Those voltages are a bit off from nominal.
We will have to see if we can match up the zeners a little better.
Do you have a 12V supply and 100, 200 or 220Ω resistors handy?

 

Those voltages are a bit off from nominal.
We will have to see if we can match up the zeners a little better.
Do you have a 12V supply and 100, 200 or 220Ω resistors handy?

Hmm, okey... can we actually change something at the zeners?

12V supply is no problem, but i only got 100Ω of the resistors you're asking..

 

I am thinking of testing the remaining zeners you have at hand with something like this:

With a 12VDC supply feeding two 100Ω resistors in series into the zener, you can measure the zener voltage at 20mA = ( 12V - 8V divided by 200Ω).

Or 10VDC with one 100Ω resistor in series will deliver the same 20mA = (10V - 8V divided by 100Ω).

Also in the same test, measure the original D410 that was removed from the board.

The zeners you now have installed are showing relatively close to nominal voltages.

D410 = 7.3 + 0.7 = 8.0V
D412 = 8.8 - 0.5 = 8.3V

(with some discrepancy of 0.2V in the measurement).

If we see some variation, I mayhave to ask you to remove the two on the board and test those too. We are looking for a pair that match.

 

I have this amp outside in my garage, it is kind of cold outside, so i let the amp sit on for like 10 minutes and measured the installed zeners again today. Don't know if it was necessary but this was the result:
D410: - 0.7 / + 7.5 Volts
D412: - 9 / - 0.6 Volts

I went outside to measure the zeners.. either i've read wrong or you have edited your post, because i only used one 100Ω resistor in my test..
If it's not okay, i'm just doing it again with two resistors.

I used a 12V battery for testing, checked the voltage on my battery under the "load" if it would make a difference.
I have three spare new zeners, and i let them stay connected some minutes, because the voltage seemed to variy a little in the beginning.

Zener 1: 12.46V battery : 8.87 Volts
Zener 2: 12.45V battery: 8.77 Volts
Zener 3: 12.44V battery: 8.60 Volts

The removed original D410: 12.44V battery: 8.60 Volts
The removed original D412: 4.02 Volts..


Don't look at the values in picture 2. it changed.

 

Sorry. Yes, I did edit the post while you weren't watching. Not to worry. Your tests are still meaningful.
With a single 100Ω resistor, you are testing the zener diodes at 40mA with a 12V battery.
With 200Ω in series, the test current will be at 20mA with a 12V battery. The zener voltage will be different at the different currents but this will still give us some idea of the variations.

Your results for D412 of 4.02V is alarming. Could this be an error in the measurement or transcription?

In any case, install the D410 just removed (tested at 8.6V).
Install Zener 3 as D412 (tested at 8.6V also).

Your setup in a cold garage is significant and noteworthy information. What is the temperature in your garage when you have to power up the amplifier and make these tests? Sorry to have to subject you to such frigid torture! The weather this past week must have been horrible for you.

I don't want you to run the amplifier for 10 minutes in these initial tests because the voltage imbalance at this stage means that some components might be drawing higher than nominal current. However, since the amplifier is cold this might not matter much. It also means that if the amplifier is sitting at very cold temperatures, our voltage readings may not be quite representative.

For now, don't leave the amplifier on for more that 5 minutes at a time. Let me have an idea of what is the temperature of the garage and the temperature of amplifier. It doesn't have to be accurate, ± 5°C will be fine for now.

After you have installed D410 and D412, take voltage measurements at

D410
D412
R428
R430
R432
R448
R450
R452
R514
+ TP502
- TP502

Let me know if any of these test points are difficult to reach.

 

Your results for D412 of 4.02V is alarming. Could this be an error in the measurement or transcription?

Sorry, i wrote "the removed" D410 and D412, i forgot to mention it was the originals removed i tested.
The D412 was measured faulty earlier, i just measured it to see.. that's why the grin smiley is there, sorry the confusion.

I have not removed the new zeners i installed at D410 and D412.
So they're not tested yet.


