Suggestions for bypassing tuner of vintage tv? - Page 1
Forum section: electronics / repair

author: jeffheath
author_rank: Regular Contributor
timestamp: December 23, 2018, 06:51:30 am
content: I hooked up a composite video cable to the base of the first video amp, and the signal was far too weak. The datasheet for the IF amp (nte1545) claims it has a video DC output voltage of 5.5 volts, and a max video DC output current of 6mA. According to the schematic, the first video amp (PN2222) expects a 4.7v video signal, as shown in the attachment. Is there a straightforward method of amplifying composite video for this, or am I opening a can of worms?

author: Nusa
author_rank: Super Contributor
timestamp: December 23, 2018, 07:16:04 am
content: Well, there is a traditional solution for this: Remember these?Then you just tune the set to channel 3 or 4 in most cases.

author: PKTKS
author_rank: Super Contributor
timestamp: December 23, 2018, 10:53:53 am
content: Quote from: Nusa on December 23, 2018, 07:16:04 amThis seems to be only a simple "BALUN" (edited) (75 to 300 matcher)Actually if I understand correctly you want to use that simple transistoras video amp...As it seems 4.7 on the base is the DC BIAS only the GAINshould be calculated by the traditional biased loaded graphI see 2 outputs but wo no load on the figureThis video amp is very simple a CLASS "A" ampw/the desired gain - trivial thing if you have the base.and btw that "TRAP" should not be used as it targetsthe carrier and not the payloadAlternatives is to use a dedicated video amp IC likeclassic HIGH BW/GAIN VIDEO AMP LM1202 from NationalNevertheless a simple NPN in class A inverter will do itPaul

author: Nusa
author_rank: Super Contributor
timestamp: December 23, 2018, 11:08:02 am
content: Quote from: PKTKS on December 23, 2018, 10:53:53 amIt'll do that too, but in TV mode it's just a pass-through, antenna to TV. The input at the top, labeled "GAME", is the composite input. Boxes like that were how all the early game consoles were interfaced to TV's. Older versions of that will only have the 300 ohm option. I probably could find a couple in my junk boxes if I looked -- they were common once. But I guess the younger crowd might never have seen them.

author: PKTKS
author_rank: Super Contributor
timestamp: December 23, 2018, 11:19:25 am
content: Quote from: Nusa on December 23, 2018, 11:08:02 amit is - you don't need to convert 75 to 75 - just 300 to 75 using the BALUN

author: fzabkar
author_rank: Super Contributor
timestamp: December 23, 2018, 11:32:26 am
content: Are we to understand that the intention is to add AV inputs to a CRT TV? If so, could we see more of the circuit diagram before and after the transistor?

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 23, 2018, 11:37:33 am
content: Quote from: Nusa on December 23, 2018, 11:08:02 amI don't think it is, this is just a switch so you can switch the input of your tv to either the antenna or the game console which has a built-in modulator.Hint: there is no power connection so this thing is pure passive.

author: jeffheath
author_rank: Regular Contributor
timestamp: December 23, 2018, 03:21:05 pm
content: So I took a look at the LM1202, and it looks like a fairly straightforward install, but I don't know what to do about "clamp gate in" which is circled in the picture. fzabkar was asking for more of the circuit diagram, so I have that attached as well. (And if I haven't been clear, yes, I'm trying to install the "yellow jack" (composite) av input in the tv, and I don't want the signal to have to go through the tuner)

author: fzabkar
author_rank: Super Contributor
timestamp: December 23, 2018, 07:47:20 pm
content: If you are tapping into the circuit at TP7 by disconnecting everything to the left of this point, then you are removing the 4.7V DC bias from Q200. This bias is provided by the IF stage. To maintain this DC bias, you would need to add one resistor between the base of Q200 and the 22V rail, plus a second resistor between the base and ground, ie a potential divider. Then remove R147 and C147, and couple your composite video signal into TP7 via a capacitor. I'm assuming that the unloaded amplitude of your composite video is 2Vpp (1.0 Vpp when driving into a 75 ohm termination), in which case it does not require further amplification.

author: jeffheath
author_rank: Regular Contributor
timestamp: December 23, 2018, 11:24:28 pm
content: I was able to achieve 5 volts at the base of Q200 with two 270k resistors, one from ground and one from the 22v rail. I then removed R147 and C147. The composite video signal (from a dvd player) is still very weak, and when I try to input a signal from a hdmi to composite adapter, I get nothing. I don't have an oscilloscope, but the voltage coming out of the video output of the dvd player is 1.7 volts, and the voltage coming out of the hdmi adapter's video output is only .6v when displaying video to a working tv. These voltages seem to correlate with what I'm seeing on the screen, telling me that the signal still needs more amplification. But without doing these mods to a working tv of the same vintage, I don't know for sure. For all I know, there could be something wrong with the set, but all the voltage rails check just fine, and the raster is bright. Suggestions?

author: fzabkar
author_rank: Super Contributor
timestamp: December 26, 2018, 10:32:39 pm
content: Working without a scope would be difficult. If I were performing this modification, I would disconnect the tuner section at TP6 or TP7, then insert your external video signal at this point. The mistake you are making is in confusing the DC bias level with video amplitude. Your multimeter can only measure the DC bias -- it will tell you nothing about the video signal.

author: jeffheath
author_rank: Regular Contributor
timestamp: December 27, 2018, 02:28:12 am
content: fzabkar, thanks for the help. The thing is, the IF amp chip just so happened to be dead, so I couldn't test the tv with a normal signal. I've ordered a replacement IF amp, and my plan is to put the circuit back as intended, and repair the set (if needed) to get a normal picture, THEN proceed with the mod. At that point, if I need a scope, I'll buy one... it's about time I should man up and have a go at using scopes anyways!

