EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: nigelwright7557 on January 09, 2020, 08:44:18 am
-
Recently bought in a Maplin 50 watt hi fi amplifier off ebay that wasn't working.
I replaced the missing obsolete parts with bits off ebay.
The amp sort of worked but was heavily distorted.
I checked all component for value and they all seemed fine.
The voltages didn't seem quite right around one of the transistors.
So I checked the transistors again using diode check and all tested ok.
It then suddenly hit me the transistor with the wrong voltages tested ok but was PNP instead of NPN !
It looks like a previous attempt to fix it went wrong.
I managed to get a new transistor off ebay and the amp now works fine.
Amps can be hard enough to fix but having wrong components fitted really makes things hard.
-
I remember that the 50W amp kit was documented in one of the Maplin magazines (as were all of their kits). I can't remember which issue, but you can find them all here... https://www.americanradiohistory.com/Maplin-Electronics.htm (https://www.americanradiohistory.com/Maplin-Electronics.htm) My guess is '80s.
Good hunting! :)
EDIT: Ahah, July 1993.
-
I remember the 50W Maplin amp very well, it was a pretty nice sounding amp but a flaw in the original design was the use of nylon nuts and bolts to mount the output transistors rather than using top hat insulators with steel fasteners. The nylon fasteners would fail, shortly followed by the output devices so if yours is still using the plastic parts get them changed out ASAP.
-
pics and schematic of the 50W Maplin amp. Nobody knows if it's a John Lindsey Hood design. He did the 75W version after.
I am familiar with the 75W MOSFET and 225W Disco kits.
-
pics and schematic of the 50W Maplin amp. Nobody knows if it's a John Lindsey Hood design. He did the 75W version after.
I am familiar with the 75W MOSFET and 225W Disco kits.
I have a few of each of those amps.
(https://i.ibb.co/RbZhrhh/maplin-3amps.jpg)