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Technics SA-300 Problems - Cause or Symptom

Anaxas

Anaxas

New Member
Hi,

Thanks for your time and hopefully someone has an idea about the issue I'm having with the Technics SA-300.

I'm new to working on receivers outside of replacing some obviously failed old capacitors.

I recently found this receiver at a thrift shop and it cleaned up very nicely - Sounded great until yesterday when I powered it on and it gave a loud hum from the right speaker (regardless of the volume setting). I immediately powered down - removed all connections and measured for DC across the speaker outputs.

At first measurement it was about 17V DC across the right speaker outputs, but while measuring the left channel it was originally 0V, but a resistor went up in flames within 10 seconds (R625 on schematic 120 Ohm 1/4 W). After the resistor flamed out , it's about 37V DC on the right speaker channel and 16V DC on the left.

I measured for a shorts on the 2 STK-0039 chips and the right channel had a short between pins 8 and 9. Left channel had no short between 2,3 or 8,9.

I'm confident in replacing the resistor and the bad STK-0039, but I'm not confident that either one of those components are the cause of the issue of the symptom of some other problem the I should be investigating.

Your guidance is appreciated.
 
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I removed both the STK0039 and replaced burnt 120 Ohm R625. Main Caps 6800 uf C701, C702 caps tested low, so I replaced them - I bridged both STK pins 1,3 and 8,0 with 1K resistors and I'm seeing:

Pin V(Left) V(Right)
---------------------------
1 , -5.08 , -5.03
2 , -37.4 , -37.4
3, 0.06, 0.10
8, 0.06, 0.10
9, 37.3, 37.4
(1)0, 5.24, 5,28

DC at speaker posts match pins 3 and 8 (60 mV on Left and 100 mA or Right).

From reading the other informative posts I replaced both the 2SA798A chips with a hFE matched pair of KSA992 (KSA992FBTA). It made no difference to the DC readings.

I'm trying to track down the 60 / 100 mV leakage / offset on the Left and Right Channels.
 
I took some measurements at the power stage
upload_2022-1-8_12-33-46.png
Voltages going into Q701 2SC1685 are 0.3V above spec and after the main capacitors the voltage is 0.7 +/- out of spec. Not sure from my limited knowledge how this would offset the voltages further down the circuit.
 
Pins 1 and 0 on the STK are also a bit high, well over the ±1½V design limit, and also not very well balanced. That ¼V imbalance could affect the output balance (DC offset) through the STKs.

The rail voltages are within the ±45V maximum limits. The fact that they are 2% over the design point is not a problem, considering line voltages can vary ±8% easily.

The other supply voltages being marginally high (1%) are also not a concern, and not unexpected since the supplies are referenced using resistors instead of being controlled more tightly by zeners.
 
Thank you for the assistance - It's appreciated.

I have the STK0039 removed, and pins 1,3 and 8,0 bridged with 1 kOhm resistors - I believe that's the reason for 5V at pins 1,0 - I read that these voltages drop to spec when the STK is put back in.

I'm not sure what resistances I'd be looking for in Q506/606 transistors, so I don't know what would be reasonable , when I put my multimeter between the C->BE (Red Lead on Collector) it reads as open. Black Lead on C -> BC the resistance seems to jump all over the place. I do see the -5v at the collector of both and -35.1V at the base and -35.8V at the emitter.

I've also been trying to track down why I see 14.8 volts at Zener Diode D603 - since the spec for that is 14.0V.
 
Last edited:
Since Q605/606 are not showing E-C shorting we can probably look elsewhere for now.

If D603 is clamping at 14.8V, is it getting hot? The higher voltage applied to the emitters of the differential pair could be affecting it, and driving up the idle current of the output module.

This unit could be a good candidate for a pair of the STK output modules detailed here: https://audiokarma.org/forums/index...x-780-and-others.721181/page-46#post-14202296
 
D603 is not getting hot - I have some STK0039 on order just trying to make sure that I don't fry them by missing something.
 
