Learning project: repair a junked audio receiver!

[de_light]

I acquired a broken ('dead') Pioneer SX-2700 for next-to-nothing. I thought it would be an interesting little project to help me learn a bit more about electronics, reading schematics and troubleshooting in general. There is no actual gain to be had out of this project, apart from saving something from being junked. Even if it isn't repairable, if I could work out what is going wrong in it, that will be enough.

Service manual: https://audio-circuit.dk/wp-content/uploads/simple-file-list/ssmp/Pioneer-SX2700-rec-sm.pdf
Schematics and boardview start from page 5

The unit doesn't turn on, but a relay is heard to click when plugged into the mains. It seems to be properly grounded. I've opened it up and given it a good clean as it was filthy. There are no obvious burnt components visible. The power button on the front panel doesn't do anything.

There seems to be a standby circuit that supply 5VDC and a backup 5VDC to IC801, so I have started my troubleshooting there. I have replaced C752 and C753 which were relatively small electrolytics that had a high ESR when I pulled them. I also replaced Q751 powering the relay as it was showing as open on my tests - I now doubt that because the relay still functioned with the original component. The relay RY751 is functioning well, with an adequate resistance showing between the coil. The single NO contact is not shorted.

I've traced some of the voltages (relative to chassis ground) and they are annotated on the screenshot below. From what I can see, the voltage across the Zener diode D753 feeding the 5VDC supply is too low - only 2.5VDC. R751 is reading 820 Ohms. Could this be a transformer issue? (Unfortunately, no labels on either the standby or main transformer).

Many thanks.

[greenpossum]

But the voltage on the other side of the 820 ohm resistor seems to be ok at 10V. So maybe the resistor or the zener or the load it supplies. Couldn't you post a sharper schematic? It's a bit fuzzy.

[rsjsouza]

Is there a way to upload the schematics to the forum? When I click on the image the new browser window asks me to install an app to see the full image.

[de_light]

But the voltage on the other side of the 820 ohm resistor seems to be ok at 10V. So maybe the resistor or the zener or the load it supplies. Couldn't you post a sharper schematic? It's a bit fuzzy.

I've only ever found one scanned version of the service manual online and I agree, it is a terrible scan. Sorry.

Is there a way to upload the schematics to the forum? When I click on the image the new browser window asks me to install an app to see the full image.

Bit strange that. Do you mean the thumbnail? Just checked it now and loads fine on mine - hosted with ImageBB. You can also access the full service manual with schematic that I have hyperlinked in the original post - it's a PDF. Schematics are on p5.

[de_light]

So, done some more probing around.

So, a little update. I have probed around and have found some interesting things. All of the inputs to the main transformer register 80VAC. Each of the outputs however seems to be very low. Outputs 3-6 are unstable: they each register between 8.5 and 9 VAC but holding the multimeter on them for a few seconds draws the voltage down to 0.

There is no input or output to the first rectifying diodes D702 and D703.

I've pulled all four visible fuses and all have continuity across them.

Any tips on what to do next?

Annotated schematic attached.

[garrettm]

Have you pulled D753 and tested out of circuit to see if it's working okay?

Also, were the AC voltages measured WRT to the "ground" center tap on the secondary side, or was it differentially across the windings with similar labels? The FL (filament?) winding should probably be measured differentially, as it looks to be floating. If you are measuring voltages WRT to earth ground, instead of the star-ground on the PCB, you won't get proper readings.

The schematic only shows one primary winding on the power transformer, but usually there are at least two windings configured for 120 Vac. This way you can place them in parallel or series for 120 and 240 volt operation. But with Japan's wacky 100 Vac mains and whoever uses 220 Vac, there might be a 100 volt tap on the two 120s to get the whole shebang (100, 120, 220 and 240 volt AC mains). Is the 80 Vac across the whole primary, or is this across one of the primary windings? It sounds low and might indicate a bad primary winding or excessive contact resistance in the path to the transformer. You may want to check that the stand-by relay has continuity when on and that it's contact resistance isn't excessive. I've replaced quite a few relays with burnt out contacts. Even if it audibly clicks, that doesn't mean it is actually working.

[de_light]

Have you pulled D753 and tested out of circuit to see if it's working okay?  Also, were the AC voltages measured WRT to the "ground" center tap on the secondary side, or was it differentially across the windings with similar labels? The FL (filament?) winding should probably be measured differentially, as it looks to be floating.

