Akai M8 Restoration – Part Two

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.

akai m8 restoration part 2
Akai M8 Restoration part II

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.

Electrolytic Replacement

All the capacitors replaced were polarised electrolytics, no bipolar types.  Shown with a blue rectangle.

Schematic below

Vintage Akai Schematic
Akai M8 Schematic – click to view full

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

head block m8
Head Block M8

Note the new pinch roller. And this one is brand spanking new.

m8 rear showing amplifiers
Rear showing amplifiers
akai m8 ready for action
Akai M8

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:  http://parts-ring.com/akai-m8-schematic-just-a-simple-tube-amp/

 

Starved Anode designs

starved anode designs

And on that controversial note of starved anode designs

The starved anode design has become fairly popular amongst guitarists because first and foremost these are highly affordable tube preamplifiers and secondly, well because they use tubes of course.

Behringer has brought out the Ultragain Mic-100 which is a cost effective instrument and microphone preamplifier. Trying to *purchase one in South Africa recently was useless, suppliers are always out of stock.  OK, I wanted two because I would have been modifying them for line preamplifiers at 250V anode supply.

The starve anode topology has been around since the beginning of time and is therefore not some new gimmick.  Read more about this on our sister website here.  In the good old days we had an accumulator pack for radio receivers and although looking back this was  archaic it did work. And work well. Better than the Note 7 at least.

Now we have armchair critics complaining about these starved anode and guitar effects preamplifiers and to be blunt, “if you don’t know what you are talking about then shut up”.  The internet is full of Behringer harbingers of doom that I wonder how they (the company) manage to make profits. A quick word on this as I am sure we are all tired of the same crusty bullshit:  Behringer did make good gear when it was manufactured in Germany. There were teething problems when they moved their manufacturing to China. They no longer have these problems or at least there is control. They do manufacture certain of their products cheaply to remain competitive.  You will pay more for equipment to be thrashed around but electronic equipment was never designed to be thrown around. Ditto to every other single company which moved their manufacturing to China. I can mention at least ten.

starved anode designs - the MIC-100
Ultragain MIC-100

The preamplifier below is not necessarily a starved anode design. In fact it is not, it can be switched to starved anode as a mode (either high plate or low plate).

Art Pro MPA 2

The reason why we are covering this is because in at least two forums some expert or other warns potential buyers off this design because of the plate voltage.

What is the Art Pro like considering I have never used the Mic-100.  In a comparison between the unit I have which is the digital version and the Focusrite 18i20 there is a distinctive “warmth” to the tone of the tube pre-amp, more so at full plate voltage with the same program material at line level. The preamplifier has variable input impedance for mic which some artists like.  From a studio perspective many home enthusiasts stick to either a full solid state design or tube.  Different strokes for different folks.

*The Mic-100 used to retail for under R800.00 at one stage. Our problem obviously is ROE.  The Mic-100 can be modified fairly simply for turntable use by adding RIAA IC amplification before the 12AX7/ECC83 tube and modding the supply rails.  Sure, there are some wishing to rather use an EF86 voltage amplifier before the dual triode ECC83 but in this case a very low noise IC costs R10.00 as opposed to the R250.00 for a tube which is fairly difficult to purchase locally.

As far as I am concerned the starve anode design does it for us.