Pioneer SX-1000TW Bias Set Procedure

anachrophile

New Member
Does anyone know how to set the bias on an SX1000TW? I have finally gotten it to stop thumping or popping (apparently, this was just a cold solder joint on the tone control board) but in the process or searching for the problem I changed many of the electrolytic caps and even the output transistors. It must need to be rebiased. But how?

Thanks.
 
Do you have the sx-1000 schematic from the database? If not, download it.

The clues are in it, the VR801 and VR802 30k ohm pots set the outputs dc bias at 1/2 vcc, with vcc = 78 volts, the output before the blocking cap (1000 uF) is 39 volts.

THEN

The pots VR6 and VR5 wherever they are, are used to set the bias, and are connected to pins 8 and 10 labeled thermistor set the bias, which the schematic shows as 20 milliamps. This current induces 14 millivolts across the 0.7 ohm resistors (ohm's law says amps times resistance is volts) .
 
Thanks. I do have the schematics, but am not very experienced. Only other item I fixed was an old Sansui 517 (lovely amps) which had nicely detailed procedures written in text and actual location of items spelled out. I'll comb through the circuit diagram and see how you figured this out!
Paul
 
Can we get the instructions for bias down to pinouts, locations and easy procedure?

OK, so I have studied the schematics and SORTA understand how MarkTheFixer came up with his instructions. Not easy for a beginner! Now I will try to reduce this to actual practice using pin outs, in case someone else has the same desire to adjust bias.

First, this receiver was not equipped as the schematics say. Instead of 2SC793 output transistors, it had 2SC897s. I replaced these with MJ211946s while hunting down the popping/thumping sound. For the same reason, I replaced output caps, too. The new ones are 1200uf/100V instead of the stock 1000uf/50V. I reckon this very slightly deepens bass response, but I did it just because those are the capacitors I could find. I replaced the two prominent 1000uf/100V "big silver can" caps with 820uf/160V (the 820 was the closest I could find at the right diameter). I figure this slightly reduces the steady state power output available (not a factor in my application). By the way, why did Pioneer choose TWO caps instead of one 2000uF? They are in parallel.

OK, first question. Do these changes affect how we set bias? My guess: no.

Next question: should I have a load on the outputs when setting bias? Does anything else matter (e.g., volume control, shorted inputs, etc.)?

Finally, the procedure:
1. MarkTheFixer said to adjust VR801 and VR802 30 kOhm pots to ½ of VCC. Output before the blocking cap (supposed to be 1000 uF, now 1200 uF) should be 39 volts.

Proposed translation: VR801 and VR802 are on the board marked W15-091 and should be adjusted until pin 9 on W15-091 reads 39 V.
Problem: I see way more voltage than that—e.g., 45 volts or so. Can it be THAT out of whack?

2. THEN MarkTheFixer advises to adjust VR5 and VR6 wherever they are (connected to pins 8 and 10 labeled thermistor) to get 14 mV across the 0.7 ohm resistors.

Proposed translation: With the receiver upside down and oriented so that the power transformer is close to your body, look just to the other side of the transformer to find VR5 and VR6, which are not on any board but located on a separate stand-off. Adjust these to obtain 14 mV across the resistors located______________.

Problem: completing the last sentence is where I get confused. There are three 0.7 ohm resistors for each channel. Which one should span 14 mV? R27, R29 or R31 (for left channel)??? Can I pick this up at any pin on the board, or must I try to figure out which resistor is which? Or can I just measure voltage from emitter to pin 9 on W15-091?
 
Sorry I couldn't be more specific at the time, all I had to go on was the schematics that are in the data base, eyes - on beats it every time.

First question, you are correct, no changes to bias setting procedure.

RE: setting bias, it is the resting state of the amp, so NO signal, shorted inputs are something the factory says usually for pre-out power in jumpers, and they actually spec 5k resistors... no don't bother shorting, just turn it to aux (no am, no fm, no phono (phono may add a bit of noise))

With an output capacitor coupled amp, and no signal, there is NO way a speaker load can influence the bias setting. So it doesn't matter, but I would go with no speaker load.

RE: the 45 volts... over the years things drift... and instead of using 39 volts blindly, you need to measure the collector voltage of the "top" transistor (Q1 for the left and Q2 for the right) on the output driver and set the collector voltage of the "bottom" transistor (Q3 for the left and Q4 for the right) to be half of the top transistor's voltage. That is because your amplified signal swings between the "top" voltage and ground, and if that voltage isn't half then one side of the signal will be clipped sooner, resulting in less output power before distortion. In extreme cases, even very low volumes can be affected.

Slightly revised translation: VR801 and VR802 are on the board marked W15-091 and should be adjusted until pin 9 on W15-091 reads one half of the voltage on pin 6 on W15-091. It just worried me that the manual shows two pin 9's and pin 9 CANNOT be shared between the channels.

As for measuring the 0.7 ohm resistors, due to many factors, in the case of bias measurements, you should measure the voltages directly at the leads of the resistor with your probes.

And I have ABSOLUTELY no idea where they actually put the resistors. You have eyes-on here...

The two 0.7 ohm resistors per channel are connected to the emitters of the transistors, the 0.7 ohm resistor after the capacitor on the output will show NO voltage across it, literally down to the millivolt level, so measure across the three 0.7 ohm resistors per channel and ignore the 0.000 volt one. Just pick one of the other two, as the same current flows through both of them.

I guess I just didn't notice that they picked (of all things) a 0.7 ohm value for the resistor in series with the output after the capacitor.
 
Much oblidged Mark, just at the point where I wanted to make these adjustments on the SX-1000TW that came from GW last month. What a great sounding receiver.

