Van Diesel
User
- Credits
- 99CR
Greetings Greedies and Goodies!
Well, like London Buses . . . you wait for ages for some kind soul to entrust you with their beloved arcade PCBs . . . and then two turn up at the same time!
Not much to initially go on with these, although there were some tantalising bits of sticky labels attached to them detailing faults. From the looks of them, some UKVac member has already had their hands on these - Phil Morris (IronGiant).
Ok, let's connect them up and see what gives!
FIRST BOARD
Ok, this one had an intermittent reset issue which was a simple fix by cleaning the legs of the 8080. I did the EPROMs too, as they had tarnished/oxidised a little also.
Further testing . . . we are getting sounds but . . . Hmmm . . . the Invaders appear to be marching without pins!
How is that possible!
It mentioned on the sticker on the board that there was a bit-shift problem . . . yup, sounds about right . . . this will be an issue with the shifter circuitry on the audio board.
I got my HP Logic Comparator on the job (a recent purchase from member 'olonguet' - thank you, Sir) and tested the eight 151 multiplexers. All tested good, so the next likely culprit will probably be the flip-flops feeding in to them.
I checked the 174s and 175s in a similar manner. My comparator was whinging straight away that there was something wrong with some of the gates but this was caused by of a few of them having floating inputs (bad design!!).
There was however, a suspect . . . I checked the IC at B5 (a 74175) and pin 11 was permanently stuck low. I confirmed a possible fix with a piggy-backed chip. Yup - Faulty.
Replaced the IC with a turned-pin socket, put a new chip in and . . .
It's got legs! Or rather, THEY have . . . (insert some appropriate music by ZZ-Top or Art of Noise . . . )
A full test of the sounds on the board showed they were all good. This PCB repaired!
I decided to do a little cosmetic touching up of the solder mask . . . I love these boards and they should be taken care of . . .
Before . . .
And after . . .
A minor thing, but it makes it look a little better to my eyes.
SECOND BOARD
Ok, before powering this board on I noticed quite a number of dry-joints and 'near' dry-joints that I had to touch up. Also, and rather worryingly . . . I saw this . . .
Yikes! Whoever had their hands on this board previously had wrenched up a lot of the tracks in the process of replacing a RAM chip and had repaired the tracks in an . . . interesting way, let's say.
I won't knock them (the repairer) too much because a) I've wrenched a few tracks off in my time . . . especially on boards that have deteriorated or aged and b) after testing all the tracks for continuity or short-circuits, that part of the board all checked out to be fine.
It looks messy . . . but it works!
Still, I tidied it up a little bit . . . I wouldn't trust those fine wires not getting pulled off in transit . . . and I applied a layer of solder mask to complete the repair.
Then I powered the boards up and the game seems to run fine at this point with no resetting.
This particular board had a Braze multi-game adapter added to it which I don't care for to be honest . . . but at least this did give me a nice menu to test each individual sound on the board.
Which neatly takes me to the next fault - This board had no sound!
A quick check of a finger placed on the inputs and . . . No buzzing - The amplifier chip had died.
There is a huge amount of solder on this part of the board, and removing the chip using my temperature controlled heat gun removed it with about as much delicacy as a dentist pulling out a tooth!
I had to use a fair deal more force than I'd wish to but it came out and quite cleanly.
The original chip used on these boards is an LM377, a 2W audio power device.
These are still readily available but have been superceded at least twice by other devices. I chose to replace it with an LM1877 which is a better design and a drop-in replacement for the old IC type.
I added a dollop of thermpath on the top of the chip, whacked the heatsink back, fully soldered it on the back of the board and it was good to go - The sound was back!
Also, I replaced the small tarantula (sic) blue bead capacitor that decouples the power supply since it makes sense to replace parts that may cause future problems at this point.
Once again I tested all the sounds and they were all fine. Even so, I did change out one of the LM3900s for a replacement in a turned-pin socket as the original device looked a little rusted.
Although now repaired the rear of the board at this point looked a little worse for wear.
I tidied up as before with a painted layer of solder mask.
Relatively straight-forward repairs . . . I was expecting to at least have had to replace some of the memory, but they all checked out to be good. Both boards were given a full two-hour stress test, and were OK.
Two boards fully working once more!
I wish to express my very kind thanks to the customer, who will be receiving his box back with the added bonus of a surprise gift inside!
