REPAIR SHOP 

 

Repair of Fender Frontman 15G

 

Fault - The amplifier was totally dead 

Removed the back cover and checked  the 500 mA PCB Fuse, this was not blown, checked the voltage at the input of power transformer at 240v but no output from the secondary at all, after removing the transformer and testing the primary winding it was reading  open circuit.

Found a substitute transformer in our second hand transformer box of the correct voltage 16v 0 16v, this replacement is slightly bigger than the original so had to drill an additional hole in the chassis as can be seen in the picture bellow, also had to attach the original wiring loom and PCB plug using soldered joints and heat shrink sleeve to the replacement transformer secondary. 

Attached the 2 original  input cables with the correct crimp connectors to the primary side cables using a 5 amp 240v terminal strip not very nice looking but safe !

Fitted the transformer and powered the amplifier up, but the 500 mA  fuse blew straight away, I suspected the main amplifier to be the problem which is a TDA2050.

The TDA 2050 is a monolithic integrated circuit in Pentawatt package, the TDA2050 is able to provide up to 35W true rms power into 4 ohm load and over 15w into 8 ohms.

Replaced The TDA2050 and the amplifier burst into life, however there was terrible mains hum with no input connected, the input jack socket should switch the input to ground with no jack plug inserted - this it was not doing causing open circuit on the input resulting in the hum problem , removed the jack socket and gave it a good clean with super 10 switch cleaner which cured the hum problem.

See below the jack socket removed for cleaning and the TDA2050 amplifier chip (a 5 leg TO 220 device)    

As can be seen from the picture bellow the TDA2050 is bolted to an aluminium bracket which acts as a heatsink - however experience with these amplifiers in the past tells me that they run very hot and require a good  heatsink, far be it for me to question the size of the aluminium bracket and its suitability for the job of keeping the TDA2050 cool but it does not really appear to be up to the job. 

Never let it be said that I leave things well alone.

(Please note the TDA 2050 Amplifier chip is not visible in the above picture as it is bolted to the front of the aluminium heatsink and is just out of view, some readers have mistakenly believed that the small transistor in-between the black and blue capacitor was the TDA 2050 however this is a JFET- part number J111and is not the TDA2050)

So in a vain attempt to improve things I retro fitted a heatsink with a liberal spreading of heatsink compound to the rear of the aluminium bracket - this should help things run slightly cooler. 

Re-assembled  and tested for a few hours - all ok

 

Conclusion - I suspect that the TDA2050 amplifier blew, blowing the fuse and someone replaced the fuse with a higher rating, this then blew the transformer, they probably then replaced  that with a correct 500mA  hence the un blown fuse in the amplifier when we first looked at it.

Also if you wanted to re-fit an original transformer at any time this would be an easy job to carry out, however the replacement we fitted is bigger than the original and all things being equal should prove more reliable and possibly give a slightly higher output as the supply voltage will not sag under load - or at least not as much..  

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