do anyone have idea why the repair man mod this amp like this

[kwm488]

hi

this is my friend 1968 marshall superbass. his amp get problem, i help him repair. but after i take out the amp. OMG. it is crazy. 8 pc of filter cap. why? and this amp no standby function. the standby switch make the sound louder only.

[Ritchie200]

Well, they have bypassed four of the original caps and just did a lazy repair.  I have seen these kinds of repairs on amps where they want to maintain an original look from the outside - showing the original caps.  However, it looks like they already changed out one of the "exposed" ones, so why not do the others as well?  I'm not sure on this one.  What problems are you having with the amp now?

Jim

[kwm488]

hi

the main 2A fuse keep blow. last week, my friend buy a new 3A fuse for replacement (because my friend can't find 2A). the result is the 3A fuse don't blow this time. when he inform me, i think the PT should blow. lucky, the power cord 3A fuse blow this time. normally, power cord should use 5A fuse.

i find out 2 of power tube will get very red color and fuse will blow. i remove 2 demage power tube, just use 2 tube now. it is fine. will keep update here. BTW, the power tube just use 1 year only.

but i still don't understand why this amp no standby

[bigsbybender]

but i still don't understand why this amp no standby

It doesn't appear that the Standby has been changed to be another function (although I could be wrong).

The Standby switch may be bad. It may have arced internally so there is always some connection (with some resistance). When you flip the switch to "On" then you would get full voltage and then the amp is louder.  Check that switch with an Ohm-Meter. Make sure there that it reads "Open or Infinity" when the switch is off.  If you get a reading in ohms the switch is shorted inside and must be replaced.

[kwm488]

but this standby switch is new. i just purchase it from metroamp one year.

[kwm488]

also, i will help my friend correct this amp back to marshall original circuit . how can i know the filter cap is dead? i have cap checker.

and i will replace all resistor to takman
http://www.partsconnexion.com/resistors_takman_rex_half_watt.html

some people told me new resistor technique is better than old resistor. before, i use RMG resistor to build my marshall jcm800. i love it much too much

[jim]

You have some work to do, my friend.  Looks at these pictures right off the Metroamp site:
http://plexireplicas.com/metropoulos-amplifiers/12000-series/
You will need to make a small circuit board like this and mount brand new filter capacitors with values similar or slightly larger than
the schematic shows.  Forget about the resistors--They are probably good, just check your screen resistors and power dropping resistors.  Then you will need to rebias the output tubes.  Use the fuse values specified in the schematic and nothing else.  I am sure that when you untangle that mess of wires you will also fix the standby switch problem.  Be careful, man..... Jim

[kwm488]

sorry, why don't replace all 50+50uf filter cap directly? i think this metroamp is another marshall plexi .

[Fresh_Start]

+1 on leaving resistors alone.  You are not doing your friend a favor by replacing parts wholesale without a good reason.  If a resistor shows signs of over-heating or its value has drifted a LOT, then you replace it.

If you were to replace them, $0.75 for carbon film resistors is silly.  Metal film resistors are quieter than carbon film.  These are available for $0.10 each at Mouser:
http://www.vishay.com/docs/31068/ccf60.pdf

Do you have a schematic for this amp?  You need to learn how to read a schematic.  You also need to learn more about troubleshooting techniques:

http://www.el34world.com/charts/fenderservice6.htm

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

Hope this helps,

Chip

[jim]

I was just suggesting that you mount the new caps in a similar manner like the Metroamp photo.  You want to place individual caps for  each stage instead of using cap cans.  You will get better stage isolation , stable voltages, and less hum if you avoid cap cans.  Leave the original cap cans in place (unconnected) so that the amp looks stock and mount the circuit board for the new caps inside the chassis
on the underside of the cap cans.  You could also mount the caps on terminal strips if you do not want to make a board.  Jim

[HotBluePlates]

and i will replace all resistor to takman
http://www.partsconnexion.com/resistors_takman_rex_half_watt.html

some people told me new resistor technique is better than old resistor. before, i use RMG resistor to build my marshall jcm800. i love it much too much

Why would you rip out a bunch of Riken Ohm resistors (which might cost $2/each or more), just to buy more resistors to take their place? I am not suggesting that the $0.70 Takman resistors are inferior, but why bother spending the extra money? It is unlikely that the change will make a noticeable difference in the amp's sound.

[kwm488]

hi
thank everyone advice

i spend 10hr to fix it amp. first, i rewire to correct superbass circult. second , change it to superlead.
i check all resistor, some is dead, will just replace it to takman. change all cap to mustard (like marshall original).

now, i order some NOS erie bias cap . will replace it , add PPIMV and send return kit

[kwm488]

and the photo with red color PTP board, which is my vintage 1978 jcm800. i mod it from PCB to PTP. of course, i will not change the RMG resistor, i think it is the best resistor. but it is discontine