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 Post subject: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 6:04 am 
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I'm new to the forums and I'm doing my first ever restoration. I'm restoring a Motorola 5X12U bakelite radio, which my husband inherited from his grandfather. It looks to be a pretty standard AA5 and was in pretty good shape before I started. I found the schematics for it on Nostalgia Air, but unfortunately since I'm new it won't let me post a link.

I read all the restoration advice from Phil's Old Radios and Fun With Tubes before I got started. First, I replaced all the paper and electrolytic caps, the power cord, as well as the rectifier (35W4) and converter (12BA6), which were missing. I believe the remaining tubes (12BE6, 12AT6, and 50C5) were original to the radio. I checked them visually for oxidized getters and tested continuity their heaters, and all looked OK. At this point, I powered up the radio with a dim bulb tester. All the tubes lit up but I got no audio--no static, no hum, nothing. Then I discovered that the 22 ohm series resistor in the B+ line had drifted up to around 70 ohms. After replacing this, it played, but reception was spotty. I checked all the other resistors and found several more out of spec, so tonight I decided to replace them and ended up replacing all the other resistors while I was at it, but there's no appreciable difference afterwards.

I tested the voltages at all the points indicated in the schematic, and most of them are within a few percent of the specified voltages. The main one that's out of spec is the plate voltage (pin 7) on the 12AT6 detector/avc/first audio tube. It's around 25V when according to the schematic it should be 45V. Also, the cathode voltage (pin 2) is around 0.19V when the schematic says it should be 0.25V, but I'm not sure that's a big deal. The AVC voltage at pin 1 of the 12BE6 converter seems a little low (-9 to -10V), when it's supposed to be around -8.5V according to the schematic. However, this jumps around a lot, even on the same station, so it's hard to be sure. All this leads me to believe the 12AT6 may be going bad, but I don't have a tube tester or any other radio to try it in.

I made a youtube video showing the audio problems I'm having but again it won't let me post a link. If you go to youtube and search for tvFrx-135mw, it should come up. The abrupt audio drop out that you hear toward the end of the video is pretty typical. It will cut in and out as I move around the room in relation to the radio. When it cuts out, it's always an abrupt loss of sound rather than the typical fading/static that i'm used to with a newer transistor radio. Sometimes it will play OK for a good while, and seems to be better after it's been off for a while. It also seems to work marginally better when I play it through the dim bulb tester rather than directly in the wall.

What do you think? Any suggestions greatly appreciated.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 9:23 am 
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Looks like you are trying to run it with the dim bulb in series? The dim bulb is for temporary use to make sure there are no catastrophic shorts, etc. Try running it with it directly plugged in and see if that helps.

With the dim bulb in series, all voltages should have checked a bit low. Are your voltage readings done with or without the bim bulb in series?

Your 12AT6 may be a little weak, but that is probably not your problem if the radio is cutting in and out.

The 12BA6 is your IF tube; the 12BE6 is the converter.

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 10:47 am 
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As Tom said remove the dim bulb.

Measure your voltages again with respect to B-

Usually any abrupt cut-out indicates a bad connection or something loose.

The sound seems ok now... but eventually you may run into SMD (silver mica disease), because of the nature of the type of IF cans it uses.

So be prepared if one day it starts making loud static crashing sounds. Then you'll need to rebuild the IF cans..
By taking out the fixed mica wafer inside (which has started allowing the silver coating to migrate from the input cap to the output cap... allowing for bursts of B + from the primary side to hit the grid of the tube on the 2ndary side).
After removing the mica disc waferx you can instead use new 100 - 120pf (externally mounted) caps.

Then after getting it running again, you'll need to re-tweak the IF slugs using a signal generator... or if you know what you're doing ...maybe tweak from an on air signal.

Good luck

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 2:51 pm 
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Thank you both for your help.

I understand I'm only supposed to use the dim bulb tester when initially testing the radio, but as I said in my original post, it seems to play marginally better when the dim bulb is in series than without it. Maybe it's only my imagination that the dim bulb is helping, but I've noticed it pretty consistently since I first powered it up. I suppose there are other variables such as my exact position in relation to the antenna that could explain it. I can make another video without the dim bulb tester to show you if you want, but the same problem is still there, maybe worse.

The voltages I reported in my original post were measured with the radio plugged directly into the wall. I just added the dim bulb while making the video for the reason explained above. As far as a loose connection, I've meticulously checked the continuity with my multimeter on every solder joint I made and everything seems OK. I agree the audio sounds pretty good when it plays but take another look at the end of the video. It just goes from full reception to complete silence and then back again. This is a nice, strong, local station so it shouldn't be a problem to pick up. In fact, the problem seems worse on the stronger stations.

