Ntc thermo switch?

[Off-Beat]

I am working on an ashdown evoii 300 ashdown bass magnifier.
According to the owner it cuts out after heating up, i for te life of me cant get it to cut out.

There is an 'ntc60 TH1' mounted to the heat sink. I have no experience with these, i guess it opens up after a certain temp to protect the output trannys?

How to i check it to make sure its working ok? When setting my dmm to continuity and reading across the 2 terminals it reads open circuit, is that my clue? Replace?

[Chuck H]

So, the amp is working but the thermal switch reads open? I think we need a schematic.

I found this schematic linked to another thread for that model. Not sure, but it looks probable. The switch you referred to is probably the fan switch. So if the fan isn't running, that switch will read open.

[Off-Beat]

I cant find the correct one but i have been using this
http://bmamps.com/Schematics/Ashdown...-schematic.pdf

[Chuck H]

Ah. Simulpost.

How are you attempting to get the amp to cut out? Dummy load and signal generator cranked up so the amp is making audio watts? Obviously it would be cumbersome to duplicate playing volumes. If it is that element causing the fault it could be a vibration issue. Try clipping your meter on with the amp playing and give it a whack while watching the meter.

[Off-Beat]

I had just put he radio through it. Tomorrow il play it with a bass guitar and back in its cab as i had it out in a cold room.

[J M Fahey]

In my schematic (EVO300/400/500) there is a normally closed thermal switch (SW1) in series with the speaker relay cutting it off only when its trigger temperature is reached.
Which is scorching water boiling 100C , WAY too high for any normal use (85C is the best compromise ) so I think it's a last ditch defense, when all else has failed.
Fan starts at 70C (thermal switch SW2) or runs almost always, but at a speed determined by NTC Thermistor TH1 , which should start around 60C and speed up with rising heat sink temperature and *should* keep temperature reasonable.

So for the 100C thermal switch to trip the heat sink must reach crazy 100C during normal use , so possibilities are :

1) non working/stalling fan

2) bad (always open) TH1
Does the fan run in your bench?
Try to trigger it blowing hot air onto TH1 .

3) working fan but air flow blocked.

I've had my share of "mystery/ghost" failures , never reproduced at the Lab, caused by guys tossing a leather jacket or a wooly pullover over the amp, partly covering ventilation slots or even external heatsinks.

Even once: an amp , playing in a small stage theater, always cut off with a full house, never with less people or on rehearsals ...... when full they fully pulled back the heavy velvet curtains so even the "bad seats" on the sides could fully see the stage and some folds reached and covered the amp back.

4) user applies some crazy loads, say 2 x 4 ohms cabinets (or even more) ... problem is amp *will* work, maybe 1/2 hour, then cutoff

5) player uses the foolish "Subharmonic Generator" , the speakers can't handle it (of course) , and show some weird impedance when driven 20/30Hz.

[Rant Mode] No high efficiency (MI) speaker can reproduce well below 80 Hz, a few reach 60Hz, all are panting and airless at 40 Hz ... and Ashdown adds a frequency halving circuit so now any 4 string Bass in the World reaches almost 20 Hz!!!!!!!! Madman of the Year !!!!!! [/Rant Mode]

EDIT: just saw:

I had just put he radio through it. Tomorrow il play it with a bass guitar and back in its cab as i had it out in a cold room.

Undistorted Radio or a few Bass bars won't be enough, owner uses it full blast on stage for hours.

You *might* have to be present at a *loud* rehearsal to see it happen (I've done that countless times).

At least: load the amp with a proper speaker simulating load, a plain resistor is "too kind" .
The simplest version is to increase loading by 50% , meaning using an 8 ohms load in parallel with desired 4 ohms , and so on.

A 4 ohms speaker will always be harder to drive than a 4 ohms resistor, because it's reactive and impedance varies all over the place, so an amp which works well driving a 4 ohm resistor can *still* overheat and cutoff if driving a "4 ohms" cabinet.

