Rotel 970 BX Repair

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#41
VR601 is a single turn 1K potentiometer. It's possible it's fried because Q617 is fried.

At this point I would remove Q617 and install a shorting link between it's collector-emitter pins. This will effectively reduce the bias through the output stage to 0. The amp should then work, but the sound will be distorted due to lack of bias.

Try this and see what happens. If it powers up (through the MBT) without smoke, you need to take some voltage measurements around the circuit and see where the problem lies. I would take the following measurements, respective to ground:

Emitter of Q613
Emitter of Q615
Emitter of Q619
Emitter of Q621
Voltage at TP1
The +VE and -VE supply voltages (collectors of Q631/Q633)
 
#42
Hi Jaycee. Apologies but not sure what you mean. Do I install another Q617 (only 1 left so will order some more!) and place a wire between the C and E pins to short it?

Also VR 601 has melted its plastic, please can you suggest an alternative to order?
 
#45
OK shorted pads of Q617 but VR 601 still smoking. Should I remove and replace with new part? Quickly tested DC at speaker terminals 45mv on left with burned transistor and 84mv on right. Not tested Emitter of Q613
Emitter of Q615
Emitter of Q619
Emitter of Q621
Voltage at TP1
The +VE and -VE supply voltages (collectors of Q631/Q633)

as suggested by Jaycee as not sure wht ground is at the moment.
 
#46
The black terminals of the speakers, for example. Or the black wire from the transformer secondary. Point E oder E2 on the PCB at the power supply capacitors.

Remove the VR601, it's defect at all. Not put the new one, because will burn also. Solder it in, when we found the bug.
 
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#50
100W is too big.
It allows too much current through.
Always start with a low bulb and after proving you don't have a disastrous wiring error you then step up the wattage of the bulb checking voltages as you go up.
the voltage getting through to the primary is very low and the resulting low secondary voltage
Click to expand...
I typically find that after the bulb goes very dim or off, that >95% of the mains voltage gets through to the primary. That means the amplifier sees ~95% of it's intended PSU voltage + a bit since it's still set for minimal output bias.

If however the bulb goes full bright I find that <5% of the mains voltage gets to the primary and the secondary is now down to <5%.
After deducting for the bridge rectifier, I find that the amplifier power supply rails can be around +-2 to 3Vdc.
Result no damage !
 
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#52
An incandescent light bulb is effectively a power resistor with a positive temperature coefficient, which means it's resistance increases as it gets hot. Higher wattage bulbs have a lower resistance overall.

The way this protects an amplifier or other circuit, is when the amp is functioning correctly with no signal, it is drawing only a small current. This isnt enough to heat the filament significantly, so the filament has a relatively low resistance. Nearly all of the power gets to the amplifier, and it operates as it normally would in it's idle condition. Good enough for troubleshooting.

However if the amp has a fault which causes it to draw alot of current, this current draw is enough to heat the filament. The filament's resistance goes up, limiting the power to the amplifier. Most of the energy is dissipated in the filament, so it lights up brightly and there is little energy dissipated in the amplifier - which stops it going up in smoke.

For a domestic amplifier I normally use a 40W bulb.
 
#53
Hi tried with the 40W bulb. Initially lights up and then goes dim within seconds. Have shorted the D617 pads as suggested but not had time to do tests yet. I will need to remove VR601 as it smokes. Will post results when I get them.
 
#55
Results

Emitter of Q613 = 1.087V (With negative dmm wire on negative speaker terminal)
Emitter of Q615 = -095.3V (“”)
Emitter of Q619 = 1.085V (“”)
Emitter of Q621 = -096.2V (“”)
Voltage at TP1 / TP3 0V (Trim pot removed as smoking)
Voltage TP2 / TP4 0V (Makes no difference if I adjust trim pot)


The +VE and -VE supply voltages (collectors of Q631/Q633) - not sure what this means.
 
#57
I kept the negative lead on the negative speaker terminal for Q615 / Q621 with the positive on the E of the transistor (all re ECB). Please let me now if I am doing this wrong. Also the fact that TP2/4 is OV and cannot be adjusted does not look good as this is the channel that is supposed to be OK.
 
#58
Results Corrected !

Hi, got it wrong. The speaker terminals are broken so was using the positive terminal! Double checked with other unit for the correct polarity.

Q617 shorted as instructed

Emitter of Q613 = 48.5V (With negative dmm wire on negative speaker terminal)
Emitter of Q615 = 47.6 (“”)
Emitter of Q619 = 48.4V (“”)
Emitter of Q621 = 48.4V (“”)
Voltage at TP1 / TP3 0V (Trim pot removed as smoking)
Voltage TP2 / TP4 0V (Makes no difference if I adjust trim pot)


The +VE and -VE supply voltages (collectors of Q631/Q633) - not sure what this means.
 
#59
for Tp1 (+DMM)-TP3(-DMM) you should measure about 8mV and for TP2-TP4 the same. Take also care of the polarity, for eample emitter Q615 should show a negative voltage.

Collector Q631 must show the positive supply voltage and collector Q633 the negative supply voltage.
 
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