Subject: Modifying a B&W TV for composite video input

From: Ian Primus <[email-masked]>
Date: Tue Jul 1 08:55:01 2003

I have read a lot about the SWTPC TV Typewriter, and I am interested in 
how people modified a black and white TV for a video monitor. I have an 
old black and white set that I thought would be fun to modify for use 
as a video monitor. Now, granted, I don't really need another composite 
monitor, I have plenty of old Apple monitors that will do the same 
thing, I just thought it would be fun, since it would be a challenge to 
find where to input the signal and how to do it. I found the PDF of 
part of the SWTPC TV Typewriter manual on a web site, and it explains 
the process, although not in great detail, and references a circuit 
diagram of what needed to be added, but the diagram is nowhere to be 
found. Does anyone have any instructions or hints that they may be able 
to give me? Has anyone done this before? Any advice would be 
appreciated.

Ian Primus
[email-masked]

---

From: John Rollins <[email-masked]>
Date: Tue Jul 1 08:55:02 2003

I would think the easiest and cheapest way is to use a plain old RF 
modulator box that will put composite video on to channel 3 or 4. 
I've taken apart one or two and they seem to have little more than a 
few coils and small parts, a far cry from the rack mounted cable TV 
modulators I just picked up(ooohh... so many trimmer caps and 
coils...). I don't know a whole lot about it, but I'm slowly learning 
as my interest in fast scan ATV increases... The converter box I have 
goes with my TI-99, I probably have one in the box with my C-64, too. 
I think you can still get them at Radio Shack. Otherwise, all you 
need to do is dump the composite signal into an AM modulator and then 
through a transmitter that the TV can pick up. I would guess these 
small boxes having only coils just use a tuned circuit for channel 3 
and the voltage from the composite signal to provide the signal. But 
then I'm just guessing, I've never poked and prodded those little 
boxes very much, I think I'll go do that now(when I find it, 
anyhow...).

>I have read a lot about the SWTPC TV Typewriter, and I am interested 
>in how people modified a black and white TV for a video monitor. I 
>have an old black and white set that I thought would be fun to 
>modify for use as a video monitor. Now, granted, I don't really need 
>another composite monitor, I have plenty of old Apple monitors that 
>will do the same thing, I just thought it would be fun, since it 
>would be a challenge to find where to input the signal and how to do 
>it. I found the PDF of part of the SWTPC TV Typewriter manual on a 
>web site, and it explains the process, although not in great detail, 
>and references a circuit diagram of what needed to be added, but the 
>diagram is nowhere to be found. Does anyone have any instructions or 
>hints that they may be able to give me? Has anyone done this before? 
>Any advice would be appreciated.

-- 

/------------------------------------\
|  http://jrollins.tripod.com/                   |
|  KD7BCY      [email-masked]          |
\------------------------------------/

---

From: Ian Primus <[email-masked]>
Date: Tue Jul 1 08:55:09 2003

Those little RF boxes don't really convert from composite to RF, they 
just provide a switch and a connection to a TV antenna input. The 
output signal from the computer is already RF. Actually, what I 
generally do with machines that have RF out only (i.e. Atari 2600) is 
to just go to Radio Shack and get an RCA to F type screw connector 
adapter. It's much smaller, and gives much better picture than those 
little metal boxes do. It just doesn't provide a switch. What I want to 
do is to actually input a true composite video signal into the TV. Of 
course, this can be done by using a broken VCR with A/V inputs and a TV 
output, but I want to modify the set as a learning experience, as well 
as to create something useful for old computers.

Ian Primus
[email-masked]

On Monday, June 30, 2003, at 09:17 PM, John Rollins wrote:

> modulator box that will put composite video on to channel 3 or 4. I've 
> taken apart one or two and they seem to have little more than a few 
> coils and small parts, a far cry from the rack mounted cable TV 
> I think you can still get them at Radio Shack. Otherwise, all you need 
> to do is dump the composite signal into an AM modulator and then 
>> have an old black and white set that I thought would be fun to modify 
>> for use as a video monitor. Now, granted, I don't really need another 
>> composite monitor, I have plenty of old Apple monitors that will do 
>> the same thing, I just thought it would be fun, since it would be a 
>> challenge to find where to input the signal and how to do it. I found 
>> the PDF of part of the SWTPC TV Typewriter manual on a web site, and 
>> it explains the process, although not in great detail, and references 
>> a circuit diagram of what needed to be added, but the diagram is 
>> nowhere to be found. Does anyone have any instructions or hints that 
>> they may be able to give me? Has anyone done this before? Any advice 
>> would be appreciated.
>
> -- 
>

---

From: John Lawson <[email-masked]>
Date: Tue Jul 1 08:55:16 2003

First - the TV itself is needed. When a unit is to hand, look up the
Sams Photofact for it - this will be the comprehensive data and
documentation package - numerous schematics, pictorials with all the
components marked, etc.  I think they're, what - $15? Dunno, but it's
money well spent.

