No bias voltage on Rotel 931 following change of output transistors

[fjadouce100]

Hi all hifi specialists
Following burned output transistors 2SB817 and 2SD1047 and cannot get original ones, i have soldered new ones 2SC2581 and 2SA1106. The problem is that I cannot adjust the bias VR and I got .04 mV only. I have verified all soldering which are ok. I get a rather good sound on my speakers I can say a better bass, but I do not want to risk playing at higher volume.
Grateful have your advice for a solution.
Regards
Alain from Mauritius.

 

[JonSnell Electronic]

Did you replace the driver transistors and the bias transistor as well?
Have you got 0v on the speaker output?

 

[fjadouce100]

Swapped transistors tests

Thanks for your reply. No I did not change any other transistors, but in the meantime I swapped the output transistors from the right side to the left side and now I can adjust the bias voltage to recommended 4 mV. But I get the same problem on the right side where I have just moved the A1106 and C2581. No bias adjustment possible.

 

[Mooly]

This is typical behaviour when transistors of slightly different characteristics are used. You are going to have to make a minor tweak to the bias controller (typically a resistor value change) to allow the required bias current to be set.

There is no danger to the amp running with low bias.

If you post an image of the relevant section of the circuit (the vbe multiplier and output stage) then we should be able to advise better.

 

[KatieandDad]

Have a look at LittleDiode – Electronic Components for All they have the original transistors listed.

 

[JonSnell Electronic]

If you replace R619/20 with a 1k 1/2W that will allow bias setting.

 

[fjadouce100]

I have checked the voltages on the output transistors. They are as follows:
Good side bias OK: Pair 2SB817 and 2Sd1047 Vbc and Vce=36V and Vbe=0.6V
Bad side no bias: Pair 2SC2581 and 2SA1106 Vbc and Vce=36V and Vbe 0 V
Is the 0V causing the problem? Sorry for that question I have only little knowledge eabout discreet transistors amp, in the past Ive been used to diy gainclones only.
thanks for reply

 

[fjadouce100]

I've just replaced R619 with 1k resistor still no bias. Will try to get an extract from the service manual soon from hifi engine. For info voltages on the speakers bolts outputs are approx 36mV on both L and R sides.

 

[Mooly]

I've just looked at the manual and would agree with Jon that tweaking R619 (assuming that channel) should allow the bias to set correctly.

You need to be very careful not to damage the amp and we normally recommend use of a dim bulb tester for any issues like this.

If you measure the voltage across Q613 from collector to emitter then you should see the voltage change as you alter the preset. You will need approximately 2.4 volts before any current will flow in the output stage. You can compare this voltage with the good channel but is just a guide to prove the adjustment control is working.

 

[fjadouce100]

Hi all, Here is the link for an extract of the schematic power section RA931
Extract Rotel RA931 — imgbb.com

 

[Mooly]

Thanks. Yes, that agrees with the diagram I came up with.

 

[fjadouce100]

One more detail: When I play CD music from low to moderate level the voltage across R629 0.22Ohm resistor is between10-20mV, but still no bias at idle. Can this info be of any help? Rgds Alain

 

[Mooly]

Music will cause a voltage to be developed across each 0.22 ohm resistor as current flows into the speaker. So that is normal to see that.

You are missing the static DC bias that is checked with no signal present.

 

[mjona]

One more detail: When I play CD music from low to moderate level the voltage across R629 0.22Ohm resistor is between10-20mV, but still no bias at idle. Can this info be of any help? Rgds Alain

Check the 220R emitter resistors for the driver transistors for conduction by measuring the voltage drop across these.

 

[fjadouce100]

Hi everyone. Have swapped the output trs in their original places. So lets as from now consider that the problem lies on the LEFT side.
Thank you Mjona: the voltages across 220R for both R625/627 are 0v . For the same measurements on the right side for R626/628 I get 0.56V.
I measured the voltages on the output TR as follows:
Left bad side: The Vbe voltage is 0V on both replacement TRs 2SC2581 and 2SA1106
Right good side: Vbe is 0.57 V.
By the way I have performed the Bulb test, every thing ok that is light glows rapidly and switches off immediately.
Why 0 volts Vbe on the new output transistors and solution if any?
tks to all for consuming yourprecious time.

 

[Mooly]

Try what I suggested in post #9

See what bias voltage you have available across Q613 and see how that compares to your good channel.

 

[fjadouce100]

Just to be sure: You mean that I place the test points on E and C of Q613 and move the VR to see any change? and compare the other one.

 

[fjadouce100]

Vce Q613 is 0.6V and does not move when I turn the bias VR. On the other good channel Vce is 2.34V and does move when bias VR is adjusted .

 

[Mooly]

So that shows you have insufficient bias voltage available and that it is not adjusting. That is quite an unusual problem tbh and one that will require great care to troubleshoot.

I would recommend you use the bulb tester at all times while working on this, and that you also (while working on the amplifier) turn the bias on the good channel to minimum.

Can you just confirm again (no need to actually do it again, just to be sure I understand correctly) that when you swapped the output transistors between the channels, that the fault moved with the output transistors ?

 

[Mooly]

Just to be sure: You mean that I place the test points on E and C of Q613 and move the VR to see any change? and compare the other one.

Sorry, I didn't answer your question. Yes, the voltage between C and E