5200 - possible RF module problem

[Lorne]

Hi folks.

 

I’m a vintage computer buff who’s been hanging around over at the Vintage Computer Forums for the last few years (hopefully, you won’t hold that against me J ).

 

A couple years ago, a fellow vintage computer collector got his wife pregnant, and had to sell his lot of vintage computer gear to pay for the upcoming medical bills, and so he could convert his computer room into a baby’s room. (poor sod).

I went in on the purchase as part of a group of three, and we split up his stuff.

Among other things, I ended up with a 4 port Atari 5200 system (still in the original box) along with a bunch of games.

Having been messing with all the other computer stuff I got back then, and other computer stuff I had already, I now thought I’d have a go at the Atari.

 

I hooked it all up (hoping to soon be reliving my youth), but I get no video – just the snow that’s on the TV anyway.

 

I’ve messed with a few computers, am good at soldering, and can read a schematic (although while I can identify a part, I don’t know what they all do or why they are there).

 

I’m here to ask for some help reviving this 5200.

I jumped onto your AtariAge forums, and read a few threads with similar problems to mine.

I’ve downloaded the Atari CX5200 Service Manual.

 

Based on other threads, and that manual, I’ve done the following:

-Tested the wall wart and I get 15.3 V output with nothing hooked up to it.

-Checked the TV/Antenna switch box, and it clicks when it’s powered on and powered off.

 

The following measurements were performed with the 5200 on, and a game cartridge inserted:

-Checked the voltage on the RF cord where it attaches to the board, and it’s at 12.65 V

-Checked the voltages at VR1 and VR2 and I get 12.6 V at the emitter, and 5.0 V at the collector

 

Also did the following:

-Unseated/reseated all ICs (and I didn’t bust a single tab taking the shield off either !)

-Replaced the 8 RAM chips with known good 4116 RAM chips (just in case)

 

Then I went on to the troubleshooting section of the service manual.

On page 4-9 for Snowy Screen Troubleshooting, I get all the way to the third box.

-“Is there +5 on Pin 3 of the RF Module?” to which I unfortunately have to answer YES which leads to:

Defective

1) L13

2) RF Module or open between Pin 2 RF module and VCC2. (I’m not sure what “open between Pin 2 RF module and VCC2 is all about, but I suspect it’s not an easy fix)

 

 

Questions:

 

Is this something that is a common failure? I’ve seen posts that mention the GTIA chip, bad CPU, BIOS and RAM as possible culprits, but nothing on the RF Module.

 

Any opinion on which is more likely – the L13 or the RF module?

 

Are either of these two parts available anywhere?

 

Is there something else that could be bad as well? IE: replace the GTIA chip while I’m at it?

 

 

All suggestions for further troubleshooting, or any help at all will be very much appreciated.

 

 

Thanks in advance,

 

Lorne.

[zylon]

Sometimes you can use the piggyback method of testing modulators. Using this method with a known good modulator will give a poor picture, but it's still something.

The same modulator was used in all 5200's, all 4 switch 2600's, and some light 6ers.

Best Electronics still has this modulator in stock new. I still have a bunch of used "working pulls" in my parts bin if necessary.

Oddly enough, the last one of these I had to replace was on someone's 2 port 5200 that gave nothing but a snow screen when powered up.

[Lorne]

Thanks Zylon - piggybacking sounds like a good idea (have to do that all the time with RAM on IBM 5150s as they soldered, not socketed, the first row to the board = PITA).

 

I checked the L13 inductor, and it's fine so it must be the RF Modulator.

I also checked voltages at all the ICs, and they're all what they should be.

 

I actually called Best Electronics yesterday to check on availability, and the RF Mod is cheap enough. He's got a minimum order value, but I have to order something else anyway, so no biggie.

Funny - he was saying that it's very unusual for the RF Modulator to go, and it's like maybe 1 in 500. I guess I'm just lucky.

I was tempted to order one of his new old stock replacement boards for $ 35 but for me, the fun's in the fixin, so I'll just go with the RF Modulator.

 

I' m glad to hear that you've seen this same problem before.

I read post # 35 in this thread: http://www.atariage.com/forums/topic/186340-i-have-a-broken-atari-5200/page__st__25

and I was starting to wonder if maybe the Service Manual had a typo in it. That post says that pins 1 to 3 on the RF Mod should show 5V, while the manual indicates it shouldn't.

I thought I was going nuts for a while there.

 

I'll let you know how it goes once I get the part.

 

Thanks again.

