Can you please check these resistors again?
They are all 2.2Ω resistors.

R515
R517
R565
R567

R516
R518
R566
R568

 

Can you please check these resistors again?
They are all 2.2Ω resistors.

R515
R517
R565
R567

R516
R518
R566
R568

Okey, here are the measurements

R515: 118Ω
R517: 15 KΩ
R565: 2.4Ω
R567: 2.4Ω

R516: 2.4Ω
R518: 2.4Ω
R566: 2.4Ω
R568: 2.4Ω

 

Ok. You will have to order replacement 2.2Ω ¼W resistors.
In the meantime we can still go ahead and do some power ON tests.

 

Ok. You will have to order replacement 2.2Ω ¼W resistors.
In the meantime we can still go ahead and do some power ON tests.

yes, i figured.. i guess the local guy might have some resistors.
I will only need two in the first place if i understand right, but i will ask for more if he has in stock.

I will have the resistors at monday if he got them, so we can take the power on test then?

I am curios to see how power on will turn out this time, we have replaced much components lately...

 

Those resistors are in the power output section. We can still do a power test without them.

Install new
Q515 2SC3182
Q517 2SA1265

Power On. Check for smoke.

Measure voltages at

TP401-1
TP401-2
TP401-3
TP401-4

+TP501
-TP501

+TP502
-TP502

 

Okey.

So i installed:

New Q515
New Q517 (even the one removed, tested ok in #73)

Power on!

R513 had literally a meltdown and R509 was glowing..
I turned off power after a second or two..

 

That's terrible!

You will have to pull and test:
Q501
Q503
Q505

Replace:
R509 680Ω
R513 330Ω

 

Just thinking, this is just the same that happened when i at first made the mistake and was trying to measure voltage at R505 when my wire leads was set at ampere measuring, could the fault i caused by making connection between GND and R505 still be there... like a component we haven't touched yet.. ..?

You know best, i was just thinking

 

There are lot of possibilities, considering how much abuse we've put that amp through.

It is possible that R515 and R517 were blown a long time ago and went unnoticed.
It appears that Q503 has gone bad.

How difficult is it to test Q501?
I would remove Q503 and Q505 and do a power ON test.

 

I will remove both Q501,Q503 and Q505 and test them before we do anything else.
So we know if some of them are bad or not

 

So i finally removed Q501, Q503 and Q505

So i would say Q503 and Q505 is bad... again(?) have i tested these earlier or did i just take them out.. hard to remember .... Q505 has also cracked!

Q503
Red to B
Black to E: 86
Black to C: 80
Black to B
Red to E: 89
Red to C: 86
Red to E
Black to C: 5 (short)
Black to E
Red to C: 6 (short)

Q505 DMM beeps (short) on every test
Red to B
Black to E: 10
Black to C: 3
Black to B
Red to E: 14
Red to C: 3
Red to E
Black to C: 10
Black to E
Red to C: 8


Q501 i am a bit unsure about the pinout on this... i found online EBC...(?) but my testing shows i might be wrong, and that it is ok...?
Red to B
Black to E: open
Black to C: open
Black to B
Red to E: open
Red to C: 812
Red to E
Black to C: open
Black to E
Red to C: 817

 

Yes, your pinout for Q501, 2SC1815BGL is not correct.
Your tests do not indicate a failed transistor.

Here is the correct pinout.

 

Okey, thanks, don't know what i was looking at.

So then Q501 is ok.
Q503 and Q505 is bad

What could have killed Q505 and Q503? Is there anything else i should test before we replace them?
I looked in the history, both have tested ok earlier.

What is our next step MrChips ?


I was at the local guy and ordered some 2.2Ω resistors, i specified they should be 0,25W.. but when they arrived, he had ordered 1W instead, he said there was no use for such small resistors as 0,25W....probably so small because of the price he said..
But when the resistors possibly can burn up, we could cause larger damage with installing bigger resistors or what?

 

Q503 and Q507 are configured in a way similar to a Darlington pair.
Why they put such low value resistors between the emitter of Q503 and the base Q507?
Normally the emitter of the first transistor is connected directly to the base of the second transistor.

The only reason I can see for putting the resistors in is so that they are able to drive two output transistors at the same time.

Why the resistors blew?
If a catastrophic failure caused both transistors Q503 and Q507, or Q505 and Q509, to fail at the same time.

Assuming a beta about 35 for Ic at 5A, the base current is about 140mA.

A 2.2Ω ¼W resistor is rated to carry 300mA.
Going to a 1W resistor increases the current capacity to 700mA.
There is no harm in using the 1W resistors. They will still blow if the transistors fail.

You can go ahead and install the new resistors. But before doing a power test I am looking to find how you can conveniently connect a load to the output stage to pull the output to 0V while we disable the output stage. The output is fed back to the preceding stages so we need to send the right signal.

 

Thanks.
Then i will use the 1W resistors.

I am currently out of new replacement for Q503 (3944)
I have one new 1535 for Q505.

But, i do have the original ones i replaced at the start from:
Q651
Q653
Q652
Q654

These are the same types. It's two of 3944
I can test these and if they are ok, i can use one of those?
Is that a good idea or will we prefer only new ones?

 

Jumper J122 is at the output of the LEFT channel.
Jumper J131 is at the output of the RIGHT channel.

Edit: We will not bother to do this:
(Jumper one 100Ω resistor from J122 to J104.)
(Jumper one 100Ω resistor from J131 to J104.)

Do not install Q501 and Q505.

Do power ON test.

Measure voltages at test points
TP401

TP501
TP502

 

Okey.
You wrote: "Not install Q505 or Q501"

Q503 is not either installed, and i have still not replaced R509, R513 or 515 and R517

Is it okey to perform the power on test you mentioned still? I just want to verify

 

Confirmed, that is correct.

Actually I meant Q503 and Q505 not installed.
If Q501 is already out of circuit, leave it off.
It wouldn't make any difference with those resistors in or out.

Install R513 330Ω
Install R509 680Ω

The objective is to disable the power amp on the LEFT channel.
There are still two power transistors Q427 and Q427 on the LEFT channel which we will assume are ok for now.

 

Edit to post #276

I have removed the following instructions:
Jumper one 100Ω resistor from J122 to J104.
Jumper one 100Ω resistor from J131 to J104.

I now see that there is already a path to ground via R535, R536, R537.

 

Okey.
So i did not do the test with the 100Ω resistor.

Q501, Q503 and Q505 not installed.
Replaced R509 and R513

Power ON

TP401:
1: -65 Volt
2: -65 volt
3: -4,7 Volt
4: - 5,6 Volt

In the middle of this testing R505 suddenly burned up.. but nothing else happened so i took the rest of measurements.

TP501:
+: +65 volt
-: -65 Volt

TP502:
+: -52 Volt
-: -63 Volt