Hi— this is my first posting at DIYaudio 🙂
Last week I picked up an Akai M8 for $100 which is cosmetically almost perfect, and I looked in the back and the electronics looked excellent, but there are multiple problems. I've just started learning electronics and have only ever put one (easy) tube amp kit together. I was told by an extremely reliable, knowledgeable source that this project is way beyond me, but since my first goal is get it working rather than try to actually understand it (of course that would also be nice) I feel positive about it. My goal is restoration to approximate factory spec feature completeness.
I've thought I'd begin by taking a sort of bill of materials of the thing, removing/testing the transformers and big parts, and it's a safe assumption that the big caps need replacing? The non-tape transport part I naturally want to restore first, then the tape part may be the thing I send out for some old timer to fix since there's likely some magic/voodoo in belt tensioning, friction on this or that, lubing, tape head calibration, etc. I can imagine….
What has been folks' experience with the resistors— will their values have significantly changed over time, or can I get by with leaving them? Any particular ones that should definitely be upgraded? All caps should definitely go though, right? The tubes test perfectly, and a visual inspection (and smell test) yield no apparent problems.
I'm getting loud hum from the unit, rev/ffwd don't work, the central spindle always spins, L channel has no activity (except noisy pot through VU and speaker), R channel only works if I push a giant line level signal through the Mic input and put the Volume at 60%. [EDIT: I get VU but that's all, no audio]
I have the rack mountable parts out of the chassis and everything looks terrific.
The R channel actually uses a two prong US wall plug for its power. It may be a lazy or stupid question, but can this be plugged right into the wall and operated safely without the tape or left channel connected?! Might be useful to teardown and restore the L channel first if this is the case so that I would have the quasi-operational R channel unit to look at.
I'm a little confused about just what I'm getting when this thing is restored since there's so much info online I'm a little confused as to what people have accomplished through mods or what features are stock. Seems like nice monoblock power amps and phono and mic stages, as well as tape. Someone somewhere said that the recording quality wouldn't be as good as that of a 1980s cassette… is that true? (I sort of don't care, I just want to preserve history and give my tube preamp something to plug in/out of, but I feel like it would be amazing to plug in a mixer and make it into the mobile analog recording studio it once was.)
Cheers
Last week I picked up an Akai M8 for $100 which is cosmetically almost perfect, and I looked in the back and the electronics looked excellent, but there are multiple problems. I've just started learning electronics and have only ever put one (easy) tube amp kit together. I was told by an extremely reliable, knowledgeable source that this project is way beyond me, but since my first goal is get it working rather than try to actually understand it (of course that would also be nice) I feel positive about it. My goal is restoration to approximate factory spec feature completeness.
I've thought I'd begin by taking a sort of bill of materials of the thing, removing/testing the transformers and big parts, and it's a safe assumption that the big caps need replacing? The non-tape transport part I naturally want to restore first, then the tape part may be the thing I send out for some old timer to fix since there's likely some magic/voodoo in belt tensioning, friction on this or that, lubing, tape head calibration, etc. I can imagine….
What has been folks' experience with the resistors— will their values have significantly changed over time, or can I get by with leaving them? Any particular ones that should definitely be upgraded? All caps should definitely go though, right? The tubes test perfectly, and a visual inspection (and smell test) yield no apparent problems.
I'm getting loud hum from the unit, rev/ffwd don't work, the central spindle always spins, L channel has no activity (except noisy pot through VU and speaker), R channel only works if I push a giant line level signal through the Mic input and put the Volume at 60%. [EDIT: I get VU but that's all, no audio]
I have the rack mountable parts out of the chassis and everything looks terrific.
The R channel actually uses a two prong US wall plug for its power. It may be a lazy or stupid question, but can this be plugged right into the wall and operated safely without the tape or left channel connected?! Might be useful to teardown and restore the L channel first if this is the case so that I would have the quasi-operational R channel unit to look at.
I'm a little confused about just what I'm getting when this thing is restored since there's so much info online I'm a little confused as to what people have accomplished through mods or what features are stock. Seems like nice monoblock power amps and phono and mic stages, as well as tape. Someone somewhere said that the recording quality wouldn't be as good as that of a 1980s cassette… is that true? (I sort of don't care, I just want to preserve history and give my tube preamp something to plug in/out of, but I feel like it would be amazing to plug in a mixer and make it into the mobile analog recording studio it once was.)
Cheers
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Hi Elliott and welcome to the forum. I know a little about this tape machine because I own one now, but do not use it. Bought it for a few dollars at auction because no one else wanted it. I also bought one new back in the mid 1960's when I was in Southeast Asia. The army shipped it home for me.
