Speccy just died - suspect problem with power supply circuitry

[hatman72]

Hi, I am hoping someone can confirm my suspicions on this? I was using my Spectrum+ with an issue 6a board and suddenly it suddenly just died while I was typing up some BASIC

With no output to the sceen or the speaker I opened it up and checked the voltages on the 4116 chips:
pin 1: -4.17
pin 8: +1.5
pin 9: +1.1
pin 16: 0

I then checked the voltages on the 7805:
pin 1: +13.3
pin 3: +1

If I am correct pin 3 on the 7085 should be about 5V so I am assuming that this is the problem, but wanted some confirmation that this is definitely the 7805, or whether anything else could cause this. I am using an original Sinclair power supply and my rubber key spectrum works fine off that.

Thanks
Richard

[1024MAK]

Either the 7805 voltage regulator has died and gone to silicon heaven, or it’s gone into fold back current limiting mode.

Is the heatsink firmly attached? Are there any signs of anything that could be causing a short circuit on the PCB?

As the DC/DC converter circuitry based on TR4, TR5 and the coil uses the +5V supply as a reference, with the +5V low, the +12V (and maybe the -5V) will no longer be in specification.

If you can’t find anything wrong, with the power off, test using the 20k resistance range on your multimeter between the +5V and any convenient 0V/GND connection (such as the heatsink tab of the 7805 or the outer of the TV/RF/UHF output on the modulator). Report back your results.

Mark

[hatman72]

Thanks Mark

Is the heatsink firmly attached? - Yes

Are there any signs of anything that could be causing a short circuit on the PCB? - Not that I can see, and this happened after I had been using the Spectrum for about 30 minutes.

If you can’t find anything wrong, with the power off, test using the 20k resistance range on your multimeter between the +5V and any convenient 0V/GND connection (such as the heatsink tab of the 7805 or the outer of the TV/RF/UHF output on the modulator). Report back your results. - The multimeter is showing a value of 1.95 on the 20K range.

Thanks
Richard

[Tiger]

As 1024mak told, the 7805 seems to went into the silicon heaven. This will the case when there is no short circuit in the 5V line. 1,95K looks ok for me a short circuit signs a value near 0. If you replace the 7805 I'll suggest to take a 78S05 which will have more "reserve power" or better a (murata) OKI-78SR-5.

[1024MAK]

The multimeter is showing a value of 1.95 on the 20K range.

Yep, definitely sounds like the 7805 voltage regulator has died.

You can either replace like with like and fit a brand new 7805 voltage regulator (any brand/make will do) or you can fit a DC/DC converter, also known as a switching regulator. These don’t need a heatsink, but are more expensive. Common makes of these are RECOM, Traco Power, or MuRata. See this topic for more details. It primarily is about fitting to a ZX81, but the same principles apply to the ZX Spectrum 16K/48K/+ boards (although for boards that are issue 4A or lower, either ensure the Sinclair DC/DC converter modification has been done or use a regulated 9V PSU).

Mark

[hatman72]

Thanks both - I'll order a replacement in and see if that fixes it ...and hope it didn't take anything else out when it failed!

[hatman72]

Hi, I'm just reporting back on this thread. The fault was indeed being caused by the 7805 voltage regulator. I replaced it tonight and it is all looking good.

Thanks for the help!

[1024MAK]

Thanks for letting us know. Great news that it’s fixed and working Have fun

Mark