Pioneer SX-727 Blown Internal Fuse - what can I do?

tusntuk2

tusntuk2

Active Member
Hello AK, thanks for looking in. I'm having trouble identifying the cause for blowing an internal 3A fuse (# 2) in a Pioneer SX-727. I have reviewed other threads here and made some changes to my unit, but the fuse blew again while on the DBT, so I'm stuck, now. Can't measure voltages if it can't function. Any ideas?

This SX-727 was actually my first recapping project, and back then I had only upgraded caps on the PCB's. I also installed new panel lights (from Dave W.) at that time, about 4 years ago. The amp has been running well since then, but the other day it went black and silent. Hate when that happens!

I read earlier threads that went through restoring the AWM-027 protection circuit, so I did that. I had changed the caps before, but now I changed all the transistors: Q1-4 with c1845, Q5 with A992, Q6 with c2383, and Q7 with c2690, per AK pros. I also changed the blue Sanyo .22 caps with Wima film. I did not change the diodes, but I can.

Other threads discussed the power supply, AWR-011, always of first importance, so I made a couple of changes there, too. I installed a new 13v Zener diode (D6), a c2383 for Q2 and TIP31's for Q1+3, with heat sinks. The 330/2w resistor did not show much PCB burn, but I boosted it to 3w and raised it off the board.

Since I hadn't opened this unit for 4 years, and I had learned more about the restoration process in the meantime, I wanted to take the opportunity to upgrade what I could while I was at it. Since several threads dealt with fuse problems, I went ahead trying to upgrade the protection circuit and the power supply, hoping to fix the fuse problem. Yeah, I know, I know, I know, I should only have done this work when the machine was operating correctly! I have gone over the work repeatedly, and I believe I did not introduce any new problems, at least I hope I didn't. But - it still blows the internal 3A fuse, and this time on the DBT. Maybe it's something in the panel lights?

Now, before firing it up again, I have pulled the main outputs, and they all test good in my cheap tester and with a DVM. I read about the trimmers dying, and I have replaced all 4 trimmers, too. I have transistors in stock (c2690, a1220) if I need to replace the drivers, Q9-12, but I stopped here to raise the flag for help.

How can I test a unit I can't seem to fire up? What would you do next?

Thanks again for your ideas and assistance!
 
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Some questions first.
  • Do you mean fuse F2?
  • What was the last work before the fuse got blown?
  • Did you change any of the panel or meter lamps?
  • What bulb (watt) to you use with your DBT?
  • What was the reaction of your DBT, did the bulb stay bright for more than 2 seconds?
 
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Yes, F2 inside. There are three fuses underneath, two are .5A and this one is 3A. Schematic shows it connecting to the lamps and to the protection board. My model does have the multiple voltage tranny.

The receiver was playing through the Aux input at the time, I made a bass control adjustment of one click and the amp died. It had been running for over 4 years until then, after I had recapped it in 2017. I did not do any of this new, described work until after the fuse blew, and I was trying to fix it.

I changed the panel and meter lamps in 2017. Got them from Wojo, a trusted AK source.

My DBT has a 60W bulb, and it has been a good friend, performing correctly through over twenty restorations.

I made changes to both the protection circuit and the power supply before trying a new 3A fuse. The DBT glowed brightly for two seconds or less, and then went dark as the fuse blew.
 
Will do, Pete. Am I measuring the transformer or looking for another short? The fuse line also connects to the protection circuit, pin 17, and a diode to ground, which I changed, too.
 
Hmmm, not wanting to stupid there, but I see that all of the points going to lamps go through the lamps to ground. I guess that's why you are looking for some resistance reading?

OK, my bare leads shorted together read about .3-4 ohm. Any ground to ground test is also .3 -.4 ohms.
I did de-solder all connections to the fuse before these readings. There is no fuseholder, per se, just short wires soldered to the fuse.

The transformer side reads .4-5 ohm to ground. The schematic shows this winding to be grounded.

The 5 dial lights read about 3.8 - 4.0 ohms, when measured at the post on top of the dial. At the fuse, the connecting wire from the fuse to the post on top of the dial gives me all kinds of weird jumpy readings! And the wire looks OK, no visible burns, breaks, bare spots to ground, etc. But it's weird I could not get a reading through this solid wire. Maybe this is suspect?

The connection to Pin 17 on the Protection board also connects to the stereo indicator light, which connects to the FM board. No stable reading there, as expected. I had replaced the diode and the cap there, D1 and C1, by the fuse connection, and I measured past these pieces.

I will open the dial and check the LED fuse lights. First observation shows no burns, sparks, bad wires, loose connections etc. The lamps look normal.
 
dont worry about transformer side of fuse but if you disconnect the lamps to rule them out . failing that disconnect pin 17 if its easier or lift one end of D10 protect circuit .
 
Have a check on both meter lamps (signal and tuning) as well. Did you replace this two as well with 8v-AC LED in fuse style? Or are these still 8v-incandescent light?
Some time ago we worked hard on a similar use until the thread starter found out, that he had mistakenly placed a fuse instead of one of the 8V lamps!

Can you post good pictures from the backside wiring of those meters?
 
I have a SX-727 here and replaced the 8v-incandescent lights with 8v-AC LED in fuse style 2 years ago as well, but figured out one very special problem?

The lamp holder used on the pioneers are short and the gap to the holder screw is very small. That was never a problem with the glass tube of the incandescent lights, but with the LED lights the backmounted smd part got in contact with the screw.

The best solution for me, was a change with flat-hat-screws I got from old harddisks as you can see here.
IMG_20190213_204948.jpg IMG_20190223_131129.jpg IMG_20190223_131822.jpg
 
I think I can disconnect Pin 17 on the protect circuit. Pete - you say to lift D10 - there are only 8 diodes on the protection circuit. D4 does connect to Pin 10, if that's what you mean.
 
I'm still trying to understand what condition caused F2 to fail. I mean, yes, it suggests a short somewhere, but how does an amp go from playing for years to a sudden short? There was lots of discussion about the various Protection circuits, too, although that problem was usually the opening or closing of the relay, and not the power to the amp. I understand better now about Bias voltage, so I zeroed in on that and the poor old trimmers, and changed the trimmers, but I can't fire up the unit for blowing the fuse. That's frustrating!
 
hold fire i just thought of something ..
if you check ohms from that diodes anode to chassis then same again with cathode . it should maybe enlighten us . all this with fuse removed .
 
Fuses can blow just from old age. Vibrations from moving in house, and transport, and Heat cycles eventually weaken the filament, and the last time it works it get weak enough to blow. Not saying this happened as you have other problems, just addressing the general question in post 14.
 
New readings -
Ohm readings from the fuse connection to the dial lamps is about 1.1ohm. They all look good, they all test about the same 1.1ohm to ground, of course they are all connected together.

The meter lamps show about 1.3 ohms or 0.0, depending on the function switch position.

The stereo indicator light connects to the FM board, so the reading is funky.

That wire connecting F2 to the Dial lamps tests about .3 - .4 ohms by itself, just like bare leads or ground-ground do.

I'll go test that D8 on the protection board
 
D8 anode/ground = 11.15K ohms. Cathode/ground = 0.L. Measured with diode in circuit.
My circuit has this change to the AWM-027 schematic - View media item 29262
Should I lift one end of the diode before measuring?
 
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