Subway Blues hum problem - when did they sort it out?

[Zoner]

Hey BB'ers, I'm looking to pick up another Subway Blues but wanted to know how I might tell if this version is subject to the thermal "HUM" problem that my previous 2 had. I used to own a couple back in the day and they had the 2 stacked input jacks on the front - this one I am looking at has a mini switch for the Bright/fat and a single input jack on the front with a half-power speaker out on the rear. Does anyone know when they solved the humming issue and which version I am looking at - early or later? Thanks. Z

 

[Boogiebabies]

The hum mod was released on paper in April of 1998. It changes the .01 Cap off the potted module/PCB to a .68 cap. They refer to it as the Mojo Module.

 

[dropped_2_c]

The hum mod was released on paper in April of 1998. It changes the .01 Cap off the potted module/PCB in the power supply to a .68 cap.

how in the **** do you retain all this info, i'm jealous

 

[Boogiebabies]

I edited the mod. I looked at the mod and it's actually coming off the Mojo Module. I don't retain anything anymore, thats why there are schematics. If it's worth noting, I make note of it.

It's to the near right of the power supply caps off the potted square.

 

[dropped_2_c]

I edited the mod. I looked at the mod and it's actually coming off the Mojo Module. I don't retain anything anymore, thats why there are schematics. If it's worth noting, I make note of it.

It's to the near right of the power supply caps off the potted square.

what all boogie models or types do you own, and i do apoloigize to the original poster if i seem to be derailing the thread

 

[Zoner]

Boogiebabies, thanks as always for your insight. That said, If I'm getting a Subway with the "fat/Bright" switch, does anyone know if that is the early or later version? Do all Subway Blues versions have the epoxy enclosed components that lead to the humming issue or did they stop doing the epoxy thing at some point? I don't have the amp to pull the chassis and see or I'd look for myself. Z

 

[Boogiebabies]

I edited the mod. I looked at the mod and it's actually coming off the Mojo Module. I don't retain anything anymore, thats why there are schematics. If it's worth noting, I make note of it.

It's to the near right of the power supply caps off the potted square.

what all boogie models or types do you own, and i do apoloigize to the original poster if i seem to be derailing the thread

3 C+'s
1 1992 DR RF-1C Chrome/Black
1 RK II Version 2
1 MK III Green Stripe
1 MK IVB

Had -

RK 1
RK II version 1
3 Channel Dual Rectifier
Many 2 Channel Dual Rec's

 

[Boogiebabies]

Boogiebabies, thanks as always for your insight. That said, If I'm getting a Subway with the "fat/Bright" switch, does anyone know if that is the early or later version? Do all Subway Blues versions have the epoxy enclosed components that lead to the humming issue or did they stop doing the epoxy thing at some point? I don't have the amp to pull the chassis and see or I'd look for myself. Z

The main circuit came out in 1994 and the mods came out in 1998.
the Mojo Module from what I can tell is covered on the entire production.
In the original schematic it only shows a Fat switch for the tone stack where it adds a 750pf cap to the 250pf cap to give you a treble voicing of .001 or 1000pf. This is the typical fat switch on the MK II, III and IV.
Along with the 1998 Hum mod, there is a Mojo Module bypass mod where they take the signal of the end of the .01 cap and connect it the the orange wire between V2 and V3. It shows the mod using the original .01 cap intsead of the .68, so I take it that .01 is fine when bypassing the Mojo Module, but .68 would reduce the hum when keeping it.

 

[Zoner]

WTF is the "Mojo Module"? If this is part of the orignal circuit I have to imagine that bypassing it will alter the sound of the amp significantly. I guess I'll wait and see what I'm dealing with in terms of the build date and hum status before getting worked up, but if it means bypassing part of the circuitry (and losing the "Mojo") to avoid the hum that is a bit of a bummer, innit. Z

 

[Boogiebabies]

They are seperate mods. You can do either. If you want the hum gone, add the .68 cap. If you want to do the Mojo bypass you can do that instead. The Module is just a series of caps and resistors covered in goo to hide the components. Nothing really defining Mojo of the Voodoo type.

