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 Post subject: Need a print, Not no mo
PostPosted: Feb Mon 06, 2017 10:58 am 
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I have a Jupiter 6T-220 six Transistor set from 1958, when I received it new as a gift, which I decided to recap, since the originals had given up the ghost.

Instead of removing the original four Electrolytic's, I D/C one lead from each, and installed replacement caps underneath the PCB. It now exhibits oscillation on every station, and although I've tried adjusting the IF cores (which have a very limited travel), I can't get it to stabilize.
I don't remember ever having this problem occur on any other Transistor portable I've worked on, since a slight IF adjustment would handle the problem, but not with this circuit.

This set also goes by the names of Constant, Windsor, Personna, and Crestline, all with the same model number, 6T-220.

I went to RM Org, and although they show this set and the other badges with the same model number, there are no prints.

Any help appreciated.

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Last edited by fifties on Feb Sun 12, 2017 6:38 am, edited 1 time in total.

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 Post subject: Re: Need a print
PostPosted: Feb Mon 06, 2017 8:17 pm 
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Quote:
I D/C one lead from each,


Why? Just to preserve the original appearance, you're going to say. Well, you've made a rod for your own back, as they say. Now you have a great blob of metal perhaps right next to something sensitive, where before the blob of metal was joined to ground or negative - then you added another cap to the other side of the board with perhaps longer leads and made an oscillator around an IF stage perhaps.

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 Post subject: Re: Need a print
PostPosted: Feb Mon 06, 2017 9:25 pm 
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When the problem occurred, I D/C three of the new caps (the forth being connected right at the V/C, and re-connected the originals, but the squealing persisted.
I carefully and under magnification checked all solder landings, but everything appears normal, no bridges.
Placing them near the IF stages was my thought as well once the squealing occurred, so I moved them to the AF stage area, but it didn't help.

I'm now wondering if an IF Transistor somehow became squirrely, so I guess I'll run some voltage checks, but having a schematic to reference would be nice.

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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 12:50 am 
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Sorry Fifties, I can't help out much on this one.

A search on the Universal Schematic Locator could not find a 6T220 model of any make. That's not good.

Allow me to ask some dumb questions: could the PCB have cracked somehow, someway while it was out of the case? Does the squeal go up and down in loudness with the volume knob?

From your brief description of the problem, I'm thinking that it might be a board issue and not a component issue. Just my two cents. I'd be very interested in knowing what you find out is causing the problem.


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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 12:58 am 
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Thanks for searching, KnobDial! The squealing occurs on each station, and does vary with the VC setting.

I'll go over the traces again, to make sure there are no breaks, but when I inspected the solder pads, didn't find anything apparent.

The circuit is most likely a generic 6 Transistor set, one Transistor for the converter/mixer/oscillator, two IF's, an audio driver and two audio output Transistors, but it's always nice to have a print to verify voltages.

The fact that it oscillated even after I temporarily restored the original caps would lead to believe that I managed to bugger up something unrelated.

Since I have another half dozen of these, I might be able to do a comparison, if I can figure out what to compare.

I'll definitely report upon (if) getting it fixed.

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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 3:44 am 
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Here are the same six transistors in a 1960 German 6V diy radio kit!
Image
Larger image→ here.

Greg.

Edit: have you seen the extension speaker?
http://www.radiomuseum.org/r/unknown_cr ... six_2.html


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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 4:54 am 
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How new is the battery? I see there is no bypass capacitor directly across the battery which is unusual - if the ESR of the battery goes up as it ages, then that's leaving the PA stage wide open to squealing.

If your radio has the same schematic, try twiddling that pot P1 from the AGC line to the negative rail - looks like a RF gain control.

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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 6:06 am 
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majoco wrote:
How new is the battery? I see there is no bypass capacitor directly across the battery which is unusual - if the ESR of the battery goes up as it ages, then that's leaving the PA stage wide open to squealing.


Agreed, I would tack a 500mfd/25V electrolytic cap across the battery and see what happens.

Brett


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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 10:04 am 
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I've been using a new, 9.50 volt battery, so that's not the issue. I double checked the PCB for breaks, but there were none.
I needed to get this fixed quick, as I have other things on my plate, so I took the easy way out and installed a .22 bypass cap from ground to the first IF. This quieted the oscillation, but as well the volume. It's now nowhere near as loud as my others of the same model, but at least it's usable until I can get back to it.

In checking my others, I discovered that one of them has a print affixed to the inside back cover. What would be the best way to enlarge it once I scan it?

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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 4:15 pm 
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Hey Richard---scan it at the highest resolution possible, then enlarge with Photoshop if you have it. If not, send the scan to me and I'll do it...

John


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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 4:37 pm 
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If your digital camera has a Macro setting, use that.

