AKAI M8 recapping

[RoyceVM]

I was wondering why there is only faint audio from the left channel.

[RoyceVM]

Originally Posted by Station X

Here's the circuit.

Have you got 260VDC across both C25's?

I located the C25s and C26s on the large can caps on both amps.
Kindly let me know how to check it step by step.
I don't want to do any mistakes.

[RoyceVM]

Originally Posted by RoyceVM

I located the C25s and C26s on the large can caps on both amps.
Kindly let me know how to check it step by step.
I don't want to do any mistakes.

The red is the + and other one is ground on those can capacitor leads. So, I will power up the unit and use DMM red on red and ground on the -ive to check the voltage, right?

[RoyceVM]

V3 (6BQ5) on the left channel is warm and heater filament is lighting, the surface temp on both these EL 84 is almost the same. Just wondering what is causing the loss of audio from the left channel that was working fine. Will any electrolytic cap failure can attribute to it?

[Uncle Bulgaria]

Don't worry about what the fault could be - it's just a logical step-by-step process until you find it.

If you've traced the circuit and verified the dual can capacitor in your photo is C25 & C26, then yes, meter between those points. It's safest to do this by connecting crocodile clips to the relevant points with the machine off, then turn it on without having to get your hands inside and risk a shock. That really takes the fun out of repair!

You can see from the diagram that the point marked '260V' on the left amplifier feeds the line that joins all the valves' anodes, providing the B+ voltage for them to operate. If this voltage is not present, we know the rest of the amplifier cannot work so the fault must be prior to this point.

[Station X]

Originally Posted by RoyceVM

I located the C25s and C26s on the large can caps on both amps.
Kindly let me know how to check it step by step.
I don't want to do any mistakes.

Each of the cans contains a C25 and a C26, four capacitors in all.

Black/Negative lead of meter to tag with black wire.
Red/Positive lead of meter to each tag with a red wire in turn.
Meter set to read DC Volts on a suitable voltage range.

That's four readings in all. Report back here with results.

You'll need to let the valves warm up fully so that the rectifier valves (6X4's) start conducting before you'll get true readings.

[RoyceVM]

Originally Posted by Station X

Each of the cans contains a C25 and a C26, four capacitors in all.

Black/Negative lead of meter to tag with black wire.
Red/Positive lead of meter to each tag with a red wire in turn.
Meter set to read DC Volts on a suitable voltage range.

That's four readings in all. Report back here with results.

You'll need to let the valves warm up fully so that the rectifier valves (6X4's) start conducting before you'll get true readings.

Here are the Initial Voltages on the can capacitors without warming:

Viewing from the rear


Left Channel - Can Cap A 267V, Cap B 251V
Right Channel - Can Cap A 265V, Cap B 251V


Here are the Voltages on the can capacitors after 8 min warming:

Viewing from the rear


Left Channel - Can Cap A 257V, Cap B 240V
Right Channel - Can Cap A 244V, Cap B 259V

[Station X]

Your test results indicate that both 6X4 rectifier valves are working correctly.

I suggest checking the other voltages shown on the circuit diagram.

[RoyceVM]

Happy News!

Thanks a lot for all your help!

I used a line in plug on the headphone and speaker outlets many times after putting Deoxit FaderLube 5 in it.

When in the line in pick up socket female socket area I probed with a plastic rod, surprisingly left channel came to life with its clarity and power.
Now both channels WORK!
That is the one left top one in the picture!

What a relief!

I will soon get all my Electrolytic caps and I thought it will be all a waste!


Now I am motivated to do all the E cap replacement.

I ordered 2 more 6X4 caps and will get those in a couple of days!

[Station X]

Poke and hope triumphs for once!

[Station X]

Originally Posted by RoyceVM

Now I am motivated to do all the E cap replacement.

I ordered 2 more 6X4 caps and will get those in a couple of days!

The voltage tests suggest that these capacitors are working correctly. If they were excessively electrically leaky the voltages would have been pulled down. So long as they don't get hot and there's not excessive hum on the audio I'd leave them well alone.

[Radio Wrangler]

How good is your workmanship?

Unless you are very good at getting the right parts, fitting them the right way round and making good solder joints free from solder splashes and dry joints, then simple statistics says that you will probably create more new faults than you will fix old ones.

Re-capping is such a popular thing to do that someone even invented a name for it. A lot of people hear of it and see it as an answer to troubles both real and imaginary. On this forum we get a steady flow of people saying "I re-capped it and it no-longer works"

It can be done, but done in one burst it is very easy to make a mistake and not find out until the whole lot has been done. This leaves the problem of finding where the new fault lies. Most keen re-cappers tend to be at the beginning stages of their restoration careers and are shot-gunning capacitors as an alternative to diagnostic skills. Any mistake or poor soldering and suddenly they are highly dependent on diagnostic skills.

The general advice on re-capping of 'Don't do it!' is founded on seeing many people who have run into more trouble than they started with.

If you must do it, do one at a time and re-test the set at each stage. If anything goes wrong then you will know where to start looking.

