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 Post subject: Dumont Manchu rebuid
PostPosted: May Fri 10, 2019 5:14 pm 
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Joined: Feb Tue 26, 2019 2:11 am
Posts: 32
Hello Gentlemen

This Dumont Manchu was a lucky e-bay find. The seller was a cleanout contractor entrusted to sell off the contents of a old home in riverside IL. He didn't know anything about it, although the original owner (a 90 year old women) was on premise told me, they bought it new in the late 40's and all the neighbors would come over to watch it. She said where it stood in this house was were it was placed after purchased in 1949. The cabinet has some minor scratches but the condition overall is very good and does not need to be refinished.
It's now completely recapped, although I didn't stuff the old cap's. Most of the cap's were plastic molded types which do not lend easily to stuffing. Note the modern heat sink with the Dale 1% wire wounds. All the original Aerovox lytics were bad. The can multi-section caps were all open, and all Aerovox axil caps were more than 3 x their rated value.
There was extensive repair work done in this set and some of it was very sloppy. There was even some Black beauty caps in there as well. It's funny but all the old paper cap's were dead on, but as far as ESR I don't know.
The picture tube was replaced with the metal 19ap4, and (see below) the picture is not bad at all. I can not seem to increase the width, and am not certain if the factory mod's were ever done after the picture tube retrofit. One of our members was good enough to send me the circuit changes for the retrofit on a RA-101 but the parts numbers don't jive with this RA106. I'm going to buy a new 6BG6 horizontal output tube in the meantime just to see if there is any difference in the width. Doe anyone out there have the circuit mod's for the RA106 19ap4 retrofit ?
Check out the pictures.


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File comment: Chassis cleaned up very well using mineral spirits
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File comment: Note the modern heat sink
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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sat 11, 2019 4:04 am 
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In most TV's changing the G2 voltage of the horizontal output stage will affect the width. Yours has a really nice picture and it appears linear, so you aren't far from getting it full. Did you try different LV rectifier tubes just to see if a few more volts of B+ from new ones might do the trick?

There weren't many of that model sold to begin with, so finding another to compare to might be difficult. I went through all the DuMont factory service documentation that came from an old DuMont dealer's service department when they closed up a long time ago, but saw nothing on modifications for that set.

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Experience is what you gain when the results aren't what you were expecting.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sat 11, 2019 4:35 pm 
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Thank you Dennis

I appreciate you looking for those mod's

Is the G2 voltage measured on the (anode) of the 6BG6 horizontal output tube ? Your right about the lack of documentation. I could not get a rider or a Sams on a RA-106. The closest I found was a Sams for the RA-105. One thing that bothered me was replacing R-165 which is tapped off the low voltage (5U4'S) and is routed to the HV sub-chassis. The original wire-wounds were gone and the replacement was around 1400 ohms. I replaced it with the value of 1280 ohms (using the Sams for the RA105) but am unsure if that is correct for this chassis. The RA-105b shows a value of 820 ohms. What do you thing of decreasing this value from 1280 ohms to around 1000 ohms for a test ? Guess it might be a good idea to pull out my RCA 1964 tube manual to check maximum voltages for tubes in that sub-chassis.
I was lucky to find a new old stock 19AP4 from the folks at Moyer which I'm going to install once I have the bugs worked out. Great guys to deal with. I tested the tube and it tests perfect. Boy did I luck out !


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sat 11, 2019 8:24 pm 
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On the 6BG6, the G2 (screen grid) is pin 8. Besides the output tube, try another damper tube.

Have you checked the screen resistor, feeding pin 8? It's not unusual for them to go up in value, in sets of that vintage.

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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sun 12, 2019 12:31 am 
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Thank you Tim I should know this (G2-screen grid). Once the chassis is out I'll check those parts.
Replacing the old 20 inch 54 degree picture tube with the 19AP4 (I think 74 degrees deflection) will probably need a couple of changes to fill the screen on the 19AP4. They are not direct replacements and I don't think they did this years ago when they did the swap-out in my set. Two things to fiddle with: 1) The Windsor model was modified by jumpering out R21 (250 ohm) in this set it is r164B). 2) Change r65 to a lower value. That should also help.
Getting the Windsor schematic (since I have the circuit modification) probably would help. Does anyone know which RA-XX model is the Windsor.

Thanks all.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sun 12, 2019 3:12 am 
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Maybe you mean Westminster and not Windsor? The Westminster is RA-101. Service info is here: http://www.earlytelevision.org/pdf/dumo ... er_tv1.pdf

R21 is on the audio chassis and has to do with increasing the focus current.

