*** M E M O R U M ***

Removed fuse, I think it drawn around 0,630amp.

I get around 14.8v at fuse

0.03ohm between R62 (left leg) and FS1 (down leg) nothing between R62 (left leg) and FS1 (upper LEG)
9.80ohm between R62 (right leg) and FS1 (down leg) nothing between R62 (Right leg) and FS1 (upper leg)
9.80ohm between R62 (left leg) and BAS at TIP2955
9.90ohm between R62 (RIGHT leg) and EMI at TIP2955
0.02ohm between 7805 ("output" leg) and COL at TIP2955

All above with FUSE F1 REMOVED and with power off...!!!

With F1 Present and power on I only get 5.25v at 7805 Input LEG. I remember it should be more ?

Those resistance readings look okay.

Thinking.....

Will add to this...

Mark

Okay, for reference, these are the voltages on one of my MTX machines:-
All are respect to 0V (I use the modulator case).

• 11.28V @ FS1 (R62 left leg, Q4 emitter)
• 10.6V @ R62 right leg (Q4 base and input to 7805)
• 4.8V @ 7805 output (Q4 collector)

If you subtract 10.6V from 11.28V you get 0.68V which is the emitter to base voltage for Q4 which is in the expected range

Do you have a meter with a transistor test function?

Other things to check are ZD2 (5.6V) mounted between the modulator and the 7812 regulator. In the event of the voltage on the +5V line going too high, it will conduct and then fail short circuit to try to save the chips powered from the +5V line.

Mark

A: 11.28V @ FS1 (R62 left leg, Q4 emitter)
B: 10.6V @ R62 right leg (Q4 base and input to 7805)
C: 4.8V @ 7805 output (Q4 collector)

A: I get 11.90v
B: I get 5.34v
C: I get 3.80v

between ZD2 I get 3.74v

It still looks like a failure of the power transistor Q4 TIP2955.

I will post a simple test circuit for testing TIP2955 transistors "out of circuit" later today

Mark

1024MAK wrote:It still looks like a failure of the power transistor Q4 TIP2955.

I will post a simple test circuit for testing TIP2955 transistors "out of circuit" later today

Mark

Oops, I never did remember to post up this circuit. Never mind

Anyway, to close out the power issue and the hot R62 resistor, the patient was brought to the UK by it's owner for Memofest 2013 (yesterday, Saturday 21st September).

"Cold tests" (meaning tests with the computer not powered) with a multimeter confirmed the same results that Crazyboss posted earlier. Measurements with the board powered were also similar

So as the symptoms still indicated a problem with the transistor that was in position Q4, type TIP2955, and Crazyboss had obtained a new replacement TIP2955 transistor, I powered up the soldering iron, removed the suspect transistor, and soldered in the replacement transistor.

Some further resistance checks to confirm that the insulators for the transistor/heatsink were okay and so to the power on...

Good so far, no magic smoke Even better, the meter revealed that the +5V was now within spec. The +12V was good, as was the -5V. And resistor R62 was no longer cooking on gas mark 9

So this problem was solved

However, this MTX board has another problem that we have so far not tracked down (yes, it defeated both Andy and myself)

When (no not if) there is further progress, I'll post up the details in a new thread.

Mark

PS, Crazyboss did not go away empty-handed thanks to our host