Sometimes it makes sense to just replace all capacitors of known suspicious types. But you need to determine when to do it. It's not something to rush into blindly.

I bought an ex-BBC Revox FM tuner which had run 24/7 for a couple of decades in a rack full of hot equipment. Testing a sample of electrolytic capacitors showed that around 80% of the small ones had lost most of their water content and had caused their equivalent series resistance to go far out of spec. It took some time to source good replacement parts from suppliers known to take precautions agsinst fake parts (a real risk, nowadays) and they were fitted only a few at a time between re-testing the set. The result was successful and I now have a reliable tuner. Some were so degraded that the thing had been lacking in bass. Using an in-circuit ESR meter allowed the state of them to be assessed without having to unsolder them.

David

[Welsh Anorak]

Agree totally. I'd use it for a while and let it settle down before doing any capacitor replacements. If it works, then there's really no point!

Also what make are the replacement capacitors? As David says, there are many fakes around so be careful.

[RoyceVM]

I bought them from DigiKey Electronics and they sell the best capacitors like DigiKey Part Number 335TTA450M-ND. I understand there are lots of bad products out there and sourcing from reputed companies like DigiKey, Mouser Electronics and all helps.

Surprisingly, it works fine for like 30 min or more with no issues.
Once in a while when I turn it on makes cracking noises that sometimes die away and at times continues. May be some capacitors in the signal path is causing this intermittent cracking sound issue.

I have done lots of capacitor replacements in the past without any issues. In my last work I totally replaced capacitors on a SABA 125 and a SABA 300 radios, and they both work fine. I used Nichicon mainly for those from DigiKey.

I am just a collector of audio gadgets for the fun of it, not for sale and not a professional repair guy. Anything beyond my capabilities I take it to a friend of mine who is an expert!

How good my workmanship? I believe it is Excellent!

[RoyceVM]

Finally, all is working fine!

I replaced all electrolytic caps. They all reading way above the specs like the 20uF 350 V can caps were reading 30. 1, 30.2, and so on.
Many 25uF/20V were like 70uF and so on.

Now no cracking and all good. Pics to follow.

The lower VU is not working, and I have no idea how to correct it.

Overall, I am so happy!

[RoyceVM]

Some pics....

[RoyceVM]

It was playing songs for 2.5 hours now and sounds so good. Even though I have several reel to reels like AKAI GX 747, GX 635D, TEACs and all I really love the tube sound of this M8.

[Welsh Anorak]

Glad it's working - these are nice machines.

It's odd yours is an Akai as I thought these machines were sold as Roberts in the US. Be gentle with the controls - you probably know this but the cams are made of soft alloy that crumbles away. Worse on more modern machines, but I have had the lower one break in an M8 which isn't easy to source.

You clearly know which capacitors to buy and where to get them from. It's always difficult when advising people on the forum as we have no idea of the level of experience of the questioner, so no offence meant by treating you as a beginner which you obviously aren't!

I would have thought the VU [problem would be relatively easy to find, though of course it could be the meter movement itself.

[Analogue man]

A version was also sold as an Ampex in the US I believe. There is at least one person on 'that' auction site who is selling replacement cams made of aluminium billet, I fitted one to my 1800SS, it required a bit of fettling before it would fit but once in works fine.

[RoyceVM]

Originally Posted by Welsh Anorak

Glad it's working - these are nice machines.

It's odd yours is an Akai as I thought these machines were sold as Roberts in the US. Be gentle with the controls - you probably know this but the cams are made of soft alloy that crumbles away. Worse on more modern machines, but I have had the lower one break in an M8 which isn't easy to source.

You clearly know which capacitors to buy and where to get them from. It's always difficult when advising people on the forum as we have no idea of the level of experience of the questioner, so no offence meant by treating you as a beginner which you obviously aren't!

I would have thought the VU [problem would be relatively easy to find, though of course it could be the meter movement itself.

Thank you! I can see the Akai M8 was made with more attention to detail and top quality back in the mid-sixties. Mostly we find Roberts brand here on Ebay and Facebook marketplace. I got it from seller in US and many of them bought it from overseas and brought it here. My Akai GXC 635D was bought from a guy who worked in the US Navy and he got it from Japan. He kept it as a treasure with no hairline scratches on it even. So, this might have been bought from outside US. This was again in pristine condition, and I wanted to give more love to it. The part I really is very little or no distortion if you increase the volume.

Actually, I didn't know about the cam problem. I know what you mean, and these are some castings with an alloy of different metals and somehow fall apart with time.

I consider myself always on the learning curve and I am still there. There is always a lot to learn, and I forgot many things I knew in the past. I love these gadgets of the past a lot and have a good collection.

When I gently turned the VU meter needle alignment screw it moves. The back tension on the hairspring is moved by the adjustment screw, right? The resistance on both VU meters were the same.

It was interesting to see brake was engaged on the supply reel axle in play mode and the axle just slips over a felt pad. First, I thought this is a malfunction and later when I checked the manual it clearly shows how it operate.

Have you found a replacement cam for your M8?

Thanks again for your advice and help!