My Westminster was converted to a 19AP4 with a new mask. I do not have a factory document detailing the change but here are the changes that I noted in my set during the restoration:

Audio Chassis: R21 - 250 ohm 10W in focus circuit removed and replaced with wire
C11 – 100mfd@150v removed

Sweep Chassis: C20 - .05 @600V paralleled with .005 @600V (Vert.)
R10 – 100 ohm 10W removed and replaced with wire (Horiz.)
C8 – 4mfd @50V removed
Horiz Out Cathode current now measures 181ma

The yoke was not changed. Yoke measurements: H: 8.4mh, 14.7 ohms V: 39mh, 63.3 ohms

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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sun 12, 2019 6:19 pm 
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Thank you Tim for this valuable info.

The audio amp in the Manchu is different than the Westminster in that the focus resister r21 is now in the main chassis. I have to look at the RA105 schematic to check on whether the changes for c11, c20, r10, & c8 are applicable being this set doesn't use 807's. The RA105 Sam's is probably as close as I can get to the RA106 chassis that I have. I also have the 105b Sam's which again is totally different from my set.
Plan to work on it this afternoon and will keep you informed as to any progress.

Thanks again

Keith


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Sun 12, 2019 7:40 pm 
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Per this info, the only difference from the 105 to the 106 is the sweep circuit due to the larger CRT.
http://www.earlytelevision.org/pdf/dumo ... er_tv2.pdf

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"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: Dumont Manchu rebuid
PostPosted: May Mon 13, 2019 5:14 pm 
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Thanks to Tim and Mr Detrola (Dennis). For the valuable information. The tech doc's are very handy for the HV sub-chassis.

I was able to increase the width by (in steps) lowering the resistance to grid G2 of the 6BG6. Measuring about 15k normally, dropped it down to around 7k. This left less than .5 inches on the left/right of the CRT to go before the CRT is filled out. (When started it was short about 1.5 inches).
My question is; will lowering this resistance (To G2) further (increasing the voltage to G2) cause damage to the fly-back ?
It looks like I'm nearly there. The 6BG6 G2 is rated at 350 dc max so that tube is safe. Probably it would be a good idea to measure the voltage at this point before decreasing the resistance again.
I'm assuming the voltage off the anode is increased when the G2 voltage is increased, thusly increasing the CRT anode voltage as well. Lowering this resistance G2 to around 3.5k might do the trick.
The focus control had to be turned c.clockwise nearly all the way to the stop to get the picture in focus (This was before any change to G2). I halved the wire-wound feeding the focus control and now the control is nearly in the halfway point for sharp focus. This change didn't effect the horizontal size though.

Thanks again for the tech support, I truly appreciate it and any feedback !
Keith


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Mon 13, 2019 9:01 pm 
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The 6BG6 is basically an 807 on an octal base.

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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Tue 14, 2019 5:34 am 
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There is a scan width problem which may be overcome by driving the output tube more heavily. The simplest way is to experiment by reducing the screen resistor value by no more than 50%. You should try to lower the value just enough to gain the extra width.

But the width problem mainly appears to be exacerbated by what appears to be the wrong video scaling format from your signal source. You have the source set to 16:9 which is incorrect. The output should be configured to either 4:3, 16:9 letterbox or 4:3, center cut 16:9. I generally prefer the center cut option as the picture fills the screen at the expense of cutting the edges which generally does not contain any action.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Tue 14, 2019 3:45 pm 
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I wouldn't be comfortable with reducing the screen resistor as much as 50%, normally a change of no more than about 20% ought to be enough.

I'd look carefully at voltages in the sweep circuit, and at the amplitude of the drive waveform. This set has a somewhat unusual design in that there is a negative supply (the two 6X4's) feeding the cathode of the 6BG6. So in this set, both B+ and B- voltages are critical and need to be close to specs. Also, the boost voltage is important, the +500 off the 5V4 cathode because it feeds the 12AU7 and can affect the drive. A vicious circle.......

There were a couple of other tricks to increase width without changing the screen resistor very much. One was to put a small value, perhaps 47pf at 6KV, cap across the damper, in this case you would put it across the 5V4 from plate to cathode. Also another trick was to put about a .005 to .01 @ 1KV cap across the width coil.

But I'd still want to try new tubes, not only the 6BG6 but also the 12AU7, both of the 5U4's and the 5V4. Not so much the 6AS7, in this set a weak 6AS7 would affect linearity the most and shouldn't make the picture narrow.