Greg.


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 Post subject: Re: Need a print
PostPosted: Feb Tue 07, 2017 10:25 pm 
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Thx guys, I tried each method. Unfortunately in trying to scan a print from the inside BC of a radio, where it's raised above the scanning glass, it comes out too dark to be legible. But, using the macro feature on the camera, I got a good image, cropped and downsized to fit within forum size requirements.

The parts and board layout for these two sets is identical, with the only difference being that the Personna uses "TEN" brand Transistors, the Jupiter with Toshiba's, of 2N designations.

My band-aid repair was a .22 disc cap between ground and the primary of the first IF, and certainly a Neanderthal approach, but it works sans squeals, although with heavily reduced volume, I assume because of the lesser IF amplification.

If you can determine what component might cause oscillation -the symptom being continual squealing across the dial and on each receivable station until exactly tuned it, and even then not stable- please advise. I cannot find any cracks on the PCB, and have re-touched all of the solder pads.


Attachments:
Personna TR-64, Jupiter 6T-220 A.jpg
Personna TR-64, Jupiter 6T-220 A.jpg [ 134.18 KiB | Viewed 394 times ]

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 Post subject: Re: Need a print
PostPosted: Feb Wed 08, 2017 2:31 am 
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Yes, a good camera works great---schematic looks good !

I'd suspect one of the .02 uF IF amp emitter bypass capacitors---but still would try a large electrolytic across battery even if new out of curiousity ! Also bridge your new electrolytics one-by-one in case one of them might be bad---it happens !

John


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 Post subject: Re: Need a print
PostPosted: Feb Wed 08, 2017 8:34 pm 
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I would also caste a very suspicious eye over that 40uF across the AGC line.

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 Post subject: Re: Need a print, Not no mo
PostPosted: Feb Sun 12, 2017 6:55 am 
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Thx Marty, but the suspicious eye belongs on my supposed abilities in the discipline of electronic repair.

I fixed it.
And I knew from the git-go that it had to be mis-wiring. I mean, how else can one screw up a simple 6 Transistor/4 Electrolytic circuit? The parts placement had one Electrolytic filter cap on top of another, with both positive leads going to ground, which is normal. At their other end, it appeared that they both went to the same solder landing as well.

I simply assumed<there's that word again, that the two 30 mFd caps were tied in parallel because they wanted 60 mFd. Wrong, as displayed in the print.

And this is where it helps to have another example handy. In doing a comparison, I found that the second 30 mFd cap was tied to a trace right next to the landing that the first one went to.

So in times like this, one waits until he is alone, and then in a most sincere exclamation, says to himself, YOU DUMB SH*T!!

So all is well, and I decided to group the eight of these that I had, for display on their very own shelf. These are all the same circuit; notice the placement of the VC knob and tuning dial. The subject is the orange-red one on the left, and the one I compared it to with the print inside, is at the back center.


Attachments:
Jupiter 6T-220, Personna TR-64.jpg
Jupiter 6T-220, Personna TR-64.jpg [ 86.22 KiB | Viewed 322 times ]

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 Post subject: Re: Need a print, Not no mo
PostPosted: Feb Sun 12, 2017 5:03 pm 
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Hey Richard---glad that you found it and thanks for letting us know ! Have been wondering...

I've done the same type of thing---very embarrassing, especially when you've told someone else about the problem and then have to apologize !

Nice looking sub-collection !

John


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 Post subject: Re: Need a print, Not no mo
PostPosted: Feb Sat 18, 2017 9:59 am 
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Check that battery positive is connected to each of the metal cans housing the
IF and oscillator coils. With a solid metal bond.

Then check the poly vari con to make sure it actually tunes the primary of the bar
antenna. Also, that the mid terminal solidly goes to the metal cans.

I is possible that if it whistles on every station, the radio is already oscillating,
with no station. Scope it with a low capacity probe. Or even sniff the radio
with no station tuned, with another radio, to see if can make that one whistle
on station.

Remember too , those IF cans have capacitors in them too. They must not have
corroded off, or broken.

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 Post subject: Re: Need a print, Not no mo
PostPosted: Feb Sun 19, 2017 2:59 am 
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Thx for the advice, Steve, but if you'll notice two posts above yours, I stated that I had fixed it, and the cause was a wiring error.


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facepalm.gif [ 274 Bytes | Viewed 160 times ]

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 Post subject: Re: Need a print, Not no mo
PostPosted: Feb Sun 19, 2017 5:25 pm 
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fifties wrote:
Thx Marty, but the suspicious eye belongs on my supposed abilities in the discipline of electronic repair.
I fixed it. And I knew from the git-go that it had to be mis-wiring...

You are not alone; this is a perfect example of why, many years ago, I got out of full time electronics repair! :?

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