During my Hypersport’s repair I discovered a problem with the display of my Commodore 1084S monitor. Notice blue missing for “3 LONG HORSE” and “7 POLE VAULT”.
This will be the third time I’ve performed a repair on this old monitor which I’ve had since the early 90s for my Amiga 500 and 2000. At first I would suspect the AV7000 Supergun as the culprit. But subsequent tests would prove otherwise.
There’s definitely a better picture with composite or svideo from my c64 and Atari 8 bits but there seems to be a problem showing blue on the RGB input even though some blue is there ( turquoise , cyan.. etc ) in other images. Adjusting the blue gun and turning up the contrast just makes the backgrounds in Puzznic more blue. Where blue is supposed to be on the screen, it seems to not be present as though the signal were being cancelled out. With no RGB signal to the monitor at all, it defaults to a blue screen instead of a white one.
Apologies for the sync bands.
I suspected the neck board transistors, so I pull out the one responsible for blue and tests with my multimeter show it’s fine. I even swap the transistors around to see if the problem would move to the other gun…. but it doesn’t.
I found this really good post on eab [ English Amiga Board ] by Loedown
IC502 TDA 3505 pins 12, 13, 14 are RGB in from Euro Connector /SCART, check voltages there to begin with when inputting a perfectly white screen. Then follow the voltages through to transistors TS604 – 606.
The schematic shows the analogue inputs go directly from the RGB connector to the Philips TDA3505 IC via some 100nf caps. Pins 12,13 & 14 are the input RGB signals. I measured 4.3v for red and green but 9.1v for blue. That seems a little too high for blue and the schematic indicates 4v for those signals.
I order some TDA3505s on eBay from atarifreakz and my package arrives a few weeks later.
Not wasting any time. I replace the chip and the picture is now perfect!
The Taito logo and blue shadow around the Puzznic logo is now visible. Notice the bricks and backgrounds are also showing up right compared with the previous screenshot.
Got this Michael Jackson’s MoonWalker PCB (Sega System18) for repair:
Board booted but sound was missing at all and game behaved strangely, you could no longer move your character whenever you pressed the buttons or being hit by enemies :
At first glance I thought the FD1094 custom CPU module was slightly suicided so I swapped it with a good one along with the i8751 MCU but with no luck.I began to think of a correlation between the lack of sound and freeze.Probing the Z80 sound CPU revealed /HALT line was asserted while the /INT was floating.The latter comes from one of the two YM3438 FM sound generator @IC88 :
I removed the chip and tested it on a good motherboard, it was bad.But replacing it made no difference, the shared data bus of the two YM3438 was still inactive.So I pulled the other one @IC87:
This is the eleventh repair log for Rainbow Islands, almost a record for a board…
PCB was in fair conditions :
But the TC0070RGB DAC module was missing and its solderside was brutally burnt during removal :
Hence, for first I soldered back a spare module and patched a burnt trace:
On power up I was greeted by a blank static screen, /HALT and /RESET lines of 68000 were continuously going high/low sign that the watchdog circuit was active:
Usually this means that main code is not properly executed due some problems in CPU/RAM/ROM circuitry.For first I focused on WORK RAMs, two Sony CXK6864 (6264 pin to pin compatible).When I piggyback the one @IC26 I got this screen:
The watchdog shut down too, this lead me to remove the chip, it failed when tested out of circuit :
Now I got always that garbage screen.Probing the two 62256 RAMs @IC-IC4 in tilemap generation circuit revealed unhealthy signals on some data lines (good signal on the left for comparison)
Piggkybacking both RAMs at same time allowed the game to boot although with scrambled backgrounds:
I desoldered the RAMs having confirm they were both bad:
Graphics was perfect now but sound was faint and scratchy:
I used my audio probe to troubleshoot the sound circuit.This revealed that sound came out from PIN12 (which is the analog output to the TL074 OP-AMP) of the YM3012 DAC already corrupted.Indeed I could measure on this pin a weak voltage :
But a bad output means also a bad input and PIN11 (the analog input) was not healthy too:
This came from pin 8 of the near TL074 OP-AMP (through a 560 Ohm resistor)
I pulled it and it resulted faulty when tested :
Fitted a good OP-AMP restored the sound and fixed completely the board.
This fantastic monitor has been on my test bench for as long as i’ve been in this hobby and its been a great little work horse.
I powered it up the other day to test something and noticed the upper half of the screen had what I would describe as thick scanlines whereas to lower half of the screen looked a little compressed.
I hooked up my NES to it and the problem was a lot more apparent
Keep you eye on Luigi when he jumps to the top of the screen. As he moves up he gets longer
Asking a couple of people quickly, they all recommended changing the capacitors. The ‘shotgun’ approach isn’t really something I like doing as I like to understand what would cause issues before I randomly poke around but monitors are not my strong point so I started ESR testing in-circuit.
All the capacitors I checked were perfect with really low ESR values and all the capacitance readings were within spec. All except for one little capacitor, C306. This is a 47uF 25v capacitor and is connected to pin 12 of the vertical deflection circuit (TDA1670A)
I tested the capacitor out of circuit and confirmed for sure it was actually bad.
With the monitor taken apart I hooked up the Muter BMR95 to the tube to check the condition of the guns
A tube can usually be identified by the label that is on the back of the tube
E2940B22 is the one we want. Next we cross reference this to the supplied manual to find which adapter we need to use.
Adapter 808 is the one we need for this tube.
All hooked up
The guns aren’t too bad and certainly no need to attempt any cleaning or rejuvenating. Sometimes this can cause more harm than good. I have a spreadsheet prepared too as outlined in the BMR 95 manual to give a better indication of the life expectancy of a tube but i’ve not included it here.
With all that done it was time to test the monitor again
All seems to be fixed again and im happy with that.
You can see the stretching better with a picture comparisson
Never underestimate the importance of a single capacitor!