So given that the plate of the 12AT6 is reading 25V instead of 45 *without* the dim bulb tester, and all the other voltages are close to normal, what do you make of that? Is it not a problem that the output of that tube is only around half what it's supposed to be? If not audio dropouts, then what type of problems would you expect in that case? I guess those tubes are cheap so I may as well get a new one and try it, but was hoping for some confirmation first.

Edit: here's another video showing the problem both with and without the dim bulb tester (search youtube for Q-pW-tKUrzc). The radio is acting up much more this morning so this is a better demonstration.

Edit 2: Tom, you correctly pointed out that I mixed up the converter and IF part numbers. To be clear, the 12BA6 (IF) was replaced, the 12BE6 (converter) is original. Sorry for the confusion.


Last edited by jblang on Feb Thu 25, 2016 3:48 pm, edited 1 time in total.

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 3:37 pm 
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Pbpix wrote:

The sound seems ok now... but eventually you may run into SMD (silver mica disease), because of the nature of the type of IF cans it uses.


This is something that I'd check in this set even if there's no "crashing" sounds.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 4:30 pm 
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Schematic link: http://www.nostalgiaair.org/pagesbymode ... 012168.pdf

There are two versions of this radio. Both have a component which combines several capacitors and/or resistors into one package. In one version, 4 caps are combined as C5A-C5D. In the other, several caps and resistors are combined as CR1A-CR1H.

In both cases, the component is involved in the plate circuit of the 12AT6, which is where you note the voltage is too low. I would replace this component with individual caps (mica or ceramic if less than 1000 pF, film cap otherwise) and resistors. An intermittent cap in this component could easily cause the symptoms you notice.

BTW, do you have a signal generator? That would help you narrow down the problem more efficiently.

Matt


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 4:50 pm 
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I have the version with the 4 combined capacitors, not the capacitor-resistor network. I had assumed (hoped) since it was ceramic that I wouldn't have to replace it. The part list for the radio doesn't specify a voltage for the capacitor array. Any suggestion there? I don't have the necessary values on hand so I will have to order them regardless.

Unfortunately I don't have a signal generator. Was thinking about ordering and building the Elenco FG500K. Would this be sufficient for radio work? I don't really want to spend hundreds of dollars on a good one.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 5:04 pm 
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None of the 4 caps see more than 50 volts. But you might as well get a higher voltage rating since they're very cheap.

For C5B and C5C you could use one like this: http://www.digikey.com/product-detail/e ... ND/2356809

For C5A and C5D, use .0022 and .0047 film caps, respectively. The usual 630V ones would be more than adequate.

The Elenco FG500K is less than ideal because it is not modulated and only goes up to 1 MHz. You'd be better off posting a wanted ad for an older tube-based unit in the classified section of the forums.

Matt


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 5:16 pm 
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Hmm, on hand, I've got two 50V 220pF disc capacitors, a 100V .0047 mylar, and I could put two .001 100V mylars in series to get the .002 one. That should work right?


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 5:26 pm 
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Yes, except that the two .001 mylars should be connected in parallel to get .002 uF.

The 50V rating is just barely adequate for C5C, and I would normally go for a greater margin... but in this case it is ok because if the cap should fail, current would be limited by R8 (470K) and no damage would be done.

Edit: I would tack in the 50V cap for C5C temporarily, and replace it with a higher voltage cap when you are able to get one.

Matt


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 5:42 pm 
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Thanks for the advice. I'll try this tonight and let you know how it goes.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 6:08 pm 
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It's fairly common for the 12AV6 plat voltage to be low, usually the 470K has drifted upward... If the 470 is good, some sets used a 330k, value isn't all that critical...

I found one radio with only about 8v on plate and that was with a good 470K, turned out the tube was a mis-typed 12BF6 or possibly 12AE6... Either those will draw far more current...


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 7:11 pm 
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The 470K resistors had indeed drifted up. One to 607K and the other to 544K. However, every resistor in the radio has now been replaced with brand-new 1% tolerance metal-film resistors, so that's a moot point. After the replacement, the plate voltage went from 22V to 25V, so there was some small improvement but not much.

I am going to try replacing the ceramic caps as suggested by skylerca tonight and will see if that helps. If nothing else helps, then I will order a replacement tube.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 10:44 pm 
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Here is a nice condition very popular Heathkit lG 102 RF Signal generator. (It's the one I use.)
You'll need a short length (3ft or so) of coax cable with a BNC connector at the generator end and alligator clips on the other end to use to connect the signal to the radio (typically through a small cap .05uf or so.)
http://www.ebay.com/itm/HEATHKIT-IG-102 ... SwPc9Wzems
Image

I also keep a low cost Freq counter connected to the RF output connector through a Tee connector to read more accurately what the actual freq is as the dial pointer isn't very exact.
http://www.ebay.com/itm/Heathkit-Digita ... SwezVWzVKm

Image

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 11:01 pm 
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The two most likely problematic coupling capacitor candidates in any tube radio have traditionally been C5D and C5A.

