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HH MA150 blowing mains fuses

Started by Melv1, September 30, 2017, 08:07:49 AM

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Enzo

Melv
Troubleshooting is a skill, an approach, and it doesn;t really matter whether it is tube or SS you are working on.  Yes or course the details are different, but it is all about isolating the problem.  Once you know WHERE a problem lies, it is usually pretty clear what needs to be done.

CLues are clues, resistors burn up in SS for the same reasons as in tube gear - excess current.

MY point is that you probably know more about troubleshooting SS than you realize.

Melv1

Cool! Thankyou Enzo, that was a bit of a confidence boost, and a clue I think. which are both rather needed in this case  ;). I will get to the bottom of this fault if it's the last bloody thing I do.  :duh

Melv1

Right then, I 've found the short causing the fuses to blow, it was R11 and R13 were shorting where they had been badly soldered.After rectifying this  I Tried the lamp limiter test and this time the bulb flickered for couple of seconds and went out, so all good there. I then turned on full power and no blowing fuses  :tu:. However I have full rail voltage at the output, that's 60vdc. Any ideas please?

g1

Post DC idle voltages for Q6&7, Q1&2.  No lamp limiter.

Melv1


Melv1

Ok I have read the voltages. Let me just point out there are two different sets of Q1 and Q2 , one set are on the board, and one set are the output transistors and are off board. I put the black lead of multimeter to ground and red lead at the Base, Collector and Emmitor at Q6,7 and Q1,2 (that is the output Q1,2), and all readings are the same at 60vdc. I then put the red lead to Q2 Collector on the board and suddenly R32 burned up and smoked. ???????

g1

Yes, I meant the output transistor Q1&2.
I think you must have had a burst of oscillation probing the Q2 on the board as R32 is a feedback loop component.
You say there is 60V on all those transistors.  I assume you mean positive DC?
Where is your -60V supply?

Melv1

Sorry I forgot to check if it was - or + volts, I think they were all plus but I will check in a mo.

Melv1

Right g1, I've took some more measurements and here they are all VDC
Q6 B=-58 C= +58 E = -58
Q7 B=-58 C=-58 E= -58
OUTPUTS
Q1 C=+59 B=-58 E=+58
Q2 C=-58 B=-58 E=-58.
I also checked the other transistors
Q1 B= -10(this was falling with time), C=+59 E=-2(steady).
Q2 B=+59 C=-58 E=+59
Q3 B+59 C=+59 E=+59
Q4 B=-57 C=-58 E=-58
Q5 B-57C-58 E=58
Also when I touch red lead to Q6 C something smoked around q1,q3,q2,c6 area but was hard to see what component as didn't smoke after that.
Any help would be much appreciated.

Melv1


g1

I'm not sure if you are making typos or what but it makes things confusing.
Q4 emitter and Q5 emitter are the same point.  How can they be different polarity voltages?
Same for output Q1 emitter and Q2 collector, unless R26 or R27 are open.

I'm assuming they are typos and the output is stuck to the negative rail.
The feedback loop puts DC to the 741 IC which makes it impossible to chase DC readings.
I'm not familiar enough with this type of amp to say whether the feedback loop can be disabled safely.
Perhaps someone else may have a suggestion.

Melv1

Well thanks for your help g1. Hmmm I'm pretty sure it wasn't a typo (I guess that means typing error?) So I will check those voltages again. Very strange.