Technics SU-8080 Stereo Integrated Amplifier: Restoration

I’ve recently been restoring vintage Hi-Fi equipment, focusing on Technics preamps, integrated amplifiers, and power amps. My first project in this series is a Technics SU-8080 integrated amplifier, which arrived in decent cosmetic condition (and with printed schematics!). However, it was covered in dust like I’d never seen before and suffered from frequent channel dropouts, rendering it unusable.

❗ The elevated voltages inside this amplifier can cause damage,

severe injury, and death. If you are not authorized or qualified to work on such equipment, do not do it. If you follow along, you do so at your own risk.

❗ Caution

All information is provided “AS IS”, without warranty of any kind, express or implied. If you’d like to use this information, you have to check and validate it yourself.

My initial plan was a complete restoration, including replacing all electrolytic capacitors and most semiconductors ( excluding the output transistors and a couple of transistors in the phone stage, provided they were still okay). And anything else that had failed or I didn’t like for whatever reason…

As it turned out, rebuilding the power supply is a major hassle – mainly because of the wiring and the old main filter capacitors’ footprints, which really call for a redesigned power supply PCB. That was more effort than I was willing to put in right now, and since this is my own amp, I decided to leave the power supply for another time – too many other projects are waiting.

Observations and component selection

As this is a hobby for me, I’m not necessarily looking for the most economical solution to get the amp running again. Therefore, I tend to replace more components than strictly necessary. This is a good opportunity to measure a large portion of the electrolytic capacitors out of circuit, getting a feel for how well they have aged.

The same goes for semiconductors. Once desoldered, I like to use my Atlas DCA Pro to generate a couple of characteristic curves (which I sometimes have to do anyway, in order to match transistors or just to make sure the replacement I selected is actually suitable). Usually, I check resistors as well, mostly in-circuit if I’m not suspicious enough to warrant more effort.

For someone repairing the amplifier, here is a list of replacements I either used or would recommend as a starting point. You have to look into this yourself though and use the information provided at your own risk.

Resistors

The SU-8080 mostly uses carbon film and metal film resistors, which are generally less problematic than carbon composition resistors, which love to drift up in value with time. I found no issues aside from one carbon film resistor with a slightly degraded/cracked case, but even that one was in spec.

That being said, I still changed one of the resistors: R416, a 680 Ohm 1W metal film resistor used to drop the -46.3V to the 24V required by the speaker protection relay. It gets quite hot: about 110°C with the top cover removed, if I remember correctly. I’d expect it to get even hotter in real usage scenarios where the amp is not idling (meaning additional heat from the transformers and the output stage) and a tuner sits above the amp. Let’s assume the coil resistance of the new relay is about 660 Ohms, one should expect about 35 mA of current, resulting in 0.83W of power dissipation into a 1W resistor. Ouch.

Power resistors don’t necessarily complain about 110°C, but manufacturers usually specify a derating if the ambient temperature rises above a certain value (e.g. linear derating from 100% of the nominal power at 70°C to 0% at 155°C for Yageo’s FMP series). So, you could come very close to the component limits of a typical 1W resistor. Also note that today’s resistors are often physically smaller than those found in vintage gear, potentially leading to even higher surface temperatures. Obviously, the manufacturer accounted for this, but I don’t like it. And larger resistors might spread the heat over a larger PCB surface area, potentially reducing the thermal stress the PCB experiences. That’s why I replaced R416 with a 3W metal film power resistor, dropping the temperatures to about 80°C.

I couldn’t find good replacements for the horizontally mounted trim pots due to their quite large footprint. To account for this, I designed a little adapter board. By the way, I would recommend not using multi-turn pots for the “power limiter adjustment” (VR350 x2, VR351 x2) to give you a chance to easily play around with the somewhat weird adjustment, unless you exactly know what you’re doing beforehand. For example, because you have tried and succeeded with the adjustment before replacing the pots.

(Electrolytic) Capacitors

I recently came across posts of a seemingly experienced person regarding a certain Technics preamp stating something to the effect of: “It’s an old unit so I changed all electrolytic caps… No, of course NOT.” This sparked my curiosity about how the capacitors in this Technics amplifier would perform.

The Matsushita/Panasonic capacitors that were also used in 1970s to 1980s Technics equipment are reported to have a tendency to leak physically (electrolyte), typically visible by corrosion on the negative terminal. For some capacitors, I could observe exactly this. Despite that, the caps will often measure like a working but aged (higher ESR) capacitor. (In other Technics gear I’ve seen actual leakage onto the PCB, degrading the PCB material, the adhesive holding the traces on the board, and the traces themselves.)

So, like those that don’t show signs of corrosion, most electrolytics in my unit retained their capacity or so it seems but indeed had a two to five times higher ESR than modern high-quality caps. This might not always be enough to result in an audible performance degradation. For me, a replacement seems absolutely worth it for increased reliability and less chance of electrolyte destroying the PCB. Some caps were a little low on capacitance, others high, or likely rather leaked a bit electrically, although I didn’t verify that assumption at the time.

