squarewave

Space Echo Trap Core Broken
« on: May 14, 2020, 11:09:54 PM »
I just broke the core on the trap in my RE-101. Who on earth has a plastic square tool suitable to turn a delicate ferrite core like that? It wasn't my fault!

But I think it was probably broken to begin with because there was 4Vpp on the R48 test point (now there's 7).

I suppose I could just hard-wire an LC trap and select the cap(s) accordingly. The coil is supposed to be 14mH:

14mH / 470p = 62kHz (indeed I measure 62kHz) with a theoretical Q of 5.5

The closest high-Q shielded inductor I have is a "toko" of 100mH:

100mH / 66p = 62kHz with a theoretical Q of 38.9

What are my chances of landing close enough to 62kHz to get the bias down to 2.5mVrms 2.5Vrms as prescribed?
« Last Edit: May 15, 2020, 03:16:31 AM by squarewave »


Whoops

Re: Space Echo Trap Core Broken
« Reply #1 on: May 15, 2020, 12:13:30 AM »
I know it's not cheap for such a small component,
but in a valuable unit like that why not buy a replacement?

https://reverb.com/item/10438545-roland-space-echo-mc-128-coil-trap-replacement-part-for-re-201-etc

https://echofix.com/products/coil-mc-128-trap-genuine-roland-nos-part-for-re-201

Sorry, it this is off topic, I know it's not what you've asked just trying to help

squarewave

Re: Space Echo Trap Core Broken
« Reply #2 on: May 15, 2020, 03:11:36 AM »
Well I found something kinda interesting.

I modeled the bias trim > record head < record amp Z and found that the 100mH inductor actually makes a very very good trap. It makes a much wider notch than the 14mH. It just barely reaches down to 20kHz according to LTSpice (0.3dB down @20k).

Also, fortunately, I misread the service manual. The target level at the R48 test point is 2.5 VOLTS rms, not millivolts.

I breadboarded the circuit, fed it with bias from the Space Echo and used a trimmer cap to zero in on the low point. I swapped in a 47p+6p8 and got down to 272mVpp. That's a 28dB improvement!

The question is, what are the benefits, of reducing the bias on the record amp load? Presumably it stops the amp from clipping? Shift the EQ down for better LF fidelity? Increase record amp gain for hotter tape? How can I take advantage of this?
« Last Edit: May 15, 2020, 03:17:25 AM by squarewave »

squarewave

Re: Space Echo Trap Core Broken
« Reply #3 on: May 18, 2020, 01:13:34 PM »
Well without the trimmer I was only able to get down to 0.98Vrms (2.76Vpp) and even that took quite a bit of work. I used a little bus board with an inductor and some handy little millmax sockets (0666-0-15-01-32-02-10-0) that allowed me to try different combinations of 2 caps:



But it's so sensitive that, even though I had reached 2.68Vpp, it jumped out of the notch after I cut the leads shorter! And it required trying many different caps. One of the ones I settled on using is from a cheap grab-bag of caps I bought from Radio Shack ~10 years ago. Having caps with bad tolerance was actually useful in this case.

In practice I would have to make a proper PCB with 2-3 SMD C0G caps and the smallest trimmer cap I can find (are trimmer caps temp stable, non-microphonic, etc?). Then I bet I could pretty easily stick the notch.

Bit it's still not clear that it's worth it. After I make a fresh tape loop, I could try de-tuning it on purpose and then see if it impacts distortion or noise or whatever.

But one thing that is worth it is making a little PCB with connectors on it so that you can easily get to the underside of the main board.


 

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