Roland GA-120 bad transistor

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

CJ

Well-known member
GDIY Supporter
Joined
Jun 3, 2004
Messages
15,664
Location
California
one of Roland's best kept secrets, this rare amp with no online schematic what-so-ever, came out in between various Jazz Chorus JC-120 models,

quiet as a mouse, lush Reverb, a good sounding EQ (not like Mesa) instead of Chorus, two 12's, a wonderful clean sound with some compression, Toshiba fets and a plus/minnus preamp supply instead of the single supply in the JC circuits,

if you find one, snag it!  they also make a GA-60 which probably sounds nice also.
 

Attachments

  • ga1.jpg
    ga1.jpg
    155.2 KB · Views: 14
this amp was cutting out on a random basis, took a long time to figure it out,

problem was a pass transistor in the power supply regulator,

this transistor had base current that drifted up and down, to the point of shutting down the plus rail,  in 50 years of electronics, i have never seen a transistor do this. usually they are blown, or they work, maybe power transistors might get leaky, but this one was really weird,

base is on the right, so we had to flip a MJE243, which is used in Peavey amps, V-c = 100, I-c = 4 amps, hfe = 40 to 180,

not a lot of heat here so the metal does not have to be against the sink, just grease up the plastic,

 

Attachments

  • ga3.jpg
    ga3.jpg
    155.8 KB · Views: 15
no schemo, so what we did is take down every AC and DC voltage coming off the preamp and power amp boards while the amp was working, then, during failure mode, we re-measured until we found the problem area, then we disconnected the wire feeding the preamp and hooked up an external DC supply to the preamp and watched the current. the current did not move, so we looked up stream which was the power supply. replaced the Zener in the plus rail circuit, no joy so we pulled the transistor and did an hfe test, this is where we noticed the base current jumping up and down like a skeleton on a hot tin roof, game over.
 

Attachments

  • ga4.jpg
    ga4.jpg
    161.4 KB · Views: 12
we did trace out a bit of the GA-120 preamp, and it looks a lot like this JC-120 preamp, only the fet was a 2SK30 or something like that, the JC uses a 2SK117,  and the GA uses a +/- 15 supply.

Later JC-120's switched to a chip preamp scheme.

also shown is the pwr supply area with the bad transistor>

 

Attachments

  • jc.jpg
    jc.jpg
    110.3 KB · Views: 19
CJ said:
.... a wonderful clean sound with some compression, Toshiba fets and a plus/minus preamp supply instead of the single supply in the JC circuits,
....
could be similar to the Roland Cube-20/Cube-40:
http://www.freeinfosociety.com/electronics/schemview.php?id=1326
 
Back
Top