The Technics SE-9600 is a high end hifi stereo power amplifier build in the mid to late 70’s - a design about 50 years old. This couldn’t be more obvious: Everything is metal, everything is massive - the chassis, the transformer, the heatsinks, all packed nicely into a 19" inch form factor. And of course it has VU meters with three ranges. Worth mentioning is the input on/off switch - I like that - and the utterly pointless output impedance switch that is capable of making your favorite music sound flat and mushy by reducing the effective damping factor.
❗ Caution
All information is provided “AS IS”, without warranty of any kind, express or implied.
❗ Caution
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.
The amplifier design
A quick inspection of the service manual shows a fairly basic class AB-design. What makes the amp somewhat more special is its linearly regulated power supply. Also, it has substantial amount of protection features:
- power supply over current protection (switches the overloaded rail off)
- protection when the negative supply is interrupted
- current limiter (for protection against short circuits/overload on the speaker terminals)
- protection against DC voltage on the output rails (not my favorite implementation though)
- over temperature protection for both heatsinks
To achieve this, Technics used only discrete components if one doesn’t count the two dual diode main rectifiers as an integrated circuit. Including the power supply and meter circuits I counted:
- 50 bipolar junction transistors
- 2 silicon controlled rectifiers/thyristors
- 30 rectifier/switching diodes
- 11 zener diodes
And that’s it.
PCBs
The amplifier has a total of seven PCBs:
SUPA2580 Power Source Circuit Board:
- +/-50.5V constant voltage control circuit
- +24V Zener regulated power supply
- Power supply over current protection circuit
- Negative voltage drop out protection circuit
- DC voltage protection and relay drive circuit
- Over temperature protection circuit
SUPA2590, SUPA2600 Driver Circuit Boards, Left/Right Channel:
- input/transconductance stage [differential input pair]
- voltage amplification/transimpedance stage with biasing [common emitter amplifier with a current source load]
- driver/current amplification stage with current limiting [emitter follower, complementary]
2x SUPA990 Thermo Compensation Circuit Board, Left/Right Channel:
- thermal compensation transistor mounted close to heatsink
SUPA2610 Meter Circuit Board:
- rectification of the amplifier output
- meter drive circuitry
SUPA2620 Meter Range Selector Board
- push button switches and dropping/voltage divider resistors
Chassis mounted components
Some other components are chassis/heatsink mounted, for example:
- socketed 48V protection relay
- main rectifier diodes
- main filter capacitors
- quasi-complementary series pass transistors for the +/-50.5V power supply [emitter follower]
- power transistors of the output/current amplification stage [emitter follower, complementary]
Initial condition of the amp
After all those years the amplifier still worked fine. Sure, it was dusty and a couple of switches were a bit noisy and so on, but it wasn’t actually that bad - and this sort of thing is to be expected anyway. However, the unit got really smelly after a couple of minutes; it wasn’t a burning smell, but rather the smell of phenolic resin boards with a ton of flux on it, typical for a lot of vintage equipment. Some might like it, who knows, I certainly don’t. Though you can get rid of the flux, in my experience a good portion of the smell (of the board etc.) will remain no matter what.
❗ Caution
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.
The following closer inspection showed that Technics used the dreaded glue as well. Signs of corrosion of the metal can transistors on the driver board with glued heatsinks show one more time why a service is recommendable, even if an amplifer seems to work fine. One other minor point of concern was R535 on the Power Source Circuit Board with parts of its body chipped away, partially exposing the helical carbon track, but still measuring fine.
I haven’t tested all the old caps in this amp, but the ones I did test were at worst aged rather than completely dead. The two huge main filtering caps in particular seem to be fine (capacitance, ESR; I didn’t perform a leakage test though). Getting exact, modern replacements is difficult if not impossible, while similar ones (i.e. same dimensions, but slightly higher voltage rating and 50% more capacitance) are very expensive. The Zener diodes in the power supply seemed to do their job as well.
My goal & the upcoming blog post series
Initially I just wanted to do a basic service. But because of the smell I quickly changed my mind about how to proceed: I decided to take on a substantially larger project by designing a couple of new boards, including SUPA2580, SUPA2590, SUPA2600 and SUPA2610.
The new PCBs shall be designed to use components that can be bought new today, but retain the option to transfer the older transistors over. Larger modifications of the circuit shall be avoided to keep the amplifiers' characteristics and to not introduce another wildcard - new silicon really adds enough uncertainty regarding performance and stability.
I decided to keep the original and perfectly working power transistors in there because there simply are no new comparable TO-3 output device available anymore. And I don’t feel like using TO-3P devices or other bodges where it is not strictly necessary. (For the series pass elements various new Onsemi devices should work just fine without significantly changing the performance, I haven’t tested this though.)
Using new components should hopefully make the amplifier more reliable longterm, and it’d even be relatively easily reverseable if I ever wanted to restore it to its original state. Worst case: It doesn’t work out and I’ve learned something…
All details will be posted in my upcoming blog series about this amplifier.