So kludge an "improved" load and drive amp at clipping with pinknoise, for at least 2 or 3 hours , serious manufacturers or Boutique makers do that overnight before shipping amps , they call it "burn in" ... sometimes literally

Mesa Boogie burning in:


Brown Note:


5x200W Cinema power amps

[Off-Beat]

Thanks for the info! The fan runs constant. When i check for continouity across the 2 terminals of the TH1 the dmm beeps. Its bad, right?

[J M Fahey]

Thanks for the info! The fan runs constant. When i check for continouity across the 2 terminals of the TH1 the dmm beeps. Its bad, right?

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"Continuity" or a "beep" has very limited diagnostic value .... measure ohms when cold and when heated with a hot air gun and then that means something.

By the way, I hope you measured "continuity" or ohms , with the amp OFF.

If ON , there's voltage applied there, enough to send your meter to the Lab in Heaven .

[Off-Beat]

Lol sorry im pretty new to alot of this. Yes with the amp off, it read less than 1ohm

[Chuck H]

First:

There is an 'ntc60 TH1' mounted to the heat sink... When setting my dmm to continuity and reading across the 2 terminals it reads open circuit, is that my clue? Replace?

Then:

Lol sorry im pretty new to alot of this. Yes with the amp off, it read less than 1ohm

For future reference the accurate meaning of "open circuit" would be no continuity. Infinity ohms. This confused me in the beginning. Especially when I read that the fan runs continuous.

Juan has the right of it. I think he may be looking too far into the extremes though. But not necessarily. If the fan runs full time once the amp is warm the power devices may indeed be running hot. Add to that the rigors of hard, but even normal use and the load thermal switch may be triggering.

And we still can't discount the possibility of a vibration problem. So do whack the amp if the thermal switch doesn't show itself before you get the chance. As a side benefit, hitting something that is frustrating you can be therapeutic as well

[J M Fahey]

First:
Quote Originally Posted by Off-Beat View Post
There is an 'ntc60 TH1' mounted to the heat sink... When setting my dmm to continuity and reading across the 2 terminals it reads open circuit, is that my clue? Replace?
Then:
Quote Originally Posted by Off-Beat View Post
Lol sorry im pretty new to alot of this. Yes with the amp off, it read less than 1ohm

Contradictory statements, both can't be right.

FWIW I trust the second, because the fan runs.

Open circuit would not.

[Chuck H]

Ah. I should have clarified that I'd arrived there by now

[Off-Beat]

Please excuse my ignorance towards those statements, i should be mor clear on what i think i know. The fan is running constant so yes obviously there is no open circuit. Am i rightin thinking the Th1 controls the speed of the fan? So its possible the fan is not speeding up when necessary?

[Chuck H]

TH1 is a component I don't recognize, but I'll assume that "T" stands for thermal and that the #60 on the component is a temperature. So yes, it probably controls fan speed. I don't think that the fan not spinning fast enough is the root problem. I think you need to check supply voltages and amplifier bias currents. Being SS impaired myself, I would need to research what should be abnormal current readings.

[J M Fahey]

TH is shorthand for THermistor meaning temperature variable resistor.

Since "variable" might run both ways,further clarification is needed:

NTC negative temperature coefficient means the hotter the less teresistance

PTC positive temperature coefficient means the hotter the higher the resistance.

FWIW most metals (copper included) are PTC and most semiconductore, silicon or germanium are NTC.

Am i rightin thinking the Th1 controls the speed of the fan?

Basically yes.

It's a very crude control, not linearat all, but roughly at higher temperatures the fan runs faster.

So its possible the fan is not speeding up when necessary?

Not "when" but clearly the fan is not enough.

Personally I'd either run the fan on all the time like many do (me included) , PC type fans are designed to run 24/7 or replace the Thermistor with a NO thermal switch, rated between 50 and 60C , so fan doesnot brun only in the studio or bedroom, where fan noise may bother but start full speed as soon as any real playing is done.

Or even the full speed fan is not enough: lint clogged filters or heatsink, amp rack mounted of a tube amp, parcially blocked air circulation, etc.

Just curious, post a couple amp pictures: guts / heatsink / fan / vents if applicable.