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In Need of SA-7800 Schematic

greap666

greap666

Active Member
SA-7800 Glowing Capacitors and a Wisp of Smoke

INITIAL ISSUE: If there's anyone out there that can steer me in the direction of an accessible schematic for the SA-7800 I'd appreciate it, I'd really like to find out where the wisp of smoke just came from.

UPDATE: Well I looked a bit closer when powering up and saw at least two caps glow a nice orangish red before emitting a nice wisp of smoke. Turned off immediately and figured, "off to the repair shop tomorrow" but I have attached an image here anyway of where this was happening.(where two small arrows are)

Will it live or should I bury it. The power and meter lamps continued to stay on when all of this was happening.

Thanks a bunch.

Bill :scratch2:
 

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All I've got in one for the sa-8800, it is very similar in design, but is it exact? I know the amp section is different but I am not sure about the preamp. By the way that board looks like the phono amp board. I would guess that the power supply regulator board has gone south (a common pioneer problem).

anyway if you want a copy just say so.
 
RE: Resistor Problem

Thanks technut but I found the issue the R60 as shown in the attached image they are both smoked.

I believe this amp was repaired one time before now. I hate to do it but I'll need to pull my working 7800 from the stack and compare the boards, some of the resistors from one channel aren't matching the other channel.

Oh well something to do in the morning.

Have a great night people.

Bill
 

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Looks like you have a manual (or something....)

The schematic is such a small fragment, it is hard to see what part of the unit it is, I'm pretty good and while it looks like the output amp, it could be the power supply for all I can tell!

If I can get what info you have, I can probably help..

If it is the power amp (and the .22 ohm resistors r62 & r64 make me think so) then q26 and q4 are strongly suspect.
 
RE: Fried Resistors

Hi Mark,

Please send me your personal email the files are too large to post here.

Thanks. :thmbsp:

Bill
 
Ok, I have the pdf's and you will be getting your DMM soon.

To learn how to test these transistors, called Bipolar Junction Transistors, read this thread

Check Q26, but in all likelyhood Q4 is shorted, or blown up...

Remove the two transistors from the circuit to test, Q26 should be easy, Q4 is the power output transistor (one of four) that forms the smallest triangle with Q26, the relay in the center of the board and itself. Just unsolder the leads carefully from the board without disturbing the mechanical mounting for now. I expect at least one, if not both to be dead.

Check R54 the 220 ohm resistor connected to the base of Q26 to be sure it is ok.

BUT this only checks and fixes the current damage, not WHY it happened. These results should be discussed before applying power to the repaired area again.

Q26 is a 2SA913 and Q4 is a 2SA1075, I'll be working on finding replacements...
 
RE: SA-7800 Caps

Thanks for all the insight Mark. I failed to mention that on C20 (100uf 50v) is a mark in a black marker as if someone found this to be the problem but did nothing about it. The mark is on the top of the cap. In addition C11 on the equalizer board has a similar mark on the top but in green marker. I don't kow if one is the begining of a trace and the other the end. Just thought I'd mention that.

As I said I have some 100uf 50's and 470uf 16's both to replace the suspect caps on the way. I do have replacements for the fried resistors but I'd rather wait to do the work once I have tha caps and meter.

Thanks a bunch. :thmbsp:

Bill
 
RE: SA-7800 Crap Shoot

I know most ppl that have read this thread are probably saying,"why doesn't he just give it to a good tech to fix". Well I said that myself but then again I said "what the hell I bought it cheap so let me try". So if I don't die from electrocution first I might be able to fix it myself, if I screw it up I could just send it to one of you folks..HAHAHAHAHA :D

Just kidding.

Thanks for all the past and future help.
 
I've seen quite a few long distance repairs from the ideas some of these guy's throw out, I've been the recipient and am always grateful for the assistance. If you're willing, and able, they'll help. If you get in over your head, they'll let you know. As far as I know, we haven't lost a member yet. :D
 
It's a nice amp, I have a couple of 5800's that other than dirty pots, have been very good to me. I'm addicted to the flourescan too. :yes:
 
Yeah I love it. As you can see in my avatar I'm a little addicted too. I have been addicted to Pioneer since my days in Europe 1975 -1981. My main amp is a SA-7800 and I need another amp on system 3 in another room so why not this one.
 
markthefixer said:
Ok, I have the pdf's and you will be getting your DMM soon.

To learn how to test these transistors, called Bipolar Junction Transistors, read this thread

Check Q26, but in all likelyhood Q4 is shorted, or blown up...

Remove the two transistors from the circuit to test, Q26 should be easy, Q4 is the power output transistor (one of four) that forms the smallest triangle with Q26, the relay in the center of the board and itself. Just unsolder the leads carefully from the board without disturbing the mechanical mounting for now. I expect at least one, if not both to be dead.

Check R54 the 220 ohm resistor connected to the base of Q26 to be sure it is ok.

BUT this only checks and fixes the current damage, not WHY it happened. These results should be discussed before applying power to the repaired area again.

Q26 is a 2SA913 and Q4 is a 2SA1075, I'll be working on finding replacements...
Click to expand...

Yep, The A913 is right around where the hot resistors are (R36 R60 R54)
 
But R60 is the one where a previous fix I assume as I mentioned earlier in this thread is where two resistors were used to get the needed value...both of those are literally smoked (black) can't even see any ID on either resistor.
 
greap666 said:
Yeah I love it. As you can see in my avatar I'm a little addicted too. I have been addicted to Pioneer since my days in Europe 1975 -1981. My main amp is a SA-7800 and I need another amp on system 3 in another room so why not this one.
Click to expand...
I was stationed at Wildflecken, '79 - '80. We now return to trying to fix the problem. :D
 
Is $.72 each a good price for some 2SA913? Found some at Centerpointe Electronics freight seems a bit high......
 
There are perfectly acceptable substitutes available, with better shipping terms. Mouser has no minimum order, and you will most likely need output devices as well.

There is a LOT of testing before we get transistors. For precisely that reason, to not pay more for shipping as we discover what else needs replacing.
 
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