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Dynaco PAS-3X troubleshoot

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Dynaco PAS-3X troubleshoot

Postby MRag » 28 Aug 2017 04:04

Good Evening To All,

First time posting/asking a question. I've noticed on my PAS-3X that the power indicator light has started flickering during use. No audio is affected, but what are your thoughts - bulb is just on its way out or maybe something deeper? Still using original selenium filament rectifiers. Thanks for any input.

-Matt
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Re: Dynaco PAS-3X troubleshoot

Postby Coffee Phil » 28 Aug 2017 07:00

Hi Matt,

Welcome to the forum.

The lamp is connected directly to the heater winding of the power transformer. It is unlikely the selenium diodes are affecting it. Look for corrosion in the socket and base of the bulb. Also look for a loose bulb and bad solder in the connections to the bulb socket.

It may be a good time to think about replacing those selenium diodes with silicon. When the fail they stink real bad and I believe they can be toxic. Back in the day one tech in a TV shop where I worked would refuse to work on a TV in which the seleniums has failed. My old Ford needed gasoline so I'd do them. I'm still walking the earth but still seleniums are nasty. If you replace the seleniums with silicon you may have to add a dropping resistor to get the tube heater voltages right.

Phil

MRag wrote:Good Evening To All,

First time posting/asking a question. I've noticed on my PAS-3X that the power indicator light has started flickering during use. No audio is affected, but what are your thoughts - bulb is just on its way out or maybe something deeper? Still using original selenium filament rectifiers. Thanks for any input.

-Matt
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Re: Dynaco PAS-3X troubleshoot

Postby MRag » 29 Aug 2017 01:19

Thank you for info. I'll dive into this week as far as potential corrosion at connections. Now getting into the replacement of selenium diodes - what is the best way to do it? I am not too sure where to start, what parts are really needed, nor the in-depth knowledge of what components would be top notch. I would like to spend the right money and keep this tank driving for a long time. So my kids can one day appreciate it too. Any ideas are totally welcome.
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Re: Dynaco PAS-3X troubleshoot

Postby solvin_dk » 29 Aug 2017 14:31

Hi

I would not chance the selenium rectifier now, just let it stay in place till it fails. It's right that the smell from selenium, is disgusting.

I have a McIntosh mx-110 from 1967 with a selenium rectifier. It stills works like new.

Why fix it when its not broken. Just my point.

Best regards,

Torben

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Re: Dynaco PAS-3X troubleshoot

Postby josephazannieri » 29 Aug 2017 15:18

Yo MRag:

As usual, I enter late. Phil is right. Hi, Phil!

I have 2 of these things, one of which I bought assembled, and the other I built from a kit and modified.

The bulb is connected directly across the filament winding, so the status of the selenium rectifiers will not affect it, unless they are both shorted. If that happens, the bulb won't flicker, it will just go dead. Your problem may be the bulb, which may have a loose filament, or a bad solder joint. The other possibility, if this thing was built from a kit, is a bad or missing solder joint at bulb socket or at the other ends of the wires that hook to socket. If you are going to open this thing up, be aware of the relatively high B + voltages (250 volts) at rectifier tube and filter capacitor can, which can give you a shock. The kind of work you will be doing will not need for you to have the power on to troubleshoot. You can cheat and disconnect one end of each selenium rectifier and cobble in with just about any silicon diode with sufficient current capacity. I'll bet Phil can suggest a replacement diode right off the top of his head. Me, I'd have to go to a catalog and look up a diode.

Finally, if it works, you could just leave it alone. You could also go into a complete overhaul. The one I bought assembled was done from a kit, and I wound up having to clean up the wiring because it was just such a haywire mess.

And good luck from that lazy old guy,

Joe Z.
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Re: Dynaco PAS-3X troubleshoot

Postby MRag » 30 Aug 2017 20:51

These responses are great. I am going to just leave it be for now since I checked all solder connections and they are good. I'll just order a #53 lamp and take the time to replace (seems to last for 50 years). I agree on the "if it aint broke, don't fix it" ideology. Thanks to all. This is extremely helpful.
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Re: Dynaco PAS-3X troubleshoot

Postby Coffee Phil » 31 Aug 2017 02:54

Hi MRag,

I'm sort of a "if it is not broken, don't fix it as well" but if a selenium rectifier is on the way out I like to get rid of it. They can smell really bad and I would err on the side of caution on their alleged toxicity. I recommend that you measure the voltage across the heater on one of the 12AX7s (pins 4 and 5) and if the voltage is less than 10 volts seriously consider replacing the nasty things.
Here is the manual: https://www.hifiengine.com/manual_library/dynaco/pas-3x.shtml

If you find the need to replace them let us know and we will recommend some silicon diodes and talk you through it.

Clean out the lamp socket with a pencil eraser as corrosion may be your lamp issue.

