May 282016

Last week I bought on Ebay an untested Polar Toneohm 850 (picture from Internet) :

For the uninitiated this is a instrument that is able to locate quickly and accurately shorts on PCBs.Device is essentially an audible milliohmmeter which, indeed, produces a tone whose frequency is inversely proportional to the resistance measured across its probes.The more you get closer to the short  and so the lower will be the resistance, the higher will be the produced tone.This is very useful when you come across a PCB with a dead short or small resistance between VCC and GND, using a normal multimeter will have no effect since you will end up to measure always same resistance values due its accuracy (most if times of 0.1 ohm ) which is too coarse.

So, I bought this kind of equipment which arrived me a couple of days ago.When I saw the package, I immediately had a bad feeling :


The box was broken and, once opened, the case of the unit too:


But worst , the unit didn’t power on at all, it was competely dead while the seller clearly stated (sending me also a picture)  that, although untested, instrument turned on showing numbers on display.

Since I needed this piece of equipment I opted for repairing it instead of asking for a refund and send it back.

I opened the unit and PCB was in untouched state, no sign of damage or burn components.DSCN3328

First of all I went to check voltages on the various test points, I could only measure few Millivolts.Thanks to the user ‘Fraser’ (thanks again!) on EEVblog forum I got schematics of this specific model so I could start my troubleshooting more confident.Design of the unit is quite simple : the 220VAC ( unit came with 120VAC line selected so  had to change it accordingly to my country) reaches the main transformer ‘T1’ being reduced to 9VAC then it got rectified by a couple of bridges and then regulated by a 7805 and  couple of 78L05 providing power for all the logic :


Again, I was able to measure more or less +1V on the inputs of the tree voltage regulators so problem was upstream, specifically in the main transformer (‘T1’ on schematics) :


Said simply, a transformer is a device made of a primary winding  and one of more secondary windings.Windings are usually wound around very high permeability ferromagnetic cores.A varying current in the transformer’s primary winding creates a varying magnetic flux in the core which is transferred to the secondary windings by magnetic induction.

So, my problem was clearly located in the windings.I removed the transformer:


I went to check windings with my multimeter and none of them gave me resistance values, they were almost all opened due the shock received during shipping!But luckily they were interrupted just near the pins, see picture of primary ones for example :


A bit of ‘surgery’ was needed using some AWG30 wire to restore all connections (part in excess of wire was cut, obviously)


Once mounted the transformer and reassembled all the pieces, the unit came back to life fully working!


Video below shows a testing on an arcade board where a 100nF by-pass ceramic capacitor was intentionally shorted:

 Posted by at 10:43 pm
May 262016

I got from New Zealand this genuine Splatterhouse PCB for a repair:


Upon boot the board was stuck on a “ROM TEST START !! PLEASE WAIT…” message displayed upside down :


From my past experience I know for sure this issue is caused by a bad ’64A1′ custom replaceable with a programmed Hitachi HD63701 MCU (plus a patched ‘VOICE0′ ROM in order to handle two custom opcodes of the ’64A1’)  following this procedure:

Namco System 1 custom ’64A1′ replacement

But in this case, as you can see from the above picture of the board,  the owner already replaced the ’64A1′ (and at same time  patched the ‘VOICE0′ ROMs using an hacked 1Mbit JEDEC EPROM in place of a non-JEDEC one ) and this didn’t fix the issue.For first I reverted the modification the and reinstalled the original custom ’64A1’.With this configuration the board successfully booted into game although some sound samples were bad.This lead me to think that there could be some problem is the data bus of the VOICE ROMs which prevented the HD63701 MCU to correctly read the patched data causing the missing boot. :


So, assisted by schematics, I went to check the involved circuit and found that pin 20 (data line D6) of the VOICE ROMs (Splatterhouse use only four of them to store samples) was not daisy-chained as it should have been:


In the above snippet of schematics you can see (red mark) where extactly trace was interrupted.I simply used a bit of AWG30 wire to restore comnection:


This fixed completely the game using both the original ’64A1′ custom and the HD63701 MCU.End of job.

 Posted by at 8:19 pm
May 222016

The 14″ TV is up now.
This one was a lot more involved.
Before starting on this one I had tested the previous PCB with this TV so I knew for sure the TV part was operating fully.

When powering this up I got no audio or video, just a black screen. Using the external A/V output also gave me nothing.
I couldn’t find anything obvious visually on the PCB itself.
Ive read a lot in the past about SNES repairs and the CPU’s seem to be a weak point. I started prodding around the CPU with the scope and even though all the voltages and clocks looked good I could see any activity.
I had a known working spare SNES which I opted to sacrifice for parts.

