Help Needed: Excessive DC Offset/Imbalance - Rotel RB-990BX Power Amp

[sakis]

Hei Mooly if you are so ""willing "" with simulator i think i will send you a few circuits to simulate for me ( second opinion is always something to listen to )

I have a few ideas id like to share but i am not that good in simulators
I have an example also that i would like to talk about Can i Pm you ?

Sorry for the off topic

Kind
regards
sakis

[Mooly]

PM sent

[dtm1962]

So guess what?

The PNP 2SA1142 measures as a NPN!!! Via Diode test and hfe test... D*MN!

Since the original NEC 2SA1142 is a green body unit.....and this was a black body unit....I should have noted...

It is a rebadged NPN from China.... D*MN!!!!

It just didn't seem like it was possible being a basic cheap transistor..

It teaches me a lesson to check each one before I put it in the circuit.

I just didn't see this coming! I'm mad at myself!

Lesson learned...

Just Move on....

I will let you know when it is all back together & working...

[dtm1962]

Originally Posted by Mooly

Thats brilliant news

Before we jump in and rebuild it all there should really be another test Its up to you if you want to do this but whether or not it is important that the bias is set to minimum before the output stage is re-connected and powered up.

The test is to confirm the vbe multiplier is working correctly as this is vital to ensure the output stage doesn't draw excess current (and I can't see there being a problem here). All this test does is confirm that the voltage across Q617 varies smoothly and correctly as the preset is turned. The 2SD1953 is an "oddball" darlington type and I'll be honest and say I'm not 100% sure what you might see here. The range of adjustment seems very small looking at the circuit values around Q617.

So what we do is this,

1. Remove the link across Q617. (Leave the 22K feedback resistor still tagged to Q617 C or E).

2. The preset VR601 is adjusted for maximum resistance. This means that the resistance between B and E of Q617 is at its highest (about 8.39K... the 6.19K + 2.2K added together).

3. We now need to monitor the voltage across Q617 C and E and in order to do this safely and with no possibility of slips I would tag two wires to Q617 and extend them out to your meter.

4. Switch on and observe the voltage as the pot is turned. It should vary from around (and this is where I am unsure) from around 2 volts to perhaps 4 volts or so. Record and post the result of the max and min voltages

5. If that is OK then return the pot to give minimum voltage across Q617.

So I performed the above test and here are the results:

Minimum voltage across Q617 is 3.25V & maximum is 3.68V when adjusting the pot.

22K resistor attached to Q617 E

3.2V drop across Q617 C & E
1.2V drop across Q617 B & E
2.0V drop across Q617 B & C

Relative to ground:
Q617 E is 0.2V
Q617 B is 1.4V
Q617 E is 3.4V

Question:

If the feedback resistor is not connected to Q617, I do get voltages near +Rail voltage (~3-4volts less than +Rail voltage)
for all 3 measurements with the .6V across the junctions (BE & CE etc.).
Is this normal or does it point to other remaining problem.... other than the
so called Counterfeit PNP 2SA1142 which is actually a NPN.

Let me know what you guys think

Thanks

[Mooly]

Ohhh a counterfeit That explains a lot.

See post #128 here (and the whole thread)
My Transistors, original or copy?

As to testing the amp and the feedback resistor connection. The feedback resistor is essential to establish the DC conditions in the amp. What you have with all the front end circuitry is really a discrete "opamp". There are two inputs, the bases of Q601 and 607 and the bases of Q603 and 609. These are the "non inverting" and "inverting" equivalents respectively to an opamp. From only the feedback resistor and R613 we can calculate the gain of the amp.

Operational amplifier - Wikipedia, the free encyclopedia

Your voltages seem "OK" in a broad sense. tbh I would have expected Q617 E (the point where the feedback resistor connects) to be a little nearer to zero.
Although 0.2 volts is low, if that ends up as a DC offset at the output of the amp then it's not really low enough...

I think at this point we say its OK. The amp is missing the "slow" output stage and its conceivable that there could, under the present test conditions, be some small HF instability upsetting the DC readings.

Lets assume its good to go


If the feedback resistor is missing then the voltages on Q617 will swing toward the rails as you have found. So no problem there. Its the voltages across Q617 that determine the bias current in the output stage rather than absolute values above and below ground. The range of adjustment seems a little small as I suspected, but thats down to the resistor values and could equally be called good design as it provides fine adjustment in the right "zone". Nothing worse than presets that are all or nothing and where a degree or two of rotation cause the current to jump wildly.

So set the preset to the end that gives the lowest voltage across C and E of Q617

What I would suggest before initially powering up after replacing the parts is to check carefully that all parts are correctly fitted and that all the caps that you lifted are replaced. Also don't forget to refit the feedback resistor back to its original location.

I would power up the fully rebuilt amp (with the bulb tester of course) initially with Q617 still shorted C and E. The amp should work perfectly like that. The only problem with no bias is that you might hear slight crossover distortion at very low volume.

[jitter]

Hmmm, wish I had posted that it had occurred to me a couple of times that it looked like NPN-behaviour in the replacement for Q613. Even the thought it being a counterfeit had crossed my mind but I dismissed it.

Well, at least you've come to this conclusion methodically. And the good news is that it indeed looks like you're good to go!

[dtm1962]

Thanks guys - Mooly & Jitter for sticking with me and my next steps are as follows:

So I:
1) will put back Q619 & Q621 (C2910/A1208) in the circuit
2) add the feedback resistor back to the emitter resistors of the outputs
3) and leave the Q617 C & E shorted
4) leave the bulb in series with AC line
5) power up & make measurements to determine correct voltages/dim light

If everything seems to be fine such as dim light & correct voltages then I will:
1) power down & remove test bulb out of AC line
2) remove the jumper across Q617 C & E
3) add #1 meter across testpoints T1/T3
4) add #2 meter across outputs to measure DC @ outputs
5) power up
6) adjust bias to 7mV

Any flaw to this plan other than maybe adding leaving in the bulb in the circuit
while I begin to adjust bias as I have other channel disconnected and the load
is less than the 60W bulb's influence on the current delivery to amp?

Let me know

Thanks in advance,

[jitter]

Looks good.
Personally, when finished, I would recheck and, if necessary, readjust the bias with the amp in normal working conditions; i.e. without bulb and with both channels connected to power supply.

[dtm1962]

Originally Posted by jitter

Looks good.
Personally, when finished, I would recheck and, if necessary, readjust the bias with the amp in normal working conditions; i.e. without bulb and with both channels connected to power supply.

Thanks Jitter.... I will move forward with this plan and I will leave the bulb in for the 1st adjustments.... just in case...

[Mooly]

That all sounds good. I would probably leave the bulb in place after removing the short on Q617. You just never know... and its a good safeguard for an initial power up with the bias generator active. If the bulb is still dim when the short is removed and then begins to glow as the preset is turned you know its looking good. At the first sign of the bulb glowing as the preset is turned, you should then turn it back down, switch off and remove the short and set it up for real.

Once the bulb begins to light then the filament starts to increase in resistance rapidly and that causes the rails to collapse. So its just used for initial testing only. Never attempt to set the bias with the bulb present because the supplies will be a lot lower than normal. Also do not have any speakers connected during any tests or adjustments.

I'm not sure what transistors you actually have fitted at the moment but if they are suitable then there is nothing stopping you testing it out now. As long as the devices fitted are adequately rated then the amp should all work as intended.

Are you happy with Q619/621 that you have and all the rest of the output stage or would you like some more simple tests to see all that part of the circuit is OK first ?