[Lorne]

Got the RF Modulator, and being almost certain that was the failed part, I just installed it rather than piggybacking it first.

Seems that isn't the failed part.

Even with the new RF Mod installed, I still get 5V between pins 1 and 3 (and the manual indicates that shouldn't be the case).

 

I'm now thinking that the "snow" was just the snow you get when there's no picture on the TV anyway.

 

Back to the Service Manual.

If I go to the Black Screen troubleshooting section, and check a few voltages and frequencies, I find that I go from B to B3 (page 4-6) and I end up at the bottom of the page where it says: Swapout U2, U3, U5, U7 & U8.

 

I'll order the parts today.

I should still be below the new board cost of $ 35 but I'm getting real close.

[zylon]

Are you getting voltage from the 2 7805 regulators? Should be about +5 or so on third leg of each.

[Lorne]

Yep, I've got 12.5V going in, and 5V coming out on both VR1 & VR2.

[zylon]

I'm wondering if your "snow" is from the switchbox not doing its complete job. It might be letting power through, but not actually completing the video circuit.

[eightbitter]

Are you sure that 4116 are proper RAM chips? In 5200 should be 4516 single voltage (5V only) chips. 4116 require dual voltage 5 and 12Volts.

[Lorne]

I'm wondering if your "snow" is from the switchbox not doing its complete job. It might be letting power through, but not actually completing the video circuit.

 

The 5200 came with two switchboxes (both click when power is turned on and off), and I get the same result with both.

Could I have two bad switchboxes?

I guess I could - I suppose that'll be step three after I replace the 5 chips that were ordered, and mailed to me yesterday.

[Lorne]

Are you sure that 4116 are proper RAM chips? In 5200 should be 4516 single voltage (5V only) chips. 4116 require dual voltage 5 and 12Volts.

 

OK - that's just plain embarrassing. It always pays to check the data sheet doesn't it ? (The devil is in the details)

 

I put the old RAM back in, and I still have the same problem.

While doing that, I noticed someone must have messed with the RAM before - there are four 4516 chips and four 4517 chips in the bank.

I'll have to dig in my pile, and see if I can't find eight known good 4516 chips.

 

I suspect I'm going to have a bunch of spare parts (some good, some maybe not) for you guys, when I'm all done with this thing.

The five chips cost me more than a new replacement board would have cost, but I don't care now - I just want to know why this thing isn't working.

[zylon]

Yep. The 5200 used different RAM chips than the other A8 systems. Using the wrong ones should have still given a solid black or green screen.

[eightbitter]

Lorne, clean cartridge socket and cartridge edge connector and try again. If no picture or no black screen - disassemble console by remove top cover and top metalic shield. Insert cartridge to socket (in proper position) and connect console, PSU and TV to switch. Turn on the console and check voltage on pin 3 of RF modulator and both pins of L13, should be 5V. In many cases console not starts when cartridge connectors are dirty.

Edited January 20, 2012 by eightbitter

[Lorne]

Lorne, clean cartridge socket and cartridge edge connector and try again. If no picture or no black screen - disassemble console by remove top cover and top metalic shield. Insert cartridge to socket (in proper position) and connect console, PSU and TV to switch. Turn on the console and check voltage on pin 3 of RF modulator and both pins of L13, should be 5V. In many cases console not starts when cartridge connectors are dirty.

 

I'd cleaned the socket and edge connectors (using a Q-tip and isoprop alcohol) on all my cartridges before I started anything.

 

I checked the RF Mod and L13. 5V is present.

I'm confused though - you say there should be 5V present at pin 3 of the RF Mod, but page 4-9 of the Field Service Manual seems to be indicating there should not be 5V.

Is there a typo in the manual?

 

New chips have arrived - I'm going to replace the old ones, and see what happens.

[Lorne]

Got it !

It's alive.

 

I figured I'd replace the chips one by one, so I could find out which chip was causing the problem.

The first chip I replaced was the GTIA, and bingo, that was it.

I've just been playing Pac-Man, which I haven't done for about 25 years - and it's a riot too.

 

Just for fun, I thought I'd recheck the voltage on pin 3 of the RF Modulator now that the machine was working with the new GTIA chip in it, and sure enough it's still showing 5V.

So, it would appear that there's a typo in that field service manual.

 

Question:

Because of the Best Electronics minimum order of $ 20, I ordered all five chips (GTIA, Pokey, OS Rom, 6502 & Antic).

You guys are the experts, so what's your opinion - should I go ahead and replace the Pokey, OS Rom, 6502 & Antic with the new chips as well?

