TR4 overheating 6A board

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RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

TR4 overheating 6A board

Post by RetroByte »

Hi all, I'm trying to repair a 6A board.

I first checked the resistances to 0v at the lower RAM, +5, -5, 12v and there was no low resistance;
so I plugged it in and resister R79 got very hot and smoked.
I replaced TR4 with a C2236 which I've found on some 4A boards; I did swap the base and collector legs around making sure that they didn't short.
I also replaced TR5 with a ZTX751.

I then tested the machine with my bench PSU at 11v with a 0.75A overload protection, and the voltage drops from 11v to just over 5v.

As TR4 burnouts are often the 4116 lower RAM problems, I removed them all (I expect some are damaged anyway), but it's still the same.

After watching JoulesperCoulomb brilliant YouTube video, I learnt that if for any reason the 12v is missing, that TR5 switches on and then TR4 can burn out.

I believe I've tested every resister, diode and capacitor in the power circuit and can't find anything wrong. I removed one end of R61 that goes to the coil and then TR4's base, just so I could test the 7805 was producing 5v, and it was about 4.8v.

I have now replaced TR4 again, with a used (but tests ok) ZTX651 but it's the same.

I have ordered some ZTX653 transistors, just in case it's still TR4 that is faulty.

Any suggestions please?
RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

Re: TR4 overheating 6A board

Post by RetroByte »

I can't properly test C44 and C45, because my tester doesn't do as high as 100uf, so I think I'll swap those two caps for new ones.
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1024MAK
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Re: TR4 overheating 6A board

Post by 1024MAK »

So, just to confirm. All the 4116 DRAM chips have been removed, yes?

So just check using your multimeter, that there are definitely no low resistance paths from the +12V, +5V and -5V to 0V/GND and to any of the other supply rails.

I’m very concerned about your comments about R79 getting hot and smoking. At 2.2kΩ it should never, ever pass enough current to do any of that. Can you show me in a picture which resistor it was, just in case there has been any confusion.

C44, C45 and C80 (or equivalent on other issue boards) should be replaced as standard whenever there is a failure of the DC/DC converter formed around TR4, TR5 and the coil.

Before powering up, lift one end of R62 (15Ω). Check TR5 and TR4 for shorts. Now the +12V and the -5V should not be supplying anything. So you should be able to power up. If TR4 dies again, the coil is the prime suspect. It may have shorted turns. Especially if there are any signs of overheating.

Mark
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RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

Re: TR4 overheating 6A board

Post by RetroByte »

Hi, thanks for replying.

*
All the 4116 DRAM chips have been removed, yes?
Yes

*
definitely no low resistance paths from the +12V, +5V and -5V to 0V/GND
All OK

*
C44, C45 and C80 (or equivalent on other issue boards) should be replaced as standard
I'll swap them tomorrow (If I have some)

*
Before powering up, lift one end of R62 (15Ω). Check TR5 and TR4 for shorts. Now the +12V and the -5V should not be supplying anything. So you should be able to power up.
I have already tried that, and there is no overcurrent or anything getting over hot.

The coil looks OK and tested the windings for shorts.

I'll let you know if there's any change after replacing the capacitors :)
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RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

Re: TR4 overheating 6A board

Post by RetroByte »

It looks like this person had the same resistor burn, and fixed it by replacing C44 and C45

viewtopic.php?f=21&t=4412&p=44644&hilit ... wer#p44644
RetroByte
Posts: 51
Joined: Mon Mar 12, 2018 1:33 pm

Re: TR4 overheating 6A board

Post by RetroByte »

I replaced C44, C45 and C80 and I now have 12v and -5v :)

Thanks for your advice!
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1024MAK
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Re: TR4 overheating 6A board

Post by 1024MAK »

Good that your making progress :D

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

:!: Standby alert :!:
There are four lights!
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer later in the year.
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