Colecovision Powersupply Question

[Tpapp54321]

A while ago I recapped my CV's power supply and replaced all of the components since most of them had died. It ended up working really well BUT I didn't have any heat sink compound so I neglected closing back up. Just the other day I finally bought some compound but I found that the Transisters were too low to be mounted back up to the Metal Wall surrounding inside the plastic shell. So I replaced the Components yet again, but now the Colecovision won't power up. I am only getting 4v on the 5v part of the powersupply but I think I Was only getting 4.5v before (or something like that). However it could also be a problem in my Colecovision as I haven't powered it up in a couple months. So my question it, Will a Colecovision still operate with 4v instead of 5v? If so then how can I raise that 4.0v up to 5v?

[+Stephen Moss]

Technically older logic devices specification for power are 4.5 - 5.5 volts for TTL and 4.5 - 18V for CMOS, if they are all operating at the same voltage it may just manage to limp along at 4V although probably at a slightly slower speed.

 

I am not familiar with the CV or its power supply arrangement but given its age I doubt it used a switching power supply or DC-DC converters therefore some of the "transistors" were probably linear voltage regulators, typically an 7805 or 78L05 for a 5V supply. An output of 4V for a 5V regulator could be caused be several things...

1) Poor regulators, some regulate are better than others - personally I prefer those made by National Semicondutor (now Texas Instruments following a buyout) wherever possible.

2) Insufficient input voltage, older linear regulators require an input of between 1.5 and 3V higher than their output (depending on type and manufacturer) for proper regulation, whereas modern LDO replacements only require the input to be 0.5V higher. Check the regualtors input voltage.

3) Something is drawing more power than it should creating VI losses in the regulators and thus a drop in voltage, if you can isolate the 5V output from the power supply check to see if the voltage rises back to 4.5 - 5V or remains at 4V, if the latter the problem is in the power stage, the former you may have more damaged components elsewhere in the system.

VI losses result in more power dissipation in the regulators which could result in them going into thermals shutdown, so if the system works for a while and stop and works again after being off for a while that could be the reason why.

 

Another thing you could try is to remove the devices from the heat sink and see if the CV powers up as normal. The metal tabs on the devices are not always connected to their 0V terminals, if that is the case it can cause problems when such devices are either connected to an common heat sink with devices that do or to a heat sink that is connected to ground.

If it does power up when you have removed all the devices from the heat sink then that may be your problem, in which case mica washers can be placed between the devices and the heat sink compound to electrically isolate the device, you will probably have to use isolators for the screws as well, when looking at then from the side they look a bit like the letter T, from the top they are circular with a hole through for the screw. Both parts can be obtained separately or brought together as part of a mounting kit.

The problem is knowing if either all or some of the components need to be electrically isolated and if the CV's design is using the heat sink as a 0V connection for any of the devices, if you can remember encountering any isolators when taking the original device out and which devices they belonged to that would help. Personally I use the non messy thermal pads rather than heat sink compound.

 

I doubt I can be of more help than that due to my unfamiliarity with the system but hopefully that is of some help.

Edited August 31, 2016 by Stephen Moss