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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Tue 14, 2019 4:48 pm 
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Than you Dennis And Penthode for the valuable advice.

Last night I changed the screen resister from 15,3k to 6.83k below 50%. Had the set running for about 2 hours and kept a eye on the 6BG6 didn't seem to be running hot, but, I don't what to risk smoking the tube and the H.O.T. . I think you are right about using a combination of the screen plus the 6KV 68mmf between the plate and cathode of the damper. I will give that a try.
I have tubes (NOS) on order and should get them soon.

meantime check out the pics from last night :


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File comment: That's the look I get when bringing home old TV's.
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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Tue 14, 2019 5:22 pm 
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The picture is looking great. However the rescaling is still producing the wrong aspect ratio. The picture is squeezed because you are displaying a 16x9 image in 4x3.

What signal source are you using?


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Wed 15, 2019 12:14 am 
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Hello Penthode Thank you for the info. I thought the picture looked funny. Probably have gotten used to it from working on my old Philco 48-2500 and making too many optical adjustments.
The Dumont is connected to a dig/analog converter with comcast cable. The unit is a viewtv at163. I'll bet that info (aspect ratio) is in the manual, I'm going to take a look tonight as this project is on hold waiting for a 6KV 68mmf cap. and several NOS tubes.
My worry is overdriving that 6BG6.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Wed 15, 2019 3:29 am 
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I shouldn't worry about overwriting the 6BG6 so lo g as it doesn't red plate. If you are really worried, measure the cathode current and confirm it does not exceed the written spec.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Wed 15, 2019 2:53 pm 
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Location: Metamora MI, 48455
The 6BG6G is just an 807 on an octal base with less RF shielding to simplify manufacturing and reduce costs. The 807 of course is just a 6L6 variant. Some 6BG6G varieties have a pretty strict 19-20 watt plate dissipation limits, a majority of them can safely handle 25 watts like an 807, and there are a handful of very late 6BG6GAs (from the 1960s) with General Electric 6L6GC "guts". Those later tubes can handle at least 30 watts plate dissipation. I've worked out which varieties are beefier than others in bench testing them for audio use.

If you're really worried about pushing the 6BG6 too hard, try to find one of the later varieties and you'll never need to worry about it again. As long as your average cathode current is no higher than 100 mA or so, both the tube and flyback should be happy indefinitely.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Wed 15, 2019 4:28 pm 
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Thank you Penthode and Benman for your timely advice !

Was easily able to change the aspect ratio. It was set as default as 16:90 changed it to 4:3 full. See the pictures.
I'm going to look for a GE 6BG6GA. I'm also going to test the cathode current with the previous G2 mod. in place, than if that value is within spec's I'm good to go.


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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Wed 15, 2019 8:17 pm 
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I think, on the box you are using, you'll need to select 4:3 pan and scan, 4:3 "cut" or "scaled" or something similar because your pic is still squashed. Otherwise you're stuck with letterboxing 16:9 content.

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 Post subject: Re: Dumont Manchu rebuid
PostPosted: May Fri 17, 2019 3:15 am 
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The squeezed image could be the fault of the broadcaster. Is the image squeezed on all channels?

With the transition from 4x3 to 16x9, a solution was to provide as side car data in the stream or as it is called "metadata" to help steer the decoder to display the video correctly. Especially for legacy 4x3 Tv's (which haven't been manufactured since 2004) there was a need to fill the screen correctly and as much as possible. A clever system of Active Format Description was included in the digital stream from the broadcaster.

The problem is many broadcasters haphazardly applied it or just didn't bother. Also DirecTV, Dish and many cable companies chose to ignore it because they wanted to differentiate between a basic Standard Defintion delivery a a higher tiered HD delivery.

The net result that if it is a satellite or cable box, it may be impossible to fix the problem. All OTA set top boxes could force a correct display and a few boxes actually obeyed the AFD to switch automatically between a letterbox in 4x3 display and display a full frame 4x3 for 4x3 content. That is of course if the broadcaster put the correct metadata in the stream!

My description here only scratches the surface of this topic. On my old sets, my preference is to always fill the screen with the correct aspect ratio. This means 4x3 is displayed as 4x3 and 16x9 has the sides trimmed off. Your display shows a 16x9 squeezed horizontally to fit you 4x3 screen.

Could you tell us the brand and rough age of the set top box? And could you show photos of other channels?


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