Unable to Lower DC Offset

[PE9ZZ]

They don't? Would you care to make a wager on this one? Do you have LTspice on your computer?

They don't. I have LTSpice in my VM. There are only two failure modes of output transistors: dead short or fully open. Usually the first. With a dead short you don't get an offset fault because everything will blow up, even in a sim. With fully open the base-emitter resistor will provide the feedback path, eliminating any offset. Minus open loop gain of course.

 

[Jose Jimenez]

They don't. I have LTSpice in my VM. There are only two failure modes of output transistors: dead short or fully open. Usually the first. With a dead short you don't get an offset fault because everything will blow up, even in a sim. With fully open the base-emitter resistor will provide the feedback path, eliminating any offset. Minus open loop gain of course.

What do you mean by "I have LTspice in my VM"? Is VM an abbreviation for Voltmeter?

 

[PE9ZZ]

What do you mean by "I have LTspice in my VM"? Is VM an abbreviation for Voltmeter?

Virtual Machine. I have no Microsoft Windows PC. LTSpice doesn't run on Linux.

Attached is my "Joepie" amp I use to torture test my wicked ideas. Maybe you can use it to show how the output transistors can introduce a significant offset?

 

[Jose Jimenez]

Virtual Machine. I have no Microsoft Windows PC. LTSpice doesn't run on Linux.

Attached is my "Joepie" amp I use to torture test my wicked ideas. Maybe you can use it to show how the output transistors can introduce a significant offset?

The is only a single pair of output transistors in the schematic, not two in parallel.

 

[PE9ZZ]

The is only a single pair of output transistors in the schematic, not two in parallel.

Why would that matter?

 

[GabrielS]

So you're not on a dim bulb tester or anything right? Those do seem low. I will go check mine.

What is the voltage on D07 and D08 on the 647 board. They are the 24 volt zener diodes in series that should give a 48 volt reading.

Other question then maybe I missed it above but does yours have the diodes on the back of the 649 board?

I am on and have been on a dim bulb since working on this unit.

On the 647 board, D07 shows 1.425v, D08 is 1.320v. Pretty far from the 48v it supposed to have.

On the back side of the board is the DV-03 (marked SV-03).

Do you feel the bias and differential transistors on both the 649 and 647 boards should be replaced?

 

[PE9ZZ]

Do you feel the bias and differential transistors on both the 649 and 647 boards should be replaced?

No. The schematic is a bit of a nightmare to read and it contains no voltages. Check if the wiper of VR401 will adjust to zero at its center position. The voltages on C11M and C12M should be -55 V and +55 V respectively. If your bulb is dim get it off DBT because it will hamper your troubleshooting.

Do both channels have the same offset?

 

[GabrielS]

No. The schematic is a bit of a nightmare to read and it contains no voltages. Check if the wiper of VR401 will adjust to zero at its center position. The voltages on C11M and C12M should be -55 V and +55 V respectively. If your bulb is dim get it off DBT because it will hamper your troubleshooting.

Do both channels have the same offset?

Agree, the schematic does leave a lot to be desired. This amp was an eBay purchase that had someone else working on it then gave up.

VR401 on boards 649 for both left & right channels have multiturn 5k trimmers replacing the original single turn 4.7k. I adjusted them so it hit 1.4k before they descended in value, so I presume that's their middle spot.

I'm now off the dimbulb and measuring on C11 1.460v/ C12 1.927v/ C13 63.4v/ C14 63.4v/ C15 0v/ C16 3.827v/ C17 -63.3v/ C18 -63.3v. Both channels similar.

Dc offset on both channels at -3.8v with neg probes to neg speaker post, pos probe to pos speaker post. Vr01 is the adjustment for zero offset but doesn't have any effect regardless of how many turns are applied.

 

[PE9ZZ]

I'm now off the dimbulb and measuring on C11 1.460v/ C12 1.927v/ C13 63.4v/ C14 63.4v/ C15 0v/ C16 3.827v/ C17 -63.3v/ C18 -63.3v. Both channels similar.

