AKAI M7 M8 / Roberts 770 Mod updates

[RodC]

My website has been rearraged, but you can find the above info in DIY. I wanted to update this thread with this info. Ill post some pics/screenshots soon.

If you are experiencing a low-level noise/hum with your preamp, follow these instructions to make sure the hum control is at its best setting.

The heaters of these preamps are AC. The designers included a hum control to minimize AC noise introduced by the heaters. These controls may have to be in different positions depending on your conditions, and will be very different if adjusted for 60Hz lines and you are using them for 50Hz lines.

The 2 potentiometers are located behind the grill that covers the front of the tube area. If you look at the front of the unit just above the meter and to the right you will see 2 pots with screwdriver slots. The top one is for the EF86 heaters, the bottom is for the 12AX7 heaters (and the other tubes).

To adjust this using a DAW, without the use of an oscilloscope:

1 Disconnect any cable going to the inputs.

2 Connect the EF86 output to your interface.

3 Turn the gain of the EF86 stage to max.

4 Record this signal with your DAW. Important: zoom in as far as you can hile looking at the waveform. Most of these noises will not even register on the meters or show up with standard recording settings.

5 While watching the waveform, vary the pot (top one for EF86) to the right and left slowly, find the point where the noise is at its minimal.

6 Move your output connection to the 12AX7 out, Repeat for the 12AX7 stage

[GregU]

Hey Guys! I've been a long time subscriber forum troller for a long now but this is my first post!

Anyway, I have some questions about the akai pres. First, Thank you so much for taking all the time to right such explicit instructions on performing these mods, they are very concise. Alright on with it. I got an M-8 for free a long time ago (I collect a ton of reel to reels), anyway the heads were shot so I decided to turn them into pres....If I only knew how to go about it. Fast forward a few years and I found Rod's site and here we are today.

First, a question about grounding. in my M8s, before everything got removed, I checked the ground paths, center pin of EF86 was connected to all of the boards rear grounds, chassis, and to the other tubes' center pins. now after the gutting, the preamps and power sections ground paths are separate. Is this normal? Or does it get fixed when the build is complete (still waiting on transformers)? Second, I 'm not entirely sure how to wire up a new 3 wire power cord to the transformer. Most of my electronics experience is with DC stuff (Mics, moding things, etc). does hot go to the 117v tap, neutral to 0v and ground to chassis? Also I just noticed that I have chassis ground (green wire) running to 0V on one and 100V on the other, do these need changed or removed?

Sorry few the noobish questions, I am new to AC type stuff...I just don't know anybody who knows anything about electronics, and I decided that I've waited long enough to build these and decided to just go for it.

Thank you guys, I appreciate any help!

[GregU]

I just figured what the deal with green wire running from 0V and 100V...the are running to the switches that have already been disconnected and just need to be removed. Oops

[RodC]

Sorry it took me a while to get back to you.

The ground for the EF86 stage is connected to a point on the lugs near the power transformer where all the other grounds are connected. You will then note that it is connected to the chassis. There are a few notes about this at the end because the wiring is a bit different on some units.

The ground for your power connects to this same point. Its not truly star grounding but pretty close.

I'm on the road this week, if this dont clear it up I'll try to take some pics sat.

[GregU]

Ok, so should I just run a wire from EF86 ground to that ground point above the transformer? Also, was I correct about running the AC line in to the transformer? Hot to 117 tap, neutral to 0V tap, ground to chassis, is that right?

Which are the cathode resistor bypass caps? You suggested replacing them and I figured that since I have this open I'll go ahead and recap all of it. Speaking of, the 25uf caps are pretty nonstandard value nowadays and kinda hard to source, could I use a 20 or 30 to replace them with?

Thanks a lot man, I appreciate it. Oh yeah, if it wouldn't be much trouble, some pictures would really help out. I was trying to look at the boris model on your site just to double check but they are kinda small.

Have fun out on the road!

[RodC]

Ok, so should I just run a wire from EF86 ground to that ground point above the transformer? Also, was I correct about running the AC line in to the transformer? Hot to 117 tap, neutral to 0V tap, ground to chassis, is that right?

Which are the cathode resistor bypass caps? You suggested replacing them and I figured that since I have this open I'll go ahead and recap all of it. Speaking of, the 25uf caps are pretty nonstandard value nowadays and kinda hard to source, could I use a 20 or 30 to replace them with?

Thanks a lot man, I appreciate it. Oh yeah, if it wouldn't be much trouble, some pictures would really help out. I was trying to look at the boris model on your site just to double check but they are kinda small.

Have fun out on the road!

Yes ground the EF86 to that same point.

I run the neutral to the 0.
The hot goes to a fuse, then the switch, then the 117 tap.
The Ground goes to the chassis.

Most of the Cathode bypass caps are the 25uf. There will be a cap and a resistor right off the cathode. I would use a 33uf for a close to stock sound, or a 47uf for a bit less compression.

I'll try to put some pics up Sat.

[RodC]

I put some close up pics in a zip file:
http://www.beyondsanityproductions.com/ ... M8Pics.zip

The link is also at the bottom of the DIY article.

I left them pretty big so you can zoom in.

[kickoldman]

Hey Rod, just wanted to tell you how much I appreciate your DIY page. I've been loving my m67, and am about to finish building my Akai pre's per your instruction. In my case, the R2R was an Akai M6, but everything seems identical to your M8 instructions so far.

One question. I really like these as low power guitar amps. In your opinion, does the sound change much in this application after throwing that EF86 stage in there? Thanks!