The schematic only shows one primary winding on the power transformer, but usually there are at least two windings configured for 120 Vac. This way you can place them in parallel or series for 120 and 240 volt operation. But with Japan's wacky 100 Vac mains and whoever uses 220 Vac, there might be a 100 volt tap on the two 120s to get the whole shebang (100, 120, 220 and 240 volt AC mains). Is the 80 Vac across the whole primary, or is this across one of the primary windings? It sounds low and might indicate a bad primary winding or excessive contact resistance in the path to the transformer. You may want to check that the stand-by relay has continuity when on and that it's contact resistance isn't excessive. I've replaced quite a few relays with burnt out contacts. Even if it audibly clicks, that doesn't mean it is actually working.

All measurements were relative to chassis ground. I've just remeasured them relative to the centre tap on the secondary side, and all are reading 0V

I've actually replaced D753 with a 5.1V Zener -  the original checked out OK but disappeared due to my poor handling.

I've checked the coil resistance in the standby relay and it meets the datasheet specs.

Interestingly, I measured the AC voltage coming in from the wall into the unit (a terrifying prospect for a beginner) relative to chassis ground - only read 80VAC (brown wire coming onto post 6 on the PCB, first entry of mains into the unit). I then checked voltages coming into the primary transformer on windings 1-4 relative to the centre-tapped ground on the secondary side - still 80VAC.

The voltage selector switch is definitely set to 220/240V on the rear.

[garrettm]

All measurements were relative to chassis ground. I've just remeasured them relative to the centre tap on the secondary side, and all are reading 0V

That's good! The new measurements imply the power transformer isn't getting power. So now we know where to look.

I've checked the coil resistance in the standby relay and it meets the datasheet specs.

The coil may be fine, but the switch contacts might actually be bad. Since the new measurements showed 0 volts across the secondary windings, we can assume the relay might be bad. So let's unsolder it and hook it up to your power supply and DMM for testing. Connect the PSU with 5 Vdc to the relay coil and then set your DMM in ohms mode and connect it to the pins that are used to switch the AC mains to the power transformer. If the DMM reads open when the coil is both off and on, you have a burnt out relay. If the contact resistance is high (more than an ohm) when energized you may need to clean it or just replace it outright. If it has extra unused contacts, you may be able to solder in some jumper wires as a quick bodge to test further.

[garrettm]

Just a quick thought. Sometimes you can pull off the plasic cover to expose the contacts. So if you don't want to unsolder it, you could simply pop the top off and look to see if the contacts are okay. And maybe take a sheet of paper and move it back-and-forth between the contacts to clean them.

Of course, you would want to disconnect the amp from the wall before fiddling with the relay!

[de_light]

This is boggling my mind, in a good way. Very impressed.

Couldn't open the case so I just desoldered it and hooked it up to my DMM and PSU as you said...
Nothing.

No click.
Infinite resistance

So confused!! Why is it clicking when it is in circuit then?!

Relay datasheet: https://www.onlinecomponents.com/productfiles/mf-om/g5p.pdf

[garrettm]

Try 12V (or whatever is listed on the body of the relay). I figured it was a 5V coil, as most of the stuff I've repaired lately used 5V.

Just realized I could read the PSU display...Doh! 5V / 16mA = 313 ohms. So by the datasheet we have either a 9 or 12V coil, depending on the "sensitivity".

[de_light]

Twelve volts works okay. Coil clicks and resistance across the contacts is zero ohms. I've taken it apart and watched it make contact.

[garrettm]

Twelve volts works okay. Coil clicks and resistance across the contacts is zero ohms. I've taken it apart and watched it make contact.

Wow, that's strange then. I can't see where else there could be an open circuit to the power transformer. Unless that fuse shown inside the dashed lines for the transformer is open, it should be getting power.

[garrettm]

Have you tried measuring the coil resistance of the primary winding?

[de_light]

Maybe I wasn't clear in my post.

I think the power transformer is getting power - I read 80VAC on the primary side going into the transformer on inputs 1-4.


Also, interesting. It looks like a power voltage selector switch was broken off from the rear (this is the SD/non-US version, unfortunately the only service manual/schematic available is for a US version).

