Falcon RF to composite mod causing cap to bulge?

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stormy
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Falcon RF to composite mod causing cap to bulge?

Postby stormy » Sun Mar 17, 2019 6:19 pm

Hey guys,

I did a mod on my Falcon a while back, removing the RF modulator, then attaching a wire from the second to last pin on the IC to a new plug for composite. The mod works fine, but it makes the 470uf 35v cap bulge. I've tried three different caps, even tried a higher 50v version but it still bulged. The bulge happens after a couple of days of use.

Does anyone have any recommendations for me to try?

Thank you,

Steve

my mobo, close up, sorry a little blurry:
IMG_20190317_180701.jpg


A standard falcon mobo just to highlight where the cap is:
030.jpg
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Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

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Re: Falcon RF to composite mod causing cap to bulge?

Postby stormy » Mon Mar 18, 2019 12:38 pm

A shameless bump in case any electronics guys missed my post :)
Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

neanderthal
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Re: Falcon RF to composite mod causing cap to bulge?

Postby neanderthal » Mon Mar 18, 2019 2:28 pm

Not really getting why one should get CV from the chip since modulator machines have it the in connector?..
Otherwise,so someone is actually using the composite output??.I've even thought about getting rid of the PAL syncing for better stability.
One will get a ugly stripey pic on CV then.

On the other hand it looks like the output cap (assuming it is C24 we are looking at),is actually backwards?
Which is really odd since its that way in the schematic aswell.Have to take a look when gonna do some upgrades and tests next time.
It could be that since you are using the CV output directly from chip it interferes with the driver Q3 making it either to shunt of or totally on.that could to the trick.Assuming of course that pin 12 is connected to something on the monitor connector.

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Re: Falcon RF to composite mod causing cap to bulge?

Postby stormy » Mon Mar 18, 2019 5:28 pm

Okay, I did not realise that the modulator actually outputs composite signal as well.

So you're saying that the cap could be the wrong way round? But I've installed it the same polarity as the original Wang cap.

If you're idea about the driver q3 being totally on, what could I do to rectify the problem?
Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

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Re: Falcon RF to composite mod causing cap to bulge?

Postby neanderthal » Mon Mar 18, 2019 6:13 pm

mzry wrote:Okay, I did not realise that the modulator actually outputs composite signal as well.

No it doesnt,but the modulator is fed with CV from the MC1377 so it can make a areial signal of it.
It is almost the same signal as on the connector,this in non modulator systems outputs composite sync instead.(one has to dis/reconnect stuff for that to happen).ie the modulator is only for making TV-antenna out.Naught else.

mzry wrote:So you're saying that the cap could be the wrong way round? But I've installed it the same polarity as the original Wang cap.

I really have to check that up on a real falcon,but looking at the schematic it looks like it?..normally inputs are considered 'ground',so making a +12V design output a 'minus'.Which brings the second question on that.You really have to have something connected on the output there for anything to happen since the cap is only used for the CV output as far as I know.Has it it been more modded than just removal of modulator and the odd CV wire out?

mzry wrote:If you're idea about the driver q3 being totally on, what could I do to rectify the problem?

For a quick fix just dont have a cap there ,hehe..But you should really check that the CSYNC (CV on mod systems) output is not grounded or something for real.Pin 12 on connector.

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Re: Falcon RF to composite mod causing cap to bulge?

Postby neanderthal » Tue Mar 19, 2019 2:22 am

Well,no day like today todo stuff that should have been made yeasterday planned a year ago..lol

Took down the old machine for a run through,and on the CV thing found out it this.(non modified xx-002 Rev D/C board on the modulator thing)
Peak+ that is the highest voltage visible on my scope.(see below for approx p-p values)
MC1377 pin 9, 4.4V
Q3 out 4.0V (this is to the minus side of cap,whatta heck??)
CSYNC(CV) out 2.3V (No load),,with 10ohm fake load I get zero based 0.5V p-p out..No load approx 1.8V p-p (plus side of cap)

