Akai M8 Restoration – Part Two
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Akai M8 Restoration – The Process
We recommend you also read our sister website’s write up on the Akai M8 – certain aspects are covered there which is not covered here.
The two amplifier modules from the M8 are both very high gain so care must be taken to keep the dressing of the interconnecting leads the same as the factory to keep hum and noise as low as possible.
All the capacitors replaced were polarised electrolytics, no bipolar types. Shown with a blue rectangle.
Schematic below
Capacitors changed:
C3 1u 150V elect with 1u 450V
C2 25u 25V elect with 22u 25V
C7 20u 300V elect with 33u 350V
C10 25u 25V elect with 22u 25V
C12 25u 25V elect with 22u 25V
C16 25u 25V elect with 22u 25V
C17 20u 300V elect with 33u 350V
C22 3u 350V elect (this was not replaced)
C25 20u (add extra 10u) elect
C26 20u (do not add more capacity) elect
None of the resistors on the tag board showed any signs of stress.
Some things to note:
C26 is a 20u+20u capacitor. This was replaced by a 22u 350V on the input side of the choke and a 33u 350V on the output side.
The 6X4 dual diode is a comparatively hard to come by device. Adding a higher capacity to the immediate output may have caused undue loading. We did not know the condition of these devices because we do not have a tester but was relevant to our restoration was to keep the original tube compliment intact. The choke reduces inrush current.
C22 was not replaced possibly the reason why the bias oscillator does not light up the neon in record mode. (or the neon is faulty which we doubt).
From the owner’s manual
Power Input Levels:
Microphone input level -55db (VR.max) at 1000cps.
Phono and radio input level -15db (VR. max) at 1000cps.
Power Output:
Head output 1mv at 1000cps.
Pre-Amplifier output 0.8V at 1000cps, impedance 10,000 ohms.
For more on dBu and dBV go to the website http://www.sengpielaudio.com/calculator-db-volt.htm
Things to know
The supply rail is about +260V (we measured 255V).
Don’t test without a load of sorts. Tube amplifiers are resilient but definitely do not like high voltage swings across the output transformer.
By replacing the rectifier tube with a fast recovery rectifier your supply rail will be over 300V. This will increase anode current of the EL34 marginally. We did not toy with this idea as we wanted to keep the original tubes.
When working on the amplifiers remember that the amplifiers are removed first and then the deck mechanism.
If planning to use only the amplifiers it would be best to modify the record/playback switching and remove the selector slides.
Turntable input goes directly to the EF86 control grid. Check schematic J3, P1 and P3 head connections.
Although these amplifiers are not of bad quality remember that this was 50 years back when they were popular. Some enthusiasts complain that the output transformers are “limp” and are of poor quality. I beg to differ. These amplifiers are rated at 6W max per channel and the transformers are more than adequate. I would never tinker with this area because to be brutally frank you could build a 100W amplifier (without PSU) for the price of just one EL84 tube. The output transformer is going to cost more. Let it rest.
The M8 also came with an important part – a capstan sleeve designed for 7.5 ips. The only one I received has a small crack running down the cylinder wall. Not known to be all that hardy (alum) they can be purchased through: http://www.oaktreevintage.com/Akai_Reel_Tape_Deck_Capstain_Speed_Sleeve_Replacement.htm
Finally
Note the new pinch roller. And this one is brand spanking new.
Sadly one of the VU meters has a dead illumination lamp. To be replaced soon. How though? – the plastic housing seems very, very tight.
Please don’t forget to read: https://parts-ring.com/akai-m8-schematic-just-a-simple-tube-amp/
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[…] Part Two – the process […]
If you do get your hands on this R2R don’t forget a liberal dose of cleaning agent to the switches, especially the record / SOS slide switch. Line out to Mosfet power amplifier sounds hugely impressive.