rel q50 sub amp problem!

[surfstu]

hi,

I have aquired a q50 sub. It powers on but does not behave and makes strange noises, sometimes loud - regardless of volume setting. The power light on the switch fades in and out of brightness rapidly. The amp looks simple enough but I am a newbie to this. The amp consists of 2 x tip3055 power transistors and another two sm3177A's and 3 other IC's. I could not find a schematic and hoped someone wiser than me may be able to inform me of what to start checking with a voltmeter?
thanks in advance
stuart

[surfstu]

anyone? there must be some simple checks i can perform here?
stuart

[surfstu]

maybe this is the wrong forum? should i be in the subs forum? have decided to unsolder the op amps and test. i hoped someone might be able to tell me if the flashing power light is a charecteristic of any particular component blowing?

[nigelwright7557]

Originally Posted by surfstu

hi,

I have aquired a q50 sub. It powers on but does not behave and makes strange noises, sometimes loud - regardless of volume setting. The power light on the switch fades in and out of brightness rapidly. The amp looks simple enough but I am a newbie to this. The amp consists of 2 x tip3055 power transistors and another two sm3177A's and 3 other IC's. I could not find a schematic and hoped someone wiser than me may be able to inform me of what to start checking with a voltmeter?
thanks in advance
stuart

Could be as simple as the power switch not making contact properly.

[surfstu]

thanks Nigel, i'll bypass the switch tomorrow and see how it goes, any idea of the next step I should take in fault finding?

[nigelwright7557]

Originally Posted by surfstu

thanks Nigel, i'll bypass the switch tomorrow and see how it goes, any idea of the next step I should take in fault finding?

Go for the simplest and cheapest things first seems a good approach.

Check your switch first, after that things might get complicated.

[surfstu]

tested switch, it is fine... tested the complimentary pair of transistors tip3055 and tip2955, checked and there is -50 V and +45V from the bottom of the two large capacitors. I presume that one of these is the rail voltage? or B+? so where would be the next logical place to look? I am still getting flickering power light and when i add any volume, it just pops and cracks?

[Sebastiaan]

Dear,

If memory serves me well, I ever repaired a REL Q50 with the same problem. The problem in this case was located in the rotary switches. Those copper contacts inside the switches are oxidated with as result sometimes a bad contact to from wiper to ground. This gives loud "bumps". the LED in the pwoer switch dims because of the huge current suckout those "Bumps" give. Those switches are cheap parts. You can order them at any general part store. Replace them. U can verify it are the switches, but rotating them up and down. If u get the bumps then, you can be almost sure.

Good luck.

With kind regards,
Bas

[surfstu]

well, i've extracted the volume pot altogether and I am still getting a highish frequency dimming of the power light, should I try taking out the cutoff pot next? anyone with any further suggestions into what fault finding procedure i should try next would be much appreciated, it seems like such a simple amp!
stuart

[surfstu]

what could be causing such a dramatic shift in current drain?