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Inline light bulb variac


The Dude

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Ok, I recently had taken an electronics troubleshooting class.  Mainly just going over basic schematics with mainly wiring and connectors.  But it did go over current, resistance, and voltages.  Some basic stuff, but made it seem simple.  Anywho it kind of got me pumped to get back into this project.  So here is what I did just to get going.

 

 So here is the latest up date and wanted to see what you all think.  Last time I worked on this I was replacing resistors and checking voltages. Everything seemed to be going good until I replaced one of the last resistors on one of the boards.  Then my light bulb came back bright and I lost my voltages.  So I walked away for a bit.  Well tonight just to get back into this, I decide to plug it in.  Well I haven't checked voltages yet, but the bulb is very dim with both cards in.  Could I have just bumped something that could have caused the short until I unplugged it and recently plugged it back in.

 

As I am stuck in a hotel room I thought I would take some time to figure this out.  Tomorrow when its not so late, I will lay out what I learned to see if I can use it on stuff like this.

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Ok plugged it in, and starting taking some voltage readings.  Everything seems to be a bit on the tad side, pushing that 10 percent side.  Any clues on what to check would be greatly appreciated.  I am thinking something along the diodes, or resistors first in the series.

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What's the voltage at the base of q8 and q10

 

Those are on the right channel, so I want to take the time to state what I had done this evening.  I thought I would see what problems are still here.  So I hooked up an audio source and speakers to see what is going on.  The right channel seems to play fine, which is where q8 and q10 are located.  The left channel is very low output and scratchy.  The corresponding transistors q7 and q9 I will check, I was able to check the base voltage on q7 which was high compared to q8.  I will have to solder a lead to q9 in order to check it.   

 

Now going back and re reading what I have wrote in the past couple threads, it seems I might have been misleading my self(along with others). I now feel the problem is the in the left board which is one that still needs the complete rebuild(makes since some what). 

 

Now if I remember right when I repaired my MC2505 as some outputs went out, and had taken a emitter resistor with it.  Once I replaced the output there was music but scratchy music.  Once I located the bad resistor and replaced it, the amp worked.  So I feel this might be the problem, but then again I am no tech.  

 

Now deafbykorns, and my past experience got me a thinking.  I took at a look at q7 and q9 which share the emitter resistor with q11 and q13.  Which there is all so 2 other resistors in those circuits.  While I wait for the rest of the resistors to come in I will check the base voltage on q9.

 

What do think, take your time.  I know I am having fun with this and feel I am learning a thing or two.

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More than likely, replace both of these.

If you have access to a scope, it would be easy to find. Sometimes the smaller signal transistors dont test shorted but leak.

Sounds like you want to learn more so find yourself a cheap scope and use a CD player and a sine wave/test disc.

This will take away all the guess work.

 

Heres a great little article about testing transistors, again, this doesnt always work (on smaller signal transistors) as I mentioned above.

http://www.allaboutcircuits.com/vol_3/chpt_4/3.html

 

I use the diode tester on a smaller fluke, it has an audible beep that tells me the diode(transistor) test is good.

 

You should also try testing the good card against the bad card, this will help isolate the problem as well

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If you have access to a scope, it would be easy to find

 

I was talking to friend of mine about this last night, do you know what the irony is.  I sold some stuff to get some cash to buy a scope, but ended coming across this and purchasing it.Duh :blink: .  I really need to get one and learn how to use it, they seem like they could really help.  Especially since I like doing this kind of stuff.   Well I will get those transistors ordered, which hopefully will come in once I get back next week.  

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I gave away a couple of Variacs, a lightbulb works better.

Your dead wrong.....a variac has many uses and is overall a better way to monitor things assuming you had one with a meter. You also have an inrush of current with a cold bulb (lower resistance).

It's simply a poor mans variac and for the novice or occasional trouble shooters.

Edited by deafbykhorns
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"Your (sic) dead wrong"

 

Right.

 

Try and start a Crown DC300A with a variac, or any Carver with a mag field supply, etc.

 

Adcom GFA 555's with the common driver transistor problem will run fine until they reach 100V or so, then blow up in your face.

 

I gave away my Variacs a long time ago, you're a hoot!

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Ok, you win, the topology of those amps need a current limiter......

I should of said "most amps"....a variac is still a valuable tool for me.

BTW, I have repaired many M and PM series Carvers and who the heck uses a DC300?

I used my last one as a boat anchor.

There's a reason why they call it "DC" in the 300 if you know what I mean.

Edited by deafbykhorns
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Ok, you win, the topology of those amps need a current limiter......

I should of said "most amps"....a variac is still a valuable tool for me.

BTW, I have repaired many M and PM series Carvers and who the heck uses a DC300?

I used my last one as a boat anchor.

There's a reason why they call it "DC" in the 300 if you know what I mean.

 

I have a 10A, 1.4KVA variac.  I'll replace the 12A fuse in it with one of a smaller rating, say a 2 or 2.5A quick blow.  I also have a small length of power cord with the hot wire split out to allow a clamp type ammeter to be used to monitor the current drawn by the amplifier. 

 

That said, for McIntosh amps (70s vintage) I'll get at the bridge rectifier, pull the fuses, remove the ps filter caps, pull all boards and start.

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