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 Post subject: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Wed 12, 2014 3:46 pm 
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Posts: 328
Location: Somewhere on Planet Earth
Hi all,

Due to a strange conjunction of stars I came across with a 1965 Magnavox console with a color TV/radio/phono combination this weekend.

Color TV started in this country in 1978, so we never knew vacuum tube color TVs down here!!!

I found this TV on a thrift store. I assume that the original owner lived in the US and just brought it here together with all his stuff when he moved to this country. There is no reason for a local to order a color TV in the 60's unless he was into splurging hehe or he thought he was not going to have to wait too long for color TV broadcast to start here :) I go for the first option.

Ok, here is the thing. The set has many missing tubes, and unfortunately I don't have any of them myself among my scrap and the market here is almost cero for this type of things. To make things worse, imports are closed so I can not bring them from abroad (unless some of you are coming here visit hehe).

I've been trying to find replacements for the missing tubes and I would really like to know your opinion about what I found.

The set in a Magnavox U45041, equivalent to the RCA CTC-15 with the exception of the H-out stage which is similar to the one on the CTC-16X.

The missing tubes are:

2AV2 (focus rectifier): which solid state diode do you recomend as an alternative? I read I should be careful because if I use a not really suitable diode and it leaks at some point it can burn the flyback out...

6JE6 (H-out): here 6DQ6 is the common H-out to find everywhere but it is weaker in specs... Have you tried another one instead so I can try to get it here?

6DW4 (damper): the common one here is the 6AX4 (not strong enough), and maybe I can get 6AU4 or 6DE4. Can solid-state diodes be used instead?

6JU8 (detector): which solid state diodes can I use to replace this one?

6GY6: I found no options...

6GU7 (output to the CRT color grids): I found 6FQ7 as an option but with less plate current for the same voltage, could there be a problem with polarization?

Any suggestions will be kindly appreciated! I can't wait to be the only person in the whole country to run a vacuum-tube color TV :)

Thanks!


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Last edited by normende on Feb Wed 12, 2014 10:50 pm, edited 1 time in total.

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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Wed 12, 2014 5:25 pm 
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Location: alameda,CA
First of all, do you have experience working on old TV's or electronics ? You need to be a little more careful with older color sets because some of the tubes in these operate on 20,000+ volts and when you get into that range we're starting to talk about the potential for X-rays. Luckily these TVs usually had shielding. But you need to be aware of that risk not to mention the higher voltages. At minimum the large electrolytic capacitors should probably be replaced. I think by 65' a lot of the non electrolytics were more solid state so you might be ok there. Bottom line: be careful.

The tubes should be the easiest part. For starters, where are you from? If you're in Europe a lot of American tubes will have direct substitutes with European tubes. Otherwise most of the tubes you mentioned are pretty common and you should be able to locate those on Ebay no problem for probably not a lot of money.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Wed 12, 2014 9:56 pm 
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Joined: Nov Wed 23, 2011 4:40 pm
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Hi Bob,

Thanks a lot for the tips.
I am new working on a vacuum tube color TV, but I've repaired vacuum tube b/w TVs and radios as a hobby all my life.

I live in Argentina, and we can not buy stuff on the internet because of "nonsense" restrictions related to payment in dollars.
I've already checked in all the stores I know with no success.... Because of that I am trying to find alternatives :)


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Wed 12, 2014 10:53 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 173
Location: Greenville, NC 27858
It appears I have all, except for the 2AV2 rectifier, which you can have for
postage. Its still probably not worth it unless you have a very good crt & fly.
These are rather common tubes here, so others probably have all, including the 2AV2.
There is/was a solid state replacement for the rectifier.
I have no idea what postage would be, or if they can be shipped without damage.
PM if interested.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 1:21 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 596
Location: Asheville, NC USA
That's an interesting console.

Does it have a phono, and radio, too? I can't tell from the pictures.

that 2AV2 should not be hard to find.

I did service on Magnavox products until 1988, and that included quite a few tube sets, but nothing like yours.

I love roundie sets, and own several that are running well today.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 4:11 am 
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Good to know Pearson, I would really like to have them (if we can find the way).

Bruce, interesting, so I guess you have a lot of experience on these types of sets.
I can see that the set has polyester capacitors so I assume they will not need to be replaced. Not being that old in age maybe the electrolytic capacitors will be fine? According to your experience, did you have to change any component due to aging?

There is a phono and a radio on the console (pics below). I am definitely amazed by the scan system of the turntable to determine the size of the record... A mechanical jewel. Also, the plate keeps running after switching it off for a long time and I didn't lubricate it yet, amazing.

The furniture itself is in great condition, but the sliding door on the phono side is missing (I can't believe someone took it out to use it for something else) but I can set there another one instead, changing also the one that covers the radio for keeping visual symmetry because it will be almost impossible to get exactly the same finish on the new cover lead.

The other really annoying thing is that it seems like a woman with long nails used the set intensively, and because of that part of the golden coating of the radio face plate is off as well as part of the inscription on the source selector knob.

