Mark III - Picture of inner workings + question

[rippingrudy]

Hi there,

This is a picture of the guts of my Mark III green stripe. Can someone tell me what the filter caps are? Are they the big blue components? Also, what are the orange ones? BTW, the date is April/1990 and seems to be signed by someone named Roth or O.Roth.

Is there any way to tell if filter caps need replacing by looking at them? Any help much appreciated.

Thanks,
Chris

 

[zool]

The big blue ones are the three on the left and the two on the right.
The larger 220uf actually has two more under the board. The noticable signs of damage are yellow goo leaking out of them. Some people say to change them every 10 years, but it's up to you. There are tons of original MK I's running original filter caps. The Orange ones are coupling capacitors to take the signal off of the preamp tubes. They basically make the AC signal ride on the DC voltage. They usually do not go bad unless they dry out and let moisture seep up the leads.

This amp looks pretty tight.

 

[zool]

Oh, it has the reverb mod too.

 

[stokes]

The filter caps are the 2 large blue "cans" on the right and the 3 on the left.Cant tell by looking but a rule of thumb by a lot of techs and I follow this ,is that after ten years they should be changed.There are a lot of people who will say they should only be changed when they exhibit signs of wear.The problem I have with this are that when caps go they often just explode and create a big mess in the chassis.If they fail suddenly they can cause other damage in the amp as well.I have seen power trannies short out from leaky filter caps causing an increase in current in the poer supply.I change the caps in my amps,and I have a dozen here in my house and a few my sons have in thier places,after 6-8 years.I have had caps explode and it is a pain in the *** cleaning that mess up.And any time I put new caps in my amp there is always an improvement in performance,the most noticable being much tighter bass response.If you do decide to change them be sure you dont use the jap crap,which is just about all except the Sprague or F&T's.I use the F&T 220mf's for the big ones on the right in your picture and Sprague's for the others.

 

[rippingrudy]

Reverb mod? That's news to me.

What exactly is the reverb mod and what does it do if you don't mind me asking?

 

[zool]

It increases the amount of reverb by adding resistance..
If you follow the red wire in the center, it goes to a 3.3K metal film resistor and the bottom is wrapped in shrink tubing.

Look on the back of the amp and you should see a red R in sharpie.

 

[rippingrudy]

There is a "R" in red marker with "MB" underneath it (also in red marker). Would they do this mod before it sold or did a previous owner send it in to have it done?

Thanks

 

[Joey707]

The "R" means the amp has had the reverb glitch mod. The glitch mod involves putting a 120K 1 watt resistor from the red reverb transformer lead to ground. Also R 141 should be a 5.6K 2 watt. If it isn't it should be changed. The reverb glitch is a slight distortion in clean mode caused by the reverb.
The other mod mentioned does increase the amount of reverb. But the red "R" written on the back of the amp means it has had the reverb glitch mod not the reverb boost mod. The reverb glitch mod is something that should be done to all MK III's that aren't already corrected. At some point they started making them that way at the factory but I don't know when exactly.

 

[rippingrudy]

Mine is dated August/1990 - wonder when the last production year was?

Thanks for the info.

 

[psychodave]

The filter caps are the 2 large blue "cans" on the right and the 3 on the left.Cant tell by looking but a rule of thumb by a lot of techs and I follow this ,is that after ten years they should be changed.There are a lot of people who will say they should only be changed when they exhibit signs of wear.The problem I have with this are that when caps go they often just explode and create a big mess in the chassis.If they fail suddenly they can cause other damage in the amp as well.I have seen power trannies short out from leaky filter caps causing an increase in current in the poer supply.I change the caps in my amps,and I have a dozen here in my house and a few my sons have in thier places,after 6-8 years.I have had caps explode and it is a pain in the *** cleaning that mess up.And any time I put new caps in my amp there is always an improvement in performance,the most noticable being much tighter bass response.If you do decide to change them be sure you dont use the jap crap,which is just about all except the Sprague or F&T's.I use the F&T 220mf's for the big ones on the right in your picture and Sprague's for the others.

Great post.

 

[stokes]

Thanks,Dave.

 

[zool]

Mine is dated August/1990 - wonder when the last production year was?

Thanks for the info.

The last of the MK III's were produced in 1998. All post circa 1990 and up were Green Stripes.