Power amp jumping into protect

[Joe bags]

Hi folks hope you are all well!

Got a power amp sitting on the shelf for a while now. Not sure what to do with it. It's a Dynacord S1200.
The problem is that it jumps into protect momentarily. I had it on test today just powered it up, no input signal or anything else going on. After about 15 minutes it jumps into protect for a second and then back to normal again. It seems to run fine after that.

Thought it might be dry joints on circuit board but I didn't find any.
Also thought it might be a leaky capacitor in the protection circuit.

Did a quick esr check on all electrolytics saw nothing strange.
Might have to look a bit closer at the protection circuit.


Anyone any thoughts on why this might happen?

Regards

J

[MarkusBass]

Might have to look a bit closer at the protection circuit.

It seems that you already know what to do. This is an amp with a quite complex protection circiut. When you understand it and perform some measurements, you'll know what is the reason for the problem. It is obvious that the protection circuit gets incorrect signals on the inputs. Common failure is caused by termistors that measure the heatsink temperature. If not them, the problem can be easily identified with few measurements of the voltages on the inputs of the protection circuit. Be aware that the heatsinks are not grounded and have voltages > 50V.

Mark

[km6xz]

As Mark said, it is a complex protection system and you have the schematic so your first task is to measure the operating characteristics to see if any are abnormal that might trigger the protection circuits. It is continously checking for DC offset, temperature, high frequency oscillation, current demands and SOAR (safe operating area) of the power transistors. Any of those sensed parameters could be triggering the protection circuit. If is the fan running on startup and then turns off after a few seconds? How often does the trigger occur and how quickly does it clear? Are you using loads when it happens and if so, does it happen when the loads are disconnected? If you monitor temperature of the heatsinks or main AC current, is anything unusual showing up around the time the problem occurs?

[Joe bags]

Thanks for the replies guys really appreciate the input.

I started off today testing the amp. It had the same issue as before. No load connected amp turned on, fans kick in for a second then cut out. Amp left to sit for ten mins then fans come on and she jumps into protect for like 2 secs then out again. Amp seems fine from then on.

So I started checking some voltages. I noticed the bias seems a little off.
A side reads 10.5mv and -8.3mv.
B side reads 10.2 and -8.9mv.

Also there was a difference in

Lim A sub = 464mv
Lim B sub = 472mv.

Then I noticed when checking the voltage on the Relais/protect pin the amp started coming in and out of protect.
I thought great I must be getting closer to the issue!!

Now the amp is in protect all the time and only comes out when I short the Relais/Protect pin the the adjacent speaker A or Speaker B pin.

Have to investigate now what this means.