Weak output from Roland RS101 string synth

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servicenote
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Weak output from Roland RS101 string synth

Post by servicenote » Wed Oct 01, 2014 5:29 pm

Hello everyone,
I'm new to Vintage Synth Explorer, and I'm very glad to find a resource like this.

I have a Roland RS101 string synth that I bought second-hand in the 80's. After that, it was in my attic for around 25 years until this past July when I got it out and hooked it up. Even at full volume, it gave only a very weak signal, although all the sliders and switches were apparently working okay. I discovered a fuse that had blown at some point (maybe years ago, maybe when I first hooked it up, I don't know), so I replaced it. I cleaned the contacts, and after working the volume slider up and down a few times, it came to life, but only for a few hours, until the sound level suddenly dropped to what it was when I first hooked it up. Since then, with the help of the RS101 (and RS202) service manual, I've recapped all the electrolytics (aluminum) and a few transistors on the tone control circuit board. I've inspected all components for any obvious external damage (there was none I could see), tested all the resistors with a multimeter (they're all okay), and I re-soldered a few loose wires. It had a 2-pronged ungrounded plug when I originally bought it; I installed a grounded plug for safety and to minimize the hum. All the control switches and sliders on it are working, but it still puts out only a very weak sound. Has anyone had a similar experience, and can you point me in the right direction toward fixing this problem?

Thanks,
Andy (servicenote)

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Re: Weak output from Roland RS101 string synth

Post by Dr. Phibes » Wed Oct 01, 2014 7:11 pm

Hi, have you checked the volume potentiometer yet?

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Wed Oct 01, 2014 8:24 pm

I opened it up, cleaned it, and tested the wires attached to it. Everything seems fine.

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Re: Weak output from Roland RS101 string synth

Post by Dr. Phibes » Wed Oct 01, 2014 8:49 pm

Did it's resistance match that specified by the manual (about 10K closed I think?)

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Thu Oct 02, 2014 4:15 am

The resistance measured around 8.03K with the volume pot all the way down. The manual says it should be 10K.

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Thu Oct 02, 2014 4:26 am

And with the volume pot fully up, resistance was 10.95K.

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Re: Weak output from Roland RS101 string synth

Post by Dr. Phibes » Thu Oct 02, 2014 2:57 pm

servicenote wrote:And with the volume pot fully up, resistance was 10.95K.
I think that may be the problem, the pot should read close to 0 at that setting. You may want to try cleaning it again - if you haven't already try some electrical contact cleaner. If the problem remains and I suspect it will you'll have to try and find a new volume pot. Finding an exact replacement may be tricky but hopefully generic substitute will be sufficient - I must confess that I haven't actually worked on a RS 101 in person so I can't say for sure.

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Thu Oct 02, 2014 4:12 pm

So, it should read close to 0 with the volume fully down, or fully up?

I'll do some more cleaning, and use some DeOxit.

By the way, I've tried removing the slider knob (by pulling it) but it wouldn't come off. How do I remove it?

[img]http://www.syntaur.com/images/3748-Lg.jpg[img]

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Re: Weak output from Roland RS101 string synth

Post by Dr. Phibes » Thu Oct 02, 2014 5:05 pm

Fully up I would imagine, that way the signal from the preamp isn't being attenuated at all.

The slider cap should come off but they can get stuck. Keep trying, sometimes the heat of your hand can help them go a bit easier I find.

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Thu Oct 02, 2014 5:26 pm

Thanks, Dr. Phibes. I'll let you know how it goes.

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Re: Weak output from Roland RS101 string synth

Post by Yekuku » Thu Oct 02, 2014 10:49 pm

As very well Dr.Phibes pointed out , it is definitely the slider.
I recommend desoldering it and opening it up for a good cleaning.
Remove dust and dirt with a soft brush, you can use a qtip with a drop of deoxit but dont overdo it because the carbon rails may wear off .Others recommend using rubbing achohol but I find it a bit abrasive on carbon.
About removing the knob you can use an angled plier like these :
Image
You can cover it with tape so that you dont scratch anything by accident.
If you bought it used , have in mind that some people have a tendency of using instant glue on knobs so that they dont come off easily. in such case, the only solution I see is to cut the slider shaft and later glue it back together
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Re: Weak output from Roland RS101 string synth

Post by servicenote » Fri Oct 03, 2014 2:53 pm

Yekuku wrote:As very well Dr.Phibes pointed out , it is definitely the slider.
I recommend desoldering it and opening it up for a good cleaning.
Yekuku.

Thanks Yekuku.
If it happens that I need to replace the whole pot, which specifications should I try to match? It's a 10Kohm slide pot, but what about, for example, the power rating or voltage rating? I don't know what that value is in the pot that I'm trying to fix (or how to determine it). The RS-101 has an overall power consumption of 9W, if that's any help. Should I look for a pot that's rated for 9W?

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Re: Weak output from Roland RS101 string synth

Post by Yekuku » Fri Oct 03, 2014 5:05 pm

Hi, I bet that the slider needs proper cleaning and it should get back to work.
Replacements are hard to find, the slider probably works with 15v but no need to worry about power consumption and voltage rating , just look for a 10k audio taper with the same footprint/contacts.
I assume that the rs101 has a mono output, modern synths with stereo outputs have dual audio taper pots/sliders.
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Re: Weak output from Roland RS101 string synth

Post by servicenote » Fri Oct 03, 2014 6:26 pm

Yes, the slider is functioning, but the signal is much softer than normal. Along with cleaning it, I should probably also make sure that the little metal wiper contacts are in good contact with the terminal strips.
And it's possible that the connections on the solder lug terminals have gone bad over time -- the solder on them is a dull grey color.

It's a mono keyboard, with one high output and one low output jack.

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Re: Weak output from Roland RS101 string synth

Post by servicenote » Fri Oct 03, 2014 8:29 pm

Here's an illustration of the resistance readings I took yesterday.

Image

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