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Author Topic: 0136 mod  (Read 2442 times)
unsolvedproblem
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« on: July 24, 2016, 08:24:51 PM »

Hi, i'm currently working on a 636-console for a studio (not mine) The owner said some channels are out of phase... I thought it should be the interconnection from computer and so on. But, now i have found out that in some channels there is some modifications: Some capacitors is removed (and bypassed), no 5532 or 34, some 2003 and then 071:s... And also, on opamps a 3.9 K resistor between output and - .... The capacitors are polarized instead, so itīs some kind of biasing resistor. Has anyone seen this before?

Of course this is the reason some channels seems to be out of phase (not 180 degrees)
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Harrison
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« Reply #1 on: August 01, 2016, 11:41:38 AM »

1) An awful lot of people don't know what they are doing.
2) This form of biasing electrolytic capacitors is used by SSL. Mixed views as to whether it's worthwhile.

Since the mods seem problematic, I'd try and put it back to reasonably stock. As we have discussed recently, there is nothing wrong with the JH600 per se, until you put the EQ in.
There is a proliferation of coupling caps that could be cut down, but the circuit blocks themselves are pretty good. And other than the stupid coarsely detented pots, the 136 modules had all the fixes on them, so why piss with them.
The one mod I would do is to change the channel level pot for a 10K (whatever)  log detented pot from Vishay/Sfernice, with 21 clicks. The clarostat ones are very low torque and thus easily moved.
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Harrison
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« Reply #2 on: August 01, 2016, 11:48:48 AM »

Sorry. Re-read your post.
the 136 boards are the eq variQ sub board.
The biasing resistor on the output of the IC is to force the IC into Class A mode. Why bother?! This, I can assure you, is not going to make the EQ any better. See another post re: retrofit sub boards.

The only caps to remove EQ wise are the 22uF coupling caps in the EQ inverting path on the module itself.

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Harrison
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« Reply #3 on: August 02, 2016, 06:39:48 PM »

Not for the first time, I'm talking complete crap!
Irrespective of what board it is, and I do tend to ignore board numbers and just describe them as 1st/2nd/3rd versions.
Anyway, the comments about the mods and forcing Class A still apply.

Hot rodding a JH600 module is much more about fine tuning. Getting rid of AC coupling caps where you can; there are far too many. When was the last time the console was recapped? How old are the switches? How reliable is it? These are far more important than forcing a perfectly good IC into Class A mode.

There is a mod out there for the Master section whereby the Mix ACNs are replaced by 990 DOAs. You might suggest that just replacing with single chips as opposed to duals could be an improvement alone. ie it could be suggested that 2 chips on one die with one dual supply does not bode well for crosstalk and cross modulation.

The 136 module has a capacitor missing in the gain section of the mic pre. It's neither on the mic pre daughter board or the main I/O board. But every version of the style of mic pre has that cap.

Changing 3 resistors on the mic pre board gives you 6dB more headroom there.
Changing the ICs on the mic pre would make a difference.
Re-doing the mic pre to a 2019/1512 or 1510 with servos. Far more effective than pissing around with Class A.
PC
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mikethebike
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« Reply #4 on: September 16, 2017, 04:39:39 PM »

Harrison,

do you have a reference/type number for the replacement Vishay 10K log pot you suggested?

thanks

Mike
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sage
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« Reply #5 on: October 10, 2019, 07:39:59 PM »

Changing 3 resistors on the mic pre board gives you 6dB more headroom there.
Changing the ICs on the mic pre would make a difference.
Re-doing the mic pre to a 2019/1512 or 1510 with servos. Far more effective than pissing around with Class A.
PC


WHICH RESISTORS ??

Sage
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