Lamp row out -FIXED

[jpd]

A row of lamps on my FT has stopped working all of a sudden (14,24,34,44 etc). I've replaced the TIP 102 Q87 and measured the sorrounding resistors and checked continuity on the board traces to J-133 and seem ok. Could my next step be a 74LS74 (U12) ???. No other lights have problems and it looked like someone has replaced the TIP 102 before me as well. Thanks for any input.

[AskJacob]

Have you checked for shorts before plugging it back together? Occasionally these transistors do let go, but more often they are taken out by a short. If the short is still there you are back to square 1.

If you need a hand and find it is the 74ls74 I'm happy to help out.

Cheers
Jacob

[jpd]

Thanks AJ, I gather you mean check for a wiring short? .I'll have a look around under the PF tommorrow. I had to replace a chip and socket in my Gilligans awhile back for a similar problem but I induced that error myself with a screwdriver short while the power was on -oops!!!. I don't remember inducing this one but I am getting older 

[AskJacob]

Just a hunch, especially if it looks like the transistor has been replaced before...

It may be a very intermittent and hard to find break/short, but at least you know what colour wire to follow...

Cheers
Jacob

[Marty Machine]

Besides the suss tranny, how does the connector look? is that 'row pin' corroded/burnt out?
(maybe pull ALL the lamps out on that row, 1 of them might still be shorted in it's filament).

Might also be worth wiggling the lamp matrix connector and note of the row comes back or not.

Also, pull the IC out, and FORCE the ic socket pin hi/lo that drives into the tranny.
This will prove that the whole tranny/connector/wiring/lamps are all OK, then you can look backwards to the suss 74ls74 IC etc....

MM.

[jpd]

Thanks MM, The short theory and removing the globes sounds like the way to go 1st. I don't really follow what you mean about the IC and forcing the socket hi/lo though??

[Marty Machine]

by 'Forcing' a pin hi (+5v) or low (0v), you are simulating what the 7474 pin would be doing to the transistor driver.
You need a small wire or clip-lead from 5v (or 0v) and attach it onto the IC socket pin where the 7474 used to be.
(the 7474 MUST be removed, while you're FORCING pins, or you risk damaging the 7474 output pins !!)

IF the transistor turns on and brings on the lamps, then you've proven everything from that pin/socket onwards is ok, and the problem is now the 7474 chip, or the signal that goes into the 7474.

Since you've replaced the Tranny, it's More than likely a shorted globe or connector issue, it pretty much always is....

MM.

[jpd]

Thanks for the input Askjacob & Martymachine . Problem is now fixed.I tested the "return" voltage from the globes and circut at the J-133 plug and the offending "row" had none at the corresponding terminal at J-133. Raised the playfield to check voltage at the various board plugs and had no V. Checked continuity between all these plugs and that was fine. I went searching through the loom from J-133 to under the P/F and found the broken wire in the loom as it lay next to the speaker grille(possibly been caught at some stage??).Soldered and heat shrunk. Many thanks for the reminder guys to always check the simplest things 1st, cheers Jeff

[Marty Machine]

Nice One mate!! sneaky wires hehehehe.

Well done!! 

[AskJacob]

Great news, and even better you found out why the transistor blew... the speaker grill probably shorted it to ground.

Nice to see you found it relatively easy too.

Cheers
Jacob

[jpd]

Ah yes - the thrill of the hunt  , It's always satisfying in this hobby when you've sussed the culprit out.

[Marty Machine]

Nuthin' better than the "i fixed that" thrill, while playing a game.....Enjoy!

[Extra Ball]

Hey Jeff funny how the answer to my problem was here, but I didnt recall this thread, and you didnt think it would relate to my issue! Its all good mate