Yamaha TA-20 guitar amplifier.

[andy smethurst]

Hi. All.
My problem is with this: solid state1968 model Yamaha TA-20 Instrumental
amplifier:. It works lovely for the first hour, then starts to erupt into sounds of thunder and lightning strikes. My thirst thoughts were, the caps are 42 years old, so I replaced all electro caps. The symtoms are still there. I changed the rectifying diodes and tested all transistors, but can't solve this problem, which is strange as there isn't too many components in this old girl.
The main and output transformers don't get hot. There is a 120 ohm 10W wire-wound resistor that does get very hot, and is situated next to one of the metal transistors heat sinks. Where do I look from here? I would love some help on this subject

[Leopard]

Re: Yamaha TA-20 guitar amplifier.

It could be a bad solder joint, though an hour seems an awfully long time for solder-related problems to appear.

You could try replacing the resistor - odds are that its value has drifted from its' original value over the years anyway.

[stj]

Re: Yamaha TA-20 guitar amplifier.

sounds like a good job for a can of freezer-spray to me.

[midibob]

Re: Yamaha TA-20 guitar amplifier.

Hi Andy,

Not much info on this on the web, no schematics either. A few people have asked over the years but not received any replies. This usually means they don't go wrong (often) which also means any fault could be a stinker!

I'm making the assumption that this is probably all discrete transistors in the PA and possibly in a class B push pull arrangement. If so, you've got the luxury of being able to isolate the stage and prove whether the the o/p is breaking down or if it's in the pre-amp stages.
If it's a more complicated complimentary pair arrangement, which could involve numerous transistors and components, then you may be talking oscilloscopes and a serious bit of fault finding.

If the wire wound resistor runs hot from the beginning then I would guess that's probably not your problem, you never say never of course!
A tin of freezer , as suggested, would be a good start though.

Have you tried http://www.diyaudio.com/ as you might find some more assistance there.

Good luck
Midibob

[andy smethurst]

Re: Yamaha TA-20 guitar amplifier.

Thanks for the replies, I have allready used a can of freezer spray with no results there. I have tested the large resistor both hot and cold, and it is spot on 120ohm.
Someone must have experienced this symptom before, it's just new to me.

[midibob]

Re: Yamaha TA-20 guitar amplifier.

Andy,

Just had a thought, are there any pots in the output stage? As per my previous comments if it's a complementary pair arrangement then there will be a pot to set the quiescent current. Now, if it's one of the miniature black plastic types they do have a tendency over the years to go noisy. Sometimes just tapping them will produce the symptoms you describe. Best to swap them out as switch cleaner doesn't really work for long.

Good luck
Midibob

[andy smethurst]

Re: Yamaha TA-20 guitar amplifier.

Thanks for the advice Midibob, yes it does have quiesent pot, a large upright one with
a plastic thumb screw and a slot for adjusting. I have cleaned and tested it and seems fine. The symptoms show more frequently now,within a few minuites, and almost blow the speaker cone out of the core. I was wondering if this could be related to the output transformer having a few shorted turns, perhaps due to the insulation (varnish) breaking down, because of its age. I did change the old wire wound resistor for a new cement version, which dose'nt get as hot as the old one, but ofcourse this wasnt the problem.

[Jon B]

Re: Yamaha TA-20 guitar amplifier.

Hi,

Are you still working on this or did you get it fixed?

Jon

[andy smethurst]

Re: Yamaha TA-20 guitar amplifier.

I have put this problem aside until I find further information. Have you any?

Andy.

[Aruba]

Re: Yamaha TA-20 guitar amplifier.

Can you post some pictures of the inside of the amp. There may be signs on there to
suggest a closer look.

[Agent24]

Re: Yamaha TA-20 guitar amplifier.

I have an Akai stereo amplifier which did the same thing then went dead in the right channel.

It uses those integrated STK series amplifier ICs and one had died

Did you try freezer spray on the transistors? I would have thought they would be the problem...

[andy smethurst]

Re: Yamaha TA-20 guitar amplifier.

Problem solved!! As this project was put asside for 3 months to regain my confidence in life,I thought i'de give it one last go before sharpening the axe.I was going to draw my own schematic,but did'nt get too far. I know that someone had replaced the Bass pot on the board,could have been 20 or 30 years ago.This was obvious from the start and the first thing I looked at.It looked good besides one of the pads was missing,due to heat applied to the old pot I suppose, but as all the pots are soldered to the board the tabs are bent over and soldered to make contact. While looking at the solder side of the board, at where the newer Bass pot was soldered you could see clearly where the pad used to be, but this was soldered fine,but thanks to the green mask on the bottom of the board,to the right of the missing pad I now noticed a gap in the mask the width of a track, leading to a polly cap, so after cleaning and using a magnifier,it appears that the idiot that replaced the pot, not only removed the pad, but the track to the poly cap. This is what happens when you come accross equipment that someone else has had a go at,and no schematic exists, lots of heart ache and head ache and a waste of perhaps 100 hrs, and a loss of confidence.This is why most techs wont touch anything that someone else has had a go at. You live and learn,but I fixed it.

[Agent24]

Re: Yamaha TA-20 guitar amplifier.

Interesting. I would have thought that a missing track would cause the fault immediately, rather than after an hour.

Any ideas on how that would occur? Because I don't understand it.

[Raggedtelecaster]

Re: Yamaha TA-20 guitar amplifier.

Hey folks, I would love to find one of these Yamaha ta-20, 30 or 60's for my home rehearsal space. Can anyone advise where I might find one?
Thanks guys. Good forum you have here.
John Booth in Ohio USA

[andy smethurst]

Re: Yamaha TA-20 guitar amplifier.

Don't bother,I had other issues with the amp later on,you get this with 40 year old components,plus the old technology.The only good thing with these amps are the speakers and the box.What I did, is scrap the amp,and fitted a roland spirit 25A amp module to the original chassis for mounting purposes,(using angle grinder) and it looks great,and sounds ten times better,and has more outputs and inputs,with no white noise or any other issues.I should have done this at the begining,as life is too short.

[Agent24]

Re: Yamaha TA-20 guitar amplifier.

Anything can be fixed, the only problem is cost.