M8 - left channel

Akai M8 Restoration

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As promised we move on to the inner workings of this single motor transport deck and tube amplifiers.

Note that we have also details on this marvellous amplifier and deck on our sister website’s pages:  Analog Ian

Restoring an Akai M8

This tape recorder came onto market about 1965 sometime (HiFi Engine lists this as 1963). It was preceded by the M7 and then afterwards, the transistorised M9.  Both these models also proved to be very popular. Enthusiasts veer towards the tube amplified version, discarding the deck.  If the deck is still in good condition I would not advise this as much of the parts can still be purchased through eBay albeit at astronomical prices or if you are fortunate enough, repair machined locally.

I purchased two of these units over the last two months. The first unit was not in bad condition, the amplifiers still worked but the head switching mechanism (mechanical) was missing. The pinch roller shows capstan burn. Capstan/flywheel belt is loose. Counter belt also loose but has enough tension. A catch was missing from the lid as well as not having the metal rear cover.

The second unit was the conundrum. The photos on Gumtree did not do it justice. When received I found it to be in many ways better than the first deck although I was going to use this for spares to get deck number one running.  Both M8s cost about 200 U$ which I felt was pretty much realistic for what I was getting.  Paying more for these recorders may leave one frustrated if there is an issue with a tube, mechanical cam etc. Although everyone sells these as being in excellent working order guaranteed there’s going to be a hiccup – if not hidden in a sea of lies maybe through shipping damage. Tubes do not travel well. This deck also had a dodgy pinch roller (dried and cracked). The heads still looked very good though.

For those buying these recorders for the tube preamp and class A single Pentodes must note that these amplifiers run off 110V and not 220 or 240V. There is an auto transformer mounted in the case which supplies the correct voltage to these amplifiers – so you will need another 2:1 step down transformer for mono block usage. These amplifiers have their own 110V (primary) isolation transformers.

Both recorders had major pinch roller slippage (as received). Back tension also felt very high. The metal cams used in the FF/REW which everyone complains about was still intact and in great condition. Cleaning and de-greasing required. Start capacitors for motors seemed to still be good, both decks had very strong motors and FF/REW pull was like new. I don’t have test tapes – this is next on the agenda but right now we are aiming to restore only one M8 and do the other one later.

At this stage I must mention that although both these machines are over 50 years old the transport mechanisms were still in very good condition (bar the rubber).  There’s a few words of caution which must be heeded when working on any vintage circuits (actually all goods running from mains power):

  • The supply rails to tube amplifiers are lethal. We are not talking about starved anode supplies but the real M’Coy.
  • Discharge all capacitors after powering up and switching off.
  • Always remove the mains plug from the outlet.
  • When working live make sure you follow the one hand rule. (keep one hand behind your back if need be).

Electrolytic Capacitors

Electrolyte dries out of older cans and these caps need to be replaced. Enthusiasts call applying a low current source at the working voltage of the capacitor reforming. In many cases electrolytics should be replaced with new units. Sometimes we can get our hands on NOS (New Old Stock) cans and because we want to keep the equipment vintage we stick to these. It’s not always the best solution. Electrolytics can blow up entirely or blow smoke and flame through the vent, not a pleasant sight, neither smell. Electrolyte is conductive and will damage other circuitry – it’s just not worth it. (a certain Chinese brand of computer power supply was notorious for catching fire so be careful of the quality of these electrolytics – and this is not to say that all manufacturers in China make shoddy products, some are excellent).

If you can, power up through a variac or have a current limiting solution like a light bulb, possibly a 60W for high voltage and a regulated supply for low voltage reforming.

Mains Transformer

The mains transformer in the Akai M8 is not an isolation transformer but an auto variety. This is a multi-tapped transformer. A while back a reader asked how to connect this to his amplifiers because the wire colouring did not match that of the diagram. The rule of thumb here would be to have an accurate resistance meter do the work for you. The highest reading between the windings on resistance check should be the 220V leg. Midpoint would be 110V. A variac or lightbulb in series is the best way to check after you have done the resistance checks to prevent the transformer from burning out. If the voltage is too high the transformer will start sizzling – they can be resilient beasts but always be very quick on the switch. Never apply power to a transformer if you aren’t sure of the winding configuration with a load applied.

Before powering up

Always clean the insides. A spider or roach resting across the terminals of a high voltage source may catch fire putting an end to your project. Smell, look and feel.  Something that doesn’t smell right is not alright. The previous owner may have cooked the mains transformer, a capacitor may have leaked, a resistor may be burnt.

Make sure the voltage setting is correct. Especially if you live in a 220V country.

Next up, some circuitry to look at:

M8 Left Channel
Akai M8 Left channel with bias oscillator

Next Page:  The circuits

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January 5, 2017 4:06 am

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