might work!!

[greatmutah]

Transformer has been sent... all indications are good that they'll take care of me

Really stoked about this - can't say enough good about their customer service. I'm gonna be a customer for life






So... a radical idea is hatching...

...gonna get the JMP-sized chassis and faceplates (chassis will ship with the replacement PT)

...rebuild the 2204 into the smaller chassis that I really wanted anyway

...either build or procure the JMP cab I really want

...sell the JCM cab and chassis/faceplates for whatever beer $$$ I can get

This way, I can incorporate all the improvements / changes I discovered during/after the first build and wish I'd have been able to do, as well as get a second build under my mitts, which I think is a really good thing.





...change my mind???

It's certainly a crazy idea, but it just might work!!

I support this.

[Hipster Salad]

Just putting this out there, the Jet City JCA800 is wedged in the JCA20 chassis

Oddly enough, I have more room to work with in regards to the JCA800 than my Ceriatone 2204. It was a super tight fit to put in another 12ax7 socket and effects loop on my Ceriatone, but I have plenty of ample space if I want to do the same with the JCA800.

[Tortuga]

Just putting this out there, the Jet City JCA800 is wedged in the JCA20 chassis

Oddly enough, I have more room to work with in regards to the JCA800 than my Ceriatone 2204. It was a super tight fit to put in another 12ax7 socket and effects loop on my Ceriatone, but I have plenty of ample space if I want to do the same with the JCA800.

Yeah - that Ceriatone looks like it's a tight squeeze.


The Mojotone layout on their small box chassis looks workable, though:



Should be plenty of room for me to locate the loop board above V1 & V2... might just pull the board and mock it up, since I can't really do much more until the transformer reaches Mojotone (Monday, according to the tracking I have) and we determine course of action from there.

Hell... I might get a right bug up my ass and start pulling everything so I can start building into the new chassis as soon as it arrives.

Gonna be ordering some things I'll be needing if I do this (in addition to the chassis, cage nuts, and chassis screws from Mojotone): wire (seriously considering solid core stuff this time), grounding lugs, tube sockets (don't want something breaking on me after the solder/resolder work), and more fuses, for starters.




BTW - thanks for the pics, Cale - that really helps me keep things clear in my mind.

Already cooking up a future amp build using the perf board and turrets like JCA did - love that kind of construction