I read 0V on the output/secondary side of the transformer, however, when measured relative to the GND output of the transformer.

[garrettm]

Just to clarify (as I can be dense at times), you measured 80 Vac across the primary winding--and not from earth ground to each terminal of the primary winding, right?

Then, you also measured 0 Vac at each secondary winding (except FL, as it is floating) WRT the center tap "ground" that runs to the star-ground on the PCB--rather than using earth ground as the reference?

[garrettm]

Just noticed you had uploaded a new pic. I believe we have found our problem! That switch looks to be missing the inner rotary contacts. Was it always missing or did it just fall apart recently. And do you have the broken bits? I find it quite odd that the standby transformer is getting power with the switch as it is... Very strange stuff.

[de_light]

I can't believe I missed this. No, the receiver never had this switch on the back - I only noticed the rotary knob was missing when I was flicking through a PDF of the user manual.

The device has two voltage selectors. The sliding switch on the right is functional. However, the rotary knob is missing and it seems to not be set to any voltage at present. I wonder why this is why I'm only getting 80VAC going in.


It's not you being dense, it's me being a beginner and getting terminology wrong! Is this what you mean about measuring 'across' the transformer (in red)? And to measure the secondary voltages, the green line refers to that. I.e. using output 4 (GND) as the COM point.

[garrettm]

Is this what you mean about measuring 'across' the transformer (in red)? And to measure the secondary voltages, the green line refers to that. I.e. using output 4 (GND) as the COM point.

I'm sure I was a bit confusing in my previous replies. So don't feel bad.

The green line is correct for the secondary side, where we use pin 4 as our zero reference (fixing the black lead to this point while moving the red lead to the other pins on the secondary side).

The primary side must be measured using a "floating measurement" where the black lead is not fixed to a single reference point. We will measure differentially using one lead of the DMM on each side of the winding. Since the primary is not a single winding as shown in the US version of the manual, we will take a series of measurements between each pair of pins on the primary side. Since the windings/taps should be in series, we need only measure sequentially between pins, moving from one side to the other. The sum of the voltages should add to 240V. If we had the manual for the international version you would know which pins correspond to taps and which are full windings. If you can find the right pins (the ends of the series connected coils), you should measure 240V input to the primary winding without having to add all the voltages together. This all assumes the selector switches are working right, as the two primary windings/taps are electrically isolated so they can be connected in series or parallel depending on the switch settings.

I suspect that the broken selector switch is acting like an open circuit and no power is actually getting to the power transformer. It is odd however, that the standby transformer is powered. If you know what the selector switch connections were for 240 Vac you could solder in some jumpers and be good to go. But without the manual I couldn't say for sure. Last thing you want to do is burn out the transformer by connecting it for 110 or 120 while powering it with 240! So some caution needs to be had here.

[de_light]

I've found this rather interesting schematic. It is for the SX-2800, the next model up from the owner's manual I originally referenced. It has the layout for the international version including the broken selector switch. The first section of the standby transformer and power transformer almost looks identical as well!

https://www.vintageshifi.com/repertoire-pdf/pdf/telecharge.php?pdf=Pioneer-SX-2800-Service-Manual.pdf

Page 8

[garrettm]

Excellent find! Looks like pins M, 3, and 4 should all be connected together on the rotary switch. You could test that's the case using your DMM in continuity or ohms (with the unit unpluged from the wall, of course).

If there is an open circuit between any of these, try rotating the switch with a screwdriver to see if the switch still has the rotary contacts. If not, you can simply solder a jumper between pins 3 and 4. If the rotary contact is still in place (as it's unsafe being broken), I'd unsolder all the wires to the switch (after marking them with a sharpie) and solder 3 to 4 with a slip of heat shrink to insulate the joint and then put some tape over the unused wires and heat shrink a cap on the end of them so the tape doesn't peal off. Then put a sticker on the back indicating it is hardwired for 240V.

[garrettm]

Looking at the new manual, the standby transformer has its own selector switch. So it makes sense why it was working.

The other selector switch is only used for the power transformer, so mystery solved.

Well, maybe not. We still haven't powered it up. There may yet be more ghosts waiting to be exorcised.

[de_light]

Well, maybe not. We still haven't powered it up. There may yet be more ghosts waiting to be exorcised.