  Then - most BW TVs from the 60s and early 70s (if tube) were
series-string type - they had no power transformer, rather the incoming
mains current was tied to a resistor string to power the tube filaments
and to diodes for the various DC voltages - this meant that one 'side' of
the mains always was connected to the metalwork of the set - thus posing a
signifigant hazard to persons **and to any other gear hooked up to the set
that didn't share the same ground polarity!!**

  It is A Very Good Thing if you can score a simple mains<->mains
isolation transformer, just 110V in and 110V out, 1:1 windings.  Be sure
its a two winding device *NOT* an 'autotransformer', which is typical of
most 'travel convertors' one buys for Journeying to dark and wild corners
of the world (Munich for instance).  If you can't find a 100Watt (or so)
1:1 unit, you can make one by getting two identical power transformers and
and attaching thier secondaries to each other, thus a 1:5:5:1 for example.
Just be sure that the transformers will carry the load of the BW set
(total watts).

  Now, using the Photfact schematics and data from the Web, here, and
other sources - locate the video and sync-separator circuits. Determine
the polarities of the signals involved, and what relative levels the
circuits want.

  Buld up an interface circuit to accept TTL level (or standard NTSC
composite analog data) and feed it at the right levels and polarity to the
video input stage of the BW TV.

  Power up both units and test.

  After the Fire Captain tells you it's all right to go back inside,
salvage what you can from the sodden charcoal mess and try to find a new
hobby that carries less intrinsic risk.

  ;}

  No but, seriously - after you have the BW TV set data, it is not a
particularly onerous task to suss out the points in the circuit where one
could ostensibly inject an external video sig.

  I might also suggest that you look into opto-isolating the video feed
from your computer, just to be safe. Letting the Magic Smoke out of one's
vintage computers is not exactly the best way for further knowledge and
gain experience, though Mr. Wilde might have disagreed with me.

  But hey - he's dead.

 Cheers

John

---

From: Ethan Dicks <[email-masked]>
Date: Tue Jul 1 08:55:22 2003

--- Ian Primus <[email-masked]> wrote:
> output signal from the computer is already RF. 

For the little ones with just a switch and a coil, that's all true.  In
the case of a VIC-20, there _is_ an external composite adapter that
hangs off the DIN-5 for power, composite video and sound.

I _think_ the one for the TI-99/4a was also a true external RF modulator.

-ethan

---

From: "Stuart Johnson" <[email-masked]>
Date: Tue Jul 1 08:55:24 2003

----- Original Message ----- 
From: "John Rollins" <[email-masked]>
To: <[email-masked]>
Sent: Monday, June 30, 2003 8:17 PM
Subject: Re: Modifying a B&W TV for composite video input

> <SNIP> -- 

The SWTPC TV-Typewriter outputs baseband video, which is also a RF (radio
frequency) signal. What it is NOT is modulated. That is why it is called
baseband. Baseband is also sometimes called DIRECT video.

The Commodore 64 puts out a RF signal on TV channel 3 or 4 (switchable),
which is usually connected to a TV set using a RF SWITCH (required by the
FCC in the US). That switch is NOT a RF modulator, it is an isolation
device, to prevent your connection of your computer to an outside antenna
and the broadcasting your computer signal to your neighborhood; that
broadcast would be considered as unwanted interference by the FCC, thus the
isolation switch.

Please not that in the rest of this post that I am NOT talking about a RF
"switch" as described above, but about an actual RF modulator vs. baseband
video.