I replaced the CPU and once again checked for signals. This time I had life but it gave up after a few seconds and still didn’t give me any output. This cycle was repeatable on resetting the machine.

I next opted to replace the ‘S-WRAM’ (work RAM) positioned next to the CPU.
Replacing this gave me audio but no video so at least I knew the game was running which was great to hear.
At this point I tried the external A/V connector again and got a good picture.

Probing the ‘S-Enc’ chip yielded no outputs at all despite all inputs being as expected. I replaced this and everything came up good. Time to reassemble and reclaim some bench space.
Thats both of these rare units fixed up.


 Posted by at 10:21 am
May 212016

A couple of friends were looking to get their SF-1 SNES TV’s repaired. Not one for letting such gems go to ruin I offered to try and help out.
There were two units in need of repair, a 14″ and a 21″.

This log will focus on the 21″ version first.

On power up with a game fitted I got audio and a very faint greyed out picture.
I had been given an RGB scart cable with these so as they have the standard A/V output port on them I thought I’d give that a go.
I got this

As you can see the sync signal is off but I could see a nice colour picture. This gave me initial hope that the PPU’s were not to blame for the grey faint picture.
Taking this unit apart was a bit of a pain with the SNES part being sandwiched in at the top but I got it out and looked for anything obvious.

(Note that this PCB image is actually from the 14″ version. The 21″ version has a very similar PCB with only the connector location being located differently. I forgot to take a picture of that one.)

There was nothing obvious so start looking at the missing sync signal first.
The answer was fairly straight forward. The RGB cable I have here uses the composite signal for its sync. The SF-1 does not seem to output a composite signal at all instead it uses the combined HV sync signal on pin 3. Hooking this up instead gave me a nice picture on my test monitor which confirmed the SNES was working great.
So whats happened to my picture on the actual TV unit?
It turns out the SNES outputs separate Luma and Chroma signals to the TV for its picture and not RGB like we initially thought.

The connector marked ‘VD’ is the video signals. The black wires are all grounds and the white wires are signal. One of the wires however is the audio output. So there is Luma, Chroma and mono audio on this connector.

Now I know the signal isn’t RGB I can pretty much assume that the ‘S-ENC’ (BA6952F) is used to generate the signals i’m missing.
Using the scope I couldn’t find any valid signals coming from the chip. In my haste I was about to write the chip off but as the output signals were really weird I traced all of the pins. I found the GND on pin 2 wasn’t connected at all and it should have been. I tried reflowing the pin but still no connected so I added a jumper wire to the GND point next to it.

I was a little nervous about patching it as if the original track (that ran under the chip) had burnt out then what had happened to do that? It could have been a manufacturing defect, Ive seen a couple of those in my time but ground connections are generally quite big compared to signal traces. I didn’t want to remove the whole chip just to check for a burnt trace so I just check resistance between GND and VCC. It looked fine so I powered up to test.

An absolutely perfect picture! Seriously this picture is really good quality.
I tested all the controls and they seem to work fine too so time to box this up.

 Posted by at 8:18 pm
May 192016

Last year after a lot of research I succeded to buy at a good price an original board of Psychic 5 , one of my favourite game of all time, which was really in mint conditions.




Some days ago I picked it up for a play and after some minutes the game developed this problem during attract mode:





After some more time the game was total yellowish:



Game hasn’t any schematics available and there are not any games known to me which have similar hardware and schematics available.

I could only go blind and try to find the reason of the fault.

So I started to short some signals of srams looking for palette changings and I found a couple of them at pos.6N and 6M, 16kbits.

The one at pos.6N had the first 4 data pins low and I was pretty sure either it was faulty or the 74ls245 connected to the data bits.

After desoldering, the sram 2018 was tested good in the programmer. That meant that the programmers didn’t use all data pins of the palette rams. The 74ls245 was good because it could drive correctly the signals without the ram installed.

I tested some TTLs nearby looking for strange signals but found nothing.

Decided to go brute force using my logic comparator and testing everything on the video board but starting with FUJITSU parts which are known to be very unreliable.

The logic comparator found a 74ls08 with all the outputs faulty



I double checked the outputs with a logic probe and they were all in the grey area (no good logic signal).

After changing it I fix the problem 100%




So FUJITSU brand proves once again to be really unreliable!