The main reason for the redesign of the PCBs was the strong smell of the cheap original boards - and I have a feeling that the Meter Circuit Board was particularly smelly, just because of the power resistors that generate a lot of heat. So I had to design a new Meter Circuit Board too.
This is a very short blog post, because I’ve already shared most relevant information in the previous posts.
❗ Caution
All information is provided “AS IS”, without warranty of any kind, express or implied.
The meter circuit
❗ 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 meter circuit board has two channels. Each channel consists of a resistive voltage divider power supply, and a full-wave rectifying drive circuit for the meters.
The driver circuit essentially consists of two NPN emitter followers with the collector connected to the positive supply and both emitter resistors connected to the negative supply via the center tap of a trim potentiometer. This trim potentiometer allows for adjusting the zero point of the meter. In simplified and potentially slightly incorrect terms one emitter follower (TR401; TR402) is fed with the positive half wave of the rectified output (D401; D402), the other one (TR403; TR404) with the negative half wave (D403; D404).
At first glance this looks a bit like a long tail pair, but the outputs are not the collectors: The meter is connected between the emitters through resistors, one of which is an adjustable trim potentiometer and adjusts the deflection of the meter. Another resistor is connected in parallel with the meter. An R-C-network on the base of TR401 and TR402 respectively controls the ballistic characteristic of the meters.
The high temperatures caused by the resistive voltage divider discolored the board substantially. And this is why the glue went into its final stage: the corrosive stage. The components’ legs did not fall off yet, but you can clearly see some corrosion on the upper leg of R424 in the picture above. (The glue already fell partially off, and was partially removed with very little force at all.)
The redesign
ℹ️ Note
The schematic in the service manual has a couple of typos or Technics changed something along the way. Always compare the components/values with what is actually installed in your unit.
I used the exact component values of Technics’ original design.
Diodes
| Type | Replacement | Comment |
|---|---|---|
| 10D1 | 1N5393 .. 1N5399 | I think I used UG2D I had on hand, but didn’t notice that these are ultra fast rectifiers. Oh well ;) |
Bipolar Junction Transistors (BJT)
| Type | Replacement |
|---|---|
| 2SC828 | KSC1845 |
Tests
I tested the circuit and it seems to work just fine, and again, the multiturn potentiometers made the adjustment super simple and quick. I adjusted the meter in the x0.01 setting, because this range is the most relevant to me. I didn’t check the absolute accuracy at medium/lower deflection, that’s something one could have done. Oh well.