In my opinion, based on personal experience, it is cheaper, easier (if you
know what to do), and certainly provides a better picture if you use the
direct video approach when connecting a baseband device like the
TV-Typewriter to a TV set. When you use a RF modulator you cause the signal
to be modulated and then demodulated (by the TV); every time this is done,
some of the information is lost in the process. When you use the direct
video, little of the signal is lost or attenuated and the full bandwidth of
the TV's electronics are available.  The lossy signal from a (cheap) RF
modulator causes the picture to be fuzzy or wavy and may not even be stable.

I used to have one of these TV-Typewriters and even modified it for 24x80
display by doubling the ram and making mods to the timing of the board.
There was an article published in one of the electronics / computer
magazines that showed how to do this, and it worked exactly as shown. I used
my TV Typewriter with a small black & white TV modified to have an RCA jack
for baseband RF input. It worked well for many years. My computer at that
time was a KIM-1, expanded with S-100 peripherals and memory, using a KIMSAI
motherboard / S-100 interface.

How one connects baseband to a TV depends on the TV itself; many small sets
have a "hot" chassis, that is to say that the chassis is connected to one
side of the 110 VAC mains, and insulated from the user by the plastic case.
This kind of TV can get you KILLED if you mess with it opened up or can
possibly blow your computer up if you connect to it improperly. This
procedure is therefore NOT recommended to the non-technically skilled. The
actual circuit can be as simple as a capacitor and a few resistors (for DC
restoration). What is done is to disconnect the TV's tuner and inject the
signal where the tuner did. Many people used a toggle switch so that the TV
or Video function could be selected as needed.

For what it is worth, TV doesn't use AM (Amplitude Modulation), it uses a
modulation technique called Vestigial Sideband, from before the days when
Single Sideband modulation was understood.

Also, RF modulators are ACTIVE devices requiring a power supply and ground
to operate; they do not derive their power from the signal; at least none
that I've ever seen do. With today's ultra-low powered devices, it may be
possible, but I doubt that anyone has gone to the trouble to design an
ultra-low power RF modulator considering that many TV's already have
composite (baseband having chrominance & luminance combined) video inputs.
Some specialty monitors, like those sold for use with the Commodore 64,
accepted separate chrominance (color) and luminance (brightness) inputs in
an attempt to give a better picture. The Commodore 64 has a special jack on
its back to connect to these monitors and uses a special cable to make that
connection.

Stuart Johnson
former electronics technician and present computer nut

---

From: Fred Cisin <[email-masked]>
Date: Tue Jul 1 08:55:28 2003

> Those little RF boxes don't really convert from composite to RF, they

The Sup'R'Mod II DOES convert from composite to RF (Channel 34).
Most of the other RF modulator boxes are channel 3, or 4, or switchable
between 3 and 4.
But some people don't know the difference between an RF modulator box and
a switch box.

But modifying a TV for composite input is a good project.

Maybe start with Don Lancaster's book(s) on the subject.

... and be careful!  You will be working near high voltages; and some are
hot chassis

--
Grumpy Ol' Fred     		[email-masked]

---

From: "Gary Dean Hildebrand" <[email-masked]>
Date: Tue Jul 1 08:55:32 2003

Ian Primus writes: 

> old black and white set that I thought would be fun to modify for use as a 
> video monitor. Now, granted, I don't really need another composite 
> monitor, I have plenty of old Apple monitors that will do the same thing, 
> I just thought it would be fun, since it would be a challenge to find 
> where to input the signal and how to do it. I found the PDF of part of the 
> SWTPC TV Typewriter manual on a web site, and it explains the process, 
> although not in great detail, and references a circuit diagram of what 
> needed to be added, but the diagram is nowhere to be found. Does anyone 
> have any instructions or hints that they may be able to give me? Has 
> anyone done this before? Any advice would be appreciated. 
> 
> Ian Primus
> [email-masked]

First you have to deterimine that the set isn't hot chassis, and has a 
transformer operated power supply.  Then get a Sams's Photofact, so you can 
find where the video detector diode is. 

Somewhere in there is a point where you can inject baseband video.  Not 
knowing the schematic, your guess is as good as mine. If you know basic 
electronics, you can look for a point where the scope video is the right 
polarity, and about 1v p-p and inject it there. 

Gary Hildebrand
St. Joseph, MO

---

From: "Hans Franke" <[email-masked]>
Date: Tue Jul 1 08:55:41 2003

> > goes with my TI-99,

> just provide a switch and a connection to a TV antenna input.

Communication error!

