AKAI AT-2400 Stereo Tuner

 AKAI AT-2400 Stereo Tuner

 

I could not resist this on ebay - an AKAI AT-2400 Stereo Tuner which obviously needed some attention. It was reasonably priced and in a condition which I believed I could turn around.

The marks you see on the fascia are apparently nicotine stains. I certainly took some time to examine all the photographs in the advert for any signs that the tuner had been abused. Apart from the nicotine stains, there are other marks, the most prominent is to the bottom right corner where the brushed aluminum has been subject to some form of impact.

I decided to buy the tuner and on arrival I determined  if the tuner was 'basically' working, which it was. The next task was to disassembled the tuner, and in particular attempt to remove the nicotine from the front.

Servicing Work Undertaken

1. Cleaned fascia, all control knobs and buttons. 
2. Switch-cleaned all contacts and potentiometers, including calibration potentiometers.
3. Replaced all electrolytic capacitors.
4. Calibrated FM and AM where possible without the use of an oscilloscope (≥150Mhz bandwidth required) and RF signal generator.
5. Adjusted the 'High Blend' mixing control to reduced the perception of stereo white/pink noise for weaker, or noisy stereo FM signals. 
6. Replaced all Power Supply Unit (PSU) high voltage rated protection-capacitors for the rectifying diodes.
   
7. Replaced PSU rectifying diodes.
8. Replaced the 13v zener diode in the PSU.
   

Pending Work: Replace the PSU voltage regulation main transistor.

Shown below is a temporary replacement bridge rectifier and new high voltage diode-protection capacitors that protect the rectifying diodes - mainly during switch-on. At switch-on, there is often a high current surge or spike where the diodes are subjected to a high rate of change of voltage (high dv/dt) and thus high current. At all times, capacitors exhibit the electrical characteristic of i=CᐧdV/dt amperes, and so these will help to bi-pass excessive transient current surge or spike.

After completing the first 9 tasks, the tuner is working fine, and looking good.

View From the Outside

The AKAI AT-2400 sits below the Trio KA-6100.

Adjusting High Blend Stereo Mixing

What is 'High Blend' as defined by AKAI?

From what I can glean from the the service manual, it appears that 'High Blend' is a technique to reduce background high frequency low level interference, or white/pink noise when reception conditions are not favourable.

To achieve this, the stereo image is narrowed with much greater emphasis placed on higher audio frequencies. Background noise is far more noticeable if the noise is stereophonic, rather than monophonic. Monophonic noise is 'in phase', in contrast - stereophonic is not.

The method AKAI employed was to mix left and right channels, but only with increasing audio frequency.

Observing the diagram below partially illustrates how they did this.

The Sanyo LA3122 IC is a dual channel high gain voltage amplifier, similar to a modern OP Amp today. The Voltage-Series negative feedback network loop from pin 5 (or 10) to pin 3 (or 12) dictates gain and applied filtering characteristics if needed.

To the left of this circuit, is the 'High Blend' mixing unit - a simple but effective capacitor circuit of just 15nF is placed across both left and right channel inputs. Since capacitive reactance is given by 1/{2𝛑fC} Ω, increasing either capacitance C, or frequency f will lower the reactance (ie, lower impedance), and so at higher capacitance or higher frequencies the audio is mixed and placed towards the centre of the stereo image.

To make this more effective, the original C14 was swapped for a 22nF polyester capacitor. I may even try 27nF at a later date?

Sanyo LA3155 and LA3122

Sanyo's LA3122 and LA3155 high gain voltage amplifiers have been used for potentially many applications including: audio phono (moving magnet) cartridge amplification with RIAA playback de-emphasis, IEC/NAB de-emphasis for tape recorder playback, an active low pass or high pass filter, and of course as a simple voltage amplifier with constant gain.

The LA3155/3122 series is centered around a three stage NPN voltage amplifier with an open loop gain of several thousand.

With reference to the LA3122, the voltage gain in Open Loop Gain configuration can be approximated to: Av ~ {(220/0.4)*(4.7/0.3)} or around 8600. This figure is likely to be subject to some considerable error, for one reason, there will later be some internal negative feedback in the form of current-shunt from pin 4 to pin 2. And another, no consideration was made of the base input resistances of each transistor.

A more analytical approach at audio frequencies would suggest that the gain will be of the magnitude: Av ~ [{(220*hie)/(220+hie)}*1/0.4] ✕ [(4.7*hie)/(4.7+hie)*1/0.3]. Here two-ported h-parameter hie is the transistor's input resistance under biased conditions, and can vary from 20KΩ to 300KΩ (may be less and more?) depending on the type of the transistor. Stage 1 is a very high gain stage, and typically hie ~ 100KΩ ... 300KΩ, stage2 is just an emitter-follower buffer, and at stage 3, we can expect hie ~ 20KΩ ... 100KΩ. Setting hie1 ~ 300KΩ and hie2 ~ 100KΩ, we arrive at an approximation of 4700; much less than 8600, and possibly more realistic? Such an amplifier is gain variable, and its bandwidth will be very restricted.

Applied negative feedback effectively desensitizes any amplifier, and in this case the current-shunt mentioned above, also concurrently applies bias to the base of the first transistor.

Whatever the default gain is, this becomes almost irrelevant. Given that amplifier voltage gain is very high and potentially variable, applying external negative feedback will finally determine the amplifier's overall voltage gain, broaden its bandwidth considerably, and offer excellent voltage gain stability. Apply external negative feedback and we have a closed loop system.

Closed Loop Gain

With the implementation of external voltage-series feedback, Rf (or Zf if it contains reactive elements) active in circuit, the gain can be shown to be Av/(1+Av*𝛃).

And if Av*𝛃 ≫ 1 (which is often desired), then the Closed Loop Gain will gravitate towards A ~ 1/𝛃. At this point, voltage gain is almost independent of open loop gain Av.

The 'beta' term 𝛃 is defined as the feedback ratio, which in voltage-series feedback topology is 𝛃=Vf/Vo.

In the particular circuit above we have -

𝛃 = Vf/Vo = 400/(400+Zf).

(This ratio is derived from potential divider action)

Simply connecting an impedance Zf (or resistance Rf) between pin 5 and pin 3 will result in a voltage gain of 

A = 1/𝛃
or

A = (400+Zf)/400. 

Sanyo's LA3122 Circuit Diagram:
Sanyo recommend 220kΩ for first stage biasing,
and is also part of internal current-shunt feedback.
Zf is the feedback impedance.

Finally, Sanyo state a maximum gain of 40dB in their datasheet example above at 1,000Hz. This can be verified both experimentally, and from 1/𝛃 or (400+39000)/400 ~ 98.5.

That is, in decibel form ...

20*LOG(98.5) = 20*1.99 ~ 40dB.

This article is subject to possible changes and corrections.
09/11/2023.

cassettedeckman@gmail.com