Phonic MAX 860 problems...

[Chrille]

Hi,
Does anyone have experience of this amp? I've spent ours trying to repair this amp!
I have several Phonic MAX 860 amplifiers and they ALL have the same problem, more or less!
This is the thing:
If I haven't been using the amplifier for a while it goes in to a "protect mode".

I'll try to explain:
When I power on the amplifier everything seems to be Ok. I connect a 8 ohm dummy load to the outputs. All still Ok.
I connect inputs to a signal source and play some music on low volume and everything is still Ok.
But when I increase the volume a bit, (at least) one channel goes in to some kind of protection mode. The peak led I lighting and the output turns of.
Now I have to turn down the volume and wait a while. Sometimes it "comes back" pretty quick and sometimes I have to wait several minutes or longer.
If I repeat this a few times, and carefully increase the volume until the amplifier goes in to clip (max), the amplifier works perfectly!
Now I can play forever and drive it to clip constantly without any problems at all! I can turn off and on and play. All Ok. I can leave it powered off a few days and power on and use it and it works perfectly.
Now if I don't use it for, say a month or so, I'm back where I started, i.e. the amplifier goes in to "protection mode" again and I have so "soften it up" again.
There is no loose contacts or noisy sound anywhere. No scratching sound in the potentiometers at all. When it work, it works 100%

Does anyone know about this?

/Chrille

[mozz]

Have the filter caps been checked? Can you check wattage draw?

[g1]

Here's the service manual:

[Chrille]

Have the filter caps been checked? Can you check wattage draw?

Hi,
Thanks for your reply.
I guess not. What capacitors are you thinking of?
Sorry, but what do you mean with "wattage draw"? Output power?

/Chrille

[mozz]

You need to have them looked at by a technician.

[Chrille]

You need to have them looked at by a technician.

Yes, that's where I come in
There are lots of capacitors in an amplifier that could be seen as "filter capacitors", so if you think of any particular, could you please point out which you mean?
(The schematic is attached by g1 above.)
I can certainly "check the wattage draw" if you explain what it means. (My technical level/experience is high but my lingual level might be a bit limited)

/Chrille

[alexradium]

it could be DC on output,check it out on relay IN when you crank the volume

[Jazz P Bass]

Agree.

Either the protect circuit is doing it's job (sensing Volts dc on the output) or there is something wrong with the protect circuit itself.

[Chrille]

it could be DC on output,check it out on relay IN when you crank the volume

Hi,
there is no DC on the output (more than 20 mV) and i guess that must be considered as normal.
Worth mentioning is that the output relay is not releasing when this "protection mode" occur, so it's probably not related to DC on output.
This also means that I can hear a heavy distorted signal on the output, but the signal is very low.

[The Dude]

Since it seems heat related, try some freeze spray. See if you can get it to come or go spraying any particular component. See if anything is getting hot.

[g1]

page 9 & 10 show the protection circuitry, one for each channel.

[Chrille]

Since it seems heat related, try some freeze spray. See if you can get it to come or go spraying any particular component. See if anything is getting hot.

Hi,
No, it's definitely not a heating problem. As I wrote, when I once have got the amplifier to work, it work 100% every time I start it, even if it has been powered off for several days. It's just when it has been unused (powered off) for, say a month or so this problem/phenomena occurs.

[alexradium]

yeah,smells like a capacitor,wherever it is...

[Chrille]

yeah,smells like a capacitor,wherever it is...

Yes, it sounds like a good idea... The "problem" is that I have to wait so long until I can reproduce the error again!

[Jazz P Bass]

Can you access the protect circuit to monitor what is happening?

I would go to the junction of R406 & C408.