So there's some hope for you that replacing all four caps in that combo-package will bring positive results.

And beyond that the replacement tube.

Failing those efforts my thinking leads us back to the IF can SMD problems. You are not exhibiting loud static crash sounds, ... but the on/off nature of the problem makes me suspect SMD especially because it comes about after a few minutes of operation and worse on strong stations.

However one thing at a time. Let's 1st follow the other steps. Fingers crossed.

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Thu 25, 2016 11:26 pm 
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jblang wrote:

Edit: here's another video showing the problem both with and without the dim bulb tester (search youtube for Q-pW-tKUrzc). The radio is acting up much more this morning so this is a better demonstration.


Sorry to say... but after just watching the first few seconds of that video.
.. I hear SMD as the most likely problem.

That would help explain why it was less noticeable when the Dim bulb was in line as the internal voltages (B+) was lower and less likely to arc across the two internal caps on the mica substrate.

So... be prepared to rebuild the IFs after you replace the C5 combo cap pack.

There are several videos and other pictures and info online showing the various types of IF can wafers and how to remove them.
It's a bit fiddly (to be sure)
... but with patience... you'll do it. And when you succeed you'll be rewarded with the greatest feeling of accomplishment... well earned!

Here's two good Videos showing one style:
https://www.youtube.com/watch?v=h1lsWaAkCGo
https://www.youtube.com/watch?v=8mS6oEMdY7k

More:
http://oldradio.ca/radio/Tech/IFmica/IFmica.html

http://www.ppinyot.com/if_transformers.htm

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Fri 26, 2016 2:09 am 
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jblang wrote:
It also seems to work marginally better when I play it through the dim bulb tester rather than directly in the wall. .

Tom Albrecht wrote:

With the dim bulb in series, all voltages should have checked a bit low.

Since the set was made to operate at 110 volts, and plugging it in directly probably juices it to 121 volts, perhaps adding a power resistor in the AC return line (off the switch) might help it's performance, aside from fixing whatever else is a problem. Here's one that I used, to drop voltage from 121 to about 110;
http://www.mouser.com/ProductDetail/Vis ... esLacR8%3d

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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Fri 26, 2016 2:22 am 
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The audio seems to be significantly improved after replacing the caps. The radio has been playing pretty well for the last 30 minutes. Youtube video: 6cMDOS0zDmM. Thanks everyone for the suggestions.

BTW, I moved the radio to the other side of the room after realizing that, duh, there's a Plasma TV on the other side of the wall from the original location--probably not the best location for reception.

Edit: The TV was off this morning when the second video was taken so shouldn't have been a factor. Just to make sure, I moved the radio back to its original location with the TV off and it's still playing well. The interference from the TV was different than the problem I was seeing before anyway. It was more of a shrill whine rather than complete audio dropouts.


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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Fri 26, 2016 4:24 am 
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The radio has now been playing great for several hours straight. I think I am ready to call this project done and present my husband with his Papaw's restored radio. If something else goes wrong down the road, I'll cross that bridge when I come to it. Next time I put in an order with Mouser or Digikey, I'll order some replacement caps with a higher voltage, especially for C5C. I'll probably also replace the two 1000 pf that I put in parallel with a single 2000 just to make it look a little cleaner.

Here's some pics of the radio, inside, outside, before, and after. Once again thanks everyone for helping out a total noob. I think I've caught the restoration bug, so I'll be scouting craigslist for my next project.

Edit: actually I had already replaced one of the caps before it occurred to me to take a before picture.


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File comment: Chassis Top
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File comment: Underneath Chassis - Before
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File comment: Underneath Chassis - After
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underneat-after.jpg [ 155.77 KiB | Viewed 2544 times ]
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 Post subject: Re: Motorola 5x12u restoration
PostPosted: Feb Fri 26, 2016 4:28 am 
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jblang wrote:
The audio seems to be significantly improved after replacing the caps. The radio has been playing pretty well for the last 30 minutes. Youtube video: 6cMDOS0zDmM. Thanks everyone for the suggestions.

BTW, I moved the radio to the other side of the room after realizing that, duh, there's a Plasma TV on the other side of the wall from the original location--probably not the best location for reception.

Edit: The TV was off this morning when the second video was taken so shouldn't have been a factor. Just to make sure, I moved the radio back to its original location with the TV off and it's still playing well. The interference from the TV was different than the problem I was seeing before anyway. It was more of a shrill whine rather than complete audio dropouts.

Well that's encouraging. It was probably C5D as the biggest culprit. Did you get to measure the plate voltage on the detector?

As a good test, leave it on for a few hours a day now for a week or so keeping it on a strong station that you can enjoy listening to. If after a week you don't have problems, you're probably good to go.. at least for a while. But eventually those IF's will start to fail... not if but when.
Good job!... esp for a noob. Where did you get your familiarity with circuits and soldering etc.?
... good luck.

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