Call Search
     

New to Ham Radio?
My Profile

Community
Articles
Forums
News
Reviews
Friends Remembered
Strays
Survey Question

Operating
Contesting
DX Cluster Spots
Propagation

Resources
Calendar
Classifieds
Ham Exams
Ham Links
List Archives
News Articles
Product Reviews
QSL Managers

Site Info
eHam Help (FAQ)
Support the site
The eHam Team
Advertising Info
Vision Statement
About eHam.net

   Home   Help Search  
Pages: [1] 2 Next   Go Down
  Print  
Author Topic: Vintage radio power-up  (Read 5871 times)
K2OWK
Member

Posts: 1071




Ignore
« on: February 26, 2013, 02:24:00 PM »

I have a national NC-98 which has been in storage for many years. I am going to power it up slowly using a Variac. Do I leave the tubes in while doing this, or is it good idea to remove the tubes before doing the power-up?

Thanks,

73s

K2OWK
Logged
N4NYY
Member

Posts: 4816




Ignore
« Reply #1 on: February 26, 2013, 03:46:56 PM »

I usually leave the tubes in for the circuits are complete. I have only done 2 or 3. So far, no sparks!
Logged
AC5UP
Member

Posts: 3927




Ignore
« Reply #2 on: February 26, 2013, 04:00:26 PM »

If you want to test the transformer first, remove the 5Y3, 5U4 or whatever the rectifier tube might be, then give it 120 vac straight up the power cord. This will juice the transformer and filaments with an opportunity to check the secondary windings. Do not worry about the actual voltages as long as they're in the ballpark... With no load they won't match the squidmatic values... But any center tapped windings should be checked for balance.

Both sides equally hot? That's good. Filaments lit? Even better.

You will see opinions regarding a low-voltage conditioning cycle through a Variac to re-form the electrolytics, but I wouldn't drop the coin for a Variac if I didn't have one. You can haywire an Edison lamp socket in series with one side of the AC line then use a 40 Watt lamp to give it a lo-volt experience. A 60 watt could be better, or maybe a 75 watt jobbie. What's the rated draw for the radio? Remember that with filaments at half power or less the B+ through the 5Y3 is going to be considerably below nominal and a weak electrolytic might be un-stressed at that level. So you really won't know if it's good until it sees full tilt DC.

Looking at it that way, doing the lo-volt startup is needless.

Especially if you assume the electrolytics are on your to-do list. Which is not a bad assumption. I've been known to test the transformer first, replace the electrolytics next, then do my first power up at full voltage. Just like I'll be doing from that point forward. If something smokes, better it should go before I finish the rebuild...........  Wink
Logged

Never change a password on a Friday                
W5JO
Member

Posts: 59




Ignore
« Reply #3 on: February 26, 2013, 04:43:26 PM »

Is there any evidence of damage to the wiring, leaking of caps both electrolytic and paper, burned resistors, frayed wiring anywhere?  I don't like the variac method, it can mask problems that come up later. 

Pull all tubes then power it up, if it blows a fuse, then you need to know why.  If it doesn't then you can install all tubes to see if the filaments light.  If they do then you should replace the filter and bypass caps.  Then check and replace out of tolerance resistors before applying full power to the entire set. 

Think it through and look at what each switch does.  I may be cautious but it is cheaper than a choke or transformer rewind.  Remember Harbach is no longer in the Amateur transformer business so that leaves only one source.
Logged
G3RZP
Member

Posts: 4830




Ignore
« Reply #4 on: February 28, 2013, 03:04:44 AM »

I check the supply lines for shorts, the transformer and chokes for shorts between windings and to frame. Look at the electrolytics for leaks, bulges and so on, put the VOM across them and look for a drop in resistance that  increases as they charge. Then pull the tubes, wire a couple of silicon diodes in instead of the rectifier tube, and bring the volts up slowly with a Variac - I usually reckon 30 minutes at each of 50 volt steps until the electrolytics are at working voltage or you have full line voltage. Check the screen grid and plate voltages at the empty tube sockets: with no tubes, they should be high. If not, look at bypass caps in plate and screen circuits. Check grid voltages in capacitively coupled stages: they should be very low. If not, change the coupling capacitor. Screens fed off a potential divider will be lower.Then remove the volts, see how fast the electrolytics discharge, remove the silicon diodes and put the rectifier tube back in. Then you can start checking things out.
Logged
K8AXW
Member

Posts: 3958




Ignore
« Reply #5 on: February 28, 2013, 10:00:07 AM »

5UP:  You no doubt have more experience than me with this "power up" thingy so I would like to ask a related question. 

As a preface, I had a small electrolytic come apart once which made a hellova mess. 

My question is, is there a possibility of an twist-lok or other large electrolytic blowing up with the sudden current surge after being dormant for years?