author: james_s
author_rank: Super Contributor
timestamp: December 27, 2018, 06:13:02 am
content: Those early consoles had built in RF modulators, the TV/Game switch was just a passive switch. External RF modulators are available though, I have one on the vintage TV at the cabin, found it at a thrift store for $2.50.Be careful adding inputs to a TV, quite a few of them used a hot chassis design so any external inputs require careful isolation.

author: jeffheath
author_rank: Regular Contributor
timestamp: December 27, 2018, 07:24:35 am
content: Quote from: james_s on December 27, 2018, 06:13:02 amOh I have a couple rf modulators, I just can't stand the thought of something being modulated, only to be demodulated again! The whole point of modulating the signals was so they could broadcast them over the air, with a trade-off of slight quality loss . My intention with this mod was to "jeep" the set, which is the name they had for turning tv's into composite monitors via injecting the video signal, way back in the 50's when monitors were really expensive. I'm not doing this for higher quality or anything, I just like muntzing things, and this could be useful for oddball tvs with weird inputs.

author: jeffheath
author_rank: Regular Contributor
timestamp: January 20, 2019, 04:17:00 am
content: As it turns out, the set was in pretty bad shape, but I was able to repair it. If anyone wants to read about that nightmare, they can click this linkhttp://videokarma.org/showthread.php?t=271230Along the way I bought an oscilloscope, and for the most part, I think I've got the hang of it. (I got an older scope so it'd be more intuitive to learn) I actually couldn't have repaired the set without it. Anyways, once the set was repaired, I went back to the mod that fzabkar was talking about with dc bias and was able to obtain a good picture from composite out from a game console. However, when I tried the dvd player, the picture was dark, but not affected by brightness or contrast controls. So I pulled out my "car" video signal amplifier, (BOSS BVAM5) and low and behold I get a good image out of the dvd player. I'm confused as to why I need to use it, because when I measure the waveform coming out of the dvd player, it looks nearly the same as the waveform coming out of the car amp (same peak to peak voltage). How can a signal be strengthened without increasing amplitude?

author: james_s
author_rank: Super Contributor
timestamp: January 20, 2019, 04:19:07 am
content: Were you measuring it while connected to the TV, or open circuit? It may be a matter of input impedance loading down the video output. The one that works may have a buffered output that isn't affected as much by the load.

author: jeffheath
author_rank: Regular Contributor
timestamp: January 20, 2019, 06:19:14 pm
content: Quote from: james_s on January 20, 2019, 04:19:07 amI've measured the output of the dvd player hooked up to nothing, the output of the car video amp with and without being hooked up to the tv, and all the waveforms look the same, around 2v p-p, and I can decrease output amplitude by turning the control. In fact, the manual for the BVAM5 states that the video input level is 2v p-p, whileoutputlevel is .5 to 1.5v p-p

author: james_s
author_rank: Super Contributor
timestamp: January 20, 2019, 06:55:30 pm
content: You'll have to measure them connected to the TV to see what's going on here. I suspect the player that is not working relies on driving a 75 ohm input, while the TV you are connecting it to likely has a lower impedance on the input you added and that is loading it down. If that turns out to be the case, I have several reels of Maxim video buffer ICs and can send you a strip of them if you want to make a buffer.

author: jeffheath
author_rank: Regular Contributor
timestamp: January 20, 2019, 07:50:41 pm
content: Quote from: james_s on January 20, 2019, 06:55:30 pmWould a circuit like the one shown work? And does it matter if I use a max4389 or max4390 instead of a max4395?

author: james_s
author_rank: Super Contributor
timestamp: January 20, 2019, 09:50:59 pm
content: I would imagine so, although you may need to adjust the value of the output resistor. I just checked and the parts I have are MAX4090EUT. I have at least 6,000 of them so I'm not going to run out any time soon.

author: jeffheath
author_rank: Regular Contributor
timestamp: January 30, 2019, 05:07:23 am
content: It turns out that I may have injured my DVD player somewhere along the line, as I attempted to hook it up to a modern tv that I have, and it produced the same results, no picture without the BVAM5 car amp, and a marginally good picture when used with the car amp,even though the video output waveform looks fine.Either way, I returned to the method that fzabkar stated, only when I first attempted it I omitted the coupling capacitor as I achieved a better picture from the game console without it. However, other signal sources simply would not work without the BVAM5 car amp. In reality, this is because my capacitor was way too small in value, (100pf) and did not do a good job of isolating the composite input. A 470uf seems to be doing the job, as well as a 75 ohm resistor behind it across the cable for input sensing on an adapter I have. So in hindsight, fzabkar's method would have worked fine, had the TV and DVD player been working, and I knew what size capacitor to use.

author: jeffheath
author_rank: Regular Contributor
timestamp: February 03, 2019, 07:33:42 pm
content: Does anyone know how I would inject AV audio? Injecting at pin 1 of IC 180 it was garbled; pin 16 sounds good, but it is already loud and when I try the volume control it just creates more static as I turn it up. Pin 9 has a similar effect, but is quieter.Sent from my POCOPHONE F1 using Tapatalk

author: jeffheath
author_rank: Regular Contributor
timestamp: February 04, 2019, 03:38:38 am
content: To me it looks like the output of the FM detector to the voltage controller is internal to the IC and can't be spliced into, is it even possible to get volume controlled composite audio with this chip?