TT no sound

[guus.assmann]

Hello,

Why not socket the LMC1992 in the 1040STE?

Then you can use that to test the other IC's. Of course you need to be able to not destroy the 1040STE.

And also, put the suspects in the 1040 and not the LMC1992 from that in the TT. If the TT is the "killer" it won't be able to get the other chip.

It's highly unlikely that the 1040STE will be killed by a broken LMC.

 

On the TT, check all spots and traces that may have been in contact with the shielding.

 

And one more thing. Check the metal standoffs of the power supply.

They can short out tracks. Specially the one on the left/middle may touch a capacitor. (I mean the metal that's away from the back of the computer.)

 

BR/

Guus

Edited January 19, 2021 by guus.assmann
Removed typo

[Randy]

will check that out too guus!

[Randy]

One thing I have noted, the VME vertical board is not attached very well, it looks like the soldering tabs that were used to hold the VME bus board to the motherboard have broken loose, not the only thing that broke during shipment when I received this!.  It does appear that those two tabs are not connected on the VME bus, and there is no ground connection to the VME cage at that end either, only to the metalwork of the motherboard/chassis so there should not be any electrical/grounding issues that I can see.  I could take the motherboard out of the cabinet and resolder that VME support board, but am hesitant as putting the motherboard back into the cabinet is always a headache due to the reset switch position.  What do you think, do I really need to resolder the VME vertical support board back in place better, is it a problem possibly with the 1992 chip going bad?  Thanks!

[Randy]

OK, got the new 1992 chips, before installing one I removed the shield and speaker from the TT and check/resoldered all the 1992 socket connections and cleaned it up.  Then tried using an old 1992 chip to see what happens and I got output on the RCA jacks (via my scope), woo hoo!  As I plugged in the RCA's to external audio, nothing, no sound and no waveform out of pins 14 and 16, back to no sound again.  Not sure why it came up briefly and then died, really strange problem that I am now thinking is not related to 1992 being bad but something else.  A quick voltage check shows 8.4 v instead of 8.2 v, and an unbalance between pins 4 and 5, 4 shows the keyboard signal going in, but nothing on 5, also 4 is 3.7 v and 5 is 4.2 volts, the divider shows 4.15 on the 2 10 k resistors, but an imbalance on the 51 k divider as noted (3.7 and 4.2 v).  I mean WTF, what is going on here?  Any ideas?  All motherboard caps replaced and quadruple-checked for proper orientation, this is just not making any sense.  I do know on my STe which has a similar sound system the voltage at 4 and 5 are exactly the same as expected, this one has me quite flummoxed!

[Randy]

On 1/18/2021 at 8:39 PM, Chri O. said:

Try removing the VME bus shield and try again ?

 

I'm looking at the Atari TT datesheets (Page#24) and I noticed this:

2,4.10 VMEBUS

The VMEbus is provided to allow for system expansion and conforms to industy

standard VMEbus revision C.1. Interrupts IRQ1-IRQ7 are interfaced and arbitrated

through the SCU chip. A global 16 MHz clock is provided to the VME bus via the

TTVIDEO chip and a74LS44latch. This provides total independence of the clock from

the processor speed. A 24-bit address bus, l6bit data bus, and control signals are

provided to the VMEbus via the MC68030. This configuration causes the main system

board to act as the VMEbus controller when a card is installed in the bus slots.

 

The Atari TTVIDEO also controls the LMC1992 microwire interface I'm wondering if there's any type of short on the VME bus shield ?

 

Thanks Chris, I'll take a look into this at some time, probably not until next month as I have been so swamped at work I haven't been able to spend the necessary time needed to troubleshoot this problem.  I did remove the VME cover and speaker, but still no sound.  With all the 1992 pins giving the same waveforms out, the only thing I am seeing different from the usual is the imbalance on pins 4 and 26, that voltage divider looks fine on one end but not on pin 4, something must be off with that I am thinking.