Pulled some capacitors - C711 was out of spec (low) .. C611 and C612 were marginal ESR. I tried pairing up another set of KSA992 to replace the differential pair without success. I'm also looking at this post for the SA-200 for DC offset issues:
https://audiokarma.org/forums/index.php?threads/technics-sa-200-how-to-adjust-the-dc-offset.239499

It appears that R614 and R613 are identical in the SA-300 and could potentially help with getting things into a more comfortable DC offset. Someone also had success with this in a SA-400

When I started this journey I couldn't make heads or tails at a schematic, there was a ton of intimidation in working on all the unit but there is a ton of knowledge all over the forums. Just as comment for the Technics gear - The boards are well laid out, the service manuals and schematics are nice and it's incredibly easy to de-solder. I have a Realistic STA-52 and it's impossible to de-solder , looks like globs of solder were used with some crazy varnish on top with copious amounts of glue for each capacitor not to mention the items are so closely packed that each resistor is installed upright and you can't shine a small flashlight easily through the jungle of electronics to see though the board to get an idea of where you should be working.
 
Update - Success!

I ordered 4 STK0039 chips - 2 marketed as original STK-0039 from an American eBay seller with a 99.9% rating at $17 each + $13 shipping ($46 Total) and for hell of it 2 "Darlington Power Pack STK-0039" from China for $12 Total (2 Chips and Free Shipping)

While waiting for the chips to arrive, I finished the re-cap / pulled all the transistors , found an issue with extremely low HFE one of the 2SC1885 chips (Q605,Q606) - I replaced both with KSC2690AYSTU - Q603 and Q604 appeared to be OK. Voltages seemed reasonable as compared to the service manual.

The American sourced chips arrived first - They appeared to be STK-0039N chips with the N removed - weighed the same as the damaged originals and the finish / mount look identical. Measured for any shorts across the pins and found none - Installed the chips and measured the DC offset and *horror* -37V on the Output - sadness and sorrow followed - I packed the receiver away for a while to mentally recover - Removed the chips and re-examined them - something was obviously wrong - The Sanyo specs have pins 1,2,3 and 8,9,10 active and are supposed to have any type of diode pnp/npn properties, but these chips had diode measurements on 1,2,3 - 7,9,10 and for good measure pin 6 to 9,10 - WTF! also nothing on pin 8. Opened a case with the seller and they are willing to refund (on ship back)

Chinese 'Darlington Power Pack STK-0039' chips arrived - seemed well built - measured for diode properties on 1,2,3 - 8,9,0 - Everything checked out. Installed them and everything worked ! - No issues

Summary:
--------------
Technics SA-300 - Worked fine until powered up one day and had a horrible noise on right channel (not volume dependent) - Measured 37V DC speaker terminal .
Resistor 625 burnt up shortly afterwards.
- Removed STK-0039 chips and had a short between pins 8-9 on right channel
- Replaced R625
- Installed 1 kOhm resistors in place of the STK chips to check voltages and DC offsets (Right channel was 110 mV and left was 60 mV).
- Removed both 2SA798A and measured for HFE balance - wasn't that great, but not really bad - replaced with matched KSA992FBTA - No real change in DC offset
- Removed both 2SC1885 chips (Q605,Q606) and found extremely low HFE on one of them - I replaced both with KSC2690AYSTU - DC offset improved to about 55 mV on both channels
- Probably wasted my time re-capping the receiver.
- Followed the posts under thread "STK0029 how to adjust the DC offset" post and replaced R613/R614 with 240 Ohm resistors and this lowered the DC offset to 10 mV - I didn't spend too much time on this.
- Removed the resistors and Installed each replacement of the 2 pairs of the eBay STK-0039s - costly 'original' ones didn't work (see above) , but the inexpensive ones (from seller adeleparts2010) worked and the receiver sounds great!