The pics (I still need to do a detailed cleaning, I only removed the dust).

Image
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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 5:12 am 
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Posts: 596
Location: Asheville, NC USA
Some of those polyester capacitors go bad after 50 years, too. When I did service, we weren't having to replace many parts due to age. But now, I'd sure replace the electrolytics automatically.

I hope that your flyback and other hard to find parts are all OK.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 7:30 am 
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Location: 07450, New Jersey
Normende:

If you decide to make arrangements with Pearson, I will send him a NOS 2AV2 at no charge to complete the package. Let me know and good luck getting that set going.

The NTE511 is a solid state sub for the 2AV2.

Regards,

_________________
Tim

"One of the advantages of being disorderly is that one is constantly making exciting discoveries." A. A. Milne


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 2:09 pm 
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Yes Bruce, I'll go for the electrolitics first and then will see if I need to change something else.
Thanks a lot Tim, very kind of you! I will contact Pearson soon to see if we can make it.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 4:02 pm 
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Joined: Aug Thu 07, 2008 1:36 pm
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Location: Orlando
that face plate is pretty typical I have several magnavox sets and they all look like that.

pull the sweep tubes (horz out and vert out)
put a meter on the B+ (there is test point on the back of the chroma board should say 405v or something like that)
use a metered variac (ac current and voltage)
start out at 10vac watch the B+ is should go to 30-40v right away
leave it be for 10 min then bump up to about 30vac
monitor the can caps for heat (should be none)
if I can get to 400v on the test point and the overall ac line current is about 1 amp (with out the sweep its low, with sweep its about 3 amps), and the can are still cool then I will shut down and put the horz and vert out tubes back in and try a full power startup.

I am not a fan of slow starting with those tube in.

Why do I do this?

(1) sometimes the cans are just fine (I have old sets with orig caps that I use as daily drivers with no problems at all that have been in use for years).

(2) I like to base line all sets, that is check to see if they work before I start messing with them. that way if after a recap is done I know its something I did. for this same reason I avoid shot gun recaps esp on sets that are easy to work on (like these).

(3) old sets can be brittle and replacing parts that are not bad is something that can cause damage.

the exception is bumble bee caps, they get replaced on site, as the are almost always bad.

I would guess the most likely bad caps in that vintage set will be in the vert sweep circuit. there are some high voltage (1600v 2000v) in the feed back loop that often are flakely making the vert sweep unstable. the 150uf 50v cathode bypass cap sometimes goes bad, but most of the one I have had seem fine.

The pcb are old so use care working on them this is why I do not like to shot gun caps on them.
other things to watch for on this and all RCA clones ctc-15 thru 17:

Power resistors on chroma board tend to crystalize the solder from heat, then become intermittant, use a good light and magnifiy glass to examine and reflow as needed.

check ground stakes for good solder flow esp chroma board

jumper wires on chroma board (runs on top of board from 6GU7 middle tube to corner near eff coil) there are 2 IIRC they corode and break but hard to see as they stay in place. a "gentle" tug upwards from the board will reveal. The symptom if they break is a tinted screen that can not be adjusted out with the screen drive pot.

there is a trace on the side of the chroma board that is close to the horz out, is thin and can break (heat from hor out most likely).
this is a ground trace and is part of a voltage divider used for taping the horz blanker tube grid voltage. The blanker tube supplies neg voltage for the brightness pot AND a safety voltage to the grid of the horz out tube. If this trace breaks, the neg voltage is too high, not a big deal on the brightness pot as you can just adjust it, but it is a big deal for the HV production as it tends to cut off the horz out. Symptom here will be low HV and width, possible HV regulation issues. tip will be the grid of the horz out tube is TOO neg more so than just the normal -50 to -60 that is due to horz osc drive voltage. This was not an issue on earlier RCA or clones as the safety function was not used.

Lastly I am not here to argue with Bruce about replacing the electros again no harm in doing it, Its just not what I would do, unless the set failed the initial startup sequence. I prob would replace them IF I was doing this for a customer as there is a higher chance of failure with 50 year old caps than new ones.

so If I was doing work for some one I would prob do the initial check out, then replace even if it worked fine.

I did not replace any caps in the color maggie radio on my youtube, but do not recall if I did on the TV.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Thu 13, 2014 4:16 pm 
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Location: Orlando
wow i just looked at that pic you have, I had forgotten that its not really a clone esp on layout, so the horz out is on the other side of the chassis, which moves that troublesome trace for the blanker tube away from the high heat source. I think they used a RCA chroma board but swapped the HV and the LV power supplies. Considering the hard time you have getting parts, I would def want to test out the CRT before putting too much time into this.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Fri 14, 2014 10:49 pm 
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Wow Dave, thanks a lot for all those great tips!

Is it common that CRTs are bad in these sets? Just to know, what can cause this? Intensive use, just aging, something bad in the circuit? If there is something related to the circuit I should particularly check to avoid any future damage on the CRT please let me know.

