Rotel RA-611 amplifier issue

[Tensop]

Hi guys,

about 6 months i picked up one of these old school amplifiers and its been running great ever since
However, ive been offered another one for $10 with an 'issue'
So i brought it home, had a look inside.. all seemed ok(these things are a pain to work with as there is shielding over all the PCBs excluding the output stage.. anyhow.. The amplifier powers up.. and works for around 5 minutes.. at which point Both channels start to fade away and as they fade the distortion goes up with it.. ive looked at some quickies and everything in it seems ok. i swapped over the traffo,caps, fed it an external preamp input to the power amplifier stage and it still does as such. voltage on the output measures the right voltage with a 20hz signal going in.. perfectly matched to my current amplifier.. they both have (almost) identical dc offset on the output.. so i'm assuming its the output transistors?


I thought it would of been heat yet it does the same thing if i'm feeding it 20hz full volume or feeding it nothing but low volume music...

weird

perhaps one of you guys could help?

 

[Netlist]

If both channels behave the same I’d start looking at the power supply.
Maybe not the main supply (looks like you checked it) but sub voltages, such as +/-15V lines for opamp’s (?) or so.

/Hugo

 

[wintermute]

Dry Joint?

Sounds like dry joint behaviour. Although maybe a bit unusual that its both channels simultaneously.

I'd be checking the solder joints around any component that generates a lot of heat, transistors, high wattage resistors etc.

Dry joints *are* heat related, effectively the hole the lead goes through (or is it the solder) expands more than the lead when it gets hot, and eventually the solder bond is broken, when cool there is enough contact to work, but once it heats up the gap appears and the solder joint goes open circuit.

Tony.

 

[Tensop]

I just noticed i wasnt bypassing the preamp stage completely.
the distortion is also being caused by their driving source, in this case... my Soundcard in my PC. it does it with both my creative audigy 2 ZS and the old peice of junk sblive. However with the working rotel amplifier(#1) it doesent do it. what's happening is the caps and surrounding tracks just before reaching the 3.5mm plug are heating up! weird behavior, perhaps current is being fed back into the source?

 

[Richard C]

Have you checked the psu voltages when the fault occurs? I agree with Netlist this should be the first test.

It may be due to a voltage regulator failing as it heats up.

 

[Tensop]

Problem fixed, there was a micro fracture running along the pcb between the transistors and their support circuitry, running the entire way along. i'd say it was stress on the pcb.
i used some laminex copper + solder to fix it

 

[wintermute]

Excellent ..... so not quite a dry joint but same concept, ie as it heated up it expanded and the tracks moved apart, breaking the contact. and probably the reason the tracks were heating up was the resistance being caused by the less than perfect joint where the fracture was.

Tony.

 

[Tensop]

Excellent ..... so not quite a dry joint but same concept, ie as it heated up it expanded and the tracks moved apart, breaking the contact. and probably the reason the tracks were heating up was the resistance being caused by the less than perfect joint where the fracture was.

Tony.

Actually it was a cracked pcb

The outputs on my soundcards have stopped cooking aswell