[FletcherMunson]

I just found a Roberts 770x and an Akai M7 at a thrift store for $20. The Roberts doesn't have a power cord and the tape mechanism doesn't work on the Akai deck. I'm interested in converting at least 1 of the decks into preamps. I've been reading plenty of information on this topic thanks to Rod C's helpful posts and schematics. The Akai powers up and everything but I get a terrible crackling and high pitched feedbacking noise whenever I turn the volume or tone knobs (with or without anything plugged into the inputs). Also, only the "pickup" input on my model works the mic and "phono/radio" input do not. Does anyone know what the culprit may be? (I still had this problem when I switched out the tubes from my other deck but I can't guarantee that these are any better as my Roberts Deck does not have a power cord) Do you think this problem may go away if I remove all of the unnecessary components as per Rod's instructions?

[RodC]

You prob have a loose connection or the large slide switch is bad. (If tubes did not correct it) I have also seen bad pots, resistors caps ect cause this type of issue.

If you decide to do the mods, it is a lot easier to work on when you are done, the circuit is much simpler.

Lots of folks have contacted me with successful stories using my directions. If you decide to do it, and you need help, email me.

[atavacron]

Blatantly piggybacking this thread, I present the Atavacron variation on the Childers Akai/Roberts mod! Though I can't compete with you guys' moustaches. Them things is super long.

I bought an early-version M7 off Craigslist for $25. I've stripped all the unnecessary components as specified by Rod in his 2-in-1 document, and robbed both the Boris and the "68K/1M 12AX7 input" note from the 2-in-1 document for the schematic design. I bought a Shure M67 (rackmount! - very useful chassis for a future project I suppose) on evilBay for $34 shipped, from which I'll be using all four M67 mic transformers.

I'll be using both an M67-coupled input and a Hi-Z input on both the EF86 and 12AX7 stages. The front panel connectors and controls are as follows:

EF86 Hi-Z Input (Akai 1/4")
EF86 Lo-Z Input (Neutrik XLR-F D-series, silver)
EF86 Output (Akai 1/4")
EF86 Gain (500K-A 13mm pot, silver knob, between Grid 1 of the EF86 and ground)
EF86 Output (500K-A 13mm pot, between the coupling cap and ground)
12AX7 Hi-Z Input (Akai 1/4")
12AX7 Lo-Z Input (Neutrik XLR-F D-series, silver)
12AX7 Output (Akai 1/4")
12AX7 Gain (500K-A 13mm pot, silver knob, between the .02uF coupling cap and ground)
12AX7 Output (500K-A 13mm pot, silver knob, between the 25uF cathode bypass cap and ground)

...and aditionally I'll be adding a fuse, dedicated grounded power cord, and pilot light to each preamp. The fuse and pilot light go in the two unused holes above each preamp's power toggle switch. The cords come out the back, and are the only rear-accessible connectors for either of the units.

But here's the big news, friends. All this crap fits on the front panel. That's right. I'm not using the same holes - in fact this is almost certainly going to require a step bit and a Dremel - but it will fit. Very exciting. So no need to rackmount the preamps with connectors and controls on the rack ears! I guess I can rackmount the complete-in-the-box pres at a later date if the case's cool factor ends up wearing on me. For now, I can keep the preamps where they are in the M7 carrier chassis (which I totally gutted), and still have space for a panel of 3/4" plywood containing a Jensen 8ohm 8" p8r. Unless somebody has another speaker suggestion for me.

I will get my chicken-scratched version of the schematic and pictures of the righteously abused front panels up in this thread at some point. Especially if someone reminds me in February.

[RodC]

Glad to see others continue the mods.

The 12ax7 stage with the M67 is really gain overboard. Boris wanted this so he could really drive the tube into distortion. You may consider a tranny with less step up or maybe a lower gain tube.

After you get all the extra crap out you can get quite a bit of stuff in there, Boris wanted his to be standard rack size so I decided to put some stuff outside and forgo drilling new holes.

Post some pics!!

EDIT/BTW I get a lot of emails/phone calls asking me to do these mods. I hate to say it but I just dont have time to do them anymore. This is the main reason I decided to publish as many details as possible.

These are gems in the rough.

I also get a lot of ppl asking about the performance of the EF86 output. The ef86 does like to see a 50K impedance, but with many of todays interfaces it will work just fine. Many are as high as 30K. If this worries you, put a cathode follower after it (or patch it into the 12ax7 stage as per my docs, this is really gain overboard), but I really like the pure sound of the EF86, mixing a 12ax7 or 12au7 in really does color the sound more.
There are also lots of hi fi folks out there that use the EF86s plate output for component level sources.

[atavacron]

Thanks for the support and info, Rod.

If you wanted to use both the EF86 and 12AX7 circuits in parallel on, say, a guitar track, what would be the best way to mult the inputs? Somehow I suspect that you'd lose a little something with just a Y cable. Some sort of Y box with a simple resistor network, perhaps?

[RodC]

Thanks for the support and info, Rod.

If you wanted to use both the EF86 and 12AX7 circuits in parallel on, say, a guitar track, what would be the best way to mult the inputs? Somehow I suspect that you'd lose a little something with just a Y cable. Some sort of Y box with a simple resistor network, perhaps?

You could use a resistor network, you may want to try combining them before the 2nd stage of the 12ax7.

[atavacron]

Thanks for the support and info, Rod.

If you wanted to use both the EF86 and 12AX7 circuits in parallel on, say, a guitar track, what would be the best way to mult the inputs? Somehow I suspect that you'd lose a little something with just a Y cable. Some sort of Y box with a simple resistor network, perhaps?

You could use a resistor network, you may want to try combining them before the 2nd stage of the 12ax7.

Well, I'm going to have two spare jacks per preamp at the top left of the faceplate once i'm done with the uber-crammed lower faceplate layout. Sweeeet, more wires. That'll be a good easy mod for later on methinks.