This is on 649 board? So C11M and C12M are dead. They should be +/-55 V. Fix the PSU on board 647, go back on DBT and retest. C16M should go close to zero. The manual has a nice writeup on how the PSU works. Voltages on both sides of R901/2 are particularly interesting.

 

[sregor]

I'm now off the dimbulb and measuring on C11 1.460v/ C12 1.927v/ C13 63.4v/ C14 63.4v/ C15 0v/ C16 3.827v/ C17 -63.3v/ C18 -63.3v. Both channels similar.

First. missing +55 on C11 and -55 on C12 which takes you back to the power supply.

However, a possible complication. The power supply and amp use a lot of 2SC945 and 2SA733. There are a lot of KSC945 and KSA733 parts which look identical, will work, but have different basing (base in the center, instead of collector). If you order KSC945C and KSA733C (the C denotes a center lead collector) you are fine, but problem is there is no way of determining what is in there without checking.
Power supply with some voltages.

Trouble shooting supply. Check to see if you have + and - 75 volts.
Make sure the 22 ohm resistors R901 and R902 are good.
Make sure Zener diodes D907,909, 908 and 910 aren't shorted.
After checking semiconductors for obvious problems (shorts, opens and correct installation and basing)
you (i hope) can adjust the +55 with VR901 and -55 with VR902 (schematic says R901 and 902, but that is also designation of the two 22 ohm resistors...)
Good luck with it.

 

[PE9ZZ]

After checking semiconductors for obvious problems (shorts, opens and correct installation and basing)
you (i hope) can adjust the +55 with VR901 and -55 with VR902 (schematic says R901 and 902, but that is also designation of the two 22 ohm resistors...)

Good catch...

 

[GabrielS]

First. missing +55 on C11 and -55 on C12 which takes you back to the power supply.

However, a possible complication. The power supply and amp use a lot of 2SC945 and 2SA733. There are a lot of KSC945 and KSA733 parts which look identical, will work, but have different basing (base in the center, instead of collector). If you order KSC945C and KSA733C (the C denotes a center lead collector) you are fine, but problem is there is no way of determining what is in there without checking.
Power supply with some voltages.

Trouble shooting supply. Check to see if you have + and - 75 volts.
Make sure the 22 ohm resistors R901 and R902 are good.
Make sure Zener diodes D907,909, 908 and 910 aren't shorted.
After checking semiconductors for obvious problems (shorts, opens and correct installation and basing)
you (i hope) can adjust the +55 with VR901 and -55 with VR902 (schematic says R901 and 902, but that is also designation of the two 22 ohm resistors...)
Good luck with it.
View attachment 3750681

Thank you for your insight and info. I've my work cut out and hope to finally arrive at a fix!

 

[tom1066]

Just disconnect the connectors to the amplifier modules/board 649 for now - That will prevent incorrect voltages from going to the output transistors and causing major problems/smoke. Hopefully that allows for being off the dim bulb tester as well. sregor and PE9ZZ have already pointed out the problem with C11 and C12 on that board - too low;

The schematic provided by sregor above is perfect for checking yours. Off the dim bulb, need to see if the zener diodes on the power supply are working. The zener diodes are where the voltages get their "reference" so start there and work from there to the c7s-c10s connectors to see what may or may not be amiss. If you have the correct voltages at the diodes but not at the c7s-c10s spots then you have to check q903/q904 and q905/q906 to ensure they are working correctly. The trim pots go bad as well - I had to replace them. It could be all the transistors are working but if the trimmers are not working then the voltages will not respond to adjustment. Just remember that C1 and C2 on the schematic should be measured in AC mode, not DC like the others. For that matter C3 and C4 are AC as well.

Let us know what your diode voltages show, my bet is they are low...on mine when 903/904/05/906 failed I had too high of voltage - basically the +/- 75 volts or so from C1/C2;

 

[PE9ZZ]

I think the zeners are OK. This PSU has completely separate positive and negative sections. Sometimes (Sansui) one voltage is derived from the other. Both sections having no output here suggests a bridge or transformer problem. So first check the +/-75 V voltages across C901/2.