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Old 6th November 2006, 11:56 AM   #1
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Default Worked first time-but not the second!

Fired up first channel of my Aleph 5 last night. No problems, hooked up a signal generator, nicely undistorted tones. Let it warm through for an hour, and saw DC offset of approx. 0.7V (plus the sweet smell of cheap speaker coil). Switched off as it was getting late in the day.

This morning, Q6-Q8 are warming up fine, but Q18-Q20 are stone cold and no sound, aaarrrggghhh!!!

A few measurments:

Vgs: Q1=3.5V, Q2=0.666V, Q3=3.55V, Q6-Q8=4.24V (each), Q18-Q20=0V.

Vce: Q4=9.46V, Q5=4.54V.

No voltage drop across source resistors from negative rail to Q18-Q20.

0.6V drop across source resistors to Q6-Q8.

Voltage across Z5=8.98V.

Voltage across R11=0V. (?)

No smoke, crackles, glowing noted from any part of the circuit.

DC offset this morning = Supply voltage!

R11 and R14 both measure okay when power is off.


Am I right in saying that Q1-Q3 will all need replacing? Any ideas what might have caused them to blow? I would really appreciate any help/guidance that anyone can offer.


It's soooo much more annoying knowing it was working last night.
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Old 6th November 2006, 12:55 PM   #2
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Here's some pic's:
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Old 6th November 2006, 12:58 PM   #3
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A child friendly power supply
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Old 6th November 2006, 01:01 PM   #4
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a family shot:
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Old 6th November 2006, 01:03 PM   #5
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A premature celebration
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Old 7th November 2006, 11:21 PM   #6
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Okay, so I just got back from work and tried it out again. Amp still doesn't warm up, but checked all the voltages again.

Vgs: Q1=3.5V, Q2=0.666V, Q3=3.55V, Q6-Q8=4.24V (each),

Q18-Q20=9.0V.

Vce: Q4=9.46V, Q5=4.54V.

0.6V drop across source resistors from negative rail to Q18-Q20.

0.6V drop across source resistors to Q6-Q8.

Voltage across Z5=8.98V.

Voltage across R11=5.5V.

DC offset still Supply voltage.

R11 and R14 both measure okay when power is off.

Any advice?

Cheers

Paul
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Old 7th November 2006, 11:53 PM   #7
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pull out little bjt critters and measure them

in your nomenclature ( I presume its from origigi A5 serMan ?)
Q1 and Q2 must have same gs voltage ; Q2 seems stuffed

besides- DC offset is full MINUS supply ?
because 9 V of Ugs for Q18-20 are twice needed;

replace both Q1 and Q2 ,and use that Variac from picture to bring it slowly;also put some 1E/2W in-line with both rails as convenient fuses and measure points during tests.

and- 8E power resistor is ,on long way-cheaper than 5 cheap spks

edit:
you have 9V across R14 just becuse Q1 must conduct current dedicated to Q2,besides his own current.
mebbe is JUST Q2 stuffed,but -in your boots- I would check pcb and solders for shorts and discontinuity..........all usuall mumbo jumbo

on the other side-with little trouble shooting adventure ,you'll certainly learn more about Alephs and electronics


EDIT II
seems that my fave board tech called Anatech is online-maybe he will giv'ya hand with your problem
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Old 8th November 2006, 07:41 PM   #8
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Thanks Zen Mod,

As you say, I have been using the original service manual. Got some matched pair irf's on order from Tarasque so I'll make a start on the other channel in the meantime.

I have got some load resistors somewhere, but I haven't seen them since my last house move. Sounds like a good excuse to get some more.

Quote:
DC offset is full MINUS supply ?
The difference (just remeasured) is 1.5V less than the minus supply.

Again many thanks.

Paul
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