While you where refering to the common (video game) switch boxes,
John did mention modulators! they are usualy the same size (ok,
the TI one is a true brick), but do what they are named, modulate
the composite signal onto a TV carrier.

The one I like most is the Amiga 520 (?).

Gruss
H.

--
VCF Europa 5.0 am 01./02. Mai 2004 in Muenchen
http://www.vcfe.org/

---

From: [email-masked] (Tony Duell)
Date: Wed Jul 2 19:38:13 2003

> as a video monitor. Now, granted, I don't really need another composite 

It's fun until you start working on a live-chassis set and you suddenly 
apply mains across your video interface circuit and yourself....

Seriously, many large-screen (non-portable) B&W TVs have a live chassis 
connected directly to one side of the mains (or worse to one side of a 
bridge rectifier connected to the mains, so the chassis sits at half 
mains voltage on average). Don't even think of adding a composite input 
to such a set unless you connect the mains input to the secondary of a 
double-wound isolating transformer. (OK, it is possible to isolate the 
video input rather than the mains -- Radio Shack did on the original 
Model 1 monitor, which has a hot chassis. But this is a lot harder to get 
working right, and a lot more dangerous to experiment with).

Portable TVs that can also run off a 12V battery _normally_ have an 
isolated PSU, but please check to be sure...

A circuit diagram of the set is _essential_. I am not sure where you are 
-- if you're in the States, get the Sams Photofact for the set if you 
can't get the original service manual. If you're in the UK, see if it was 
covered in 'Radio and Television Servicing'. 

Then all (!) you need to do is find the video detector stage and feed in 
your composite signal at the output of that stage. Hopefully the sync 
separater takes it's feed after that point (it's almost certain you can 
find a point where this is the case) so you really can feed in a 
composite signal and expect it to work.

-tony

---

From: [email-masked]
Date: Wed Jul 2 19:38:29 2003

> apply mains across your video interface circuit and yourself....

Yep.  Nasty.  Even with isolation still can get that ZAP!.
 
> mains voltage on average).

Every one you described for non-isolated sets are like this.  Every 
one, I'm referring to cheap color sets old or new.   The - side of 
rectifier bridge is "hot ground" in those.  Very rare few I seen had 
one diode and that was in old citizen sets usually 13" to 19".  This 
is really downright cheap.   In those with non-isolated sets that 
does have inputs, always have small signal isolation transformers and 
or optical isolation.  The others that is both hot/cold chassis be it 
via flyback transformer like this one hitachi and about 7 yr old.  
Other kind via SMPS is very common.  Using mains isolation 
transformer is *extremely rare*.  In so many sets in 1 
and half years of sets (cranks out about 30  sets every 5 days) I 
worked on, only ONE did by design and that was really ancient Samsung 
32" tv.   So do your self a favor buy a newer used or new sets and 
hook up things without modifying that set or be hackish on old stuff, 
splice it on that internal mains wires a isolation transformer 
and bolt it inside that set.

> isolated PSU, but please check to be sure...

13" is considered portable also and almost always are hot chassis by 
design.  In RCA 13" AC/DC tv it's hot chassis, to run on 12V RCA 
puts in an DC to DC converter!  I'm talking about those that is made 
from 1991 onwards.  ( chassis TX8xx series)  But what nice about this 
13"  RCA has no B+ regulator and the pix is very stable because of 
direct control of horizontal pulses to regulate power in flyback 
itself.  Hence very little horizontal size swelling or jaggies with 
bright parts.  The later 13" RCA tvs gained mono audio and video 
input after it got the SMPS but basic design of pervious 13" RCA 
mostly unchanged.

By contrast I have seen so many other sets that doesn't have good 
handle on the HV regulation the pix width swells with bright parts 
and shrinks with darks.  This drives me nuts.  Those have same 
chassis but has pincushion feature added (it is a module in RCA) had 
better handle on this but not 100%.
If the vertical edges isn't straight (more of a flared or jaggies)  
that what I mean by this poor HV regulation.
 
> A circuit diagram of the set is _essential_. I am not sure where you are 

From original makers is asolute best please.  Especially from RCA 
much better info and also have writings, and RCA also put out 
training manuals for particular chassis.