There sure are!

Well, I jumpered a wire across 3 and 4 and plugged it into my dim bulb tester. Flipped the switch, lo and behold....the device lit up! I almost fell off my seat. It was magical.

Before we go further (since the whole purpose of this is education):

1. How did you know from looking at that additional schematic that 3 and 4 were the required leads to be jumpered? 3 goes to winding 3 on the primary side and 4 goes to 5.

2. If there was an open circuit, why was I getting a voltage reading of around 80 Volts going into the transformer?

3. Even when the device turns on, I'm still getting only 3V at D753 on the startup circuit - I'm confused as to why the device is able to start when the voltage is low!

Just to let you know what I've done - instead of just capping of the redundant wires, I've removed them entirely. Except a purple wire, that leads from a point marked '3' on the PCB to winding one on the primary side of the power supply - nothing works at all if that's removed. I've also left the grounding wire too.





New problems!

The devices turns on, but only for about two seconds. It will then rapidly turn off. Sometimes, the display will flicker. Other times, it will 'hang' (i.e not respond to any input). I'm no means an expert, but just from tinkering around with computers, it *feels* like an intermittent voltage supply or a low voltage situation. I also hear from time to time a high-pitched wine or hum coming from the volume pot. The protection relay RY561 also clicks 'off' (I presume) after about four seconds.

[garrettm]

Well, I jumpered a wire across 3 and 4 and plugged it into my dim bulb tester. Flipped the switch, lo and behold....the device lit up! I almost fell off my seat. It was magical.

Nice! We're finally starting to making real progress.

1. How did you know from looking at that additional schematic that 3 and 4 were the required leads to be jumpered? 3 goes to winding 3 on the primary side and 4 goes to 5.

Well, you are on 240Vac mains, that implies the two primary windings must be in series for proper operation. So I simply traced the lines in the diagram and it checked out. It was convenient that the rotary switch diagram was already drawn with the 240Vac connection in place. The M pins on the switch are simply a jumper, so if the inner contacts fell out, pins 3 and 4 would be left disconnected and thus require a jumper. Or alternatively a jumper could be soldered at the transformer directly, to connect the two primary windings in series without a bunch of wires running around.

2. If there was an open circuit, why was I getting a voltage reading of around 80 Volts going into the transformer?

Well, the hot and neutral lines were connected to the ends of each primary winding. Hot switched by the relay and neutral always connected. So WRT earth ground, you may have measured some residual voltage, but there was no current passing through the transformer's primary winding due to the open circuit at the selector switch. This is why a differential measurement at each pair of pins on the primary side would have helped. You would have seen 240Vac between the ends of the two separate primary windings, but 0Vac between any two pins on each individual winding, as the two primaries were electrically disconnected.

3. Even when the device turns on, I'm still getting only 3V at D753 on the startup circuit - I'm confused as to why the device is able to start when the voltage is low!

The standby 5V rail only powers a small portion of the circuit, so its effect on the total operation of the unit is considerably less than that of the power transformer side. So it would make sense that it should still sort of work, even if the 5V rail has a fault somewhere (assuming everything else checks out okay).

Just to let you know what I've done - instead of just capping of the redundant wires, I've removed them entirely. Except a purple wire, that leads from a point marked '3' on the PCB to winding one on the primary side of the power supply - nothing works at all if that's removed. I've also left the grounding wire too.

That works too. I believe the purple wire goes to the relay on the PCB controlled by the standby circuit. So it would make sense that the amp wouldn't work without it.

The devices turns on, but only for about two seconds. It will then rapidly turn off. Sometimes, the display will flicker. Other times, it will 'hang' (i.e not respond to any input). I'm no means an expert, but just from tinkering around with computers, it *feels* like an intermittent voltage supply or a low voltage situation. I wonder if it could be related to some dodgy capacitors?

I suspect there is a short circuit loading the zener diode on the 5V stby circuit. We just need to look at the schematics and see what is powered by this rail and do a little poking around. Most electrolytics fail open circuit, so it could be a shorted transistor that’s causing the loading. If we measured high AC ripple, that would imply there are bad electrolytics. But the voltage is low, so we definitely have a loading issue.

[de_light]

Nice, thanks. I'm going to start poking around. I might start by pulling Q752.