Studiomaster Mixdown PSU bypassing, recapping & mods hel

[paulrichards7]

Hi, so Ive just picked up an old Studiomaster 16-4-8 in great condition. Its just a bit noisy, Id like to recap it and mod it.
I have some info such as:
I went through a few of these around 20 years ago. Installing local .1 uf bypass caps on the power rails does help. All feedback loops need to have a small cap across, most stages had them. Use Wima FKP-2 or MKP-2 polyprop caps in the EQ, especially the hi mid and hi shelf. Opamps can be the older OPA 2134 BurrBrown stuff, the LT1358, the OP-275, the AD 8599 (with a brown Dog adaptor) or the National LME49720/LME49860, balanced outputs can use the National LM6172 for great output drive. Also check out the new AD8510 series, a BurrBrown fet input sub for the OPA134 series. Mic pre transistors can be 2SA1316 or 2SA1083's to lower front end noise and add some HF air. A nice big Power One HDD-15-5A will be a good power supply if you add the small 1/2 amp 48 volt supply for phantom. Then you can play with opamps all day long. The summing amps in the master should be the National LME part due to it's large open loop gain, = a clearer mix buss.
Jim Williams

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I started by the power supply caps; major improvement in noise and hum. I found out that most original caps were rated low, like 16 volts for a 15 volt power supply, so I used 25 volts throughout (except where the design called for higher voltage).

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Replace with the same type of capacitor to the one you remove. So, if you remove a 10uF or 22uF tantalum capacitor, you do NOT replace it with an aluminium electrolytic!
2. Neve designers were restricted back then on the size of capacitors and, in the case of the power decoupling to the 183/283 (fitted to the frame of the module), fitted the largest value that could fit in there... usually 640uF. I personally recommend fitting the largest capacitor that you can now fit in there... like a 4,700uF which may be the same size now as the original.
There is nothing to be gained (and, indeed factors may go against you in extreme cases) by using a much higher voltage electrolytic capacitor. You should fit one as close to the operating voltage as you can. 35v is probably a good, top limit, value.
3. As for capacitors on circuit boards... remembering the warning about substituting tantalums for other types... there's no harm in fitting bigger value capacitors if they will fit. If it's decoupling the power rail, it will decouple slightly better. If it's decoupling between two amp stages or an emitter bypass, it will improve the bass response marginally.
Geoff Tanner

Here are the links to pics:
http://i390.photobucket.com/albums/oo345/paulrichards7/PSU16-4-8.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSUschem16-8-16.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSUtracks.jpg
http://i390.photobucket.com/albums/oo345/paulrichards7/PSU.jpg

Now as I dont know anything about electronics and gonna have to rely on your guys's knowledge. I have attached the schematics for the PSU, a pic of the front of the PSU and one of the PCB track. If you could point out which caps should be replaced and where the bypassing should go, I will break out my soldering iron and do the hard work.
Thanks so much for this
Paul

 

[tommypiper]

Well, who wrote the short part about the power supply caps, was that you? I can't tell if you've started on it or not.

I have the same beast. Be very careful taking it apart. It's not easy. Lots of wires to disconnect and if you don't document it will get confusing.

I don't know what to say about giving specific info. It's not really a paint by the numbers thing. Everyone will give slightly different advice. I don't have time to take photos and mark them up to show you what to do. I'm afraid you'll have to take your own photos and post them here and ask questions as you go.

Everyone will tell you need an oscilliscope to measure output for oscillation once you start swapping opamps. Opamp swaps are rarely straight forward.

If you have the money, I would send the thing to Jim Williams instead of trying it yourself, if you really want to have his mods.

If you want to do it yourself, I would not start with bypassing the PSU caps. If the PSU is working, leave it for now. You could replace the big caps in the PSU in your photo if you like. But leave it at that for now IMO.

Replacing the EQ caps would be a safe place to go next. After that, you're in the zone of trial and error and you'll need to test as you go and check for oscilations, ete, and it's very easy to get tripped up.

If noise is your first concern I would not start modding. I would replace caps and check grounds, etc. As stock that desk is not noisy. You don't need to mod it to reduce the noise, just repair it.

 

[paulrichards7]

bump

 

[paulrichards7]

The Latest
So Ive just put her back together and did a bit of testing
There only seems to be 2 channels that are quiet
The rest are either noisy, not working and the scratchy pots seem worse off then when i first got it
I took it all apart and lubed everything usun Deoxit faderlube
There is a hum coming out the left hand side of the headphones
All the channels right/left configuration seems to be backwards
Is there any way I can take out each channel and check it with some kind of scope to see where the problems are based
I have a laptop running Tru RTA which has a scope and frequency sweeps etc
I gotta get this baby working. Soon
Thanks
PAul

 

[paulrichards7]

Latest(17/10/08)
After drowning all pots and switches in Servisol Super 10 I have gotten rid of intermittentness on the channels i have tested
I am still getting, what sounds like to be white noise and maybe 50hz spikes at 50, 150, 200, 250, 350, 450 & 650hz
Ive knocked up a crystal earpiece contraption my friend told me about and on 2 of the channels ive traced the noise, which starts at the collector of the 1st transistor(http://www.toshiba.com/taec/components2/Datasheet_Sync//50/6451.pdf) from the line input. My friend said that I should check the caps around it to see if thats the problem as the transistor should either work or not. No in between.

 

[tommypiper]

Well, something has gone awry if you're getting L R reversed, and noise. Again, cap replacements, grounding issues, wires got reversed, and opamp signal path troubleshooting... confirm the PSU is working to spec... etc.

You should be able to trace a signal through each channel and find where it might go awry... if you're sure you want to use the board in future then might as well replace all electrolytics while you have it open...

You need to find a systematic approach to troubleshooting it or you'll be guessing and replacing parts without knowing what the problem is.