Pioneer SX-1980 Power Supply Issue

Working on a Pioneer SX-1980, power supply assembly board AWR-154. The unit works with good strong output but there's a hum so that's the main symptom.

It's always more difficult to follow behind someone who "rigged" one or more repairs on this board. Keeping that in mind, I'm dealing with changed parts.

As of right now I don't have the voltages I should have on one side. The positive side of the board is fine, I'm getting 30v to 35v on pin 9 and 12. The problem is with the negative side where I'm only getting -10v to -13v on pins 8 and 11 when I should be reading -30 to -34 volts.

Transistor Q213 was bad. When a good transistor is installed (On-Semi), R213 will blow and Q213 will fail. I thought that FET Q207 might have been compromised so it was replaced, same problem. Diodes D210/D211 both test good and Q209 was replaced, Q215 & Q217 both are good, even checked with new parts and completely out of the circuit, same problem. I've also double checked the foil traces and don't see anything shorted. So I'm missing something and need help to figure out what's going on and how to find the problem.

basicblues said:

Working on a Pioneer SX-1980, power supply assembly board AWR-154. The unit works with good strong output but there's a hum so that's the main symptom.

It's always more difficult to follow behind someone who "rigged" one or more repairs on this board. Keeping that in mind, I'm dealing with changed parts.

As of right now I don't have the voltages I should have on one side. The positive side of the board is fine, I'm getting 30v to 35v on pin 9 and 12. The problem is with the negative side where I'm only getting -10v to -13v on pins 8 and 11 when I should be reading -30 to -34 volts.

Transistor Q213 was bad. When a good transistor is installed (On-Semi), R213 will blow and Q213 will fail. I thought that FET Q207 might have been compromised so it was replaced, same problem. Diodes D210/D211 both test good and Q209 was replaced, Q215 & Q217 both are good, even checked with new parts and completely out of the circuit, same problem. I've also double checked the foil traces and don't see anything shorted. So I'm missing something and need help to figure out what's going on and how to find the problem.

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My first inclination would be to check the zener to see if it is damaged. Then I would be looking at C217 electrically leaking, it could have been subjected to over voltage and damaged during one of the failures, then the zener replaced - but not the cap.

I would check ALL the parts at the same time (even the polarity on the electrolytics..), and I would remove Q215 & Q217 until I had the -80v working correctly. That way there are only 3 electrolytic caps connected in the regulator that can cause problems.

What happened to the -80v as R213 was being destroyed?

C221 would be just too much of a load if it is leaking.

C217 leaking would pull too much current through Q213. And drop the apparent zener voltage.

C219 leaking would put too much voltage on the base of Q213, turning it on hard. D213 could be popped. Then either Q213 pops or the voltage is throttled down. Depends upon the condition of the rest of the circuit.

markthefixer said:

What happened to the -80v as R213 was being destroyed?

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Don't know for sure what was going on with the -80v when I saw the R213 start smoking I shut it off immediately.

I had the SX-1980 back on the bench again yesterday, mainly to test for bad parts. I have correct replacement caps available for C217/C219/C221 plus another zener diode for Q207 and thought that I had everything I needed but I didn't have the diodes necessary for D213 which is bad. So I may need help finding exactly the right diodes to order.

It's a little confusing with the D213, the schematic I have shows (1) diode but (2) diodes are actually installed. So I am wondering if I have a later updated circuit SX-1980. The supplemental service manual (page 120 of the .pdf SM file) outlines changes to these diodes in this circuit with additional diodes D218 & D219 added in series with D213 & D214 if I'm looking at schematic correctly.

markthefixer said:

I would check ALL the parts at the same time (even the polarity on the electrolytics..), and I would remove Q215 & Q217 until I had the -80v working correctly. That way there are only 3 electrolytic caps connected in the regulator that can cause problems.

Click to expand...

Good call! I was excited to get back to the board to check just what you suggested (the -80v) and I had inadvertently already removed Q215 & Q217 while checking other parts but never checked pin 18 for the -80v, now it'll be a few more days until I get the parts I need.

Back to the diodes, the only cross reference book I have is an ECG. To get started, I used the ECG guide to get an idea of the voltage for diodes until I could get to my computer to hopefully cross reference to something else better (not a big fan of ECG). Lets see how far off I am (this time).

The WZ-177 or MZ-177 crossed to ECG5077A or NTE5077A. (1w-18v)
The WX-240 crossed to ECG5081A or NTE5081A. (1w-24v)

Considering the supplemental service manual change, the added D218/D218 in series with D212/D213 would bring the voltage to 36v, correct? If so, I was thinking of just using a single (1w-36v) diode such as ECG5085A or NTE5085A.

Quite a coincidence that ECG and NTE have the same basic part numbers... not! I'd like to find something else but no problem using NTE if necessary but I'll be looking this afternoon for alternative parts but if you have a quicker cross reference source I'd apprecaite your input on these (2 or 3) diode values.



