Build Thread:MS76

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rtron

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May 16, 2014
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Ok, well I think I'm going to try a switchable slow attack mod. Think I'll add two (one for each channel) DPDT (on-on-on) switches to switch in either a 33k or 68k resistor to be in series with the 22k attack pot (probably just cut the middle wire from the header and put it there (so between the side chain rectifier diodes and the attack pot). Switch down (shorting wire mode) will be stock 1176 (RC value ranges from 100 us - 4.9 ms), switch middle position will be slower attack (RC value ranges from 7.4ms - 12.2 ms) and switch top position will be slowest attack (RC value ranges from 15.1 ms - 19.9 ms). I'll report back if that explodes or sounds awesome or whatever.
 

TwentyTrees

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Ok, well I think I'm going to try a switchable slow attack mod. Think I'll add two (one for each channel) DPDT (on-on-on) switches to switch in either a 33k or 68k resistor to be in series with the 22k attack pot (probably just cut the middle wire from the header and put it there (so between the side chain rectifier diodes and the attack pot). Switch down (shorting wire mode) will be stock 1176 (RC value ranges from 100 us - 4.9 ms), switch middle position will be slower attack (RC value ranges from 7.4ms - 12.2 ms) and switch top position will be slowest attack (RC value ranges from 15.1 ms - 19.9 ms). I'll report back if that explodes or sounds awesome or whatever.

I did a similar thing on my MS76 - swapped the 22k pot for 250k to give way more range. Gives around 9 or 10ms max attack, going by how the original 1176 speeds were calculated. I also used a pot with a switch at the slow end of the pot rotation, which switches in an additional 500k or so resistor for a "slow" mode of about 30ms for situations where you want the transients left intact. Sounds great!
 
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rtron

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May 16, 2014
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I did a similar thing on my MS76 - swapped the 22k pot for 250k to give way more range. Gives around 9 or 10ms max attack, going by how the original 1176 speeds were calculated. I also used a pot with a switch at the slow end of the pot rotation, which to switch in an additional 500k or so resistor for a "slow" mode of about 30ms for situations where you want the transients left intact. Sounds great!
Sweet!
 

ho-fi

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Not much to follow up on, until you report back/try the suggestions!
:)

Gustav
Yes, sorry Gustav, I was very busy...
But finally I found time to measure the voltages and some were quite off.
CH1:
1.5V measured: 4.4V (next to wire going to ratio-board)
1.5V measured:4.2V (left from BC107)

CH2:
1.5V measured: 4.3V (next to wire going to ratio-board)
1.5V measured:4.2V (left from BC107)

Let me know where to look for an error.
 

Gustav

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Jun 4, 2004
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Yes, sorry Gustav, I was very busy...
But finally I found time to measure the voltages and some were quite off.
CH1:
1.5V measured: 4.4V (next to wire going to ratio-board)
1.5V measured:4.2V (left from BC107)

CH2:
1.5V measured: 4.3V (next to wire going to ratio-board)
1.5V measured:4.2V (left from BC107)

Let me know where to look for an error.
Im sorry, but it is unclear to me what your problem is, and what you are measuring!?

The last post from me you quoted was in July...

Also, did you read the thread covering ground from then to now?

Gustav
 

James Meder

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Aug 19, 2012
Messages
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Hi James, I'm still certain we have the same issue. Apologies for not posting more, I have also been busy with work.

I have made some progress with my unit, and I can now get it operating correctly, although the fix is a bit unusual... I'm hoping you would be up for seeing if you get the same results with your unit.

With my unit in 2:1, the meter is buried to the left, and there is very little output. If I physically touch the top of the BC107 next to the 6-pin connector (I've highlighted it on the attached image) the meter springs back to zero where it should be. I can send drums through, and now the unit is compressing properly at all ratios, provided I keep my finger on the top of that transistor.

I know it seems unwise poking around with your fingers, but this component only has 15V DC on it, it won't cause you any harm.

Hello all, just checking back here. I quoted c1184701 above to give a refresher because I know it's been a long while. My unit does something very similar to this as I wrote about previously. Has anyone else experienced this?

I've tried calibrating numerous times, have checked resistor values and have correct voltages everywhere on the board. I'm at a loss of what to do, but here are some interesting findings related to the quote above.

With an oscilloscope I have oscillation reading at 50k on transistors T17 and T7. T7 is the one c1184701 was referring to - see photo for reference to where these are on the board. When I touch one of the transistors the oscillation goes away. This will effect both of them - like if I touch T7 the oscillation will also go away in T17. This oscillation is on all of the transistor leads with the largest reading being on the collector. The transistors measure ok, but could they still be faulty?

Another thing I noticed is that there are a few electrolytic capacitors on the +30v rail going to ground that are rated for 25v. Such as cap15, cap11, cap5 and there might be more. I think this could be suspect after having to replace the undervalued tantalums already.

I noticed that the psu schematic has a +24v rail located in the +30v spot so maybe the 25v caps were sufficient in one iteration of the design? I imagine it could have been confusing for someone putting together the parts for the kit :(

If anyone has any ideas to pursue, I'd appreciate any help. Thanks!
 

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Gustav

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maybe the 25v caps were sufficient in one iteration of the design?

Unfortuantely, no

The tants were a mistake on my part - grabbing a replacement, because the usual was out of stock, and "unfortunately" verifying it as working in a build, while it was obviously on the fringe, and off for some.

The caps you are refering to have been the same part for at least 100 kits, and specs are good for their placements. They are not seeing 30V. 24 t

Are you coming into expected range, when you hold T7?

Gustav
 

unexpectedbrunch

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Sep 16, 2020
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USA
Just wanted to chime in and say I'm in a similar boat... I haven't gotten back to it in a while and had been meaning to troubleshoot further before posting, but thought it might be helpful to know there are others i.e. me. Somehow one of the trimmer resistors broke when I tried calibrating it again so I have to add another and get back to things. Here's where I left off:

Things seem to work as they should except on ratio 2:1 and 4:1(I believe- I have to double check it on 4:1 again, but my best recollection is that both lower ratios were experiencing issue). I went through and thoroughly checked component values on main board and switch boards, including resistors. Everything's right, with the tantalum capacitors replaced with higher voltages as instructed. I checked surrounding voltages, although quite every one(was next on my list). All checked were accurate. Touching that same transistor yielded the same result- where suddenly it was functioning as expected on the lower ratios as long as my finger was on the transistor. Without it there it was doing something erratic like swelling/intense volume drops/rises. Apologies for the lack of clear information and I'll post again once I troubleshoot again. Hopefully there will be some kind of clarity I'll be able to bring to the table too...

It *feels* like something simple is happening like a ground connection not being made somehow. Everything else does seem to be working properly apart from the lower ratio(s).
 

karloff70

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Aug 2, 2007
Messages
281
Location
London
Ok, so after a very long break I am going back in to finish this thing. Seems a lot has happened.

First question: when replacing the 6.8/16V tants with 35V, do they have to be tants or could they be film caps as long as they are 35V?
 
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