Technics SU-VX 700: Teardown & Service

The Technics SU-VX 700 is an integrated audio amplifier from the early nineties. It delivers 2x 90W into 8 Ohms and is specified with a THD of 0.007% (20Hz .. 20kHz, rated power). In my opinion it is one of the better Technics amplifiers. Whereas the sound of an amp is always subjective to a large degree there are some design aspects that are a bit more objective. What I particularly like about the SU-VX 700 is mostly related to its simplicity and repairability:

  • Many other Technics amplifiers use one or two hybrid ICs in the power amp section. With mostly no stock available from reputable sources, a blown hybrid IC is a likely death sentence for an amp. In contrast, the SU-VX 700 uses 2SC3280 (NPN) and 2SA1301 (PNP) bipolar junction output transistors in a TO-3PL package (looking similar to TO−264 and TO-247 transistors). Although it might be difficult to find original Toshiba transistors, it seems like there is an asian brand still producing a transistor with that exact number. However, I would likely try the very similar, but more robust complementary transistors MJL3281A (NPN) and MJL1302A (PNP) made by onsemi.
  • The amp has a very clean layout with three large PCBs and a few small auxiliary boards. It is very easy to work on. (There is a small hatch in the chassis that provides a partial access to the underside of the main board. It might be useful in some cases.)
  • Most connections – in particular those after the capacitor block – are not soldered, but instead use connectors.
  • It doesn’t have the infamous corrosive glue all over the PCBs – might not be old enough. Actually, it seems they didn’t use glue at all. You decide whether that is a good thing. I don’t mind.
I like the clean layout of this amp. It’s certainly not as nice as some dual-mono amplifiers, but it’s very easy to work on. Also: Have you heard of Oxygen Free Copper transformers and X-Pro Twin capacitors? Now you did. You’re welcome!

Of course there are other properties and characteristics that are less than ideal. A few of them are:

  • To me, the power supply seems a bit weak: The single transformer gets really warm even when the amp is idling. (Only the big brother, the SU-VX 800, has dual transformers. This one also has multiple output transistors.) It uses a proprietary capacitor assembly with the stupid marketing label “X-Pro Twin Capacitor” which make the bulk capacitors unnecessary difficult to replace
  • There are no separate transformer windings for the preamp stages. This means dropping the voltage from +-54.8V (nominal) to +-18V and +-9.5V. Obviously, this increases the power consumption and creates a lot of heat in the amplifier unnecessarily. Some of the series pass elements and resistors reach temperatures as high as 82°C at 25°C ambient temperature and – if that wasn’t enough – are located in proximity to electrolytic capacitors. A heat sink would have been a great idea. Not as great as two more windings, but still.
  • It uses two hard-to-find ICs, the high voltage input amplifier in the voltage amplification stage (AN7062N) and the protection IC (AN7073). (The opamps are all in dual-opamp DIP-8 packages with standard pinout and can be easily replaced.) Most potentiometers – including the 4 gang volume potentiometer – would surely be hard to source as well, maybe somewhat harder than for some other amps.

After more than 30 years, the amp shows the usual signs of its age: Two speaker relays that sometimes need a special invitation to do their job, a few scratchy potentiometers and dirty switches.

The mainboard with both speaker relays removed (left). The discoloration of the PCB material clearly marks the hotspots

Finding a suitable relay was a bit more complicated than anticipated, mainly because of poor availability and obsolescence of many relay types in general. My requirements:

  • Matching footprint and configuration (DPST or DPDT; the latter means cutting off two of the pins)
  • High current capacity (8A)
  • Low wetting current (Gold plating)
  • Matching coil voltage (24 V)
  • Matching coil resistance (~1150 Ohm)

My strict requirement to use a relay with the same coil current might seem odd – usually a lower current shouldn’t be a problem and in many cases a slightly higher current wouldn’t be a big deal either, right? The reason behind it is Technics’ rather interesting implementation of the speaker selection and relay drive circuit:

  • The following speaker combinations can be selected via a rotary switch:
    • Off
    • Set A
    • Set B
    • Sets A + B
  • There are two speaker relays, one for each speaker set (relay A, relay B); they handle both the speaker selection and protection at the same time.
  • The “current drive” indicator LED, that indicates that the amplifier is not in protection, is also connected to the speaker switch.

Depending on the speaker selection switch position, the following happens (simplified):

  • Off: The “current drive” indicator is driven by a not-in-protection signal. A series resistor limits the current.
  • Set A: Both the “current drive” indicator and the speaker relay for set A are in series and driven by a not-in-protection signal. A series resistor limits the current.
  • Set B: Both the “current drive” indicator and the speaker relay for set B are in series and driven by a not-in-protection signal. A series resistor limits the current.
  • Sets A + B: The “current drive” indicator and both speaker relays A and B are in series and driven by a not-in-protection signal.

The main reason for this comparatively complex switching circuit is to minimize the power dissipation while using the rather high +54.8 V rail. The series connection with the LED means that a significant increase in the coil resistance would reduce the current through the LED, making it dimmer. Since the LED isn’t all that bright to begin with, I don’t like that. Although the LED should not get brighter with a higher current (because of another implementation detail, I won’t go into), I didn’t want to increase the current further for multiple reasons, including a higher power dissipation in surrounding components.

I selected a Weidmüller RCL425024 relay. It is one of the few relays I could order cheaply and that meets all my requirements – except for the coil resistance, which is 1440 Ohms instead of the original 1150 Ohms. Adding a 5.7 kOhm resistor in parallel with the coil solves this small issue.

The so called “Operation PCB” with the hard-to-find high voltage input amplifier IC (DIP18)

Since replacing the relays and servicing the pots and switches requires an extensive disassembly anyway, I decided to do a full re-cap while I’m at it – except for the “X-Pro” bulk capacitors which measured fine. Almost all other caps where still in spec or very close, some even had a slightly lower ESR than the new ones. However, two of the caps closest to the hot series pass elements on the main board were on their way out (capacitance, ESR). What a surprise…





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