The AKAI AA-1030 Stereo Receiver
Back in the late 1970s I drooled over many HiFI seperates, knowing that I could never afford many of them. However, sometime in 1978 I bought a Rotel RX-402 receiver for £128 from COMET.
More recently I've been looking into buying and reviving some old classic receivers, and in particular the AKAI AA-1030.
This is a heavy 13Kg, 30 watts per channel stereo amplifier, and tuner. Although 'HiFi Choice Receivers' review back in the late 1970s found that this AKAI was able to deliver around 47 watts per channel into an 8Ω load.
There were other AKAI receivers for sale on ebay, other similar models were: AA-1050, AA-1135, and the AA-1150; eventually I took the risk and chose the AA-1030.
As always with purchasing second hand gear, there are risks, so one can be certain that both repairs and modifications will have to be made.
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| Original Advert |
Opening Up the AA-1030
Dust had settle mostly on the power amplifier side where the venting was situated, there was also signs of rusting.
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| AKAI AA-1030 Class A-B Amplifier Board, this shot was taken after some initial cleaning. |
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| Main Amplifier - at top left, Main 6800uF electrolytic capacitors (for amplifier) - bottom left, Power Supply & Regulator Unit: right side. After initial dusting. |
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| After cleaing the face plate, and repairing the broken antenna switch - top right. |
Initial Repairs
- Clean circuit boards throughout - partially completed.
- Switch clean all potentiometers, switches, and contacts.
- Broken antenna attentuator switch (fixed immediately)
Electrolytic Capacitors:
- Replace all power supply (PSU) electrolytic capacitors.*
- Replace all power amplifier electrolytic capacitors.*
- Brushed aluminium face plate requires some very careful local 'sanding' and cleaning with warm soapy water.
- Loud output at headphone socket, with excessive noise and hum. (fixed, see below)
- Random low-level burst noise in right channel, sounds like discharging. (fixed, see below)
- Inspect main Class A-B amplifier and set biasing currents. (done, followed service manual advice, but the SM is a little abiguous!?)
- New lamps needed for tuner display. (replaced with DC powered LEDs, may alter the arrangement later?)
- Inspect FM Tuner section, and the Front End, may recalibrate at a later date?
Loudspeaker Protection Circuit
During the process of switching on, and off, both headphones and the loudspeakers can experience a transient surge of charge. In most cases this doesn't present any problems but can be unpleasant to listen to. In time this could cause damage to a loudspeaker or connected headphones, which is why 'modern' amplifiers incorporate time-delay relay and circuit which disengage the Class A-B 'push-pull' output stages from both loudspeaker and headphones when the amplifier is switched on or off.
Unfortunately, the AKAI AA-1030 does not facilitate this kind of protection. This was not uncommon back in the early to mid 1970s.
Capacitors*
There was no obvious call to do this, but as the amplifier is very old, why not?
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| For now, I've replaced the 6800uF units with KEMET 10,000uF capacitors that serve the main Class A-B amplifier. I will return new 6800µF units very soon to the circuit. | |
I actually replace them again later with 2 x 6800uF SamWha. Just look at how small the new designs are!
The electrolytic capacitors for the PSU were also replaced. I don't believe there is any need to populate non-audio paths with expensive 'audio capacitors', so I didn't.
Random Burst Noises in Right Channel
A random burst-like noise was picked up in the right channel. I suspected that this may be coming from the tone control board which serves the volume, bass, and treble controls?
There are four 2SC1222 NPN transistors here, and all were replaced with KSC1845FTA types. This completely resolved the problem.
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A warning here: the pin out configuration of the 2SC1222 and the KSC1845FTA were identical, and not different as suggested in a data sheet for the 2SC1222. Since I followed the advice of the C1222 datasheet my wiring had put the base and collector the wrong way around - all volume/bass/treble controls did not work. It was only then I suspected that the old 2SC1222 datasheet was incorrect?
On-semi data for the KSC1845-xxx
The KSC1845 probably biases slightly 'earlier' than the KSC1815.
Regarding transistor current handling capacity, there is no problem with excessively high quiescent collector or emitter currents during standby.
Observing the circuit diagram above suggests that the quiescent current through R5 will be approximately (12.9v-6.3v)/1500Ω, or 4.4mA.
In theory, if during high level audio stimulation, TR1 is in its 'saturation' region (unlikely), then VCE ~ 0.2v, the current in R5 will be approximately (30v-0.2v)/(3900+1500) or 5.5mA. This transient case is well within the limitations of the KSC1845 at 50mA.
Headphones Issue
Since 1976/1977, it appears that one or more individuals have been inside this unit as it had both a name and date written in; it was dated as '29-12-89'.
At some point in time, the output of the headphone amplifier circuit had been modified, I became aware of this to my cost, the output was far too high. Infact I blew the right-channel internal diaphragm of the headphones I was using during either switch on, or switch off, although I suspect it was during the 'off' state?
So, what was happening?
Examining the output of the Class A-B push-pull amplifier, and how the headphones are integrated into the circuit will explain.
Looking solely at the headphone circuit (marked in red), we see at the headphone socket a series 330Ω, 2 watt rated resistor. With or without the louspeakers switched in, the 330Ω resistance is there in circuit to lower the audio voltage across the headphone load, all thanks to potential divider action.
Typically, back in the 1970s, 8Ω to 16Ω 'impedance' headphones were not uncommon. The headphones I used for monitoring the audio here were the AKG K-55, which I think are approximately 33Ω in impedance. However, this was not the reason why the headphones were ruined. (Note: the AKG K-55 phones were later repaired with new drivers).
During the last 'service', a very low valued resistor was soldered in parallel with AKAI's own 330Ω resistance. The resistor markings are difficult to read, but an ohmmeter suggsted that the overall resistance of this parallel combination was less than 3Ω, yes, that's 'three' ohms!?
This explains why the volume and noise were so high, and why a headphone driver blew when the amplifier was 'powered down' after a few times.
During 'power down', the large electrolytic capacitors will discharge through the amplifier and a momentary imbalance will occur between the complementary push-pull NPN/PNP pair giving rise to a voltage 'blip' at the output. As there's no effective potential divider in the output stage, the headphones absorbed the effects of a high voltage discharge surge.
I later removed this idiotic 'modification' and inserted a 680Ω 3W resistor on its own. All is now good, no noise, no hum, and no damage to any headphones!
The original 330Ω specification is electrically more suited to lower impedance (8Ω-16Ω) headphones.
Facia and Lighting
As can be seen the unit is now clean, but not devoid of scratches. Careful fine grit sanding has been applied in some areas, but care must be taken not to 'ruin' the facia completely by incorrect application or any application.
New LED Lighting (22/08/2023)
Three white LEDs are now affixed into the original filament lamp holders, and now powered by a DC source. The DC source being tapped off the original 15v AC (rms) primary transformer windings, then fed to a simple full wave rectifier, and a 680Ω 2W resistor is placed in series to act as a voltage dropper.
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| The 15v AC primary winding terminal point. This was the original distribution point where all three lamps received their 15v AC voltage supply. |
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| DC LED Display Circuit Each LED is running at about 11mA, which is sufficiently bright. | |
Shown above is a fused (200mA) AC to DC full-wave rectifier for the excitation of the three display LEDs. Although provisional, it is likely I will keep this circuit, and later make the arrangement more electrically secure.
The AA-1030 is working well - both AM/FM tuner, and amplifier.
Tuner: AM/FM section .... completed.
It is worth mentioning that there are no component reference marks on the circuit boards, so precautions must be taken when re-capping or replacing transistors.
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To be updated and probably corrected without notice.
29/08/2023.
