• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Trying to repair Pioneer SA-1490

R

Rocky_714

New Member
Hello all,

I’m exploring a possible repair on a Pioneer SA-1490 receiver I got from a friend who insists it was working for him before giving to me. I’d like to save it from the recycle pile but it’s not working now.

Two sentences about me. Although trained as a EE, I’ve leaned on more of the theoretical and DSP software side for most of my career. I can (mostly) read a schematic but attaching that to practical and the physical realities gets tricky for me. I have little practical experience.

So, here is the symptom. At turn on, the delay for turn on transient settling flashes a light on the front panel. The input selector light is on the last selected input and after a few seconds the flashing light settles to on. However, the top part above a cosmetic crease in the panel, nothing lights up. This part houses graphic equalizer tone controls (I’m 99% sure they should). Behind that front panel is only one circuit board for both top and bottom parts.

To debug, I started at square one and tried to verify the power connections going to that circuit board. The rest of the description references p.6, coordinates B10 of the schematic, of the online viewable service manual at

https://archive.org/details/manual_SA1490S_SM_PIONEER_EN

The connector CN3 (coordinate B10 of the schematic) goes from the power board to the board behind the front panel. I was able to verify that pin 1 of that connector has power (measured at C503, 5.3V versus the 5.6V it should be, which is probably not fatal). However, a close relative of pin 7 is the output of the rectifier (coordinates C11, right above capacitor C511) where it shows it should be 20V, measures next to nothing. All the fuses are okay so that’s not the problem.

Here’s where I need the practical help. I’m not sure what to do next. Does the fact that the near 5V supply looks good mean that the transformer is probably good and the fault is elsewhere (meaning that the capacitors supporting that rectifier may be at fault)? Or could that part of the transformer (those particular taps) have failed because historically/likelihood that’s what would have happened? If it is the transformer, is replacement unlikely making repair impractical?

I apologize if this is the wrong place for this kind of post but I’m not sure where else to turn. If there are recommendation for a better place (forums), please make them. If there is guidance for this repair here, I certainly welcome them. If I can do my part to lessen the burden on our landfill/recycling and get a receiver out of it, that’s all the better. Thanks in advance.

-Rocky_714
 
Register to hide this ad
Thanks for the reply, Pete. I checked the AC voltage across 11 and 12 and sure enough, it was pretty much zero. I guess that means the transformer has failed? Are there any other explanations? There looks to be a wonky 220/110 voltage selector in back cellophane taped in back of the unit but my guess is if there were any input problems, I wouldn't get even the 5 V on the low voltage side? Is that a fair assessment?

So, does that make it pretty much dead unit? Do I have any options here? Not a huge deal at this point since it didn't cost anything, just a lost opportunity.

Thanks again for taking a look and for the help.

-Rocky_714
 
Amps with remotes, always fun to troubleshoot. I'll download a schematic and see what's there, it's possible you have a relay operated circuit to turn off that circuit with a separate transformer. Very likely switched if it's a fluorescent display.

Got one, 5.6V is from a seconday transformer that is "always on" .

With power off check for continuity in the transformer wiring for that part of the circuit, might be open (inwhich case you need another transformer if you can't fix it).

I'll keep looking to see if there is a main power on/off relay somewhere.

edit:

Found it. Switches the transformer on and off, so if you have power to the rails (13&14) but not the 20V section (12 &13) you have a broken wire, cracked connection, or an open transformer winding.
 
Last edited:
To HPMguy:

Thanks for taking a look. The equalizer in the top half of the front panel is accompanied by a fluorescent display that is in the part that is not working. However, that is high(er) voltage part (18-20V), that is the part that is driven by the main transformer. The lower voltage part (5V) has it's own smaller transformer on the same board as the rectifier electronics and that seems to work OK. I posted the link to the online service manual with the schematic in the earlier post because I thought it would be easier and because I didn't know if the forum allowed attachments but I have a PDF of same if allowed and you want it. Just let me know.