Are they liable to die one day too, so replacing them would prolong the life of the 5200 ?

 

Thanks,

 

Lorne.

[zylon]

I would replace them as they fail. ANTIC is the next most likely to fail. In fact, I have a 4 port that needs one currently. I'll get one on my next order.

That is a typo on the manual. Usually +5 volts on pin 3 is good.

[Lorne]

Well, I dunno.

I just turned it on for the third time, and it's back at a solid screen (with a slight red tinge to it).

I tried a few different cartridges, and I still get the same thing happening.

 

I took some measurements based on the Service Manual, and now I'm at page 4-2 of the trouble shooting guide where I get a no to the question "Is there a 3.37950 MHz signal on Pin 35 of U3?".

The result of that seems to be "Open between U5 Pin 29 and U3 Pin 35" (I don't know what that's telling me), and the solution appears to be: "Repair or Replace".

Repair or replace what, would be my question?

[eightbitter]

Repair broken track or replace GTIA or ANTIC .

I think that you should carefully check middle pins of VR1 and VR2 whether any broken or bad contact with GND (without supply!). Disconnection this pin causes the appearance 7.5V instead 5V on output. It's hard to find when you don't know about it.

[Lorne]

Repair broken track or replace GTIA or ANTIC .

I think that you should carefully check middle pins of VR1 and VR2 whether any broken or bad contact with GND (without supply!). Disconnection this pin causes the appearance 7.5V instead 5V on output. It's hard to find when you don't know about it.

 

There's no broken track.

I checked continuity (with power off) between U5-29 and U3-35 and there is continuity.

 

I've been suspecting VR1 & 2.

I even left the machine on for a 1/2 hour and checked their voltages again in case they were putting out more voltage than they should be, and they weren't - I was getting 5.01V and 5.02V.

I think at 86 cents each I think I'll just replace the damn things.

 

I'll have to replace the GTIA again, and I'll probbaly order another spare Antic while I'm at it.

 

Another thing that has also had me wondering:

The wall wart (power supply) puts out 15.3V with nothing hooked up to it.

Is that normal?

It's rated for 9V output.

Are your wall warts putting out more than 9V with nothing hooked up to them?

I know the VRs are supposed to drop that voltage to 5V but maybe when I first turn on the power, a litle more juice is getting through than should be?

 

Thanks again,

 

Lorne.

[zylon]

13-15 volts is normal for an unloaded power supply. A bad ANTIC will give a solid black or green screen. Turning them off and on rapidly will sometimes "jumpstart" a bad ANTIC.

[Lorne]

13-15 volts is normal for an unloaded power supply. A bad ANTIC will give a solid black or green screen. Turning them off and on rapidly will sometimes "jumpstart" a bad ANTIC.

 

 

Hmmmm... I will give that one a try.

 

I suspect however that it's probably the GTIA again, as I tried the new ANTIC and still had the same result.

 

It's just an observation based on my very limited experience (like only this one) with these things, but they certainly seem more finicky than computers.

[Lorne]

Here's an update.

 

I ordered a new GTIA and a new Antic (along with a new board as well - the multiple parts orders and their associated shiping costs were killing me, and I thought what the hell).

 

I replaced the two voltage regulators (VR1 & VR2) which wasn't as easy as it should have been.

I don't like the way they put that electrolytic cap right in the way of the screw holding VR2 in place, nor do I think it is a good idea to stick an elecrolytic cap next to a heat sink.

It was a PITA to get at VR2.

I removed the cap, even though I suppose I could have removed the heat sinks with the VRs attached to them instead.

 

Then I replaced the GTIA.

Tried it, and no go.

 

I replaced the ANTIC as well, and all is now working as it should.

I still don't know exactly what caused the problem in the first place, which was actually my desire and intent, but it's working, so I guess I shouldn't tempt fate.

 

 

Thanks for your help guys.

 

 

Now, I've got some spare parts for cheap here (cheap = shipping plus something):

I'm going to keep the new board, so I've now got a used 5200 board with:

8 new RAM chips

a new RF modulator

new VR1 and VR2

a new GTIA and a new ANTIC

 

In addition, I also have a new unused:

Pokey IC

6502 IC

ROM OS IC

 

and a probably working RF modulator that I thought was bad, but maybe isn't.

 

 

Do you two (zylon or eightbitter) or anyone else want this stuff?

[zylon]

Antic chips are funny like that. i still have a 4-holer waiting on one. When you install a new IC, it's a good idea to let the system run a good half hour after it's operational.

Shoot me a good deal, and I might bite.