Ibanez TSA-15H Power cuts out

mustang76

Member
2010-08-14 1:44 pm
I bought a used Ibanez TSA-15H. Great little amp and works really well. It has one fault though. The power will randomly cut out, i.e. the power indicator as well as all sound goes out.

This will normally happen while the amp has been on for a while, but it has happened shortly after power up. sometimes it works fine with no power cuts.

Any ideas? I was wondering if it might be the the regulator feeding the preamp heaters and power light that's failing? It also looks like it might have a thermal fuse on the mains section?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Without seeing a circuit its all guesswork. Can you observe the heaters going out when it fails ? What feeds the power light ? Is it an LED off an HT rail or is it a neon off the mains etc.

It could literally be anything at this point, even a loose wire/fuse in the mains plug.
 

mustang76

Member
2010-08-14 1:44 pm
I haven't been able to see whether the heaters go out, as it happens relatively quickly and normally comes back straight away.

The power indicator is an LED powered from circuit that powers the preamp heaters and the Tube Screamer circuit.

I have attached a copy of the schematic which should help.
 

Attachments

  • TSA15HTSA15SCHEMATIC.pdf
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Mooly

Administrator
Paid Member
2007-09-15 8:14 am
OK, so the power LED going out is down to the output of that regulator disappearing.

It could be a problem local to the regulator (a dry joint etc). The acid test is for you to observe/measure whether any of the other rails disappear at the same time. If so then its a transformer/mains input problem. If not then it is local to that regulator.

If the 6V6 heaters stay lit its a local issue. If they go out its a true power supply problem on the transformer/mains side of things.

If the problem is so intermittent that you cant measure anything then you could rig a test LED + resistor up and connect it across C40 in the power supply. That would stay lit if the fault were local to the 12 volt regulator.

Its worth looking at that 12 volt regulator and making sure there are no dry joints.
 

mustang76

Member
2010-08-14 1:44 pm
Thanks for the replies and suggestions.

I have used it with a few other power cords, and the problem persists, so it likely isn't a cord problem.

OK, so i had a chance to test and see if I could replicate the fault again. I had it set up for about 20 minutes until it cut out. I could clearly see that all the heaters were off, preamp and output valves. Fortunately it didn't come straight back on, so I was able to check.

Switched it off and back on again, and it worked fine again for at least another hour without issue before I turned it off again. This shows how intermittent the problem is.

From Mooly's comment, it does seem that the problem is the mains side of things. Not too much in there, so it narrows it down to, transformer, power switch, fuse, thermistor (?), AC inlet socket and connections...

What would you suggest be the best / first place to start looking?
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
It could be anything from the wall socket to the transformer itself, but most likely:

1/ There is a thermal fuse shown. Is that a automatically resettable type ? If so then it could be tripping out. A simple AC voltage measurement across the device would prove beyond doubt if it were faulty.

2/ Do a thorough visual check of all connections and the security of the fuse holders. A one in a million fault like this can be caused by a faulty fuse whereby the fuse wire is not secured to the fuse end cap internally. I've come across that just once in literally 10's of thousand of repairs when working as a service tech.
 

Pema888

Member
2016-08-22 9:31 pm
It took a while, but finally got the time to follow up. Meanwhile I used the amp many times and sometimes the issue is there still ... Totally unpredctable.
I openened the amp and found the NTC thermistor who most likely is the cause. After tearing of carefully the black shrink foil I could read the item code: it is a SCK 253. Will try to find and order it and go from there.
On other forums I read that people with similar issues on similar amps solved it by putting a wire instead of the NTC, commenting this thermistor is not needed. Well, so far I trust the designers of this amp put this in for a safety reason.

PS: on the biggest trafo i noticed a label saying "built-in thermal fuse" but i concluded this one must be of the "single use type"; once blown no automatic reset, while the NTC on the board is of the resetting type:rolleyes:

I keep you posted .... Let me know your comments in case of relevant news, please!
 
Hello, just wondering if there were any fixes found for this issue. I have the same amp and it has the same issues. Thanks!

TSA15 Combo Amp: Working on mine now... happy for some but in my case it's def not the Poly Switch (TH1) as i jumped it and it still cut out.

now i'm chasing down a possible bad fuse... F6 (and/or holder) is a acting a bit suspect. If that doesn't do it the next process will be to chase dawn a bad ground connection somewhere or a bad solder joint...

not my first go over with this problem... :boggled:

:sigh: alas the woods! is there no sky!