Phil

MRag wrote:These responses are great. I am going to just leave it be for now since I checked all solder connections and they are good. I'll just order a #53 lamp and take the time to replace (seems to last for 50 years). I agree on the "if it aint broke, don't fix it" ideology. Thanks to all. This is extremely helpful.
Coffee Phil
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Re: Dynaco PAS-3X troubleshoot

Postby KentT » 05 Sep 2017 14:24

Also, if you don't have a copy of the Dynakit PAS-3X assembly manual, I'd get one, it has very useful schematics, and troubleshooting information in it.
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Re: Dynaco PAS-3X troubleshoot

Postby MRag » 20 Oct 2017 19:05

Hello Again All,

So I finally did the replacement of the selenium rectifier as per CurcioAudio - went fine. I'll even post some pictures eventually. But during the replacement I did notice a bubble on the positive seal on both of the electrolytic capacitors. Do you suggest I replace those as well? Maybe those are the weak link causing my power indicator lamp to still flicker? For all I know those are the originals.

IF I replace these electrolytic caps, I see their value is 2,000 MFD or uFD, but the main question is what voltage value should these be? I cannot find anywhere in the schematic or understand what is correct. Any thoughts are once again much appreciated.
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Re: Dynaco PAS-3X troubleshoot

Postby josephazannieri » 20 Oct 2017 21:32

Yo MRag:

If there is a bulge in the center of the capacitor, it will probably need replacing.

Undo the screw in the center of the rectifier and drop the selenium rectifier out. Then you can look directly at the capacitors. You should see a marking on the side of capacitor saying 2000 mFd, xxx volts. You may have to rotate the capacitor to see the markings. Replace it with same number or greater volts. As a guess, I would say at least 50 volts should be enough. You are looking at filament voltage, which is about 12 volts maximum.

And good luck from the old guesstimator,

Joe Z.
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Re: Dynaco PAS-3X troubleshoot

Postby Coffee Phil » 23 Oct 2017 18:25

Hi MRag,

I agree with Joe. If you are seeing the case of the caps bulging or bubbling they should go.

The caps for the heater supply are axial leads so easy replacements are still made. They will be physically smaller the the '60s technology caps. Here is a link to the manual: https://www.hifiengine.com/manual_libra ... s-3x.shtml

Sadly the parts list does not give the voltage rating for the caps. As Joe said it should be printed on the cap but 50 Volts should be overkill and will still likely be physically smaller than the OEM parts.

I'm hoping that when you did the selenium to silicon rectifier conversion you checked the heater voltage and added a resistor to make it reasonable.

The lamp is fed from AC upstream of the rectifier diodes and capacitors so the caps affecting the lamp is unlikely, however if the caps have significant leakage which varies I could conceive of how they could do that, however first check the soldering to the socket and the cleanliness of the contacts in the socket. Also you can monitor the AC voltage across the socket as the lamp flickers. If it is steady or rises as the lamp dims, replace the socket.

The Twist lock can electrolytic caps for the B+ supply are of the same age as the ones in the heater supply so they may be approaching their end of life as well. Exact replacements for those may be available but they will be speciality items and therefore spendy. I would favor newly made replicas for those as opposed to NOS as the seals can fail over the years.
Some folks gut the old caps and put modern caps inside the case to preserve the look. I have been known to do that.

Phil

MRag wrote:Hello Again All,

So I finally did the replacement of the selenium rectifier as per CurcioAudio - went fine. I'll even post some pictures eventually. But during the replacement I did notice a bubble on the positive seal on both of the electrolytic capacitors. Do you suggest I replace those as well? Maybe those are the weak link causing my power indicator lamp to still flicker? For all I know those are the originals.

IF I replace these electrolytic caps, I see their value is 2,000 MFD or uFD, but the main question is what voltage value should these be? I cannot find anywhere in the schematic or understand what is correct. Any thoughts are once again much appreciated.
Coffee Phil
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Re: Dynaco PAS-3X troubleshoot

Postby macshifi » 23 Oct 2017 19:44

I have 2 PAT 4 preamps, both originally had neon bulbs inside the power switch, and the one I had originally built from a kit in 1975 would flicker, too. I'd say you have nothing to worry about regarding lamp flicker, but I have also heard that selenium rectifiers give off toxic fumes when they go up in smoke, so I would suggest replacing those when possible, just to be safe.

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Re: Dynaco PAS-3X troubleshoot

Postby Coffee Phil » 23 Oct 2017 22:18

Here is a link to selenium rectifiers: http://www.wikiwand.com/en/Selenium_rectifier

In the replacement section it discusses failed seleniums as being malodorous and highly toxic.
I'm pretty sure it true. I can attest to the malodorous part as I replaced plenty of them back in the day, but thankfully I'm still on top of the grass and I'm fairly old.

Phil
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Re: Dynaco PAS-3X troubleshoot

Postby kb007 » 23 Oct 2017 22:32

Since you're alraedy replaced the selenium, you're on the way.

If it was me, I'd replace all the caps, and likely all the resistors with some nice Vishay or others. Probably time to swap out all those nasty cheap rca connectors too. But that's only if it was me... ;)
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