Fixing bad video on a 2600 JR Unicorn

[OldPAL]

Got a Unicron from ebay. Initially I could not get any video upon power up. After testing the power supply, cables, and the voltage regulator successfully, I decided to fiddle with the pot on the RF modulator with my hex screwdriver. It started to turn and I did get some video but now it broke and released this graphite-looking pieces. I don't think it turns anymore.

 

What is the most logical course of action here? Is there an easy fix for this pot? Or do I go with composite mod knowing that the machine is working? This is the only reference I could find online regarding modding a unicorn. https://theokoulis.com/index.php/2018/09/27/the-unicorn/

 

Ps. I was hoping it landed on a different frequency around channels 2 and 3 but reprogramming TV did not work. 

[OldPAL]

Nevermind. I am an idiot. I used an auto-switching RF box. As soon as I used a simple cable it worked.

[OldPAL]

OK, as I was closing it up I heard rattling inside and the rest of the carbon pieces came off. Now I have no video. Is there an easy fix for that pot like replacing it with a resistor? I don't need the adjustment.

[OldPAL]

After some search, I found out this part is called variable inductor coil. On my board it is part L6 while on others it might be marked as L201. What specs do I need to follow? I could not find this info on schematics online. These parts are sold with different inductance levels (0.25 - 0.41 uH, 1uH, 2uH etc). Another discussion I found was talking about 15-20uH range for the VCS which sounds too far from what's on the market. Am I missing something?

 

Edited September 20, 2021 by OldPAL
Typo

[OldPAL]

I ordered these three from Mouser. They have slightly different resistance/impedance. 

 

542-5300-15-RC
5300-15-RC
15uH 10%

 

542-77F150K-RC
77F150K-TR-RC
15uH 10%

 

871-B78108S1153K
B78108S1153K
15uH 610mA 0.6ohms

 

 

 

 

 

 

[ChildOfCv]

If they are the same size, you can even remove the ferrite slug from the new one and then pop it into the old one.  The bottom of each coil is open, so you can transfer the slug through that end.

[OldPAL]

1 hour ago, ChildOfCv said:

If they are the same size, you can even remove the ferrite slug from the new one and then pop it into the old one.  The bottom of each coil is open, so you can transfer the slug through that end.

Yeah, unfortunately the ones I ordered are fixed, not adjustable. The cores are wrapped tigth.

[ChildOfCv]

Oh.  Well I ordered some variable inductors a while back that were almost identical to those.  I assumed those were the ones you bought.

 

https://www.mouser.co.uk/datasheet/2/597/uni10-463412.pdf

[OldPAL]

On 9/21/2021 at 8:36 AM, ChildOfCv said:

Oh.  Well I ordered some variable inductors a while back that were almost identical to those.  I assumed those were the ones you bought.

 

https://www.mouser.co.uk/datasheet/2/597/uni10-463412.pdf

They look identical, but their inductance go up to 1.5uH max. Documents I found keep talking about 15uH. I did not want to mess with it. Someone used a fixed one successfully, and I assume you used one successfully so maybe inductance is not that important. Or maybe it is not the core that decides inductance.

[ChildOfCv]

1 hour ago, OldPAL said:

They look identical, but their inductance go up to 1.5uH max. Documents I found keep talking about 15uH. I did not want to mess with it. Someone used a fixed one successfully, and I assume you used one successfully so maybe inductance is not that important. Or maybe it is not the core that decides inductance.

I would want to see empirical evidence of readings taken before trusting what someone says about its actual value.  What I know is that the different colored tunables are colored based on the number of copper wire turns around them.  That is how their base inductance is determined, by diameter of the turns and number thereof.  The slug then adds inductance depending on how much of it is within the coil's plane.  But if you have a tunable with a certain diameter and a number of turns, then the replacement with the same diameter and number of turns will work in its place.

 

I have successfully resurrected one of these tunable coils by transferring the slug just as I noted.  I can't guarantee that will always work, but it was the same application.

 

If you find a sufficiently tunable TV or just the right inductance, then yes, a fixed inductor will work.

[OldPAL]

On 9/22/2021 at 11:23 AM, ChildOfCv said:

I would want to see empirical evidence of readings taken before trusting what someone says about its actual value.  What I know is that the different colored tunables are colored based on the number of copper wire turns around them.  That is how their base inductance is determined, by diameter of the turns and number thereof.  The slug then adds inductance depending on how much of it is within the coil's plane.  But if you have a tunable with a certain diameter and a number of turns, then the replacement with the same diameter and number of turns will work in its place.

 

I have successfully resurrected one of these tunable coils by transferring the slug just as I noted.  I can't guarantee that will always work, but it was the same application.

 

If you find a sufficiently tunable TV or just the right inductance, then yes, a fixed inductor will work.

I listened to your advice and bought 2 pieces of adjustable RF inductors from ebay. I removed one of the cores carefully, and put it in the inductor in my JR unit. It worked! 

[skintigh]

Hi @OldPAL! Could you tell me the serial number (and color of the numbers) of your unicorn? I'm compiling a list here: https://forums.atariage.com/topic/309869-compiling-2600-jr-unicorn-1-chip-serial-number-list-please-help/

Thanks!

 

 

[OldPAL]

Hi @skintigh I ended up selling my collection due to housing situation couple years back. However, it was listed on the unicorn JR list (not sure if it was yours or another one) as it was picked up from an ebay listing.

Edited January 8 by OldPAL