DYNACORD - SL1800 et DSA8209 limit problem

[mmm999]

Hello all
I would ask about two amplifier rack unit (SL1800 and DSA8209) that when I increase audio level, the limit led light, the relay open and close and the fan accelerate. so it repeat this cycle as the audio level is incresead. I share with you the schematic of other audio amplifier model but it share the same schematic part with my two amplifier
thanks for help

 

[barry]

When you overdrive the amp, the limiting circuit works. So? What's your question?

 

[mmm999]

not really overdive the amp, from 20% of audio level the amplifier make a sequence of ON-OFF-ON-OFF.... for fan, led limit, relay of audio output.
and the audio sound is not good

 

[KlausST]

Hi,

From all what you tell : it's overdrive.

Do you have (lend) a scope to verify the output level?

Klaus

 

[mmm999]

Hi KlausST
same think if I don't connect a speaker.
no I didn't make scope in output.
but for test I remove the high voltage powersupply (+60 / 0 / -60)
just I connect the low voltage (+15 / 0 / -15)
so when I rise audio level the limit light.
I changed the AOP name I100A (datasheet CL100-CP2200) but in my audio amplifier have other name (but saùe R111, R112, Q103...) because the sound when enter to I100A it is distored in output and the audio level change from 0 to fixe signal level (when i change volume the input change but output no). when i changed the audio quality is more good and I can change level audio. But I forget something, when I listen to continius BEEP (because I send 440 HZ sinus signal) the audio level rise and fall without I made any change.
for the same audio amplifier the other channel still not working, in first time the other channel have a problem, when no sound in input the output is in 60V. so the amplifier open the relay, light LIMIT and PROTECT, and switch ON fan for high speed, so I changed :
- Q356 (2N3904)
- Q357 (2N3906)
- Q315 (MJE340)
- Q314 (MJE350)
so now the voltage level is 3.9 V (not in zero) but not working when I put sound

 

[FvM]

I think, the description isn't very clear. You have two amplifiers that behave in the same erroneous way? The cases are unrelated except for the fact that they are both Dynacord devices with some circuit similarities?

Protection circuit will be usually triggered if the amplifier exposes DC output voltage to protect the speakers. In so far switching output relays can be expected with many PA faults. It should not trigger with healthy amplifier, even if it's overdriven.

Your attempts to fix the problem seem rather arbitrary, at least you didn't clarify why you decided to replace these parts.

--- Updated Jul 22, 2020 ---

It should be possible to distinguish fault in preamplifier/limiter and powerstage. But a PA fault may trigger the limiter and fam preamplifier failure.

 

[mmm999]

You have two amplifiers that behave in the same erroneous way? The cases are unrelated except for the fact that they are both Dynacord devices with some circuit similarities?

YES !

Protection circuit will be usually triggered if the amplifier exposes DC output voltage to protect the speakers. In so far switching output relays can be expected with many PA faults. It should not trigger with healthy amplifier, even if it's overdriven.

In general description, the amplifier is protect trigerred when : speaker exposed to DC voltage, high temperature, HF

Your attempts to fix the problem seem rather arbitrary, at least you didn't clarify why you decided to replace these parts.

I changed the transistors because it is shorted,
I change the operationnel amplifier because the signal in output is not correct