Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 11:26:47 AM

Login with username, password and session length

Recent Posts

 

Yamaha TA-20, wedge, 1968, ss amplifier

Started by Andy smithurst, July 13, 2010, 09:57:01 PM

Previous topic - Next topic

Andy smithurst

Hi' I have a yamaha 20w ss amp, yes the old wedge shaped one
with the strange planar type speaker.It's in excellent external condition,and sounds great,but dose have a bit of hum.It also has tremolo and reverb that work.The trouble is,after about 1hour from turn on, it suddenly gennerates loud horrific sounds similar to thunder and lightning strikes.This happens every time,even with nothing connected and all pots at zero.I must have spent 20hrs searching for a schematic for one of these but have only found promos.I recapped it,but problem remains.The only parts that get hot is a 120ohm 5w resistor,the transformer only warm.There are no IC's and the only diodes are 2, for the rectifier.I changed them as well to no avail.The 2 power transistors are the same NPN,(is this a linear design) I changed these with plastic substitutes,again everything sounds sweet until that 60min mark.this is definately a heat related problem. I've used a can of freezer on it, perhaps replace 120 ohm  resistor with a 10watt.?    :loco                         

DJPhil

Sounds like you've made a lot of progress already. I'm a bit of a noob, but I'll try. :)

Do you have access to schematics? What's that 120Ω resistor doing for a living (bias, etc.)? Is the racket from the amp after warm up volume dependent?

If it were me I'd start with the power side. You're probably looking for a bad connection or heat sensitive component, but it won't necessarily be hot itself and may just be responding to a temperature change from a nearby component. If you have freeze spray you can puff it about until you hear a change, and you'll know roughly where to start. I'd be starting in the bias network and feedback network.

Replacing the resistor probably won't help you much, it'd just treat the symptom and not the problem.

Hopefully that helps a little, perhaps gives you a direction to test. :)

Enzo

Thermal problems are not necessarily in parts that get HOT.  CAn be, but often is not.  As the entire thing warms up, the temperature of everything climbs, and the silicon in transistors warms up.   Just as you can have a 500v cap that is leaky at 450v but not at 350v, you can get a small part that is fine at 28C but not at 30C.  SO don't limit your search to things that get really hot.  Check them too of course, but check everything else too.

I don't know that model or have its papers, but you might call Yamaha in Buena Park and see if they do.  They will likely want to sell you a copy of the manual for it if they have it in their files, but really, $10-15 is worth it to not have to screw around.   Their manuals tend to be very complete, with board layouts and trace art, schematics, mechanical blowups, parts lists, and any service procedures like bias adjustment.

You have two power transistors, but I bet there are a bunch of small transistors as well.  Did you freeze everything on the board, or just the parts you felt were getting hot?

Jack1962

Call Yamaha Parts Department at 714-522-9958 , give the the info on your amp (serial # etc) and they will send you a diagram if they still have it .

                                                 Rock On