SX-525 Refurb

[33&athird]

The two transistors form a differential amplifier, a strange beast with two inputs, two outputs, and a propensity to compensate for the other transistor.

A cool useful circuit. One thing to note in this application, is that one of the inputs of the differential amp is a feedback line from the outputs. This helps control everything quite nicely- the amp design on the 525 is really neat:yes:

Usually things like this are learned in an electronics school lab, with a pile of really cheap transistors at your side...

Yup, small set of 2n2222's, 2n3904, and 2n3906, get you where you need to go to figure this stuff out. Can't tell you how many dead transistors were floating around our devices class, or half-functioning ones. Once got one in a circuit, took forever to figure out.

And to Rockmore, man...i'm feeling you, this has to be far more than you expected. The PS isn't a huge deal, pretty typical for those to be some work, but amp boards can be a mess. The similar case I remember from recently was a phase linear amp, with, sure enough, bad transistors in the driver board...

 

[Rockmore]

Mark, 33, thanks.

Will be waiting on your replies to get the Mouser order in ASAP.

Please confirm also 512-ksc2073tu for the PS Goat recommended.

I have spares I ordered originally for the TIP42cg0s-nd and TIP41cgos-nd, and a monster block cap for the 6800uf 63v, but thats it.

 

[markthefixer]

Spares of anything you bought are always a good idea. Especially when their cost is less than the postage for getting them later. I tried to guess on a list, and failed. Probably you could omit getting duplicates of the output transistors.

I have bags of harder to find spares secured inside of a lot of my personal equipment (as it requires work).

 

[Rockmore]

Let me think on it, we have until Sunday evening for the Mouser order.

I WILL be thinking about this. I'm not a big football fan... although if the Bears were in the fray, I might work up a little enthusiasm...
But I'm not willing to sit through it just to see the clever commercials.

So other things (like this) will occupy my thoughts.


Usually things like this are learned in an electronics school lab, with a pile of really cheap transistors at your side...

OoooooKkkkkk... glad I got up at 4 in the morning to see nothing else, eh.... and with no confirmations here...

I ordered all the transistors again for the amp, with spares. Trying to go through this mess of a thread, I CAN TELL YOUright now this is some of the problem. Besides the 992's, like I said, I replaced all the transistors exactly like the originals. But every single one of them are ECB, not just the 992's.

4 of 512-KSA992FBU
2 of 512-KSA1013YBU
6 of 512-KSC2383YBU

 

[goat67]

Rock

You may want to get this also. Might not be needed but what the heck.
Power supply
Q1 2SD313 bce
512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54

Hey do not get discouraged you are close..... I bet the 992s will fix it.

 

[33&athird]

Hey do not get discouraged you are close..... I bet the 992s will fix it.

:yes:...it's a good thing to keep in mind that all this work will proabably guarantee this receivers working for the next 20-30 years.

 

[Rockmore]

Rock

You may want to get this also. Might not be needed but what the heck.
Power supply
Q1 2SD313 bce
512-KSC2073TU npn to-220 bce 150v 1.5a 25w 4mhz 40-140hfe $0.54

Hey do not get discouraged you are close..... I bet the 992s will fix it.

I did! I took your recommendation. Ordered 2 of them. Only thing I knew 100% for sure as they are BCE and it's marked with the letters B C E right on the PS board lol. I was just listing the others on the amp as they are ALL ecb...

 

[Rockmore]

Got replacements.

Replaced the sd313 with KSC2073TU on the PS.

Replaced the 4 992's.

Replaced a single KSC2383YBU that was in backwards.

Been through the amp, ps, and every other board three times.
Every transistor is in correctly. Nothing looks shoddy or bad. All solder joints cleaned up and look good.

Everything exactly the same as I stated in post #73.
On powerup, a slight audio to the speakers. Slightly touching the volume creates a extreme amount of distorted volume. Nothings changed.

 

[goat67]

Rockmore
Can you please summarize by board the parts that you have replaced.

I know you unsoldered the boards this is a concern for me one wrong wire or bad solder joint and who knows.

Lets start over

What are the symptoms?

Does volume effect the sound is the noise always present in tape phone tuner both channels?