Thanks to the community for all their posts and to Watthour for guidance. Very much appreciated!
 
fyi, just in case those replacement 0039s dont work out of both prove to be chinese, in another thread, it is found that the stk050 replacement in the pioneer forum DOES in fact work and are iron man robust in the weak sister SA3xx series technics
 
I found this thread and others extremely helpful. I have also been working on an SA-300. I ended up doing about the same steps as Anaxas and thought I landed in a good place including getting DC offset down to about the 20mV range. But then after playing on speakers for a few minutes there was some popping. I turned things off and investigated and it appears that one of my new STK0039's had shorted. I replaced them with yet another pair and tried again. Ended up finding that resistor R625 had also burnt. Replaced that, and for the most part the left channel is back in business with everything measuring good and DC offset around 20mV. But the right channel has DC offset of 360mV. So something else is still bad. I see differences at the left and right differential (which I had previously replaced with KSA992 pairs). Q601 B2 is 138mV but Q602 B2 is 360mV. For kicks I re-replaced Q602 with another KSA992 pair but it didn't help. I'm still pretty new to all this and I've tried to figure out how to track down where the difference is coming from but I'm just not sure what to do next. Could use some help.

For the record I have:
- Fully recapped the amp board with a combination of UPW, UKA, MKS caps
- Replaced Q601, Q602 differentials with KSA992FBTA pairs
- Replaced Q603, Q604 current mirrors with KSC1845FTA
- Replaced Q605, Q606 pre drives with KSC2383YTA
- Replaced R613, R614 with serial resistors to go from 470 ohm to 280 ohm. This got DC offset down to 20mV (but currently only on the left channel)
- Maybe shouldn't have done this with STKs, but after my first round of getting things working I did a load test with dummy load resistors to test power output. It worked well and was indeed getting 35W but maybe that test is what burnt something out.

Now could use some help figuring out what else broke that is leaving me with 360mV DC offset on the right channel. Thanks!
 
Brian, are you sure that your 2SA992 pair is correctly mounted on the right channel? I ask this because it looks like there is no current flowing into R616 and R618, since the base voltage of Q602 (on the feedback side) is exactly the same as your output offset voltage. It looks like the base and emitter of that transistor are inverted.
 
I double checked how the Q602 pair is mounted and it is correct. Verified the base(s) and emitter(s) are not swapped. It's mounted exactly the same as Q601. Also, before I re-replaced Q602, the pair that I had there before were measuring the same. But that same pair was previously measuring good, the same as Q601 before I blew R625. So I think blowing R625 affected something else. Regarding R616 and R618 I'm getting about the same 360mV on both sides of the resistors. But with power off and measuring resistance they are to spec at 47K and 56K respectively. I also went through the entire board looking for any other blown or out of spec resistors but they all look good.

Here's what I'm getting on the differentials:

Left Channel
Q601
E : 0.7V
B1 : 130mV
B2 : 138mV
C1 : -35.8V
C2 : -35.9V

Right Channel
Q602
E : 0.8V
B1 : 224mV
B2 : 358.8mV
C1 : -35.9V
C2 : -35.6V
 
When one makes a dual transistor like you did, a common mistake is to mount the two flat faces together. It's so tempting. Then, one transistor has its emitter connected to the base of the second transistor (collector is center on those transistors). Please, double check this because it is not normal that you have no current in R616 and R618. Does R620 still measure 10 Ohm? What voltage do you have on both sides of R625 and R620?
 
ecluser said:
When one makes a dual transistor like you did, a common mistake is to mount the two flat faces together. It's so tempting.
Click to expand...

there is nothing wrong with mounting the faces together as long as you put the leads where they are supposed to go. it's not that hard and i wouldn't call it a mistake either.

2sa798 via 2-ksa992

Annotation 2022-09-18 003102.png
 
My ksa992 pairs are mounted side by side with the one on the left flipped. So if you look at them together the left shows the round side and the right shows the flat. This puts both emitters in the middle. So the pins left to right are B, C, E, E, C, B. The E's are connected together. I checked again that the pins are in the right place.

R625 measures 118 ohms and has -37.85V on the left (towards the back of the unit) and -36.08V on the right.
R620 measures 10.5 ohms and has -36.15V on one side and -36.08V on the Q606 emitter side. FYI I'm getting the same -36.08V the left channel Q605 emitter.
 
I suspect a possible short (or large leakage) in C610. Remove C610 and see if that corrects the offset. The amp will not oscillate if this 1pF capacitor is removed.
 
I really appreciate the help on this. I removed C610 but it does not correct the offset. Getting 233mV on Q602 B1 and 365mV on B2.
 
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