To Pete:

I'm not opposed to taking a shot at what you suggest with the transformer since there's not much more else to lose but I've certainly never done anything like that and it sounds ominous. The windings themselves would be impenetrable so the break I'd be looking for would have to be near the surface, that is, from the inner windings to the connectors that lead outboard toward outward connections, I expect. I would think that I'd have to be pretty lucky for the break to be in these limited inspect-able locations. Also, I'd need to get the wirewrap and removal tool since that's what connects the wires to the board. Also, I've not ever seen a thermal fuse on a transformer before. I assume they're provided on the larger transformer like this one. I imagine it's on the underside and not immediately visible without removal from its mount because I don't see it now. Do I have all of this generally right?

To both:

Since the transformer hacking seems like a last resort kind of thing to, let me know when there's nothing left to try before I take it on. Thanks.

-Rocky_714
 
Typically the thermal fuse will be close to the surface, hidden under the wrapping material. I have no idea just where it is (if it's on that circuit only, typically they are for the whole transformer). Anyway, check for a broken wire -- should have zero ohms between the solder trace on the board and the soldered connection at the transformer. Ditto for a cracked solder connection on those wires.

Check to make sure you have power on the rails ( main rectifier and big caps) -- transformer is switched on and off by Relay 501.

And, if the equalizer display is all that 20v supply drives, the amp may work just fine without it.
 
Here’s what I did. I first performed a continuity check between the output wires of the transformers (not the connection posts or the transfer wires) and the connecting posts of the board (not the wire wraps on them). It was tricky to touch transformer wires between the end of their insulation and before the connecting post and transfer wires. And even though they tested good, I re-soldered those connection wires on the output side and on the ones input side as well. No change in behavior.

I went to check the inputs and outputs of the big transformer and verified across certain input terminals (there are 7), I get 120 VAC. Across no pair of output terminals did I have any AC voltage. I must conclude that the transformer must be dead at this point. I have to go on the hunt for the thermal fuse.

This looks tricky. The wrapping material goes around the bulge on the long side of the transformer and under the metal case on the short side. Under the wrapping material but over the bulge looks to be a short, limited piece of cardboard. What is the strategy at this point? I want to get this straight before I do something irreversible. I would think that an Exacto knife cutting at the edge on both sides of the bulge to remove the wrapping material and the cardboard. If the thermal fuse doesn’t present itself on the one side, do the same on the other side. Does this sound right? And if all this turns out to be successful, replace the wrapping somehow. Thermal tape of some sort?

Or do I have to do a full removal of the transformer before I operate on it?

Thanks again for the help. I’m looking at this as a learning exercise even if it doesn’t work out. There’s nothing lost here and only something to gain if it works.

-Rocky_714
 
Well, folks, I’m sorry to have to say it was a total failure. I started by trying to make entry into the transformer in a nondestructive way. The outer band seemed to be copper, or at least some metallic composite. I clipped one edge on the output side so that I could pivot it away from the transformer core and windings. The cardboard underneath came away easily after that. However, underneath that was multiple layers of blue vinyl or plastic stretch wrap like you’d find protecting your weather sensitive outdoor equipment. I started cutting into that, carefully I thought. Before I knew it, I cut into wiring leading to the output posts. Even then, I could find no thermal fuse.

At that point, I decided to remove the transformer and open it up and destructively try to find the thermal fuse. I opened both sides removing all covers on both the input and output sides and I did not see a thermal fuse. Since I figured the effort was destined for failure, I sort of lost interest. If there is a thermal fuse in that transformer, it’s internal and I wasn’t going to be able to get it despite my efforts.

Anyway, thanks to those who helped along the way. Even if it wasn’t successful, I gained sorely needed experience and started to learn some of the practical side of the art. Thanks again.

-Rocky_714
 
You must log in or register to reply here.
Back
Top Bottom