SA-7800 Help Request

[anonyme-x22]

Hi,

After a recap of the power supply except one little and the big ones, transistors changes, some burned resistance on the power amp in one side, as recap for the two side except the two 2.2ùF . The amp has worked for approximatively 1h30. After that strange clip noise before shut off.

I've now changed all the caps on the power supply!
Still don't turn on.

One thing which is strange: on the power board , the cap on one side are filling themself when applying diode tester on the caps, but not on the other side, it's seems always shortcuted even after retiring the diode and big caps in this side. If i retire the negative (orange) cable of the other side on the power board, the caps seems to fill again!

Any idea. is it the power supply, or something else?

Thanks in advance,
Rgds,

 

[mbz]

First up, download the service manual from hifiengine, need to register, it's safe/no spam etc...
Please reference parts preferrably by their part number, eg, Cnnn, or the 2200/63V psu cap....prefer Cnnn though

I don't recall any protection system on the SA-7800 that would block power-ON. Check main fuse.
I assume the display is off.

 

[anonyme-x22]

Hi,

I'm already using it for the recap.
Ok for the references, but all the chemical caps on the power supply were changed.
Main fuse are good, if i turn on the unit for about 10s, then i turn it off, i noticed that the board is hotter specially near the C29/C30 (220ùf/80V).
But i will check them again.
The display stay off. (Perhaps i should recap the display part...)

Rgds,

 

[anonyme-x22]

erratum

 

[mbz]

C29/30, part of the regulated power supply, some 3W resistors in the area.

Hope you have replaced the bias trimmers, preferrably with the multi-turn type.
Known issue with failed trimmer and blown outputs with NSA.

 

[mbz]

a recap wont repair everything

Agree, the display is just a light show. Better to spend your time tracking down issues.

Main fuse are good, if i turn on the unit for about 10s, then i turn it off,

OK, so it is possible to power on the amp, but you get no display. Why do you turn it off? Does it get abnormally hot? C29/30?
Does the relay click 4-8 seconds after power ON?

 

[anonyme-x22]

Hi,
No, i haven't replaced the bias trimmers.
Do you have a reference for good ones?
I turn it off, because i don't want it on for the moment when searching the issue!
Does not seem to go abnormaly hot.
No the relay don't click anymore.

Thanks in advance,
Rgds,

 

[mbz]

The old trimmers have some history of failing open when people tinker
with them.

The sm calls has VR5,6 at 100ohms and VR7,8 at 68K which is an odd
size, 50k did the job for me, alternatively go for 100k, note wiper
and one leg tied together.

mouser parts
100, 652-3296Y-1-101LF
50k, 652-3296Y-1-503LF

Give me a minute to get some test points, need to do a basic health
check of the power supply. The protection relay not clicking may indicate
a failure in the protection circuit or excessive Vdc on the spkr line.

 

[mbz]

Following should be pins on the power supply assembly, the board
with the 4 main caps, fuses

Pin 22 -???Vdc
Pin 15 or 16 or 17 +48Vdc
Pin 19 or 20 or 21 -48Vdc
Pin 29 or 30 +46Vdc
Pin 31 or 32 -46Vdc

You should check for abnormal Vdc on the spkr lines.
Check at the bias test points on the power amp board,
left channel TP1 or TP2
right channel TP3 or TP4
expect mV

All measurements with respect to ground.

 

[mbz]

Maybe consider asking a moderator to move this to the Pioneer forum for better visability. They have superior knowledge of such amps
and of their short comings like heat build up on the phono board...

 

[anonyme-x22]

Hi,

First,
Thanks for all the information.
Regarding the relay, it has switched before the power failure.
That's why i'm pretty sure that the bias are good, and the power amp card is good.
If the failure was at the power amp board, the amplifier should power up and the relay blocking, like before i fixed the left channel on the power amp.board.
I think my issue is on the power supply board!
But all the component seems good. The only things i haven't tested are the two transformers.
How can i check the transformers. they have worked, but perhaps one has died died, specially the small one!

Ps: how can i contact a moderator in order to move the subject in the pioneer part?

Thanks in advance,
Rgds,

 

[mbz]

The voltage measurements are required to determine current health. They will test both power suuply and power amp.
I'm not interested in the bias for the moment, just the dc offset, they are measured at the bias post
If the power supply measurements above are good then the transformer is good.
Hit the REPORT link in any post and add note requesting a move.

 

[anonyme-x22]

Hi,

Pin 15 or 16 or 17 +48Vdc tested between chassis and pin 15 -16 -17 =>0V
Pin 19 or 20 or 21 -48Vdc tested between chassis and pin 19 - 20 - 21 =>0V

I assume the transformer is out.
Is it important to rewire it like it was, or just soldering is sufficient ?
Is there a tool to wire the cables like they were?

Thanks in advance,
Rgds,

 

[mbz]

Give me a minute to digest the measurements.
Please do not remove the transformer or undo any of the wire wrap.

 

[mbz]

Your post is a little confusing.
What was the measured voltage between pin 15 and chassis
What was the measured voltage between pin 19 and chassis

 

[mbz]

0Vdc at pin 15 points to fault in regulated supply Q34, 36, 38, D31, 32,,, and
Fuse FU2, remote chance rectifier D27,28 and transformer

0Vdc at pin 19 points to fault in regulated supply Q35, 37, 39, D33,,, and
Fuse FU3, remote chance rectifier D29,30 and transformer

What was the voltage at Pin 22, also Pins 29 and 31?

 

[anonyme-x22]

Hi,

Between chassis and Pin 15 => 0V
Between chassis and Pin 16 => 0V
Between chassis and Pin 17 => 0V
Between chassis and Pin 19 => 0V
Between chassis and Pin 20 => 0V
Between chassis and Pin 21 => 0V
Between chassis and Pin 29 => 0V
Between chassis and Pin 30 => 0V
Between chassis and Pin 31 => 0V
Between chassis and Pin 32 => 0V
Between chassis and Pin 22 => 0V

Rgds,

 

[mbz]

Thanks for that.
Something not adding up, in earlier posts you talk about powering the amp on and noticing C29 and C30 getting hot.
Voltage measurements suggest a major power supply failure.

Next step would be to measure the AC voltage, black probe connected to chassis, red probe on fuse FU3/Pin 40
Repeat for fuse FU2/pin 41. The fuses are next to the small transformer.

 

[anonyme-x22]

Hi,

chassis to FU3/Pin 40=>0V
chassis to FU2/pin 41=>0V

Wait, the Ac fuse behind the device has blown!
Will try to get ones.

Rgds,

 

[anonyme-x22]

Hi,

I put a new one
On power, little smoke, the pin 32 cable has been disconected during the power on!
NB: the pin 32 cable broke, and i used soldering on it, perhaps to few. the soldered cable was versy short!
The smoke seems to come from the relay but not sure.
The fuse is still good, do you think i should reconnect the pin 32 cable, and try again?
Does not seem something has blown!

Rgds,