JVC doesn't do much on this just an schematic, troubleshooting charts 
and raft of adjustment instructions in slim floppy books.  Same thing 
with Samsung, marginally better but their TV isn't my taste since it 
tinkered too much with circuit that it is hard to troubleshoot.  
Samsung liked to use philips ICs which is lousy and using AKB for 
CRTs.  we don't like those!  This kills the useful lifetime of sets 
because tube is still useable when AKB kept it shut and cannot be 
bypassed bec of under digital control via that jungle IC programmed 
by microcontroller based on contents of eeprom IC.
I had blank samsung tv that like this and it was not too old to 
cause akb to blank tube and found out later on another similar 
samsung while adjusting corrupted eeprom data the pix blanked out 
then popped back on when value reached within range.  Now I know why!

RCA shamefully started using AKB in ATC113 series.  :-P  I think it 
has schematic that might have a way to turn off AKB.

> composite signal and expect it to work.

This is dandy in really ancient sets with older discrete IF stages.  
Sets in last 10 years have moved on to one jungle IC with SAW filter. 
In other decent sets, tuners pipe the IF right to the Y/C circuit 
(either analog or digital comb) and feed Y/C to the jungle via 
switches if set have inputs (Y/C is same thing as s-video). 
Component video (three jacks) is fed directly to jungle itself.  Just 
this year few brands makers finally released sets (my thought) super 
jungle.  I call this "super jungle" it has microcontroller & jungle 
in one DIP package.  Actually made my day much easier to repair 
those, less complex. But i know this isn't great for others who like 
to hack but those always have composite inputs (those yellow RCA 
jacks) so this is less of a problem IHMO and especially JVC with 
pincushion makes great HUGE composite monitors with those vivid, 
sharp CRTs (many used RCA CRTs !!)  (25", 27" without those swelling 
problem.

I really loathe Philips with that all in one SIMM module.  It has 
jungle, microcontroller, audio processing & input switches in few 
tiny SMD ICs all unprobeable bec it's so damned tiny and stuffed 
into worst place without getting bitten or no room for probes.  And 
it's 100 bux part and is bit high failure rate.  The mainboard is 
only way u can probe around.  Their philips schematics is locked and 
unviewable and must pay big bucks to own key for each CD to view 
only on ONE PC, installing same cd on diff PC gives different key #.  
RCA and others isn't like this.

Cheers,

Wizard

---

From: [email-masked] (Tony Duell)
Date: Wed Jul 2 19:38:31 2003

> Yep.  Nasty.  Even with isolation still can get that ZAP!.

True, but the mains is (IMHO) a lot more nasty than the EHT to the final 
anode...

> one, I'm referring to cheap color sets old or new.   The - side of 

In the UK, just about all colour TVs (even small ones, 14", etc) have 
what's called a SCART socket. This is a 21 pin connector carrying 2 
channels of audio in and out, composite video in and out, RGB in (which 
uses the composite input for sync), blanking, etc. It turned out to be 
simpler and cheaper to isolate the PSU rather than all those signals

Still watch out if you're repairing the set. Some sets put things like 
the horizontal output stage on the mains side of the isolation barrier 
(which means the yoke has the horizontal windings 'hot' and the vertical 
windings isolated!). But at least the signal circuitry is isolated.

Just about all small B&W TVs over here (for the last 20 years) have a 12V 
B+ line. They will run off a car battery or an internal (isolated) mains 
PSU. Again, modern sets (even small ones, I saw a 5" one the other day) 
have 'AV inputs' (audio and composite video).

> does have inputs, always have small signal isolation transformers and 

Other than an old, partly valved B&O colour TV, which had a transformer 
isolated audio output (and no other I/O), I have never seen isolating 
transformers for the signals on a UK TV.

> transformer is *extremely rare*.  In so many sets in 1

Most UK TVs have an isoleted SMPSU these days

> direct control of horizontal pulses to regulate power in flyback 

Are US TVs really so far behind European ones? We've had fully regulated 
supply lines, including EHT, for many years... Over here we'd not accept 
a TV where the picture changed in size with the average beam current!

> >From original makers is asolute best please.  Especially from RCA 

Sure!

> Samsung liked to use philips ICs which is lousy and using AKB for 

The advantage of Philips ICs is that you can generally get full data 
sheets on them. And there's nothing wrong with them if correctly used!

> only way u can probe around.  Their philips schematics is locked and 

I can rememebr when Philips would send you a service manual free of 
charge if you asked nicely.... Oh well :-(

-tony