If you want to get this fixed you need to provide the info we need to help you.

You may be in over your head on this one I am sure it is something simple but
it is very hard to fix over the internet.

BTW I have an SX-424 that works and is collecting dust

 

[Rockmore]

Rockmore
Can you please summarize by board the parts that you have replaced.

I know you unsoldered the boards this is a concern for me one wrong wire or bad solder joint and who knows.

Lets start over

What are the symptoms?

Does volume effect the sound is the noise always present in tape phone tuner both channels?

If you want to get this fixed you need to provide the info we need to help you.

You may be in over your head on this one I am sure it is something simple but
it is very hard to fix over the internet.

BTW I have an SX-424 that works and is collecting dust

Go through this thread Goat- I think I've already stated what I've done, and used the exact recommended parts. Already have given you all the info you needed- go through the thread.

A brief summary again is:

-Receiver worked just fine (just old and tired) to start with.
-Replaced the caps on all the boards except the tuner.
-Replaced the sd313 transistor, the 2 zener diodes, and 2 resistors on the PS board.
-Replaced the transistors, caps, and 4 wattage resistors on the amp board.
-Replaced the two smaller and one huge block cap on the chassis.
-All the voltages seem to be in range from you guys and are as stated before.

The two boards I cut the wires on and re-soldered are just on the PS and amp boards. Had them all clearly marked, and have gone back through them 3 times. They are all back exactly to the same posts on each board correctly, including to the 3 block chassis caps. Have traced all the wires many times over to ensure each ends are still correctly connected and soldered solidly.

Only thing I havent done is replaced the all of the 2383's and 1013's for the SECOND time... Just the 992's like you Goat kept saying multiple times.

What are the symptoms (again)? See post #73 and and previous.

"...Does volume effect the sound is the noise always present in tape phone tuner both channels?...":
-See post #73. It's not noise. Turning the unit on, there is slight sound out to the speakers with the volume all the way
down. Barely turning the volume up it is very loud and distorted. Doesnt matter what input source (tuner, tape, etc.). Yes, it's both channels. As if the amp is running without a volume control and very tinnie loud raw and distorted. All the pots work just fine- balance, trble, bass, volume.

 

[33&athird]

Oh dear...I was really hoping those transistors would be it:sigh:, at this point it seems it's far and above what anyone would have expected...

Almost sounds like a feedback circuit issue, but I don't know...i'll think about it, maybe try running a sim of the boards, and see what I come up with:scratch2:

 

[Rockmore]

Marktf

Mark-

You sure 863-TIP42CG pnp to-220 bce 100v 6a 65w 3mhz 30-75hfe and 863-TIP41CG npn to-220 bce 100v 6a 65w 3mhz 30-75hfe at 65w were OK and adequate replacements

instead of
512-KSB596OTU pnp to-220 bce 80v 4a 30w 3mhz 15-240hfe and 512-KSD526O npn to-220 bce 80v 4a 30w 3mhz 15-240hfe at 30w (since they were not available at the time)?

-Also, if you turn the volume up to say about 1/10th from zero, the right channel completely cuts out and dies from distortion, and the left channel barely audible from distortion. This, again, is all like a guitar or other amp where you have the gain maxed out at 500% of acceptable listening levels. I also put back in the three stock 40 year old chassis caps (1 3300uf 63v and 2 2200uf 35v). No change to this current situation and problem, only that it brought back the bass and fidelity

 

[markthefixer]

I have been thinking on this. It could be the preamp rather than the power amp. I have to think about the caps you changed back in the power amp versus the fidelity change and what that means, could be a separate issue....

Did you ever get the paper service manual from Rick at Stereomanuals.com? If so, did you save my address, to send a photocopy of it to.

There's some stuff on the board schematics I'm forced to guess on because of lack of board interconnect information, plus it's lack severely hampers me from advising on tests....

What is the lowest full scale AC voltage your meter can read? I may end up answering my question by rereading thew thread, but that's later on...

All things considered, it may be better having it, a service manual, my bench, my instruments and myself all together at once.

 

[Rockmore]

No I never got a service manual. Had the schematics.

