Colecovision Power Supply Repair - +5V Regulator Replacement Question

[bradd1978]

I have a Canadian Colecovision power supply that won't output +5V. I reflowed the solder on the regulator but it still didn't work. When checking the voltages it appears the Ground was on one of the outside pins, and with your average 7805 its the middle pin. Upon closer inspection, the regulator had the markings "ADT TIP 41A".

 

My question is with regards to replacing it, can you use an everyday 7805 reguator in its place? From testing the voltages it seems the pinout is different, or am I wrong?

 

I've looked up this issue but no one has specifically said anything about a different pinout. They just state they "replaced the regulator" and it was working just fine.

 

Anyways, let me know if anyone has a similar experience :-)

 

Thanks in advance,

Bradd

[+5-11under]

I've never opened a CV power supply, but a TIP41A is a transistor.

[bradd1978]

I looked up the datasheet and it shows it in the form of a transistor and a voltage regulator. That's what was puzzling me

 

I googled and saw someone repair a US version supply and they said they bought a replacement on Amazon and all was good. That's what made me believe they used just your average 7805 as replacement. But I'm only assuming

[Dr. D.]

I've never opened a CV power supply, but a TIP41A is a transistor.

 

It is a power transistor in TO-220 package (like the 78xx and 79xx regulators). Any device with 3 terminals can be made in TO-220 form.

 

The one CV power supply I tried to disassemble many years ago was completely filled with clear (yellowing) epoxy, making it a solid block, and the organic solvents I tried didn't touch it. So I don't know what the actual circuit is. However, if there is no actual 5V regulator in the circuit, it is quite possible that (for cheapness) it's actually an UNREGULATED power supply, with a tap off the transformer at around +6V, that relies upon the current load from the CV to pull it down to the correct voltage. It certainly is a linear design, not switching. Most of your cheap wall transformers are like that: you have to test them under a load to actually see the advertised voltage, otherwise you see something greater. Remember, V=IR.

 

Coleco was known to be cheap with power supplies. The voltage regulator circuit on the ADAM disk drives has poor little 78xx regulators screwed to the back frame for heat sinks, and they will burn your fingers if you touch them. The board artwork on the ADAM printer power supply suggests that the original design for the power transistors was similar, with 2N3055s substituted later because the current draw was too much for TIP41As. (The regulators for some of the voltages are in DIP packages, LM723s I think.)

 

*Dr. D.*

Edited August 5, 2014 by Dr. D.

[mightywiz]

they use:

2 x tip41a

lm7905 for the -5v supply

 

usually in a power supply the electrolytic caps are what go bad first, they reduce the output voltage and cause increased

current draw. hopefully to pop the fuse and not destroy components.

 

there are I think 3 large capacitors and 4 small ones, I would replace them all and see if that fixes you problem.

especially if the voltages are just low, you should have +12, +5, and -5 and ground. but you have to check

them under load because unplugged from the colecovision they probably will read correct.

 

if you totally missing all voltages then it's the fuse or the transformer has opened.

 

the 2 chips on the circuit board are also voltage control chips, but they usually blow the top off or crack when

they go bad.

 

if your missing a single voltage then you need to do ohm meter in diode mode to test the transistors with?

 

I'm a component level repair tech and this stuff is easy for me to repair if I have the unit in front of me, but

with you having it I can only point you in the right direction.

Edited August 5, 2014 by mightywiz

[bradd1978]

Thanks for all the info guys :-)

 

I tested the power supply but it wasn't plugged into the system. I got all voltages but the +5V. Ill test again with it plugged into the system.

I'm still curious if I can straight up swap the regulator for a 7805 without issue?

 

On visual inspection, the chips look good and the caps too

[iscout62]

Just to clarify.

A TIP41a is not interchangable with a LM7805 Vreg.

 

You need to figure out if your transformer tap is providing voltage to input that side of your +5V circuit.

If you have voltage at the transformer, work outward and find the offending component. That transformer is a mess, with leads going all over the place, good luck.

 

If you want a new supply, a European ebay member has them.

(I also have some DIY supply units)

[bradd1978]

iscout62, Thank you :-)

 

I figured it can't be interchanged but thought I'd ask. Yeah these power supplies are a mess for sure. After some further probing, I'm pretty sure the regulator is dead so I might just order some TIP41A's

 

Thanks again

[iscout62]

If you are interested,

I have a old coleco PSU that is dead on the +12v side, if you want the internals to fix yours I'll shoot it to you.

-Cheers

[bradd1978]

Thanks:-)

I'm in Canada so not sure how much shipping would be. I'd even take the internals minus the transformer. That seems to be the heaviest part

[iscout62]

PM me your details.

I'll put it in a cheap post package tomorrow.