So yeah,you might have stumbeled upon a real bizarre design miss by accident there?
Some speculation on that.
1. Old TV:s/monitors/etc that actually have CV used to have a own cap in the input making this miss not notable back then.
2. 'modern' scarts and the likes with CV built in tend to have a fake load on the input (like a 75 ohm or something) to emulate old inputs,the built in multichips are very high Z often(impedance).
3.Pretty few people ever use the CV output today making it a really odd one to find.
4.It takes a bit time and maybe your wire onto pin9 on MC1377 to fry up a cap for real?(dont really wanna try that out since have no proper 75 ohm load,where are my boxes with testresistors)

Anyhow,this feels like a big lovely deucy as the old STE LMC1992 thing that made the -12dB mute..Have to really recheck tomorrow again :)
Meanwhile,,gonna fix my CF-adapter so the card actually gets a proper reset from system..weird damn PC ? POR(power on reset crap)
Oh yeah,should maybe aswell take a picture of the oddity that makes some CF cards bug out me thinks.

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Re: Falcon RF to composite mod causing cap to bulge?

Postby stormy » Tue Mar 19, 2019 8:02 am

neanderthal wrote:Well,no day like today todo stuff that should have been made yeasterday planned a year ago..lol


Wow that's brilliant, thanks for going into so much detail. You obviously know a lot more about electronics than me, could you suggest a fix I could install? Thanks again
Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

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Re: Falcon RF to composite mod causing cap to bulge?

Postby stormy » Wed Mar 20, 2019 8:27 am

It turns out that on the STE, MSTE and Falcon the schematics are all wrong and the capacitor c24 is in backwards. I've had this confirmed by another reputable Atari technician outside of this forum as well.
Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

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Re: Falcon RF to composite mod causing cap to bulge?

Postby neanderthal » Thu Mar 21, 2019 11:13 am

yup,,just saw that and was reminded of the thing.
Mainly thinking about the reason why no one noticed before?
After all I know that I run my falcon (and other ST/E models btw) on 'modern' screens that might only use a resistor for load on input.
But it is a rather big cap so having 'only' 4V backward on it via some 150 ohms might not fry it up,I quess that your driver transistor Q3 is maxed or something due to using the direct connection from Mc1377 making the DC level on the cap much higher?(have to check up on how NPN transistors work again..long time ago,hehe)

So yeah turn the cap around and use the ordinary CV instead of Mc1377 pin 9 out might be a good idea :)
Altho I would check that it is actually used backwards with a multimeter just to make sure that there is no board revision there they fixed that(even if unlikely).

Wonder if this have todo something with the odd RGB sync problem with my STE on my flatscreen teli,have to check that up aswell since looks like pretty much same design.

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Re: Falcon RF to composite mod causing cap to bulge?

Postby stormy » Thu Mar 21, 2019 1:31 pm

neanderthal wrote:yup,,just saw that


When you say use normal CV and not pin 9, where do I take that from? Could you point it out for me thx
Falcon ct60e 060 - 256mb ram - phantom bus and dsp accel // Atari TT - Thunder and Storm IDE 64mb ram - Lightning VME - USB LAN - Ati Mach64 2mb // Atari STM 4160 - 4MB ram - 16Mhz exxos 1.5 booster - Ultrasatan

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Re: Falcon RF to composite mod causing cap to bulge?

Postby joska » Thu Mar 21, 2019 2:22 pm

I guess he refers to the composite out signal on the video connector.
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Re: Falcon RF to composite mod causing cap to bulge?

Postby Greenious » Thu Mar 21, 2019 4:03 pm

CV is Composite Video

and yes, he's referring to pin 12 (I think) on the Falcon monitor port.
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Re: Falcon RF to composite mod causing cap to bulge?

Postby neanderthal » Thu Mar 21, 2019 4:03 pm

Almost,composite video yes is short for CV,but pin9 on MC1377 where you connected the extra odd video directly from chip.(meaning dont use that).
CV on connector is pin 12.Or simply use a adapter where you can have that connected,

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Re: Falcon RF to composite mod causing cap to bulge?

Postby Greenious » Sun Jun 09, 2019 9:40 pm

Good work on this Neanderthal. I'm just sad to see you don't get proper cred for this discovery, as "another reputable Atari technician outside of this forum " copied your work, as much else.
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