As replacements, I chose Panasonic caps and the FR series specifically if available – to keep everything a bit closer to original. And hey, Panasonic caps are considered high quality.

I noticed that my two power limiter boards were populated with different caps from what the service manual stated. As always, don’t trust the service manual blindly.

Semiconductors

Diodes

For typical signal level applications, 1N4148 and BAV20/BAV21 diodes (higher voltage rating) are good candidates. Since both the MA150 and MA162 have a voltage rating of not more than 75V, the 1N4148 will do just fine.

As a replacement for the 10E1 diodes, I prefer the 1N5399 over the 1N4007 due to its higher current handling capability.

The S1RBA20 is a bridge rectifier in an obsolete package. Not a big deal, really, but it’s a bit more work to build the new bridge with 4 discrete diodes (1N5399 .. 1N5399 should again do the trick).

The main power supply uses 3.5A rated S3V20 diodes. I’d probably try the 5A rated standard rectifier diodes like the BY550 series. “Try” because I haven’t properly checked the clearance.

I have quite a lot of 1.3W (2%) Zener diodes of the BZX85 series in stock, so that’s what I used to replace the old Zener diodes. Certainly not always required, but why not. Yep, there is a reason, why one wouldn’t use those diodes, but just didn’t pay enough attention to the datasheet until a forum member on audiokarma hinted at this potential issue: the I-V-curve is quite different, which is even much more visible for lower voltage types (I think it has something to do with Zener effect vs. avalanche): With the 1.3W variant, at a low current one might end up in right in the middle of the “knee”.

Transistors

ℹ️ The pinouts of the replacement transistors might not match the

pinout of the original transistors. In the majority of cases where the pinouts do not match, the transistor has just to be rotated by 180°; sometimes the pinout might be completely different. Always double-check how to install the transistors correctly.

At one instance, I thought I heard a popping sound coming from the speakers like the one to expect from a noisy transistor. I haven’t heard it a second time during my various tests. Nevertheless, it would be consistent with the fact that this amp uses the 2SA798A double transistors for the differential amplifiers, which are known to cause issues over time. Also, I noticed that the DC offset wasn’t as stable as I wanted it to be. This improved significantly after replacing the 2SA798A transistors, which by the way weren’t nearly as tightly matched as my almost perfectly matched and thermally coupled pairs of KSA992FB I replaced them with. Note that the FB gain range is “end of life”. Buy soon, or you possibly have to use F or FA range transistors (or a completely different type).

There is no modern replacement for the output transistors that come in the original TO-3 package and closely match the specs of the old ones. I would assume that MJ21193G/MJ21194G, MJ15023/MJ15025 and the likes work just fine, but haven’t tested it. With some modifications, it might be possible to use TO-3p style transistors. Maybe one of those comes closer to the originals. That I haven’t researched enough and certainly not tested either.

All in all, I achieved pretty good results in my limited tests (no phono just to name one example). Instead, I ran reasonably long tests (still heating up the heatsink quite a bit), including driving the amp multiple times into clipping (both at 4 and 8 Ohm). That being said, with and without the transistors replaced, the transition from clipping into the “normal” region wasn’t as clean as I’d like. This behaviour seems to be a bit more pronounced after the transistor replacement. In my opinion, this isn’t too significant of an issue because during actual use my amp will never come anywhere close to clipping. Still, I don’t like that, tbh. Other than that, I haven’t observed any issues like instability/oscillations etc.

Speaker Protection Relay

After cleaning the relay contacts, the relay seemed to work properly again. Since it seems virtually impossible to access the relay without fairly extensive disassembly, I just replaced it with a new Omron MY2-02-DC24 relay. (A possible heavy-duty alternative seems to be Finder 55.12.9.024.0000, slightly higher, but may still fit. Definitely check first, also the coil resistance.)

Restoration

The SU-8080 was a pretty typical restoration – disassembling the amp, changing many components, de-fluxing the PCBs, cleaning all switches, connectors, and the chassis of course, adjusting the amp, and reassembling it. I documented many, but by far not all aspects of it in a ~20 minutes video. If you like, head over to YouTube to get a few impressions:

https://youtu.be/siOn07GVEGc (external page, YouTube)

One remark: The adjustment instructions for the power limiter circuit are a bit lacking in my opinion. I reckon one has to use a 4 Ohm load, otherwise I don’t think it’s possible to get the current required to trigger the limiter circuit and without limiting the output power with 4 Ohm loads at the same time.

Conclusion

The amp is up and running again. Admittedly, the biggest issues were the dirty switches, a bit boring. But I was able to gain some experience with amplifier circuits and obsolete, discrete components ;) And I actually enjoyed it quite a lot. So much so that I’m in the process of repairing, servicing, and rebuilding a couple of other preamps, amps, and receivers.

After finishing some other projects, I might rebuild the power supply. I consider to replace these poor speaker terminals while I’m at it.