I see. Thanks.

 

I don't blame you @atferrari

On another project I read over 90 pages to see what someone attempted to do only to find out much later that he just started replacing components without regard to the value required. But it was an good reading of the user modifications to this one preamp.

 

Please lift one end (or both) of R460 and R462.
Repeat the measurements at R428 and R430.

I cannot find R460 and R462 in the photos. Also R459 and R461.
This is a case where photos from two angles would be desirable, not too close up, as wide angle as possible but still able to read the part IDs. For example,
1st on post #464 works well.
2nd photo is readable too (and is helpful when needed to see the wider view).
3rd photo is not necessary.

 

I don't blame you @atferrari

On another project I read over 90 pages to see what someone attempted to do only to find out much later that he just started replacing components without regard to the value required. But it was an good reading of the user modifications to this one preamp.

I do not recall any story in any forum starting with parts replacement and ending successfully. At least not declared by the OP to "close the case". Done here myself.

 

I understand it's hard to see the number on every component on the pictures, i have taken some new pictures from various angles i hope you can see some more numbers on.

I lifted one end of R460 and R462 and repeated the measurements

First measurement is towards heatsink if it matters.
R428: -0.3V / +5.8V
R430: -3.1V / -9.4V


R461 and R459 will be down to the right in the #3 picture
R460 and 462 are around the end of the black heatsink you see the legs are lifted at picture #2

 

I do not recall any story in any forum starting with parts replacement and ending successfully. At least not declared by the OP to "close the case". Done here myself.

It didn't end. The TS just left. It was a few years ago in 2017. I went over to the forum where he got the mods to read more about the equipment and one day I saw his name and asked if he ever fixed it, he said no, he just sold it as he was frustrated troubleshooting it and especially so when he realized he installed the wrong parts in places (different values). It happens, but I managed to gather more info about that preamp and modifications.

 

Install new components if you have them.

R401 560Ω
R402 560Ω

IC402

Q424
Q426
Q428
Q430

Reinstall R460, R462

Apply power. If smoke or any lamps glow, make a note and turn off power.
If none, measure voltages at
TP401
TP502

indicating order of pins measured.

 

I would add to make the TP measurements according to the service manual, not to ground.

On edit ... TP401 on the schematic is called TP451 in the description page. The adjustments are correct though.

 

So i installed all new parts besides R460 and R462 which measured ok.
R401 560Ω
R402 560Ω
IC402
Q424
Q426
Q428
Q430
Reinstalled R460, R462

Power ON.

No smoke or glow from lamps.

Have not measured values yet because of my question under.

I would add to make the TP measurements according to the service manual, not to ground.

On edit ... TP401 on the schematic is called TP451 in the description page. The adjustments are correct though.

By this do you mean measure between + and - on the pins as in the manual, picture attached

 

Yes, measure voltage between two pins + and -.
This gives the differential voltage (difference voltage) and is more accurate.

It does no harm to measure each pin with respect to GROUND. This will give the absolute voltage.
Do this measurement also. I would like to know the absolute voltage.

We are expecting (hoping) all readings to be close to 0V.

At TP401 (aka TP451) note that the LEFT channel is disabled and the measurement is not important at this point.

 

Okey!

Measured TP502 + to - : +0.1V
+ to GND: 0V
- to GND: -3.2V

Measured TP401 RIGHT channel + to - : -0.7V
+ to GND: -2.5V
- to GND: -3.3V

 

Okey!

Measured TP502 + to - : +0.1V
+ to GND: 0V
- to GND: -3.2V

Measured TP401 RIGHT channel + to - : -0.7V
+ to GND: -2.5V
- to GND: -3.3V

Your results are very encouraging!

However, this does not seem correct:
Measured TP502
+ to GND: 0V
- to GND: -3.2V

I would be expecting to see the the two values to be about 0.1V apart to match your differential measurement.

 

Your results are very encouraging!

However, this does not seem correct:
Measured TP502
+ to GND: 0V
- to GND: -3.2V

I would be expecting to see the the two values to be about 0.1V apart to match your differential measurement.

Good
Okey, so we have a problem there then..... i tested again with the same result just now...

 

Good
Okey, so we have a problem there then..... i tested again with the same result just now...

So you tested again and got the same result.
TP502 + reads 0V
TP502 - reads -3.2V

Yet
TP502 + to - shows 0.1V

Can you see the inconsistency?

 

So you tested again and got the same result.
TP502 + reads 0V
TP502 - reads -3.2V

Yet
TP502 + to - shows 0.1V

Can you see the inconsistency?

i don't think i have enough knowledge to understand...... but if i look at the measurement at
TP401 it seems i can add the 0,7 volts between the pins to one of the measurements i will get almost the same at both sides to GND. Is that what's not adding up? Since +/- are the differential voltage, you mentioned above...

 

i don't think i have enough knowledge to understand...... but if i look at the measurement at
TP401 it seems i can add the 0,7 volts between the pins to one of the measurements i will get almost the same at both sides to GND. Is that what's not adding up?
Sorry, it's a shot in the dark

No. It has nothing to do with TP401.

We have two points on TP502, + and -.
Let us name + as TP502A
and - as TP502B.

Let us assume that your DMM has an input resistance of 10MΩ and should not influence the reading.
(This could be the problem and we will soon suggest a way to fix this.)

So we have three connection points

TP502A
TP502B
GND

TP502A to GND gives reading A.
TP502B to GND gives reading B.

TP502A to TP502B should give reading A - B.

There are only two explanations I can conceive:

1) The DMM is altering the reading,
2) TP502A is not connected to the circuit the way we expect.

 

With POWER OFF,
DMM set to lowest resistance range,
measure resistance between TP502 + and -.

Do the same at TP501 + and -.

 

NIce progress guys.

On edit: I would recommend two tests on each point. One as normal, negative lead on ground, positive on the test point and reversing the polarity of the meter (pos lead to ground, neg lead to test point).

 

I did the same test, TP502 to ground with another DMM, got the same value. -3.2V

Measured resistance at TP502 and TP501 with power off:

 

NIce progress guys.

On edit: I would recommend two tests on each point. One as normal, negative lead on ground, positive on the test point and reversing the polarity of the meter (pos lead to ground, neg lead to test point).

I will try that.