When I have powered up old radios, I use a variac but I also maintain a ramped up pucker factor out of fear of hot flying crap.
Logged
AC5UP
Member

Posts: 3927




Ignore
« Reply #6 on: February 28, 2013, 12:45:46 PM »

...is there a possibility of an twist-lok or other large electrolytic blowing up with the sudden current surge after being dormant for years?

Survey Says:   YES!

But there are two things to consider when assessing risk:  Probability and Severity.

While the severity of a mini-nuclear blow out is potentially high, IMHO the probability is low.  Remember that it takes a fair amount of moxie to steam a twist-lok can to the point of rupture and the chances of the secondary winding / rectifier combo in a typical receiver having the Joules to make that happen are low. Diathermy machine, different ballpark. What is more likely is the little black rubber plug in the phenolic cap on the base blowing out / drying out far enough to release nasty goo from 1962..........

On a related note, many moon ago I tore down a hi-amp computer power supply. Beer can sized electrolytic felt like it had a golf ball inside when I pulled it. Mr. Fluke sez it's wide open, best guess is the innards globulated in the heat of catastrophic failure.

To look at it you'd never know there had been a problem...   Grin
Logged

Never change a password on a Friday                
K8AXW
Member

Posts: 3958




Ignore
« Reply #7 on: February 28, 2013, 06:41:08 PM »

5UP:  Copy that.  Many thanks.  I feel just a little better now.  I always step back some when bringing on up..... sometimes cover the unit with a heavy towel..... I know the lil cap made a pretty big mess so didn't want to have to clean out a chassis from one of the big twist-locs.

'course, always concerned about the eyes.

Logged
G3RZP
Member

Posts: 4830




Ignore
« Reply #8 on: March 01, 2013, 12:53:01 AM »

In the days when some of the development labs at the Marconi establishments at Chelmsford were literally in wooden huts, there was one of them with, in 4 by 2 roof beam, the can of an electrolytic was embedded. It had exploded on power up.......I believe it was a 350 volt 1 amp supply.

Usually, they expel their guts through the end with the terminals.

Using the silicon diode/variac approach, I recently restored some test gear dating from the mid 1950s with the original electrolytics. The waxed paper capacitors were, however, beyond redemption.
Logged
KB4QAA
Member

Posts: 2444




Ignore
« Reply #9 on: March 01, 2013, 07:23:15 AM »

Once you are confident of the transformer, I'm a 'Full Power" man.  I see little to be gained by fiddling at lower voltages.

If their is serious doubt about the capacitors they should be replaced.  If there is serious doubt about their condition and you aren't replacing them but fiddling with low voltages, they won't get better, and you are just playing Russian Roulette every time you turn the radio on.

The cost in time and money of replacing old capacitors is a lot less than having a tired one blow later down the road.

bill
Logged
G3RZP
Member

Posts: 4830




Ignore
« Reply #10 on: March 01, 2013, 09:12:19 AM »

Electrolytics DO re-form, and provided they haven't dried out, don't need replacing. If you just wallop them with full volts when they have been out of service a long time, they can then blow because of the high leakage current until they re-form. At one time, it was common practice to re-form electrolytics that were in store for  more than 18 months or two years.

Heat is the big enemy, because it dries them out.
Logged
KD0REQ
Member

Posts: 1006




Ignore
« Reply #11 on: March 01, 2013, 09:59:06 AM »

back in the 60s/70s, Mallory required all their dealers to return electrolytics on the shelf for two years, constantly exhorted them to rotate stock, FIFO.  they didn't want an old cap to wreck their name, and in the day, radio/TV servicemen would buy an extra part or two for common chassis so they'd have one on hand when the next bad set came in.

so the shelf life is definitely longer than two years, but shorter than X.

if the train is going 50 mph and is 3 minutes late at Poughskeepsie, calculate the mass of the sun.
Logged
G3RZP
Member

Posts: 4830




Ignore
« Reply #12 on: March 01, 2013, 12:22:07 PM »

But is Poughskeepsie the best or worst place to live in the US? Apparently surveys 6 months apart come up with different answers.....
Logged
K2OWK
Member

Posts: 1071




Ignore
« Reply #13 on: March 01, 2013, 03:06:32 PM »

So back to my original question. I am reading that leaving the tubes in there sockets during a slow power up using a Variac will not hurt anything?

Thanks,

K2OWK
Logged
G3RZP
Member

Posts: 4830




Ignore
« Reply #14 on: March 02, 2013, 03:01:44 AM »

The rectifier tube will be running at a low cathode temperature, and any current being drawn could strip the cathode. That's why I temporarily substitute silicon diodes.
Logged
Pages: [1] 2 Next   Go Up
  Print  
 
Jump to:  

Powered by MySQL Powered by PHP Powered by SMF 1.1.11 | SMF © 2006-2009, Simple Machines LLC Valid XHTML 1.0! Valid CSS!