AC voltage on my DMM? 2. Has 2, 20, 200, 700.

BTW: there was no change at all from changing the 992's, or any of the transistors. Only thing that ever happened was when "we" put in the 33v zener on the PS to finally get audio out.

 

[EchoWars]

I read the whole thread from start to finish, and I got no bloody idea what might be wrong. Probably a little late to say that this project should have been tacked in steps.

Massive distortion & big output with little input makes it sound like the feedback loop is broken. I'm looking at the oddball muting for the pwr amp and trying to see if anything there could account for it, but I don't see it (do check that pin 17 on the power amp has +50 volts on it, elsewise things will certainly be weird). The problem with failure mode analysis is that, although there's only one way it works correctly, there's a 100 ways for it to go bad.

Truth be told, this is a matter of going back and checking your work...polarity of caps, orientation of transistors, quality of soldering (looking for shorts from a solder ball), etc etc... I go no rabbits to put out of a hat here.

Rockmore, sorry I had a tough time finding this thread. Weird, because either my searches didn't show it, or I looked right past it. But...it's super easy to give someone a simple link: http://www.audiokarma.org/forums/showthread.php?t=269939

 

[33&athird]

Almost sounds like a feedback circuit issue, but I don't know...i'll think about it, maybe try running a sim of the boards, and see what I come up with:scratch2:

Massive distortion & big output with little input makes it sound like the feedback loop is broken. I'm looking at the oddball muting for the pwr amp and trying to see if anything there could account for it, but I don't see it. The problem with failure mode analysis is that, although there's only one way it works correctly, there's a 100 ways for it to go bad.

Worth noting; the feedback on this amp is kinda weird. from the amp module ouputs(schematic; pin 12&11 on the amp board), the signal goes through c17&c18 (2.2uF@50v). Signal then goes to a RC circuit of a 3p cap (c20&c19), in parallel with a 18k resistor (R16&R15). The feedback chain then goes to the base leg of Q3&Q4, which are in the differential stage (the 2sa726). Hard to convey in words, but the schematic shows it pretty clearly.

Truth be told, this is a matter of going back and checking your work...polarity of caps, orientation of transistors, quality of soldering (looking for shorts from a solder ball), etc etc... I go no rabbits to put out of a hat here.

Sadly, may be the best approch, there are other ways, but it's really hard to troubleshoot things like this. Maybe it means something that both channels act the same- the 525 does have a line-noise filter (I think) that relates both channels.

 

[goat67]

EW some more info from other threads on this unit

http://www.audiokarma.org/forums/showthread.php?t=271222

http://www.audiokarma.org/forums/showthread.php?t=270094

As Paul Harvey says now for the rest of the story.......

 

[Rockmore]

Thanks fellas- Im trying to get a full service manual now.

Youre the experts- so keep thinking on it if you will. I cant tell you the stress involved since Ive been at this since right after Thanksgiving.

Between suggestions and new posts over the past several weeks and this week everyday, I have already been going through each and every single thing I physically did. Every single solder joint, wire that I cut and re-soldered, every cap etc I replaced, etc. I'm telling ya, it's maddening and I cant find a single thing wrong. I mean it. Ive been through it so many times it almost looks new (the boards and everything else). I cant see anywhere there is something wrong, especially on any of the circuit boards. For the heck of it, I have even re soldered every single joint on every board. The only thing left untouched is the tuner. There is no cap, diode, transistor, resistor that is in backwards or anything. The only thing never mentioned is the 3 general purpose diodes on the PS I replaced, Yes, the cathode and anode orientation is in correct LOL. Sometimes I just sit there and stare at the boards top and bottom closely to see if I can spot anything wrong underneath a magnifying glass even....

EXCEPT: for those of you with a 525 at hand, when I say I replaced all the caps except the tuner, I did. IF you flip it over underneath by the selection pot there is 6.3uf 100V cap with insulated leads I replaced. Its not on a board. And just to the upper left of that underneath the tuner there is a 100uf 16v cap with insulated leads I replaced thats attached to the underside of the tuner on the board.. Sorry to be so base and generic, but thats where and what they are. Those are the only out of the ordinary things I replaced besides all the parts recommended. Since Im not worried about the tuner or the quality of it, never gave it a thought if if makes a difference.