In my test set up, I'm playing it through AUX, driven by a good TEAC CD player, and Roy Orbison is coming through with just a ton of depth (for lack of a better description), through little EPI 100s 3 feet off the ground. From grunt base through the wispiest highs, this 1970 model (with a lot of dust, looks original in there) is a cap coupled sleeper.:nono:

The GW purchased Pioneer SA6800 sounds much more clinical, and although the audio power between these two is about the same, the music sounds just plain better through the not yet adjusted SX1000TW.

Middle shot shows the underside of the unit, with the two pots on the W091 board, set to get 1/2 VCC on pin 9 on each side, for symmetric amplification.
This unit was 80.0 VCC and pin 9 was at 40.1, both sides (no adjustment needed).

Then set idle currents (left shot), CCW on these decreases drop across the 0.7 ohm emitter resistors. This unit, after 10 minutes, was in the 5 mV range before adjustment.

Shot on right is the setup where the SA 6800 gets basically outclassed by the 1970 vintage cap coupled SX1000TW.
 

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gentlemen, your contributions thus far have allowed me to take the next step in this SX-1000TW restoration and actually follow the service manual and fine tune the voltage levels to ideal conditions.

I initially recapped the power supply and Main Amp Unit and replaced the transistors to MJ211946s and listened to it for years like that with my near field book shelfs and never noticed an issue.

Having looked at the voltages on the preamp Pin 6 = 90.3V! and I couldn't get the pot to bring Pin 9 to 45V (maxed at 44.5V). I also noticed that Pin 3 = 41.8V (B+)..... Then around back I noticed that the Mains voltage selector was set to 110VAC...

20180930_190358.jpg

After switching to 130VAC input (my area usually sits at 122-124Vac), I was able to read Pin 6 = 76.8V, Pin 9 = 38.4V (1/2 of Pin 6), and Pin 3 = 40.8V..
I also adjusted the current through the power transistors to be 20mA... or 28.6mV across 0.7Ω resistor.

20180930_195741.jpg

I did notice in the service manual Page 21 "Main Amp Unit" shows 82V on pin 6, mean while the power supply that feeds Pin 6 is showing 78V, and 1/2 of 78 = 39V, Pin 9.

Also, I was wondering, on page 22 looking at Main Amp Unit Pin 2 shows a 400mV in a box, mean while I read almost 1-2mV... Does the box indicate that it's the voltage with an 8Ω speaker connected?
 

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There should be a legend area on the main schematic with different box types in it and explanation for the box. It's in the Bottom Left Hand Corner of Page 22. 400mv(input voltage) @ 50W/8ohm (full power). This is how many mv should be at the input of the amp, when cranked up to 50W (full power into 8 ohms) at the speaker outputs.

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There should be a legend area on the main schematic with different box types in it and explanation for the box. It's in the Bottom Left Hand Corner of Page 22. 400mv(input voltage) @ 50W/8ohm (full power). This is how many mv should be at the input of the amp, when cranked up to 50W (full power into 8 ohms) at the speaker outputs.

View attachment 1295871

Cool, that makes sense. So if it's a signal voltage then it should be measured in mVAC right?
 
Gonna resurrect this because I'm trying really hard to figure out what's going on with this Pioneer. I'm getting the following:
Left side pin 6: 172
Left pin 9: 86
Right pin 6: 171
Right pin 9: 84

I've searched this out because there's left channel distortion and quite a lot of heat difference between sides when feeling the dissipators. I'm trying to figure out if it's a dying transistor or maybe I need to recap?
Also, I should note that the only way I got those to 1/2 v as suggested in this thread is by turning the pots full over.
 
If I'm reading your post right ,those measurements are from the main amp board.Pin 6 is to high it should be 78-82v .Your measuring Pin 6 to chassis ? . What do you have on pin5 power supply board ? You should at lease recap the power supply for now, the rest later. I just finished with 3 1000tw's and those pots were all turned full,on all of them .
 
Hi all! Well I have used the thread info to set the voltages on my Allied 395 which is basically an SX1000. I recently rebuilt the main amp board, replacing most components as there was a power issue when I got it which was smoking resistors and transistors on one side. I also replaced the 4 main filter caps, .7kohm resistors and variable resistors in the main body. All voltages set well (although reading through I see some discrepancy as to what the voltages across the .7 resistors should be). However, once I introduce a load it all goes to crap. I get a pulsating heartbeat sound through the speakers and the voltage values jump up and down. So any ideas on what I screwed up?
 
Hi all! Well I have used the thread info to set the voltages on my Allied 395 which is basically an SX1000. I recently rebuilt the main amp board, replacing most components as there was a power issue when I got it which was smoking resistors and transistors on one side. I also replaced the 4 main filter caps, .7kohm resistors and variable resistors in the main body. All voltages set well (although reading through I see some discrepancy as to what the voltages across the .7 resistors should be). However, once I introduce a load it all goes to crap. I get a pulsating heartbeat sound through the speakers and the voltage values jump up and down. So any ideas on what I screwed up?

make your very own, new thread.

Tell us exactly which components you replaced, WHAT you replaced them with, and conversely - which transistors and electrolytic caps are still originals (as you got it) and what are their parts numbers...
 
make your very own, new thread.

Tell us exactly which components you replaced, WHAT you replaced them with, and conversely - which transistors and electrolytic caps are still originals (as you got it) and what are their parts numbers...
Done! Thanks for the advice.
 
Resurrecting this one because I am a little unclear on the settings for the VR5 and VR6. Is that a variable or is it going to be the 20mA... or 28.6mV across 0.7Ω resistor?
My main volts are 82-ish volts and I set VR801 and VR802 to 42.5V. I currently show24.1 and 30.9 mV on them now.

Do I need to just adjust these to 28.5 mV?

Thanks from this newbie.
 
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