Van
Van Diesel2021-01-26 15:02:54
Well, like London Buses . . . you wait for ages for some kind soul to entrust you with their beloved arcade PCBs . . . and then two turn up at the same time!
Not much to initially go on with these, although there were some tantalising bits of sticky labels attached to them detailing faults. From the looks of them, some UKVac member has already had their hands on these - Phil Morris (IronGiant).
Ok, let's connect them up and see what gives!
FIRST BOARD
Ok, this one had an intermittent reset issue which was a simple fix by cleaning the legs of the 8080. I did the EPROMs too, as they had tarnished/oxidised a little also.
Further testing . . . we are getting sounds but . . . Hmmm . . . the Invaders appear to be marching without pins!
How is that possible!
It mentioned on the sticker on the board that there was a bit-shift problem . . . yup, sounds about right . . . this will be an issue with the shifter circuitry on the audio board.
I got my HP Logic Comparator on the job (a recent purchase from member 'olonguet' - thank you, Sir) and tested the eight 151 multiplexers. All tested good, so the next likely culprit will probably be the flip-flops feeding in to them.
I checked the 174s and 175s in a similar manner. My comparator was whinging straight away that there was something wrong with some of the gates but this was caused by of a few of them having floating inputs (bad design!!).
There was however, a suspect . . . I checked the IC at B5 (a 74175) and pin 11 was permanently stuck low. I confirmed a possible fix with a piggy-backed chip. Yup - Faulty.
Replaced the IC with a turned-pin socket, put a new chip in and . . .
It's got legs! Or rather, THEY have . . . (insert some appropriate music by ZZ-Top or Art of Noise . . . )
A full test of the sounds on the board showed they were all good. This PCB repaired!
I decided to do a little cosmetic touching up of the solder mask . . . I love these boards and they should be taken care of . . .
Before . . .
And after . . .
A minor thing, but it makes it look a little better to my eyes.
SECOND BOARD
Ok, before powering this board on I noticed quite a number of dry-joints and 'near' dry-joints that I had to touch up. Also, and rather worryingly . . . I saw this . . .
Yikes! Whoever had their hands on this board previously had wrenched up a lot of the tracks in the process of replacing a RAM chip and had repaired the tracks in an . . . interesting way, let's say.
I won't knock them (the repairer) too much because a) I've wrenched a few tracks off in my time . . . especially on boards that have deteriorated or aged and b) after testing all the tracks for continuity or short-circuits, that part of the board all checked out to be fine.
It looks messy . . . but it works!
Still, I tidied it up a little bit . . . I wouldn't trust those fine wires not getting pulled off in transit . . . and I applied a layer of solder mask to complete the repair.
Then I powered the boards up and the game seems to run fine at this point with no resetting.
This particular board had a Braze multi-game adapter added to it which I don't care for to be honest . . . but at least this did give me a nice menu to test each individual sound on the board.
Which neatly takes me to the next fault - This board had no sound!
A quick check of a finger placed on the inputs and . . . No buzzing - The amplifier chip had died.
There is a huge amount of solder on this part of the board, and removing the chip using my temperature controlled heat gun removed it with about as much delicacy as a dentist pulling out a tooth!
I had to use a fair deal more force than I'd wish to but it came out and quite cleanly.
The original chip used on these boards is an LM377, a 2W audio power device.
These are still readily available but have been superceded at least twice by other devices. I chose to replace it with an LM1877 which is a better design and a drop-in replacement for the old IC type.
I added a dollop of thermpath on the top of the chip, whacked the heatsink back, fully soldered it on the back of the board and it was good to go - The sound was back!
Also, I replaced the small tarantula (sic) blue bead capacitor that decouples the power supply since it makes sense to replace parts that may cause future problems at this point.
Once again I tested all the sounds and they were all fine. Even so, I did change out one of the LM3900s for a replacement in a turned-pin socket as the original device looked a little rusted.
Although now repaired the rear of the board at this point looked a little worse for wear.
I tidied up as before with a painted layer of solder mask.
Relatively straight-forward repairs . . . I was expecting to at least have had to replace some of the memory, but they all checked out to be good. Both boards were given a full two-hour stress test, and were OK.
Two boards fully working once more!
I wish to express my very kind thanks to the customer, who will be receiving his box back with the added bonus of a surprise gift inside!
Van
Van Diesel2021-01-26 15:02:54
