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SX-1250 Getting 2 Volts Between Pin-9 and GRND - Should be Zero

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DaDima

New Member
I just finished recapping my SX-1250.
I am setting the voltages following the instructions on page 38 of the service manual (Amp Board #048, Stabilizer Board #106)

I'm getting 2 volts between Pin-9 and GRND (it should be zero) on both the Left and Right Power Amp boards. The readings are erratic, but averages at 2 volts.

Turning the VR pot does not change the reading by much. I replaced the VR pot on one Amp Bd., no change in readings.
I do have 100 mV between Pin-7 and Pin-19, which is OK per specs, on the Power Amp Board (#048)
I do have 65V between Pin-17 and Pin-12 (GRND), which is OK per specs, on the Stabilizer board (#106)
I do have 65V between GRND and Pin-16, which is OK per specs, on the Stabilizer board (#106)

Also, I wonder if any of this is related to my speaker relay NOT switching 'on'. I put in a new relay, it didn't work either.

Please give me advice on how to make the 2 volts become zero and ...
Please give me advice on why the speaker relay is not switching 'on'.

If these procedures have already been discussed, please point me to the right direction.

Thanks, guys
 
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Welcome to Audiokarma.

The reason why the relay is not energizing is because it's preventing the 2V of DC offset from reaching your speakers. That's exactly what it's designed to do. Once the cause of the DC offset is corrected, the relay should pull in.

Did you replace any transistors or just the electrolyic caps and trimmers? Since the same problem is appearing on both amp boards, it's likely that the same installation error was made on both boards. Check the orientation of C2 in particular since it could affect offset if it's in backwards.
 
Disconnect the pre-out/power-in jumpers on the rear panel and recheck this reading.
DaDima said:
I'm getting 2 volts between Pin-9 and GRND (it should be zero) on both the Left and Right Power Amp boards.
Click to expand...
 
Thank you all for your help.

After posting this thread, I went ahead and changed 10 transistors in the stabilizer board. However, I did not change the 3 big ones that was attached to the heatsinks (Q1, Q5 & Q9).
I also changed all the transistors in the Protection Ckt board (6 transistors).

When I put it back together and powered it on (I use both a Dim Bulb Tester (DBT) and a Variac), the speaker relay kicked in after 2 minutes! It did so after a creepy hum coming out of it - almost like an undecisive, slow motion kick in. :)

I tested the voltages and I got the voltage lower but erratic between zero and 0.5 volts. I switched it off, took it out of the DBT, I switched my DBT into direct mode (i made my DBT so that it has a 3-way wall switch, I can select BULB or DIRECT so that I don't have to plug-in and unplug the unit from its outlet), then powered it on slowly using the Variac, it did the same thing - after the 1st relay kicked in, the speaker relay kicked in after 2 minutes but has a noisy in-out hum. I touched the relay and verified that the relay is humming in and out. I did visually see inside the relay that it switched on. I think it's humming because of the jumping voltage 0 to 0.5 volts.

After a while, after it warmed up and making repeated on/off tests, I'm back to square 1.
The speaker relay is not engaging.
I still have 2 volts on both L&R Power Amp boards.
I still have 100mV on both L&R Power Amp boards (rock steady, which is good)
I still have the two 65 volts on the test on the Stabilizer board (rock steady, which is good)

I tried getting the 70 volts you requested, and here is how I got it (let me know if I botched this procedure):
I saw the 70 volts in the schematic diagram near the big caps. I switched my tester into DC-200, I poked the neg lead into the chassis and I poked the pos lead into the orange, then the red leads to the big caps of the Left channel. I got +/-68 volts.
This is with 120V AC coming into the unit (I measured these inside the unit where the power cord enters the chassis, I adjusted the Variac to get this 120VAC exact with the tester).
Let me know if there's another way of getting the 70 volts.

I will check C2's polarity installation as you have suggested and I will get back to you guys.

My Pre-amps work though - FM, AUX - I put RCA lines from the pre-out terminals and hooked it up into another stereo unit and there's music coming out.
I haven't recapped the pre-amp section. I have only recapped the power section - Power Supply board, the Left & Right Power Amps, the Stabilizer board and the Protection Ckt board. I also replaced the 4 big 20,000uF caps.

I have not changed any diodes in any board too.

Once again, I appreciate your responses. I know I'm very close.

Thanks, guys.
:)
 
Btw, I did use two 5.1kohm jumpers in the Power In RCA terminals when I was getting these readings - per Service Manual (page 38)...
 
Did you replace all caps on the stabilizer board? You should also replace all zeners, and there are three resistors that NEED to be bumped up in wattage and moved further off the board.
 
'Funny you mentioned the hot resistors on the Stabilizer board, I moved all the 5 resistors (that are 2 watts and above) and moved them to the chassis. I replaced them with chassis-mounted resistors with heat sinks with higher watt value. :)These are R13, R14, R15, R16 & R18. The board did have traces of scorch marks from the resistors running hot after all these years. :)

I do have the parts for the diodes and zener diodes. I will hold off on replacing them until I made another check on parts values, polarity and soldering. I'll take voltage readings of all the connection pins.