The only other thing I could possibly think of is is a few posts back to Mark about going with the 65w transistors instead of the 30w. All of this probably doesnt make any difference but thought I would mention them.

Sooo... Ill check for 50v on pin 17 on the amp and post the results. But as always just waiting for any other solutions and ideas, and waiting for a service manual. Thanks again guys and pleas keep this thread alive.

(Receiver and chassis has been naked for so long dont even remember what it was like to have it whole, working, and good looking anymore...).

Addendum: I'm getting exactly 50.6vdc from pin 17 on the amp to chassis ground. The right channel is stronger and louder with distortion always.

 

[Rockmore]

I read the whole thread from start to finish, and I got no bloody idea what might be wrong. Probably a little late to say that this project should have been tacked in steps.

Massive distortion & big output with little input makes it sound like the feedback loop is broken. I'm looking at the oddball muting for the pwr amp and trying to see if anything there could account for it, but I don't see it (do check that pin 17 on the power amp has +50 volts on it, elsewise things will certainly be weird). The problem with failure mode analysis is that, although there's only one way it works correctly, there's a 100 ways for it to go bad.

Truth be told, this is a matter of going back and checking your work...polarity of caps, orientation of transistors, quality of soldering (looking for shorts from a solder ball), etc etc... I go no rabbits to put out of a hat here.

Rockmore, sorry I had a tough time finding this thread. Weird, because either my searches didn't show it, or I looked right past it. But...it's super easy to give someone a simple link: http://www.audiokarma.org/forums/showthread.php?t=269939

Ya, I wrote a note to the mod a while back asking if they could rename this thread to SX-525 recap or something so it could be easily identified to anyone.

-Seems we've been through the 100 ways bad so far...LOL.. so hopefully all you guys collaborated expertise can knock this bear out at 101..lol... :thmbsp:

 

[markthefixer]

This is against my better judgment. My instincts tell me to run screaming away from this. Instead I am sticking my neck out.

Whatever was done to cause this fault, it has happened apparently to both channels. There are few points of intersection of the circuits that a common single fault could cause this. Thus it is a single error, repeated twice. An error of understanding.

Not an insult, just a statement of fact: something you did the same on both channels caused this. Sorting through all the changes in print is next to impossible. Especially when there is a terminology gap. It will be a long and frustrating experience for you (never mind MY state of mind). It might not even be your fault - for example there could be an error in the silkscreen printing of the transistor symbols on the board. Pioneer WAS just starting to get their act together back then....

We all have our areas of expertise, I stick to doing tech stuff, and it ain't as simple as it looks. Lots of pitfalls. I don't know a tort from a tortilla, and would get myself in a lot of trouble fast if I dabbled in THAT black magic... ( I've had to tangle with bad employers, The IRS, State revenue departments, pro-se small claims court (lost that one), been a witness, and a juror and in every instance expended considerable effort, sweat and intestinal distress over long periods of time to end up coming out even or slightly ahead... I'd calmly (and rather) face an exploding amplifier then THAT arena again, you can have it...)

Usually I run a lot tighter oversight of repair threads, steering the other guy around the pitfalls he doesn't see, or need to. Thus things go smoothly. This time I allowed my emotions to back me off of the thread very early, and it went rushing along without me "steering".

Tests will be necessary that you just don't have the test equipment for.

Thus my recommendation: when you get the service manual from Rick at Stereomanuals.com, pack it and the sx-525 together and send it to me. I will endeavor to keep costs down. I'm NOT doing this for the money, nor the practice. You are lucky that I also have a sx-525 here that I can use as a reference for the factory original state. Should speed things up considerably.

I don't think you'll get a better offer anywhere.

I have a line of equipment waiting for work that is between 6 months to a year long, and somewhere in there (soon i hope) I will slip this unit onto the bench during a parts procurement delay.

Let me fix the gear, you lock up the bad guys....

P.s. If you ship it to me, leave off any knobs over glass, although a 525 is light enough that packing won't be critical for it's survival.