New 48k 6A - power supply issue

[stansteph54]

Hi together,

I have bought a 48k issue 6A. Nothing on screen when power on so I opened it.

Bouh it has been already repaired.. with more or less finesse

I started checked the tension lines which were out and a few second after having started, I noticed that TR4 was almost burning..
TR4 has been already replaced by somebody and is now a BD139

the coil looks ok after my controls
the voltage on the 7805 are already out with about 1.2V on each side !

[1024MAK]

Most likely one (or more) of the 4116 (or equivalent) “lower” DRAM chips (IC6 to IC13) has failed and is drawing too much current from the +12V supply.

As there is no easy way to isolate the +12V supply, if it’s not obvious (due to one or more of the chips getting hotter than the others), the only practical way to isolate the DRAM from the +12V is to cut the +12V track that feeds the DRAM.

Mark

[stansteph54]

Thanks a lot Mark,

Do You think it could also explain crazy voltages values on both sides of the 5V 7805 regulator ? They are each between 1.2 and 2V ! but TR4 is still burning anyway ! thanks again

[1024MAK]

Ah! That’s not good (the 1.2V on the 7805).

The power arrangements are:

The external PSU is an unregulated DC type. The nominal voltage is 9V, but at less than full load, the actual output voltage will be higher. It should be around 11V to 12.5V (approximately) with no load, but depending on your mains voltage may be higher or lower than these values. It is not protected against short circuits. But if the transformer gets too hot, a thermal (temperature) fuse will blow.

On the computer PCB, the +9V feeds two things (not including the capacitors used for decoupling/filtering/smoothing):

The 7805 voltage regulator - a series pass element type. As long s the input is between +7V and +25V it should produce a +5V regulated output (it should go into foldback current limiting if it either gets too hot or the load attempts to draw too much current).

The DC/DC converter (discrete component switching regulator), formed from the “coil”, TR4, TR5 and various resistors, capacitors and diodes. It’s important to note that it uses the +5V supply as it’s own voltage reference. If the +5V supply is out of specification, the +12V from this DC/DC converter will also be out of specification.

Also, on unmodified issue one to 4a boards, if TR4 is not oscillating, the +9V supply will leak through the “coil” to the +12V line. Hence, depending on the fault, either owners think the +12V is a little low, or a short circuit on the +12V can cause the +9V to be dragged low. This should not be an issue on the issue 6A board. But do check that capacitor C80 is okay.

The DC/DC converter provides the +12V supply required for the 4116 (or equivalent) ‘lower’ DRAM chips and the +12V supply for IC14, the colour encoder chip. The DC/DC converter also provides the VERY IMPORTANT -5V bias supply for the 4116 (or equivalent) ‘lower’ DRAM chips. Without this supply, they may be damaged if the +12V supply (even if out of specification) is present.

Either way, given that you say you only have 1.2V on the input to the 7805, you need to work out why.

I suggest you desolder the 7805 AND TR4. Test for shorts, then powering up the board. The +5V, +12V and -5V rails should all be zero volts.

Apart from a multimeter what other test gear do you have?
Do you have a regulated bench PSU? Does it have an adjustable output voltage and current limiting?

Are IC6 to IC13 already in sockets? Or are they soldered directly to the board?

Which chips/components look like they may have been worked on/replaced?

Mark

[stansteph54]

Good evening Mark,

First thanks a lot for your kind help and time already spent for me

I have a multimeter and also a vintage oscilloscope in the basement to investigate.

But seems the architecture of the dc/dc bridge is k not usual for the tests compared to some others 8 bits machines for which you could plug and listen to ram chips..

Anyway here are my first answers :

« I suggest you desolder the 7805 AND TR4. Test for shorts, then powering up the board. The +5V, +12V and -5V rails should all be zero volts.

>>will do

Apart from a multimeter what other test gear do you have?
>>a vintage 2 ways analog oscilloscope


Do you have a regulated bench PSU? Does it have an adjustable output voltage and current limiting?

>>I do gave some vintage lab PSU, I think one of them has current limitation

Are IC6 to IC13 already in sockets? Or are they soldered directly to the board?

>>only IC8 and IC13 are in sockets see new pictures

Which chips/components look like they may have been worked on/replaced?
>> TR4 by a BD139
>> 2 upper RAM’s are also in sockets, see pictures


In the end, I think I have to put all lower rams in sockets ?
what references do you recommend me to order for TR4, TR5 and TR6 replacement?

Many thanks again !