The +2volts is happening on both Left & Right Amp boards consistently - it's jumping around at 1.5V to 2.2V. Also, turning the trim pot does not change the value on both boards. One amp board has the original trim pot, the other amp board has the new 25-turn bourne trim pot - same results.

I traced all the components that's on the path of the 3 legs of the trim pot and resoldered those too. I tested all the resistors on both boards (with one leg desoldered) and they are all good and they are amazingly accurate at either 1% or spot-on (after all these years, credit to those Japanese manufacturers of 45 years ago) :)

I checked all the capacitors' polarity and they are installed correctly.

All the capacitors for the following boards were changed: Power Amp L&R, Stabilizer, Protection Ckt, Power Supply and the 4 big 22,000uF caps. The pre-amp boards I have not touched yet.

I'll move on to the zeners next.

Thanks. :)
 
I’m testing a zener diode (see photo).
The “bar” on the tester’s schematic is opposite that of what’s on the diode.
the diode does not test if installed the opposite way. I know diodes goes only in one direction.

Is this how the “bar” should be read and installed?
Is the tester wrong?
 

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I am making some progress :)
I disconnected both power amps one at a time and tested them.
The right channel power amp measures zero and 100mV - which is good. The unit also starts normally with the DBT and Variac. The first relay kicks in, but the protection relay does not kick in, though. Maybe because the the left channel power amp is disconnected?

Now, when I switch on the unit with the left channel power amp plugged in alone, the first relay does not kick in and the DBT is always bright. This indicates something is wrong.

With both left & right power amps connected, the first relay does not kick in and the DBT is always bright BUT it "pulses" every 3 seconds - sorta weakens its brightness then goes back to bright every now and then.

I noticed that the left channel power amp board has some diodes replaced by the previous owner. Diodes D7 and D11 are different than the originals.
Can you help me track down replacement diodes for my SX-1250 - the diodes are basically the only ones that I have not finished replacing because some part numbers are not available:

Power Amp (048)
1S1886 - D3,D4,D6,D7,D10,D11
1S2471 - D8, D9
1S2473 - D5 (512-1N4148)

POWER SUPPLY (107)
SIB01-04 - D1, D2, D3, D4
SIB01-01 (1S1885) - D5, D6

STABILIZER (106)
WZ-240 Zener - D1, D3 (512-1B5252B)
WZ-061 Zener - D5 (512-1N5234B)
1S2473 - D2, D4, D6 (512-1N4148)
S5151R - D7, D8 (Big Old Pioneer Stuff Built to Last?)

PROTECTION CKT (091)
1S2473 - D1, D2, D4, D5 (512-1N4148)
1S2471 - D3
S5151R - D6 (Big Old Pioneer Stuff Built to Last?)
S5151 - D7 (Big Old Pioneer Stuff Built to Last?)

If you guys can help me out with the part numbers for these diodes or if you can point me to the right direction, I would appreciate it very much.
Thanks, guys.
:)
 
It works now!

After replacing all the resistors and transistors on the Left Power Amp board, I finally got the 2nd relay to kick in - releasing the speakers.
However, the Left channel is very weak. Using all the sources - FM, AUX, etc. produce the same weak output to the speakers.

1. The speaker is good - when I switch leads, the other speaker becomes weak.
2. It's not the pre-amp or flat amp (volume, balance, loudness, etc.). I connected the pre-out to another amplifier and the output is equal strength out the speakers, the balance control is perfect too.
3. I connected a DAC into the inputs of the "Power In" (amplifier section) - the same weakness to the Left channel is present.

Some of the diodes on the Left Power Amp board are the only ones I have not replaced. Maybe this is the cause.

Will other boards (other the Power Amp board) make this effect?

Any suggestions would be helpful.

Thanks.
 
Guys, this 2-volt mystery of the DC offset has now been solved and it's now at Zero. The 2nd relay (speaker protection relay) has now kicked in and, finally, after 2 grueling weeks, there's music running through the speakers, although the left channel is crappy.

The trick is to isolate which amp board (left or right) is causing the non-zero dc voltage. The dc offset shows up on both boards and somehow the unit balances both. An error on one power amp board will show up on the other board when measured. I noticed this when, after working on the left channel board, the DC offset went down to 1.5 volts on the left amp board, AND, it also reads 1.5 volts on the right amp board which i didn't remove or touched. So, isolating one power amp board one at a time will isolate the problem.
To do this, disconnect all the connectors (front and back connectors) of one power amp board, power the unit up and take measurements on pin#9 and GRND.
When I did this, I found out that the right power amp board HAS zero on the reading, but the left power amp board has the 2 volt reading.
So, not knowing what component is causing it, I just went ahead and replaced all the remaining old parts that's in that board - and it worked.
I figure that even if a component tests OK on the tester (resistors, transistors, capacitors, diodes, etc.), these tests are low-voltage and low-current tests. These parts might fail when real-world voltages and current passes through them. After all, these are 45-year old parts.

To tackle this next problem of having a weak left channel, you can follow this link to the new thread I created:
Pioneer SX-1250 Weak Left Channel After Re-Cap
https://audiokarma.org/forums/index...sx-1250-weak-left-channel-after-re-cap.929041

Thank you, guys. And keep helping out the other guy. Thanks.
:)
 
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