I have no CRT tester unfortunately. My excitement of having a valve-stated color TV working will make me stand a reasonable issue in the CRT..... I hope this will not happen, but for me it will be great anyway to be able to see something on the screen! :)

Thanks again, I will try to get the tubes somehow.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Fri 14, 2014 10:55 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 596
Location: Asheville, NC USA
The CRT in your set might have been replaced at some point. It looks really good from the front, with no halo effect around the edge of the screen.

Can you find a sticker on the CRT that would tell you who built it?


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 4:26 am 
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Joined: Aug Thu 07, 2008 1:36 pm
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Location: Orlando
They can just get weak, go to air (gassy), I had one that one of the filament was out the other two fine.

Chances are it will be ok, but its always nice to know. If you look at the guns look for blue metal (bad lots of heat, lots of run time), brown is not as bad, and shiney is good. This is not definitive but I think there is some correlation between how the gun looks right at the filament and the overall use of the CRT. Some can become gassy and still look new (rebuilds sometimes are like this).

The upside is often combos like this were not the daily driver/baby sitters but more special occasion relegated to the living room and thus rarely used but for entertaining guest.

It clearly was TOTL considering the doors that conceal the CRT, I always liked the way Magnavox used the stereo amp for the TV sound (most other brands used a switch to connect the speaker to the on board audio amp from the TV). Those old stereo amp both the tube and the later astrosonic really did sound nice, esp when the speaker system used the 12 or 15" mains with horns and a cross over for the highs.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 7:37 pm 
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Below is a pic of the label on the CRT and a pic of the neck.
I checked for colored metal as Dave suggested but I just saw shiny cylinders.... Maybe I am not looking at the right place or so far I am being lucky?

About the sound, I would have been much happier if the set have had a valve-stated amplifier, but the radio and amp on this set only have transistors. Anyway the sound is acceptable but according to my particular taste it lacks of treble. I can try to equalize the amp or add a tweeter (there is plenty of space in the front to do this, but in general I am reluctant to modify original configurations so maybe I will just change a capacitor to boost high frequencies).

Image
Image
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Last edited by normende on Feb Sat 15, 2014 7:56 pm, edited 2 times in total.

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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 7:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 596
Location: Asheville, NC USA
The picture tube you have did not come with a safety glass bonded to it.

Channel Master tubes were pretty good, IMHO. I sold many of them, and only had one bad one in the bunch.

I would replace the capacitors that you will find at the speakers themselves. They are non-polarized crossover capacitors. I'd do this before trying to reinvent the audio section of your amplifier. I think you might be pleasantly surprised.


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 8:08 pm 
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Wow, it's great to hear than Bruce!!!


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 9:26 pm 
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Joined: Aug Thu 07, 2008 1:36 pm
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Location: Orlando
Indeed don't short change those old astrosonic amps, they can sound very nice. I assume you set has the 12" cone side firing and horns that point forward?

My favorite opening sequence is the old Hawaii 5-0 tv series. Mine has the astrosonic solid state amp and sounds great. With respect to treble, yes they seem lacking when listening to the radio, but if you try the phono I bet you find it just about right.

I don't know if there was a difference in pre-emphasis in today's programming vs back in the 1960's (pumped up bass today).


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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 10:14 pm 
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You are right Dave. That's the loudspeaker's configuration on my set.

I will follow your and Bruce's advice. I will not modify the amp. I will replace the caps on the loudspeakers and see what happens. If you mention that in your set the radio sounds a bit like mine but not the phono maybe in the circuit there is a capacitor right at the output of the radio section that could be changed. I don't have the schematics for the radio.

The phono cartridge unfortunately doesn't deliver almost any signal... After playing with it a bit to the sides back and forth I can hear something coming from the left side, but setting the volume control to maximum.
Though I will not be very happy with this, if I want to be able to use the phono I will have to replace the cartridge for another one. Will try and see, if it really makes the difference I will have to do it. I have some contemporary Sonotone ceramic cartridges that were good enough to not damage the records.

Image


Last edited by normende on Feb Sat 15, 2014 11:01 pm, edited 1 time in total.

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 Post subject: Re: Alternatives for tubes on a Magnavox color roundie?
PostPosted: Feb Sat 15, 2014 10:33 pm 
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Joined: Aug Thu 07, 2008 1:36 pm
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Location: Orlando
Yes sometimes the cart can get weak, and yes you need a pretty high output for it to work right. I have been lucky, most of the time mine have been ok but I did have a dead one once, think it was an old grundig.

Your CRT guns look excellent. Hopefully the set was a living room queen that did not see a lot of use.

You should check the output from the cart a scope would be nice about .5v PP IIRC. Or just try touching the terminals at the cart with a metal object you should get a pretty loud hum, that would prove the wiring is good from the cart back thru the amp.

On one set of mine an RCA with a tube tuner and amp I did change the RC filter in the de-emphasis circuit for a more pleasing tone. Doing this at the FM de-emphasis vs the amp allowed the phono to play properly (if I did it at the amp the treble would be too great in the phono mode. I cant recall exactly how I did it since there was a FM and FM stereo, but it worked well.


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