Akai M8 Amplifier – pre 1970 – all tube

The Akai M8

Now here’s an open reel tape recorder which gets a lot of publicity. One needs to understand that most of the positive publicity comes from the EF86, ECC83 preamplifier and EL84 output stage.  If one can pick up a unit in working condition for about R1 000.00 (these were about R300-R350 new in the 60s) then it works out as a very cheap alternative to building your own low powered tube amp. But before we all start our engines and run to OLX, Gumtree or eBay remember the following:

a) Look up the Roberts equivalent units on the net e.g. the 770X series was the M8. The M9 does not use tubes, neither the 1800 etc. Buying a reel tape recorder for the sake of it can prove to be an expensive and costly mistake – in many cases the machines up for sale are shot in one way or the other. An M8 in exceptional working order is going to set you back about 200 U$ and then shipping is an extra U$ 200 through eBay. And there are plenty of them around.

b) If you have never heard the so called “tube” sound maybe this is what you need to do first.  Sometimes it’s not always cut out to what people say it is.

c) If you then decide to go ahead, what do you want to do with it?  Will it become only a mic pre-amp, a modular stereo amplifier, will you trash the deck or restore? We see just one module, modified for two mics input only going for about $1 000. And they sell!

First and foremost note that Akai mechanical parts are notoriously hard to find.  Secondly if you do have a working deck in excellent condition then why trash it – these have become collector items. They are 50 years old now.

Akai M8 Roberts 770X rubber parts kit

The kit above covered idlers, capstan and tape counter belt and wonder of wonders, a brand new pinch roller. The parts cost R400.00 (this is super cheap) and the shipping about R500.00 through eBay’s global shipping program. (this should mean it will get here). Pinch rollers are hard to come by but bear in mind that many Akai models share the same roller.  (Top secret).

So I went and purchased an Akai M8 off Gumtree for R1 000.00. The lower head cover was missing, the box was in bad shape and the pinch roller had a crease or indent in it where the capstan had been running against it – the process is always to switch the motor off, there is a slide switch for this purpose. In any event…

Slowly applied 60V DC from a variable PSU to the high tension section of the amplifiers to form the still original electrolytic capacitors and then with a 100W bulb in series with the live input applied power. No shards of electrolytic anywhere, things looking good. Then  apply full power by removing the current limiter and still no explosion. Lights on, action, camera.  High tension at 230V. Very little residual hum. Microphone input worked, much distortion.

Next I purchased another M8 off Gumtree and managed to drop the seller from R2  000.00 to R1650.00. My intent was to get one good condition M8 out of the two. This one looked the worse for wear but strangely enough besides using one round knob to control the record and play function and a volume control knob missing this one was amazingly enough still in very good nick except for the pinch roller which was hard as a rock.

Before moving on I do need to show you something which is after all the very reason why we discard the mechanical parts and use the electronics as raw amplifiers.

Head Block – Akai M8 Roberts 770X showing cross field head – Akai patent

Although both these head blocks are filthy they are actually in a remarkably good condition.  (once we have cleaned them of course). The giveaway which makes the bottom image head block look in such a disgusting state is not rust but the tape oxide from lots of use and no cleaning.  I have cleaned tape heads and path for the last 45 years with meths (denatured alcohol) and have never had a problem. Purists will say you need to use isopropyl. By all means use what ever floats your boat.

Akai M8 Restoration – What is recapping?

Although I do believe that some of the older equipment on the market used a better quality of electrolytic capacitor compared to the common and garden variety purchased at your local electronics convenience store it is sometimes advisable to replace all the electrolytic capacitors in the circuit.

Electrolytic capacitors do have a tendency to dry out and become ineffective.  In this case we removed and replaced all the electrolytics in the high voltage section as well as the cathode bypass capacitors for all the tubes.  We kept the split reservoir capacitor in place to act as a mount for the two individual capacitors in the smoothing section using a 22u 350V pre- choke and all the rest 33u 350V after.  Bypass capacitors 25u 25V were all replaced with 22u 25V.

Part Two – the process