Technics SA-5760 - one channel repair
- Thread starter ViperZ
- Start date
Oldsansui441
Super Member
Nice to hear you have it up and running. I didn't look at the full schematic either sorry, l assume it is using that 2SC828 for temp compensation instead of a more traditional STV type diode?
Yes, that's right. My mistake was that I checked that that transistors are not shorted and did 3-way junction test. I never noticed that 619 and 620 check out different from each other, and that 619 is connected to my channel instead of 620.
Looks like bias settled at 14mv on dully hot channel.
I need to spray some contact cleaner into selector switch, but so far so good. Driving Minimus 7 speakers easily lol.
Looks like bias settled at 14mv on dully hot channel.
I need to spray some contact cleaner into selector switch, but so far so good. Driving Minimus 7 speakers easily lol.
Oldsansui441
Super Member
Ahh fair enough, yes you would have moved on after using three way diode test. They can do some strange things sometimes, test ok out of circuit but then fail in circuit due to higher working voltages etc.
Nice one
.Haha, it probably sees those as large headphones.
14mV stable on repaired channel, 12-12.5mV on the other channel. DC offset is 2-3mV on the original channel, and I set it to 12-14mV on the repaired channel. I just felt more comfortable with variable resistor being closer to the middle - I could get it to 0, but it looks good for now 
Hopefully will button it up and start showing it off tomorrow.
Oh, and these automotive festoon bulbs give off so much heat. I only had to replace one bad one. This is probably the only receiver where I would consider going with LEDs. I think I bought some "AC-capable" LED assemblies for this receiver a few months ago, but forgot about them.
Ah, found them.
https://www.ebay.ca/itm/391001424580
Hopefully will button it up and start showing it off tomorrow.Oh, and these automotive festoon bulbs give off so much heat. I only had to replace one bad one. This is probably the only receiver where I would consider going with LEDs. I think I bought some "AC-capable" LED assemblies for this receiver a few months ago, but forgot about them.
Ah, found them.
https://www.ebay.ca/itm/391001424580
Generally, I only worry about trimmer position if it is dramatically different from one side to the next. Half a turn difference might (or might not) be cause for concern, a few degrees to an 1/8th turn....meh.
LEDs might be nice. Some receivers look great with them, some don't.
LEDs might be nice. Some receivers look great with them, some don't.
I assume DC offset pots are off from each other (not by much obviously, but noticeably) since output transistors are totally different in both channels. Would that make sense?
Everything else is pretty much identical, except for that one transistor that I replaced yesterday.
Ran it for a few hours with Minimus 7 speakers off FM radio and CD player. Playing great.
I wonder if there is a way to clean the volume control. It seems to be sealed in plastic enclosure; some positions at higher volume are scratchy. I worked the pot back and forth, but still want to clean it hopefully.
I wonder if there is a way to clean the volume control. It seems to be sealed in plastic enclosure; some positions at higher volume are scratchy. I worked the pot back and forth, but still want to clean it hopefully.
Eastham
More Class-A than ever!
Perhaps you could drill a hole into it with a dremel or melt a hole with a soldering iron.
If it is one held together with four screws on the corners, you can take the screws out and gently pry it open just a little to get some deoxit in it.
Just variations between components in the input stage for the most part.
Oh lawd! Great thread and a massive effort. An SA-5770 just landed here and after reading this entire thread, I’ll probably take a real simple look into the speaker protection board, etc. before delving into it any further. Yes, an upcoming Winter project perhaps?
It powers up... but zero sound or noise via headphones or speakers & no overload alarm light on.
It powers up... but zero sound or noise via headphones or speakers & no overload alarm light on.
Start with the basics - check all voltages from power supply (the service manual schematic is very detailed); check if output transistors are blown or not (i.e. DC on the output before protection relay), then start checking protection circuit to see what's causing it not to engage.
I kept my 5760 - it's a great receiver.
You will get more help if you start your own thread.
Technics liked to call these little bjt's thermal compensators. Which should help with idle current drift during the hot times. Interesting writeup and the trials and tribulations of debug. I just got my greasy little hands on a SA-1000 and will be putting together a parts list to give this big guy a nip and tuck. I'll probably start a new thread to update during the edit process as well as show a few pictures of the piece. I've seen the guts of a few pioneers SX-1980,1280,1010, etc. But have yet uncovered a technics. So I"m bound to find some surprises. Quick glance at the schematic makes it apparent that I have seen some of these blocks before.
wiseshanks
New Member
Wanted to come on here and thank you for this thread! I have one of these on the bench right now and like to always do a cursory check on AK before I delve into an unfamiliar unit. I have very similar issues at the moment with the amplifier board. What did you end up doing for the 2SA798’s?
many of us like to use gain matched ksa 992, thermally coupled together. there are other options.
just type 2sa798 into that little search box in the upper right corner and you can find at least 100 threads on it.
buy this version of the ksa992. it has kinked leads for easier assembly and come on a tape strip, meaning that they come from the same batch and are easier to match.
buy at least 20. alot of us buy by the 100s.
you will need some way to measure for matching. a mini component tester from amazon is common. a peak atlas dca55 is another.
some match hfe, some vbe. i match both. i have learned that vbe is the more important number to match. ask 5 people, get 5 opinions.
make sure you get the pin out correct. this is the simplest way to assemble them.
wiseshanks
New Member
Thank you for this! I have a DCA55 so matching pairs should be ok. I’ll definitely order a good amount in to have as dry powder. I’ve used a pair of the surface mount versions of the 2SA798’s from eBay and they worked well. I figured there was a simpler way to go about it.
Similar threads
- Sticky
