roberts 770x mods?

[ubertar]

I found the preamps from an old roberts 770x recorder on my street.
It works, but only when going in through the phono in. It's louder if you go through a step up transformer first, but there's plenty of gain either way. Anyone know any good mods for these to make them better as mic preamps? Or have any ideas why the mic and line inputs wouldn't work?

Also, there's a faint noise, like a nose whistling or short wave radio noise. Is this the filter caps going bad, or maybe something else? It'll probably benefit from a recapping anyway.

Here's a clip of a gong recorded with it. The mic is a sm57.

www.ubertar.com/creot/gong.MP3

[RodC]

I have done lots of mods to those preamps. The secret to those is that there is an EF86 stage in it for the tape head. Most ppl overlook that stage. The EF86 can sound real good if you mod it for mic level input.

Do a search for EF86 (6267) and you will see that it is used in some pretty high end stuff. I like the sound of it more than most 12ax7s out there.

Most ppl just get the preamps and use the Mic in and the Line out, that only uses the 12AX7 portion of the unit.

The first thing I do is strip out all the unnecessary parts, that step alone makes them sound a lot quieter.

With my most common mod I use the input of the EF86 and then put an output after 1/2 of the 12AX7 or after the 2Nd for more gain. The EF86 alone isnt enough gain without a transformer.

These units have a ton of natural compression that colors the sound if your not careful. (They have small cathode bypass caps and tube rectifiers)

I cant listen to your MP3 but what you are describing could be a bad tube also, does it rev like a car and make clicking noises also?

I'm at work right now, if you are interested I will put some schematics up and some descriptions tonight.

[ubertar]

No clicking or revving. The noise is pretty faint, and only happens in one of the preamps.

strip out all the unnecessary parts

I'd love to do that, but have no idea what's necessary or not. My electronics knowledge is pretty basic.

Which is the input of the EF86? Do you get that by using the phono inputs? I'd imagine there's an eq part of the circuit for that, too.

Natural compression can be a good thing...

If you could give me more details about how to do this (input at the ef86, output after the 12ax7 (both sides-- i like gain) I'd appreciate it. Thanks for the help!

[RodC]

Hell yes natural compression is good! Thats one reason a lot of guitar players like tubes and older amps, they dont know it, but its a large reason.

I will get you as many details as I can. Just be careful, as with any electronic equipment there are high voltages in this unit. If you dont know what you are doing please dont get in over your head and hurt yourself.

As for the inputs I change the wiring for all the jacks on the front and add an XLR jack. Some purists are going to hate me, but there are a ton of units out there that have the tape portion worn out. I think we are giving these units a new chance at recording.

[ubertar]

Don't worry. I've recapped a tube preamp before. I can discharge old caps safely. Anything else I should watch out for? Thanks again, I'm psyched to get this thing going. I still can't believe I found this thing on the street. On my block, no less. I hardly had to carry it anywhere.

[RodC]

There shouldnt be any large caps in there to worry about, just make sure everthing is unplugged when you are working.

Go to http://www.beyondsanityproductions.com/ scrool down to the bottom I put a link there for you. This is some schematics and a pretty good doc describing many mods. I have this URL masked and I some times have probs with links to it.

IMPORTANT, please ignore my studio website, I havn't had time to update it in forever, I do much better work now. (But still no where as good as many on this site!)

I have been so busy recoding!

I will get you some good old tube schematics for the EF86 tonight.

[joeysimms]

That's some great info Rod, thanks for sharing.

[RodC]

Couple more things incase you didnt know....

The Robberts were the US name for the Akai Reel to reels. The 770 series were the M7 and M8 Akai, so the link I made is: Akai M7 Schematics.zip

Each of the preamps are self contained, they have their own power supply and transformer NOTE, IMPORTANT. The transformer is setup for 100V The transformer in the main unit is used to change the selected voltage to 100V GOOD NEWS, there is a 117V tap on the tranformer for each unit, make sure you change this. (Its in the docs)

Also, the most important:

One of these units was used to play The Ride of the Valkyries in Apocalypse Now.

However, it really didnt do much:
When Kilgore calls for the soldier to start The Ride of the Valkyries, the soldier starts the reel-to-reel tape. Unfortunately, none of the tape actually touches a playback head. Instead the tape is wound underneath a tension bar and on to the take-up reel.

[ubertar]

Thanks Rod, that's awesome.

Check this out, it's amazing:


http://www.vintagetips.com/2005/07/weir ... dosis.html

[RodC]

Ok, I added some EF86 data, I still have to dig up this one pdf.

This has some data I saved from the Svetlana website, but now they dont have the links up for the old schematics.

I saved them a few months ago. I put all the files with the schematics in a zip file and added a link at the bottom of my site.

The pdf is the standard data sheet, the webpage and .jpg files are the good stuff. They appear to be streight out of the Mullard "Valve Circuits For Audio Amplifiers" textbook, I have seen these same examples on other websites.

If you notice, the schematic in figure 1 is nearly the same as the Akai preamps. The major differences are the value of Rin (this is the one you will want to change to adjust the input impedance) and the catode bypass cap is much smaller than the suggested 220 uf! They are only 25uf. See my notes about how this affects compression, or search some groups.

beyondsanityproductions.com

[RodC]

I added the final pdf to the .zip file on my website.

[BENDYmusic]

Hi, I am also looking to do some modifications on an Akai branded version of this same tape machine. I tried your link but the site is blank. Thank you very much for sharing your research and knowledge. I was planning on using some input and output transformers pulled from some ADM cards, the input transformers are UTC with 50, 200, and 500 to 15k ohms. Do you think something like this would work? Thanks for all your help.

-Andy

[RodC]

Crap, they must have taken it down when we all tried to dl it.

I updated my EF86 Data.zip file. If you go to my site and get a new copy of this file you will see the ef86.pdf.

I havn't tried too many different transformers, but the ones you describe sound like they should be in the ballpark.

I liked the sound of mine without the transfomers, but I havnt given up on this option.

[BENDYmusic]

You say you are using it without input transformers? With microphones?

BTW, this thing makes a pretty rad guitar amp with no modifications...

Andy

[RodC]

You say you are using it without input transformers? With microphones?

BTW, this thing makes a pretty rad guitar amp with no modifications...

Andy

Yes, and yes.

Just look at the schematics, some ppl like transformers. I'm still looking for a transformer that I like better than the EF86 without the transformer. Im sure someone here will find one some day and send me one free for all my efforts (LOFL)

You should try the guitar throught the ef86 stage. If you use it unmodified you are only using 1 stage of the 4 existing stages in this amp. (1/2 of a 12ax7) most simple circuit you could built!

I have one I built for my son that matches the EF86 stage to a guitar impedance and he can then select 1 of 3 outputs and 2 inputs (one at in at the EF86 and one at the first stage of the 12ax7):

IN - EF86 - out
(optional IN) - 12ax7 stage 1 - out
12ax7 stage 2 - out
Or the speaker out using the powertube stage.

You can really get some different sounding distortions, but he is always looking for a better clean sound. It was fun to build.

Richie Blackmore made lots of recordings with these preamps.

I have got to get some better recordings posted. I use these mostly to mic guitars and drum overheads. The natural compression make them sound pretty good as OHs and clean guitars.