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Genetically Modified: Roberts 770X -> SE Bamboo Mono Blocs

Alaska Bill

Alaska Bill

Member
Acquired a non functional, tar encrusted Roberts 770X tube reel to reel recorder (~same as Akai M-8) and after an extensive web research decided to recycle it as a pair of mono blocs as these seemed to be well regarded. Goal is to maintain full original functionality (Phono, Line, and Mic In with Pre Amp and Speaker Out); this requires being able to activate the reed switch.

Project is complete now except for updating the schematic and making some decals where the original lettering has rubbed off. Have a pretty detailed list of what I did to it as well as some pictures if there is any interest.
 
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Very nice. Is that a metal case with look of bambo or is it bambo? I have my M-8 still in tact but plan to break it down some day. Would be interested in your mods and impressions.

Oh, another question. Is there an RIAA equalization circuit when using for phono input? I recall mine sounded fine with phono but thought it sounded like there was no RIAA. It's been a while.
 
The case is bamboo. I used bamboo flooring material which is 3 layers of bamboo. Milled the tongue and groove off and edge joined them to make wide enough boards.

My understanding is the phono input has the RIAA equalization intact and it sounds like it to me.

My impressions:

Power is a measured 5.4 left and 5.9 right watts with a 1 kHz Line In sine wave signal using a scope to determine the point just prior to the onset of clipping

It is a quiet amp. With the reed switch in the “Line In” position (tube 6267 switched out of the circuit) and no input signal, there is very little audible hum at full gain. With the switch in the “Phono In” position, running through 6267, and no input signal, the hum is increased but not real significantly. Physically removing 6267 did not result in lowering the already low hum when running Line In

Recapping made very little audible difference

Vinyl (Phono In) sounds very good; with the tone control at max, there is plenty of treble; I like it better in the lower half of the control

CD's (Line In) sounds great to me.
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All in all I am extremely satisfied with how it turned out and the sounds. Will put together a work list of the mods.
 
The work list:

  • Remove connectors J1, J7, J8, J9, J10, J11, J12, and J13, along with associated resistors, capacitors, and wiring
  • Remove tube 6AR5, socket, choke and associated resistors, capacitors, and wiring (tape equalizer circuit)
  • Remove mechanical linkage associated with the reed switch
  • Remove the record light and the two latch type push buttons (retain for modification and later re-installation)
  • Install new 1 amp fuse in each front panel
  • Install the latch push buttons after modifying to incorporate a slave cable to actuate the reed switch (the “in” position is for Line In (6267 out of the circuit) and the “out” position is for Phono In)
  • Re-cap all polarized electrolytic caps. Gut the single can containing the original combo of two power filter caps and install the two new ones and close up to retain original look
  • Add capacitor to the two position tape speed control to make it flat response in one position and a slight rise in high frequency response at the other position
  • Clean the pots and reed switch
  • Tube test remaining tubes 6267, 12AD7, 6BQ5 and 6X4 to verify condition
  • Scope output and adjust heater hum trim pot for preamp 6267 (EF86) and adjust heater hum trim pot for 12AD7
  • Scope and determine power output before start of clipping
  • Scope and adjust VU meters to read 0 at start of compression / clipping
  • Fabricate wood case
  • Update schematic
  • Make front panel water slip decals for the push button switches and to replace the faded lettering

Have yet to update the schematic (for the items I removed) and haven't made the decals but the rest has been completed.
 
I thought long and hard about taking the VU meters apart to calibrate them. Then I put that in the to hard to do basket and instead just added a very small trimming resistor on the incoming leg to the VU meter. It added about 9k ohm if I remember well.
 
Thanks for the kind words guys.

A couple of additional pictures. The one is the amps as removed from the recorder before the extra's were stripped out. The second shows the gray cable that was added to be able to move the white tab on the reed switch by pushing the button on the front panel. Button in and it is setup for the input signal to be Line In or Mic. Button out and it is setup for the input to be the phono.

Right now am running an ipod nano of MP3's into the Mic input and that seems pretty nice.

Got to thinking. Could you run wireless headphones from the ipod i.e. it would have the bluetooth dongle plugged into it. Then place each mono bloc on top of its speaker at the chosen room location and take the headphone signal and feed it into the Mic. Would have a minimalistic system without wires between the source (ipod, CD, receiver, phono, etc) and the mono bloc. Or would this be too noisy with the wireless transmission? Anybody ever try this?
 

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Alaska Bill said:
Got to thinking. Could you run wireless headphones from the ipod i.e. it would have the bluetooth dongle plugged into it. Then place each mono bloc on top of its speaker at the chosen room location and take the headphone signal and feed it into the Mic. Would have a minimalistic system without wires between the source (ipod, CD, receiver, phono, etc) and the mono bloc. Or would this be too noisy with the wireless transmission? Anybody ever try this?
Click to expand...

You mean something like this? I don't think you'd get noise, but wireless headphones loose a bit on quality, or so I'm told.
Personally I think it's a great idea! Should be a fun project.

Awesome work, by the way.
 
Very sweet!!
I had the same set and sold them to Squidward as my plate was so full I'd never get to them. I know he and dnewman04 both have interest in these amps.

Well done :thmbsp:
 
Thanks Guys. In the searching I did see the thread started by Squidman's and got a lot of good information that Tracker-X posted in that thread. A couple more pictures.

My water slip decals (not shown) will have to be re-done. I got the paper and printed it and put it on only to find out I had ordered paper with a white background where I wanted it to be transparent. :(
No since getting older if you don't get smarter. So now I get to try again.
 

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Nice job! :thmbsp: I have a pair, mostly modded, that are sitting unused right now. I wish I could build cabs like yours, those are sweet. :smoke:
 
Great pics! I have a Roberts 997 that looks very similiar . I had heard about using the amps out of the R to R but yours is the first I've actually seen. I had planned on keeping mine intact but the R to R is noisy and I may be forced to try it your way . I am incouraged by what you have accomplished but I have too many projects to even think about doing something like that now. Curious as to what speakers you are driving ? I am going to see how well the will drive a pair of Klipsch Heresy's once I find some jacks that will fit the speaker outputs. john
 
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