Zx 48k after the ic7805 other problem
Hi everyone, i am Tiago from Portugal, and i am trying to repair a Zx Spectrum 48K.
the machine didn't do anything, no image, no sound, nothing. The PSU is fine, i used it with other 48k and works fine
I open it and with a multimeter i checked the ic7805, and it had less then 1V.
I bought a new one and did some soldering work (don't have much skill) and i now have 4,95V that i think it's ok, it shows the same reading as a other working spectrum i have. So, i didn't have any image on screen and i do now have something on screen, but it doesn't look good.
the image i get in TV is this:
so after the ic7805 what i should do now? Is it RAM? ULA?
how can i check if it's one of those? Thanks a lot
the machine didn't do anything, no image, no sound, nothing. The PSU is fine, i used it with other 48k and works fine
I open it and with a multimeter i checked the ic7805, and it had less then 1V.
I bought a new one and did some soldering work (don't have much skill) and i now have 4,95V that i think it's ok, it shows the same reading as a other working spectrum i have. So, i didn't have any image on screen and i do now have something on screen, but it doesn't look good.
the image i get in TV is this:
so after the ic7805 what i should do now? Is it RAM? ULA?
how can i check if it's one of those? Thanks a lot
Post edited by tiago on
Comments
Check that 5V(9), -5V(1) and 12V(8 ) are all present at the lower RAM ICs (the 8 in a row on the lower left). I put the 4116 pin numbers in brackets after the voltages.
If the -5V or 12V are missing then there's probably a fault in the voltage converter circuit and probably one or more of the 4116s have failed. You'll need to isolate the 4116s from their power supply lines (cut tracks or remove them from the board), repair the voltage converter circuit and re-connect/re-install the 4116s after checking that none of them have shorted their power supply connections.
If all the voltages are present, then your Spectrum has probably got a slightly easier to repair fault. It would be useful to see the display as it could give a clue to what is wrong.
let me try to post the link:
picasaweb.google.com/lh/photo/Wb_ox5OTdGvB7vmkjIyakNMTjNZETYmyPJy0liipFm0?feat=directlink
see if you can see it now, or is it the forum that cuts the url later? am i allow to put a url?
Carry out the checks I described in my previous message. I think you're going to be in need of some replacement 4116s or some alternative lower RAM arrangement.
Thanks
If a working Spectrum is important to you, I recommend you seek help from someone with more experience of working on electronic equipment. In particular, it's easy to do a lot of damage when removing the RAM ICs if you don't go about it the right way. I could give you some tips on what I've found works if you decide to have a go.
http://www.worldofspectrum.org/forums/showthread.php?p=624138#post624138
If you get anything other than an open circuit or very high reading, I would suspect TR4 has failed. It's unlikely to have done so for no good reason though and so the cause must be found and rectified or a replacement will die too. Common causes are faulty 4116s shorting their power supplies, failing/failed electrolytic capacitors (I recommend replacing all of these in a Spectrum that still has the originals) or a mishap with the expansion connector. If TR4 has failed, you should also test TR5.
with the mutimeter i mesure the RAMs, pins 1,8,9 against the 16, is that it right?
in my working zx i had very near values, with a +/- 0,3 difference, it runs ok.
With the other 2 ZXs, i have very low values on many RAMs, they are not ok....
so what can i do now? How can i now that it's the only problem, it i replace them, i could have other problems in other components?
The rams in one of the zx are in slots, so it's more easy to change them, but in the other one, i have to solder...
what do you advice now?
Thank you so much
-0.04V ,+2.24V and +4.94V on pins 1, 8 and 9,
when i remove all rams, and messure the slots, i have
-0.05V ,+1.85V and +4.94V on pins 1, 8 and 9,
dont understand why pin 8 has less without the ram in place....
I should now go to TR4 and TR5, should i replace both? or just TR4 first?
thanks
If you have a bench PSU that has a current limit facility then I recommend using that (set the limit to about 700mA) to power the Spectrum. The limit may active for a instant at power on while capacitors charge, but if it doesn't stop limiting straight away, then turn the power off. This will usually stop the transistors from burning out if there is still a fault in the circuit. I also hook up a 'scope to TR5's collector so I can immediately see whether it is oscillating as it should; prior to the issue 3b circuit, there should be a sawtooth waveform at this point.
If then any of the 2 voltages is still not ok, I always replace both transistors.
After that I test the RAM ICs for short circuit.
Ater that I replace the RAM ICs one by one, checking the voltage every time.
Also feel the temperature of the ICs with my fingers.
If any of the RAM ICs fails, then I always replace all of them..... I'm a bit paranoid...
My skills don't allow me to much more then desolder and solder and mesure from multimeter... i can try to replace TR4 and TR5.
I know that replacing them doesn't mean it will work, the problem could be in other place. But if the price is low, i can try it...
so,
1) what is the price of TR4 and TR5?
2) what is "the diode test function of a DMM" sorry for my ignorance :-)
http://www.elexp.com/t_test.htm
and it seems to give a reasonable explanation. Remember that TR4 is an NPN transistor and TR5 PNP.
It's normally possible to use a resistance range of a DMM, but many have a special mode for testing semiconductor junctions that shows the forward voltage drop when the junction under test is conducting. You should find 0.6-0.9V for each junction when forward biased and open circuit reverse biased.
Neither type is especially expensive, though you may have to try a few different suppliers.
I don't think I've ever needed to replace a TR5, but I have replaced quite a lot of TR4s.
you said:
ZTX651/652/653 for TR4
ZTX751/752/753 for TR5
searching on web i found that TR4 is more reference to ZTX650,
is it the same?
thanks
As Zorn said TR5 in most cases is not the reason for problems. But if you need a new TR5 you can use nearly all pnp silicon transistor you find, also BC557 or similar will do the job. Check if Diode D15 (in 12V line) acts as diode.
If the RAMs are not in sockets I would not prefer to do that.
Greets Ingo
This has happened to me a couple of times. Often this also results on one or more of the 4116 memory modules going bad.
well i bought:
ZTX653 for TR4
ZTX753 for TR5
I desolder them with a normal iron solder,
and solder the news ones.
It worked !!!!! I am so happy !!!!
Thanks to yout all, you are the best !!!