Let me start by saying that the electronics are pretty nice. They each have a single ended power amp using EL84s. And they would serve as a nice microphone amplifier in a sound system of some kind. But that's about the only good thing I can say. The transport is a real dog. Did you get the slip-on sleeve for the capstan necessary for the higher speed?
It's only one motor to begin with. Turntables are rim dirven by rubber tires which may be hard and dry by now. But the worst part is the way they develope holdback tention for the supply reel. It's a chinchy friction clutch with a felt pad sandwiched between two plastic discs. As the felt polishes the plastic pieces, uneven slippage (cogging) begins which causes flutter. A real cheap and dirty way of doing things IMO.
This machine, originally sold as a Roberts 770, was probably meant to mimic an Ampex 602. But it's in no way even close to it's equal. It also uses an unconventional bias arrangement with a seperate bias (crossfield) head. The system works but never became popular. I have no idea if any parts are available for this fifty year old machine.
Let me start by saying that the electronics are pretty nice. They each have a single ended power amp using EL84s. And they would serve as a nice microphone amplifier in a sound system of some kind. But that's about the only good thing I can say. The transport is a real dog. Did you get the slip-on sleeve for the capstan necessary for the higher speed?
It's only one motor to begin with. Turntables are rim dirven by rubber tires which may be hard and dry by now. But the worst part is the way they develope holdback tention for the supply reel. It's a chinchy friction clutch with a felt pad sandwiched between two plastic discs. As the felt polishes the plastic pieces, uneven slippage (cogging) begins which causes flutter. A real cheap and dirty way of doing things IMO.
This machine, originally sold as a Roberts 770, was probably meant to mimic an Ampex 602. But it's in no way even close to it's equal. It also uses an unconventional bias arrangement with a seperate bias (crossfield) head. The system works but never became popular. I have no idea if any parts are available for this fifty year old machine.
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A few people, including myself, have pulled the transformers from these for RH84 amps.
Very enjoyable project
http://www.diyaudio.com/forums/tubes-valves/158330-akai-m8-rebuild-restore-will-work.html
Very enjoyable project
http://www.diyaudio.com/forums/tubes-valves/158330-akai-m8-rebuild-restore-will-work.html
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Thanks for the response and the schematic— I have an original paper schematic which is white lines on black background, and I think they might even be different revision from these.…
Wait… 😱
DOH! This this is an M4! What do I do?! I didn't pay attention at all to the branding, I bought it based on the photo and links to info about the M8. Fortunately the guy who sold it to me is a friend and this machine belonged to his grandfather. Not much info on the M4 out there it seems. I guess I overpaid, but then again I haven't actually given him the money yet…. 😛
I just realized all this when I saw these schematics didn't seem like the ones I have.
Welp, anyone care to tell me if this thing has decent guts to it like the M8 or do I need to put it back together and return it?
Wait… 😱
DOH! This this is an M4! What do I do?! I didn't pay attention at all to the branding, I bought it based on the photo and links to info about the M8. Fortunately the guy who sold it to me is a friend and this machine belonged to his grandfather. Not much info on the M4 out there it seems. I guess I overpaid, but then again I haven't actually given him the money yet…. 😛
I just realized all this when I saw these schematics didn't seem like the ones I have.
Welp, anyone care to tell me if this thing has decent guts to it like the M8 or do I need to put it back together and return it?
I remember this also being an interesting read when I was wondering what to do with my M8.
AKAI / Roberts Preamp Mods
AKAI / Roberts Preamp Mods
That looks great!
Now that this is no longer an M8 I'm thinking maybe I really should ditch the tape part and mod this thing all I want. Hard to find much info on the M4, I just had noticed that the tube complement in the M8 schematic differed from what I tested the other day. I DO still need a phono/mic/power stage, and a fun project, so I think I'm going to soldier on uninhibited….
Can you post pictures?
I've had a couple M8s from which i have salvaged the amps. At one point i was considering restoring one but gave up after investigating their performance.
dave
I cannot be stopped… if they don't sound as good as my current phono/power stage then I'll use them for some kind of musical application!Right now I have a 50L6 kit preamp/headphone amp (from Old Stock Audio, a project I was involved with slightly, though I am not an employee BTW) going to a 1985 harman/kardon receiver (if I'm using the turntable I have to bypass the preamp and use only the HK). I'd love to have an all valve system, just for the fun of it.
Here's a pic I took this morning when I still thought I had an M8. I can take some of the transformers if you think they are different from the M8's?
Thank you for taking an interest. I love this stuff but know nothing.
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The chassis are nice too. M4 is substantially similar but uses 6AU6 in the front-end. You could easily strip it down and just use 1 6AU6 > EL84 in an RH84 variiation. Make a right nice little amp.
dave
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Excellent, thank you for the encouragement. If it turns out that the important/unique stuff still checks out then I can begin going through it all tomorrow.
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