Make a note of the orientation of the VR402 pot labeled RCH VAMP ICQ.



With a small flat screw driver blade, turn VR402 all the way counter clockwise.

Take the following voltage measurements listed below.

Turn VR402 all the way clockwise. Repeat the voltage measurements.

Adjust VR402 until the VAMP RCH test points are close to 0V (if possible) or voltages at R450 and R452 are close to 0V.
VAMP RCH test points are shown in RED below.
What is the number of this 4-pin test point as printed on the PCB? Is it TP401 or TP451?

This is just a preliminary check. If we can manage to find a 0V setpoint, we will come back and adjust this properly.


Repeat the voltage measurements.

D410
D412
R422
R424
R428
R430
R444
R446
R448
R450
R452
R514

+VAMP RCH (shown in RED below)
-VAMP RCH (shown in RED below)

+TP502
-TP502

Edit: added R422 and R424 to the list.

 

What is the number of this 4-pin test point as printed on the PCB? Is it TP401 or TP451?

It's TP401
I will get to the testing later

 

So i begun testing.. power on, and i turned VR402 all the way CCW.

I started to measure all testing points.. i did not get through all before i noticed a humming sound from the amplifier...
..and suddenly R513 went up in flames .. so i turned it off... tried to turn it on again.. then R401 started smoking so i turned it off afraid of what it would burn next... ... so if i turn it on again now i think R401 will smoke even more... what should i do?

Here are the measurements with VR402 all CCW before it started smoking.. i don't know if they mean something at all since i did not get all the measurements.. but here they are..

D410: -0.3 / -0.3 V
D412: -8.5 / -0.3 V
R422: +64.3 / +64,9 V
R424: -64.7 / 64.1 V
R428: -2.8 / +7.4 V
R430: -4.9 / -8 V
R444: -2.9 / -2.9 V
R446: -4 / -3.7 V
R448: -4.3 / -3.6 V
R450: -3.7 / -2.9 V
R452: -3.7 / -3.7 V

 

Very strange.
R401 and R513 are in the LEFT channel.

Presumably, you turned VR402 fully CCW which controls the RIGHT channel.

R401 should not even be in circuit because the cable at CN302A has been disconnected?

 

Yes i have not plugged in anything
What could have happened...

I am not totally(!) sure about R401 was the smoking part, but it looks like it have been hot and the smoke came from that area. To be totally certain i have to turn the power on again.. if i dare...

 

No. Don't do anything yet until I get a chance to analyze this.

 

Hey guys ... is this an old pic and you have already replace Q420?

 

Yes, it is an old pic.

 

I still have not had a chance to go over all the data collected so far but here is something you can do in the meantime.

With the POWER OFF, measure the resistance across these resistors (without removing the resistors):

R421
R423

R422
R424

Take the measurement both ways, i.e. across the resistor leads from left to right, and then again from right to left.
Expected resistance is 560Ω for all resistors.

 

Measured:

R421: 566 / 565Ω
R423: 569 / 568Ω

R422: 561 / 561Ω
R424: 565 / 564Ω

 

We are going to take a different approach.

Remove YOUR jumpers at
J222
J223
J224
J225

Replace R401, 560Ω
Replace R513, 330Ω
Set VR402 back to the original setting.

Power up and check for smoke.

Measure voltages at

TP401-(1)
TP401-(2)
TP401-(3)
TP401-(4)

+TP501
-TP501

+TP502
-TP502

Edit: By jumpers at J222-J225, I meant your hook-up leads and not the wire jumpers on the PCB.

 

Yep, so i removed my hook up leads.
Put VR402 back to original setting
Replaced R513 and R401.. original R401 is much smaller then my new resistors, but in the manual it says 0.25W like the other ones.. so i guess it's right..

Power on!

Humming sound...about two seconds and C426 went up in smoke.
After that, no more smoking and i took the measurements..

1 TP401: +65,5 Volt
2 TP401: +65,4 Volt
3 TP401: +64,7 Volt
4 TP401: +63,1 Volt

+TP501: +65,3 Volt
- TP501: +65,3 Volt

+TP502: +62,5 Volt
-TP502: +63,2 Volt

 

That is terrible.

Maybe there is something wrong with the GND connection.
Check for accidental shorts on the bottom side of the board, especially around R401 and Q401.

Remove the bad C426. Don't replace it for now.

Put back the hook-up jumpers at
J222
J223
J224
J225

Power up and try again.

 

GND connection to PCB is a screw going to the chassi, it's also holding PCB so it's there.
So this suddenly happening is a mystery if i get it right?
We thought we had stabilized LEFT channel and suddenly it's unstable again...
I have looked closely for shorts, but i can't seem to find anything wrong.


I noticed that R402 looked a little burned too.. but i measured it to be 566Ω, so i thought i'll give it a try.

So i removed C426.
Put back my hook-up jumpers on J222-J223-J224-J225 to J401

Power on!

It went a couple of seconds, before both R401 and R402 started smoking, then R401 burned up and right after R402

After that, nothing more were smoking..


Before..

 

Something is very wrong.

R401 and R402 are 560Ω input resistors on the LEFT and RIGHT channels respectively. One leg of the resistor is connected to GND via your hook-up jumpers.

In the failure just before this last failure, your hook-up jumpers were removed and hence the resistors were in effect not connected to anything at this point.

The other leg of the resistor is the input to this stage of the amplifier which nominally is at 0V.

At this point I cannot see what would cause high current to flow in R401 and R402.

What is the circuit board resting on while you are testing?

I am going to wait for some input from other members, @JoeJester for example, at this point.

 

hmm.. the circuit board is in it's place in the amp chassi everytime i'm testing.
I'm taking it in and out to replace components if they are not availabe at the "service hatch" underneath.
It stands on it's original plastic legs well abobe the metal chassi.

I turned the amp on today by a mistake without putting back the hook-up jumpers today at first.
Nothing smoking then, but when i put on the hook-up jumpers and turned it on it happened as described.

I can see under it by the service hatch, but i will remove it again and check really well......... so what could be wrong, a short? or is there some connection to gnd? what's most likely?

Looking at the "history" i replaced R414 and R416, then the board was out, but i had the power on measuring a lot of points after that... so i donæt understand what could have gone wrong

 

This is a previous photo of the solder side of the PCB showing the connections to R401 and R402.

It is a mystery as to why both R401 and R402 would be destroyed even when one leg of each resistor is left floating.
We need to see a closeup of the pads at the connector circled in red at the area around R401 and R402.

 

R401 and R402 are 560Ω ¼W resistors.

According to my calculations, for R401 and R402 to be destroyed so quickly, they would have had to have a lot of current flowing through them, of the order of 100mA.

Subjected to only 28V, they would have 50mA flowing, equal to 1.4W.

56V /560Ω = 100mA = 5.6W

Somehow R401 and R402 were subjected to high voltages in excess of 20V.
The mystery is that one leg of each resistor was floating (not connected to anything).
Can you think of anything that we are not seeing that might have happened?

 

So i measured the legs at R401 and R402 today, they have both +64 Volt on one of their leg's.....

I will take out the PCB carefully and look for problems..

 

I have taken out the circuit board and carefully inspected it.
I cannot seem to find anything wrong on the solder side....
See pictures, the points that have been hot is R402, the one upper left is R401

On the component side, i did find something that i have not seen earlier, so if it happened now or earlier, i'm not entirely sure..
Q423 is damaged.. but i suppose it would only cause a break..