Quick Specs:



NTE5077A
Nominal Zener Voltage Vz@Izt Volts = 18
Zener Test Current (Izt) Ma = 14
Maximum Dynamic Impedance Zzt@Izt = 20 ohm / Zzk@Izk = 750 ohm / 0.25mA
Maximun Leakage Current IR = 5 uA @ VR = 13.7 volts
Max Forward Voltage VF = 1.2 volts @ IF = 200 mA

NTE5081A
Nominal Zener Voltage Vz@Izt Volts = 24
Zener Test Current (Izt) Ma = 10.5
Maximum Dynamic Impedance Zzt@Izt = 25 ohm / Zzk@Izk = 750 ohm / 0.25mA
Maximun Leakage Current IR = 5 uA @ VR = 18.2 volts
Max Forward Voltage VF = 1.2 volts @ IF = 200 mA

NTE8085A
Nominal Zener Voltage Vz@Izt Volts = 36
Zener Test Current (Izt) = 7.0 mA
Maximum Dynamic Impedance Zzt@Izt = 50 ohm / Zzk@Izk = 1000 ohm / 0.25 mA
Maximun Leakage Current IR = 5 uA @ VR = 27.4 volts
Max Forward Voltage VF = 1.2 volts @ IF = 200 mA

Haven't read all of this thoroughly, but the 1980 power supplies always need a full rebuild/recapping. Also, if you need to replace the 35V zener diode (which you usually need to do), replace it with a 17V and an 18V 1/2W wired in series (this is a Pioneer recommended modification).

EchoWars said:

Haven't read all of this thoroughly, but the 1980 power supplies always need a full rebuild/recapping. Also, if you need to replace the 35V zener diode (which you usually need to do), replace it with a 17V and an 18V 1/2W wired in series (this is a Pioneer recommended modification).

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Based on what I've seen so far with this board a total recap is certainly in order. Any recommendations on the diode part numbers WZ-177 or MZ-177 other than what I've already outlined might be helpful. I'd like to place an order for what I need tomorrow so delivery would be here by the weekend.

The 35v zener diode is definitely bad, the 17v and 18v zener diodes were actually installed as you describe for the 35v, so I that's why I was thinking that this particular unit may have been a later build.

Since a rebuild/recap will be happening as soon as the final parts arrive, any other bulletproofing of parts that I might need to change or be aware of before I place my order?

basicblues said:

Based on what I've seen so far with this board a total recap is certainly in order. Any recommendations on the diode part numbers WZ-177 or MZ-177 other than what I've already outlined might be helpful. I'd like to place an order for what I need tomorrow so delivery would be here by the weekend.

The 35v zener diode is definitely bad, the 17v and 18v zener diodes were actually installed as you describe for the 35v, so I that's why I was thinking that this particular unit may have been a later build.

Since a rebuild/recap will be happening as soon as the final parts arrive, any other bulletproofing of parts that I might need to change or be aware of before I place my order?

Click to expand...

I don't know where you plan on sourcing expensive and hard-to-find Japanese diodes in Arkansas, but I simply recommend a plain 1N5247 and a 1N5248. I also like to replace Q210 and Q211 with tough On-Semi MJE15032 & 33. If the Q206 or Q207 current-source FET is bad, finding a replacement that will last more than a week is a problem. With the help of some guys at DIY Audio, we came up with a current source that is a lot more reliable: http://www.diyaudio.com/forums/showthread.php?s=&threadid=92904

EchoWars said:

I don't know where you plan on sourcing expensive and hard-to-find Japanese diodes in Arkansas.

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Finding any audio related electronic parts in Arkansas is certainly a challenge, everything has to be ordered so I don't bother looking anymore. Your dialed in on what I'm really asking now and that's where and what parts to buy that would be consider decent quality replacements or upgrades.

EchoWars said:

I simply recommend a plain 1N5247 and a 1N5248. I also like to replace Q210 and Q211 with tough On-Semi MJE15032 & 33. If the Q206 or Q207 current-source FET is bad, finding a replacement that will last more than a week is a problem. With the help of some guys at DIY Audio, we came up with a current source that is a lot more reliable: http://www.diyaudio.com/forums/showthread.php?s=&threadid=92904

Click to expand...

Thank you very much for this link! When I read your first post it sounded like you were describing the unit I am working on, almost exactly. I feel I need to re-group and take another look at the parts I have on hand, most of which are On-Semi, and locate what parts I need to locate. Otherwise, I'm just watching paint dry waiting on the next failure if all the weak links aren't corrected. Is this the worst design of the SX-1980? That's what I think everytime I look at this circuit section.

Time to do some homework and work on a parts list, I may need your help deciding or locating a particular part.

The serious hum issue I had on the 1980 in the DIY Audio thread was due to a busted ground wire and disappeared once I soldered it to the proper ground post. Still, the board needed a full rebuild to be reliable.