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Pioneer SA-7800 sound problems

A

Alesij

New Member
Hi I bought this great amplifier for a good money in non-working condition... It actually works. At least it turns on and plays.

But it distorts sounds very much and sound is not crispy nor have any "wow" in it. I was able to output the audio signal into the video recorder so here it is - this is how it sounds on left channel, very very distorted, right channel plays good but not completely ok, like i said, no WOW, even my cheap Marantz plays much better.

I already ordered caps for total recap, I measured some caps in the amp and they are not so good anymore, so recap is needed for sure.

But my question is - does this sound like caps problem only? Is here anyone with experience with this amp who might tell me if this could be another problem?

VIDEO:
 
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No, I wouldn't call that a cap issue, more like a bad connection/solder joint or very tired transistor, or power supply or,,,

That said a recap would be advisable. Replacing the bias/dc offset trimmers with multiturn types is mandatory.
Check R104 (10 ohms). There are some good posts on the Pioneer forum, do a search, there are heat issues on the phono board.

Suggest,
- clean all pots, switches
- check all solder joints using magnifying lamp or similiar, google "dry solder joints" for pix
- check for any hot spots and charred/discoloured resistors
- check bias and dc offset for general housekeeping, it won't be the cause of the distortion
- check output via headphone, also distorted
- if possible run the SA as a separate, ie, use pre-out/main in with different pre/power amp to localize the distortion.
Some people use their ipod/iphone/ixxxx to connect into the main in, but please use minimum volume on ixxx.

I did a recap on a SA-7800, it's a nice amp, in the same ballpark as the Kenwoof KA-7100 and Yamaha CA-1000 maybe +/-10%.
It's certainly not a SA-9800, but a good honest performer.
 
Thanks for your input! :) I cleaned internals because it was very dusty, i will clean the pots today, I already checked solder joints and resoldered some of them, but they seem to be in good condition, all resistors seem to be good too, I will have to measure some of them to be sure, but it seems to be ok.

Output on headphones is also distorted

I will try to run SA as separate today and make some more measurements...

I just hope tranzistors are ok, they are very hard to get these days :( most of them are discontinued and alternatives are obsolite too :( sigh!
 
Add in dirty speaker relay to the possible causes...

Feed the pre-amp out into another amplifier's input like aux and see if it is still distorted.
 
Yeah relay could be the cause for sure... :) I will check and clean as many things as i can today and will report back, thank you for your responses...
 
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so... I cleaned everything, I resoldered many many joints that didnt look ideal... And - it is better... a little bit better... It is cleaner now, but distortion is still present and is still very bad :) well... time to find where the actual problem is :)

So I connected my mobile phoen straight into the power amp and - distortion is there :)

Where do I start with the power amp repair? well... Measurements measurements measurements

edit: oh and btw R104 is completely ok
 
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I assume you don't have an oscilloscope, so distortuion will be difficult to track down. In this case a start would be to,
- check power supply voltages
- replace known problematic transistors
- replacae some caps (in hope)

On the power amp board check voltages at the followung pins
Pin 1 +48
2 -48V
9 +46
10 -46V

I don't recall any problematic transistors in the SA7800.
Check caps on any sleeve shrinkage...
 
well my oscilloscope is on its way, if i will not be able to track it down without it, i just wait for it to come and continue :) thank you mbz
 
check of idle biases will maybe reveal something .
i listened to the recording and something sounds like its in overdrive .
please tread carefully with this amp as its NSA design . do not twiddle bias pots before renewing them .
i know as my sa-8800 blew a channel on me .
being as its both channels look for common things such as supply voltages and grounding .
 
idles are on 30mV and 23mV its actually not bad so much, i made some other measurements on various places and its all actually pretty good, nothing really out spec here... well...
 
without any load connected, left channel is showing signal on display, probably noise, but when i connect headphones or speakers, noise on display stop but i can hear low volume of this distorted noise in headphones... right channel is almost clean, but it gets distorted from time to time too, but very rarely and not as much as left channel do...
 
check with using one input at a time into aux .
i suspect poor grounding on the inputs .
are your bias readings really ma ? and measured where ?
 
so.... power transistors on right channel (on that better channel) are heating much more than on left channel where they are pretty much cold...

So i desoldered power transistors and measured them and they seem to be completely ok. thats probably good news...

I desoldered few transistors around the left channel and did some measurements on them too and they all seem to be ok ... but yeah, how can i be really sure?
 
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a tried to resolder and clean all the grounds aroudn amplifier, i did recap of the power supply and power amp and the result is that speaker relay wont open anymore, the distortion is now worst so protection curcuit wont even let it open...
 
measure voltages where r61 r63 join together ..also same with r64 and r62 . these are the 0.22 ohm emitter resistors .
 
I might have found some problem, one side of amplifier was working very hot so i checked all resistors for heat and resistors R59 and R57 were running very hot (but they were not bad by just looking at them) so i measured them and there it is, they are dead. Any suggestions on how to continue? Is this problem of that resistors or is there some bigger problem causing this all?
 
i would next install new bias trimmers and set bias exactly how it says in the manual .
if you keep the old trimmers in it is very likely one might go open circuit whilst adjusting . less than one second of being open the outputs transistors will be destroyed .
whilst you are waiting for the trimmers to arrive check every resistor and every transistor . replace if necessary
 
sigh... replacing transistors is something i was hoping not to do :( i have pretty much no idea where to get them...
 
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