 

[Chrissmoth]

Hi Boogie-Dudes

I’ve got this problem – seems like a simple fix to change one capacitor & I’m reasonably nifty with a soldering iron – but what about all this potting in the way?

And for that matter – which potted section is the 0.01 cap under? I have two, one at the front (tone control etc side) & one at the back (speaker connection side).

Is there any easy way to remove this potting without damaging the circuitry underneath?

Has anyone got any experience in carrying out this procedure?

As an aside I’ve got a wire that appears to have been chopped off. it is connected to what appears to say GY2.3 and ends approximately 1” later in free space – any ideas?

This is the 2 input Bright / Fat type Subway Blues

Any guidance would be greatly appreciated.

Here’s a picture of what we’re dealing with:

http://i21.photobucket.com/albums/b294/vandalism/SubBluesBoogie.jpg

 

[Chrissmoth]

Nevermind - I found & replaced it No more hum

Here it is

 

[LooseChange]

Boogiebabies, thanks as always for your insight. That said, If I'm getting a Subway with the "fat/Bright" switch, does anyone know if that is the early or later version? Do all Subway Blues versions have the epoxy enclosed components that lead to the humming issue or did they stop doing the epoxy thing at some point? I don't have the amp to pull the chassis and see or I'd look for myself. Z

there is a Mojo Module bypass mod where they take the signal of the end of the .01 cap and connect it the the orange wire between V2 and V3. It shows the mod using the original .01 cap intsead of the .68, so I take it that .01 is fine when bypassing the Mojo Module, but .68 would reduce the hum when keeping it.

Hi,
My first post.

I would like to try both mods to see which one works best.
Can you give me more information on how to bypass the Mojo module?

Also, is this epoxied stuff on the distributed schematics? What is being hidden?
Thanks!

 

[Chrissmoth]

I tried the Mojo bypass whilst waiting for the new capacitor to arrive & it does work - I wanted to be able to use the effects loop - so I put it back how it was & replaced the cap when it arrived.

Hope this helps

 

[LooseChange]

Thanks for that!

It appears, looking at the schematics, that the mod jumps over the solid state effects send and puts the signal directly into V2b. So, my guess is the Mojo Module is the circuitry for the effects loop.. Is this right?
If it is, why are the potting it??

 

[Chrissmoth]

Yep - that's it.

When I bypassed the Mojo module I kinda thought that things didn't sound as good as they did before - that might just have been my imagination tho...

But the intense hum after about 40 minutes use was gone.

Sounds great with the replacement cap modification.

Lord knows why the potting is there - I've started hacking away at it this morning - i think it was just there to hide the components - not really serving any purpose - it's quite soft & comes away reasonably easy with a craft knife - just like carving beef - I won't recomend doing this til I get to the end & I'm not really sure if it's worth the effort.

Happy Easter by the way.

 

[LooseChange]

Let us know what you see under the goop. I think you will find exactly what we discussed. It's possible they gooped that part of the circuit to protect the delicate SS stuff... Sort of doubt it.

 

[carlsoti]

I've got a rocket 44 that hums, pretty much only on the clean channel. Anyone know if these mods will kill the hum on it?

 

[Chrissmoth]

Carlsoti

The Subway Blues hum is REALLY loud - not just the normal background hum that most amps have - so I don't think this mod is for you.

The way to quiet an amp is to use insulated Input jacks to seperate the signal ground from the chasis ground (although they are connected at just one point further down the line)

There's some good stuff about this here:

http://www.aikenamps.com/StarGround.html

 

[Ken j]

The later version of these did not have the potted section on the circuit board. Also they did not have the hum problem. New board pic: http://s188.photobucket.com/albums/z75/Ken_J_photos/?action=view&current=IMG_0614.jpg

There is another current thread in the Vintage section worth reading http://forum.grailtone.com/viewtopic.php?t=27632