Peavey TNT 115 output reduction

[lowell]

After playing at gig volume for a bit the amp output keeps reducing. I'd imagine this is intentional in the design, but it seems extreme and is the reason it was brought in. "The amp volume fluctuates over time." I played it for a while and definitely found the volume to go down quite a bit. I thought perhaps the bias was too hot, but the bias is set with diodes, not a "rubber" diode and pot.

I noticed the triad and thyristor on the speaker output. And what seems like a jfet limiter on the PA input as well. I'm quite unfamiliar with how triacs work or how to go about testing it or monitoring what it's doing, and what it SHOULD be doing while the amp is heating up. The jfet seems to be controlled by the +16v rail, which is confusing. But I guess it's there to keep the preamp from outputting signal should the 16v rail go faulty?

Looking for any advice on how to go about troubleshooting this issue. Thanks!

[nosaj]

After playing at gig volume for a bit the amp output keeps reducing. I'd imagine this is intentional in the design, but it seems extreme and is the reason it was brought in. "The amp volume fluctuates over time." I played it for a while and definitely found the volume to go down quite a bit. I thought perhaps the bias was too hot, but the bias is set with diodes, not a "rubber" diode and pot.

I noticed the triad and thyristor on the speaker output. And what seems like a jfet limiter on the PA input as well. I'm quite unfamiliar with how triacs work or how to go about testing it or monitoring what it's doing, and what it SHOULD be doing while the amp is heating up. The jfet seems to be controlled by the +16v rail, which is confusing. But I guess it's there to keep the preamp from outputting signal should the 16v rail go faulty?

Looking for any advice on how to go about troubleshooting this issue. Thanks!

Make any difference with DDT enabled or not?
Does vol drop though the headphone output also or DI?

nosaj

[Enzo]

Before all the circuit stuff, have we checked the FX loop jacks and if it has them a preamp out and power amp in jacks? Also does it have an insert jack?

The output triac is there to SHORT across the output when fault conditions exist - hopefully to blow the fuse and stop the amp. It is called a crowbar circuit, and I doubt it is your issue.

[g1]

The jfet seems to be controlled by the +16v rail, which is confusing. But I guess it's there to keep the preamp from outputting signal should the 16v rail go faulty?

No, it's just a standard quiet turn on mute. 16V line is just a convenient place to sense power has been turned on. Q513 mutes the power amp till C539 charges, which should be long enough to prevent any turn on thump while the supplies stabilize.

[lowell]

Thanks for the input. Enzo anytime one of those jacks was an issue on other repairs the sound has completely cut out so I didn't think to check 'em. The output power/volume just slowly decreases over time while playing.

I'll check with/without DDT. Will also try to take some voltage/current measurements cold vs hot.

No, it's just a standard quiet turn on mute. 16V line is just a convenient place to sense power has been turned on. Q513 mutes the power amp till C539 charges, which should be long enough to prevent any turn on thump while the supplies stabilize.

Pahty

The output triac is there to SHORT across the output when fault conditions exist - hopefully to blow the fuse and stop the amp. It is called a crowbar circuit, and I doubt it is your issue.

Pahty

[g1]

Pahty?

[lowell]

Party slang... Rock n roll.. gotcha. Sorry for the confusion haha.

[lowell]

So I ran the thing at 100w into 4 ohms for about 30min. No issues whatsoever. No difference with DDT on or off. The output stayed consistent. The transistors went up to about 60 deg Celsius. The hottest one was 60...others were mid 50s. Mains at 100w was about 2.5A.

The tweeter is blown. Where do you all source tweeters, horns and diaphragms from? Also do you typically have to retrofit the replacement? I find alot of options for replacements but they usually require drilling new mounting holes and sometimes even a larger baffle hole. Thoughts? I cannot find this exact tweeter online and emailed and called Peavey yesterday but yet to hear back. Still wonder what your process is. I do what I gotta do to fix things... but generally prefer not to have to modify the equipment and steer it too far from stock condition.

[g1]

You won't see any DDT action with a constant signal, you need a burst or look at the attack of a plucked string.
Parts Express has a fair bit of tweets etc.

[Enzo]

Doesn't this just use a piezo tweeter? Like the old Motorola/CTS/whoever piezo with the three screws holding the motor to the back?

[J M Fahey]

DDT action is subtle:
rise input signal bit by bit (say: 100 - 200 - 300 - 400 - etc. mV) and output will proportionately rise, say 2V - 3V - 4V - and so on.
At a certain level, say 24V into 4 ohms (which you did NOT reach), output will stop rising even with increased input: that is the DDT Limiter vworking.
It *should* be something between 120/150W|RMS , the amp rating.

OR:

drive amp until output visibly clips, engage DDT, output should automatically lower a little so it barely clips-

In any case, below the trigger level, it must do nothing.

[lowell]

Thanks guys. I figured at a certain point the DDT would kick in...but was hesitant to put out the required wattage for fear of meltdown. Not sure the amp is designed to handle constant full output. Thoughts? My dummy load is 300watts.

Enzo yes it's a cheesy little piezo. It's blown so not sure of ohms ... Or wattage for that matter.

If the amp is 150w and the tweeter has the voltage divider before it (about 28ohms) that means it should be a minimum of 29% of 150 or 43watts... so 50 watts.

At 8ohms
8ohms/(28+8) = 29%

[The Dude]

Those piezos are very high impedance and no DCR. They don't have a voice coil. It's basically just a flapping crystal. You can't prove it bad with an ohm meter. First, check it by directly hooking it to an amp at low volume to make sure it's not something else. If it's bad, any piezo will do. They're all over the web. If you can't find the right physical size horn, you can often take the motor off of something else and replace just that part.

[olddawg]

If it's bad MCM has a lot of Piezos, cheap replacement speakers, etc.

[The Dude]

FWIW: MCM is now Newark.