Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Dec Tue 12, 2017 7:09 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 8 posts ] 
Author Message
 Post subject: Eico 950 Capacitor Bridge
PostPosted: Nov Wed 22, 2017 6:36 pm 
New Member

Joined: Nov Wed 22, 2017 6:01 pm
Posts: 9
Hi: Let me first say that I am a beginner in repairing/using tube electronics (or electronics at all), so please bear with me. I purchased an Eico 950 some years ago with the intention of repairing it. Finding myself between projects, I decided to repair some of my stockpiled test gear and try to work on a couple of AA5's I've collected over time.
The first thing I noticed when disassembling the 950 was that the 1629 tube was burned out, showing some molten metal droplets on the side. I ordered another 1629, and all the capacitors for the 950. I replaced all of caps, and one resistor that tested high. I did not replace any others. When I brought it up on a Variac, everything seemed to work well. I had ordered a precision capacitor to use as a calibration guide, and the 950 seems to measure that very close, almost exactly.
My thought was that perhaps the 1629 was burned out due to a faulty cap. However, after playing with the 950 for a while (checking the old caps with it) I heard a fainting buzzing or rattle. Just a couple of seconds, but noticed the eye tube seemed to flicker slightly during this buzz.
Concerned that I was about to cook another 1629, I shut it down.
Would anyone have a suggestion about where I should look to find this problem?
As stated, it is labeled a 950, but the pot that is shown as a 1 megohm is labeled 100,000 ohm. From what I have read on here, that makes it a 950B maybe?
Any ideas would be greatly appreciated.

Thank you.


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Nov Wed 22, 2017 11:34 pm 
Member

Joined: Nov Wed 30, 2016 7:35 pm
Posts: 414
more details, and clearer descriptions, will yield better assistance. You should be able to look up the manuals for both the 950 and the 950b.. try the bama website, others may be out there as well. That should get you into the right version of the instrument. However, it's also entirely possible that some changes were made by someone in the past. Try and match up the instrument with the schematic and go from there. Posting the link to the schematic here would also help us help you


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Nov Thu 23, 2017 12:20 am 
Moderator
User avatar

Joined: Jan Thu 01, 1970 1:00 am
Posts: 10253
Location: Latham NY
Post some pictures.


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Nov Thu 23, 2017 1:49 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 3381
Location: Monterey California USA
Maddeningly, I have found 950B's that were actually 950A's or whatever the earlier version was, and the bottom line is that you can't go by the front panel model number as far as picking the right manual.

It may just be my experience, but I have yet to run across a bad 1629 and only rarely a dim one. That does not mean yours wasn't bad, it means that because it was a kit, it is possible some hodad wired it incorrectly.

Worrying about the bridge capacitors seems a waste of time today. You can get a digital readout capacity meter for that, a cheap Chinese import, and no need to mess with a hobby grade bridge. The 950 is good for leakage tests and reforming electrolytics (the old ladies are screaming now, because I mentioned reforming electrolytics.) Measuring capacity is usually only needed for testing unmarked junkbox parts. Measuring capacity of parts in an old radio won't tell you much as to whether they are good, and all paper capacitors will not be anyway. Just as consumer grade electrolytic capacitors from the 30s-70's will all be shot by now in some important respect.

I have found the weak link in the 950 is the wirewound voltage adjust pot. They burn open and then there are tears before bedtime. Simplest fix is a 10 position rotary swich with resistors across the steps. Guidance offered if you get to that situation.

_________________
WB6NVH
California Highway Patrol Radio
Bell System Mobile Telephone History
http://www.wb6nvh.com


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Nov Fri 24, 2017 1:48 am 
New Member

Joined: Nov Wed 22, 2017 6:01 pm
Posts: 9
Thank you for the replies. I will take some pictures, try to match the unit with a likely schematic, and get back with you.

Thanks again, and Happy Thanksgiving!


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Nov Fri 24, 2017 3:15 am 
Member

Joined: Jun Fri 19, 2009 6:34 pm
Posts: 7197
Location: Long Island
The Eico 950/950B capacitor testers were enormously popular products and there were a lot of production runs with changes from one to the next. You can't always go by the front panels or other cosmetic details; some of the older style blue panels have 950B circuitry behind them. Add to that the fact that previous repair efforts did not always appreciate the fact that the 950 was a different animal than the 950B; more than a few second and third hand owners replaced parts and wiring thinking Eico provided the wrong parts, the original owner didn't know what they were doing, or the 950B circuit was better than the 950.

From the schematics we see that in the original 950, the leakage voltage pot is 1-megohm and the series resistor is 30-k ohms. In the 950B, the leakage voltage pot is a 100-k ohm, 4-watt monster and the series resistor is 3.3-k ohms. Eico got complaints about the leakage test circuit on the original 950 (which was designed in 1950) being unable to cope with the high-uF electrolytic cans that were only then starting to become popular in TV sets with voltage doubler power supply circuits. The 950B circuit delivers a lot more current and can handle big capacitors, but if you short the test leads out accidentally, it will start sending up smoke signals pronto.

The symptom you describe, brightness of the eye tube flickers, may be due to faulty insulation in the leakage voltage pot. They sometimes break down and arc to the metal case of the pot. You can pop the back off the pot in some cases and spot the burn spot in the phenolic insulator strip.

What did you use for C1, BTW? It is rated at 8-uF @ 525-VDC, and there is not a lot of margin on it as it may run as high as 520 volts in normal operation. 525-volt electrolytics are not very common, and 450-VDC caps would be over their limits.

_________________
"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Dec Wed 06, 2017 1:11 am 
New Member

Joined: Nov Wed 22, 2017 6:01 pm
Posts: 9
I couldn't find an 8 mfd, so I used two 16 mfd with 600 V ratings in series.

I'll try to look at that pot.

Thank you.


Top
 Profile  
 
 Post subject: Re: Eico 950 Capacitor Bridge
PostPosted: Dec Tue 12, 2017 1:32 pm 
Member

Joined: May Wed 16, 2012 6:58 pm
Posts: 255
Location: Eastlake, Michigan
Had a bad leakage pot on mine. Found a Centralab WW-104, 100K 5 Watt wire-wound pot. Physical size is about half that of the original pot.
Seems to work fine.

Internet search turned up these:

http://www.idealsurplus.com/centralab-w ... tentiomete ($25 minimum order)

https://www.amazon.com/s/ref=nb_sb_noss ... lab+ww-104


Top
 Profile  
 
Post New Topic Post Reply  [ 8 posts ] 

All times are UTC [ DST ]


Who is online

Users browsing this forum: No registered users and 16 guests



Search for:
Jump to:  


















Privacy Policy :: Powered by phpBB