Project: Technics SA-800

[Super Noob]

I should restore the pictures... I will.

It been a minute or two since I did this rebuild, but sometimes the bias rotation is reversed (from what the service manual shows) on some channels in my experience. (may or may not be the case on the SA-800)

Those multi turns can require a lot of dialing to get them to start passing mV. I suggest you do this on the DBT. Once you start to get bias voltage on each channel (Amp Board) back it off and go to full 120VAC and start dialing it up again.

If you get the leads on reversed, it will show negative voltage, but don’t worry about the voltage sign - dial to specs.

If you can’t bias voltage (mV), then you will need to start checking for proper voltage starting at the power supply. Are you getting out of protection (relay click)?

 

[tinkerer]

The pictures would be nice I cannot view them in your post.

The relay clicks and both channels play, but the heatsink is getting quite hot...too hot. I haven't replaced the trim pots, they are still single turn originals. I get no Mv response from turning either of the pots. So far I have found TR911=2SC945 shorted and replaced, R970=1.5K ohm burned and replaced, R704=15 ohm burned and replaced. I'm thinking the problem is in the power supply. That's when I discovered that the brilliant designers thought it would be a good idea to put the power supply regulators on the tone board, which requires an almost complete disassembly of the front to get to the tone board. I'll probably push forward because I've got a bunch of time already invested, but right now I'm just staring at the unit, bad mouthing the design engineers at Technics.

 

[Skywatcher]

The pictures would be nice I cannot view them in your post.

The relay clicks and both channels play, but the heatsink is getting quite hot...too hot. I haven't replaced the trim pots, they are still single turn originals. I get no Mv response from turning either of the pots. So far I have found TR911=2SC945 shorted and replaced, R970=1.5K ohm burned and replaced, R704=15 ohm burned and replaced. I'm thinking the problem is in the power supply. That's when I discovered that the brilliant designers thought it would be a good idea to put the power supply regulators on the tone board, which requires an almost complete disassembly of the front to get to the tone board. I'll probably push forward because I've got a bunch of time already invested, but right now I'm just staring at the unit, bad mouthing the design engineers at Technics.

Replace those old trim pots! They, like carbon resistors, drift way high over time and you cannot get the BIAS current low enough. Measuring the POT leads end to end reveals their resistance was high. I just rebuilt one of these myself. The original "complaint" was it ran too hot. But of course the heat cooked the caps to death. New trim pots and the BIAS dialed right in and ran nice and cool afterwards.

 

[tinkerer]

Super Noob and Skywatcher you're awesome. You nailed it.. I changed the pots and bingo, dialed in 13mV and the heat came down. And yes the opposite pot from where I attached the probe made the adjustment. Kind of a curve ball because I didn't think changing the pot had helped until I tried adjusting the other pot..

Really appreciate the help.

Didn't mean to hijack this thread.

Thanks Super Noob for documenting your rebuild and nice work.

 

[Skywatcher]

Super Noob and Skywatcher you're awesome. You nailed it.. I changed the pots and bingo, dialed in 13mV and the heat came down. And yes the opposite pot from where I attached the probe made the adjustment. Kind of a curve ball because I didn't think changing the pot had helped until I tried adjusting the other pot..

Really appreciate the help.

Didn't mean to hijack this thread.

Thanks Super Noob for documenting your rebuild and nice work.

It is relevant information if someone else reads this thread for information. All is well.

 

[Super Noob]

Hello Super Noob. I have tried setting the bias on a Technics SA-800 according to SM instructions with no success. Seems to be an odd way of setting the bias. Is the SM correct? Or is there another way to set the bias? Thanks.

Pictures updated.... pretty sure the VRs identified on the SM are opposite.
If you bias is NOT moving, move to the other pot and see if moves bias.

 

[1JGP1]

Holy Shi#! They are opposites! Here I am cranking the Trim Pot per the manual and (as I come back to double check the first one) I am over 200mV of bias & the receiver is getting hot/ transistors are "buzzing"...I drop a thermometer on top to see 50 Celcius on the top of the vents on the cabinet before I shut it down. Thank god for this post, now I am 13mV on one side and 39mV on the other...I might have a bad "trimpot" as I've got it turned right down/ if I move it a hair more it jumps the opposite way.

Regardless, I am guessing I am sitting better right now - I am going to check temp and see if it's running cooler now than pre-adjustment.

 

[Skywatcher]

Holy Shi#! They are opposites! Here I am cranking the Trim Pot per the manual and (as I come back to double check the first one) I am over 200mV of bias & the receiver is getting hot/ transistors are "buzzing"...I drop a thermometer on top to see 50 Celcius on the top of the vents on the cabinet before I shut it down. Thank god for this post, now I am 13mV on one side and 39mV on the other...I might have a bad "trimpot" as I've got it turned right down/ if I move it a hair more it jumps the opposite way.

Regardless, I am guessing I am sitting better right now - I am going to check temp and see if it's running cooler now than pre-adjustment.

That's why you watch the voltage as you turn the pots to be sure you are going the correct direction. If your trim pots are original, change them immediately! They go bad, period. Use the same value and style, but use the Bourns Cermet (blue) ones and never look back. I forget the "value", but I used the single turn type. The SM mistakenly has the direction backwards because it is as viewed from the rear (bottom) instead of the front (top). The Bourns 3386H style single turn pot will turn the same direction as the original pot.

 

[Lui]

Hello Super Noob and everyone else,

I recently got an SA-800 and I'm about to embark on the same restoration process, I just have one question. Do you happen to remember if you were able to reach all of the components without having to undo/remove the tuning string?

Thanks in advance!

 

[Matt03221]

Going through the same process myself. I noticed no one answered your question Lui. I'm sure you've figured it out by now, but for anyone else who may want to embark on the effort. Yes, you can get to everything without removing the dial cord.

 

[RVT1K]

I wonder if one of those is my old unit??
I bought one brand new when I was in HS but sold it when I was in college.
That was my first bit of "real" stereo gear and I had nothing but fun with it.
And I don't care how much better a digital tuner may be, I LOVED that huge flywheel!

 

[Daniel Stan]

Hi!
Great work man!
Awsome device! Wish I had one...
I am a collector myself (a horder as you call them).
I have lots of them and, as you said, hard to let them go...
So far I have
Technics: V8; V9; V10X; V90D; V707; 8080; 8088; 8099; 5460
Hitachi SR-903
Denon POA 2200 (PRA 1100; 1200)
Yamaha AX-930
Rotel RB-870BX
Onkyo A-8690
Kenwood KA-7020