I had the amp inside earlier, but because of some work inside the house i took it out in my garage, we've had about -10 celsius for a couple of months. Have not thought of mentioned it earlier. But that will be the temp the amp also has had for the last months.
No problem, just need to put some clothes on.. we're at 3 plus today. hurra


So.. i will install the "original" D410 and zener "nr.3" and measure the points you've given me.

I will not run the amp for too long, roger that.

 

Be clear on what you mean by "original" D410.

"original" to me means the one that came with the amplifier.

I want you to use one of the newly bought 1N756A (for both D410 and D412) so that they are matched.

 

Be clear on what you mean by "original" D410.

"original" to me means the one that came with the amplifier.

I want you to use one of the newly bought 1N756A (for both D410 and D412) so that they are matched.

I mean the same as you with original, the one that came with the amp.


OK, then i will remove the new installed D410 and D412 zeners, and measure them too before i do something else.

I will post the measurements when i've removed them.

Would that be the right thing to do?

 

The idea is to find two of the new IN756A that show closely matched zener voltages, presuming that the test measurements are made at reasonably same conditions (same voltage, current, and temperature). But let's not get too fussy about this. Close enough is good enough. This is not a mission to Mars.

 

Hehe, i understand.
Measurements are coming

 

So now i've removed the new zeners at D410 and D412, let's refer to them as Zener 4 and 5

Zener 4: 12.48V battery: 8.75 Volt
Zener 5: 12.48V battery: 8.67 volt

 

Pick two zeners that come close to giving the same voltage.

 

I guess that will be zener 2 and zener 4 ?

Do you agree?

 

Your call. Looks ok. Go with it.

 

I've been away for some days, but i'm back at it!

So i installed Zener2 -D412 and Zener4 -D410

Power on!

Measurements:

D410: -0.3 / +7.8 Volt
D412: -8.5 / -0.3 Volt
R428: -2.8 / +7.4 Volt
R430: -5 / -8 Volt
R432: -2.9 / +5.7 Volt
R448: -4.3 / -3.6 Volt
R450: -3.7 / -3 Volt
R452: -3.7 / -3.8 Volt
R514: -3.2 / -4.2 Volt
+TP502: +0.1 Volt
-TP502: -3.8 Volt

No points were hard to reach.
There is 0 degrees celsius in my garage today

 

DC bias voltages are a bit off. We'll have figure out why this is the case.

 

Measure voltages at:

R414
R416

Then I am going to ask you to remove R414 and R416 and measure both resistances.
Then I want you to put those aside.

Now select two new 100Ω resistors that match closely in resistance and install these at R414 and R416.

Power on and measure voltages at

R414
R416
R428
R430
D410
D412
R514
+TP501
-TP501

 

Measured voltage at the resistors without done something:

R414: -0.2 / -0.2 Volt
R416: -0.2 / -0.2 Volt

Removed R414 and R416
Measured out of circuit:
R414: 101.4Ω
R416: 100.8Ω (This is an original one)

So i tested a bunch of my 100Ω resistors, varying from 101 to 101.6
I found two who measured exactly 101Ω , lowest i found.

Then i installed two new ones at R414 and R416 (101Ω)

Power on!

Measuring:

R414: -0.2 / -0.2 Volt (varying up and down, measures just as much 0.3 as 0.2)
R416: -0.3 / -0.3 Volt
R428: -2.8 / +7.4 Volt
R430: -4.9 / -8 Volt
D410: -0.2 / +8 Volt
D412: -8.6 / -0.3 Volt
R514: -3.2 / 4.2 Volt
+TP501: 0 Volt
-TP501: 0 Volt

 

False euphoria! when I saw 0V at TP501.

Sorry, I meant TP502. We already tested LEFT channel TP501 is at 0V.

R514 should be reading -0.5V/+0.5V

I have to look over your results and get back to you.

 

False euphoria! when I saw 0V at TP501.

Sorry, I meant TP502. We already tested LEFT channel TP501 is at 0V.
R514 should be reading -0.5V/+0.5V
I have to look over your results and get back to you.

hehe, sorry, yes i know we have been testing TP501 but i thought we should do it again since you wrote it

I went out again and measured TP502

+TP502: 0 Volt
-TP502: -3.5 Volt

hmm.. what could be causing this..