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Inline light bulb variac


The Dude

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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's a good idea, I have a ac amp clamp I could use for this purpose.  

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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.

 

 

Bingo! I use a 1-1/2 amp quick blow and change it out to 4 or 5 for the "burn in" test (depending on the amp). "To each his own", I guess its not that much different than changing out light bulbs......

 

Duder- Back to the subject, what did you find?

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Duder- Back to the subject, what did you find?

 

Lets re evaluate the complaint or issue.  When plugged in and audio output from the left channel is way low, when you turn the gain up things get scratchy.  Now mind you that when I plug into the inline light bulb.  The dimness might be ok to you, but to me an amateur it might be a bit much.  So I pulled the left card and compared the brightness of the light bulb to when the card is in.  It all seems the same, so that go me thinking (after I rebuilt the whole card).  Maybe the problem I have isn't in the card but in the gain pot.  So I started doing some research and I came across another thread with the same problem.  So I dissected the pot. to find that a good portion of the contact material was worn down.  So I re assembled the pot and turn it past the bad part, turned my audio source down to not blow my speakers.  Now everything sound loud and clear.  So tonight I am going to run to rat shack and see if they have a 250k ohm volume pot to verify all of this.  If this is the case I will be on the search for some decent replacement pots for the gain. 

 

I wasn't going to post my findings until I proved myself, but what do you think.

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Is log taper similar to audio taper.  I picked up 100k ohm audio pot, I installed it and everything was working.  However I thought I should keep the right channel equal so I replaced that one to.  I all so replace a make shift 3.6k Ohm.  But now I have a problem, there is now a feedback sounding noise coming from the right side that wasn't there before.  Along with one of the heat sinks getting hotter then normal.  So back on the bench tomorrow to find the problem.  The good news of all of this is, I am all most there. 

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Your heat sinks getting hot are the transistors biasing on, thats a problem

See below definition of taper

 

Resistance–position relationship: "taper"

Size scaled 10k and 100k pots that combine traditional mountings and knob shafts with newer and smaller electrical assemblies. Note the "B" designating a linear taper.

The relationship between slider position and resistance, known as the "taper" or "law", is controlled by the manufacturer. In principle any relationship is possible, but for most purposes linear or logarithmic (aka "audio taper") potentiometers are sufficient.

A letter code may be used to identify which taper is used, but the letter code definitions are not standardized. Newer potentiometers will usually be marked with an 'A' for logarithmic taper or a 'B' for linear taper. Older potentiometers may be marked with an 'A' for linear taper, a 'C' for logarithmic taper or an 'F' for anti-logarithmic taper. The code used also varies between different manufacturers. When a percentage is referenced with a non-linear taper, it relates to the resistance value at the midpoint of the shaft rotation. A 10% log taper would therefore measure 10% of the total resistance at the midpoint of the rotation; i.e. 10% log taper on a 10K ohm potentiometer would yield 1K at the midpoint. The higher the percentage the steeper the log curve

Linear taper potentiometer

A linear taper potentiometer (linear describes the electrical characteristic of the device, not the geometry of the resistive element) has a resistive element of constant cross-section, resulting in a device where the resistance between the contact (wiper) and one end terminal is proportional to the distance between them. Linear taper potentiometers are used when the division ratio of the potentiometer must be proportional to the angle of shaft rotation (or slider position), for example, controls used for adjusting the centering of the display on an analog cathode-ray oscilloscope. Precision potentiometers have an accurate relationship between resistance and slider position.

Logarithmic potentiometer

A logarithmic taper potentiometer has a resistive element that either 'tapers' in from one end to the other, or is made from a material whose resistivity varies from one end to the other. This results in a device where output voltage is a logarithmic function of the slider position.

Most (cheaper) "log" potentiometers are not accurately logarithmic, but use two regions of different resistance (but constant resistivity) to approximate a logarithmic law. The two resistive tracks overlap at approximately 50% of the potentiometer rotation; this gives a stepwise logarithmic taper. A logarithmic potentiometer can also be simulated (not very accurately) with a linear one and an external resistor. True logarithmic potentiometers are significantly more expensive.

Logarithmic taper potentiometers are often used in connection with audio amplifiers as human perception of audio volume is logarithmic.

Edited by deafbykhorns
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Could resin cause a short, which ever. I cleaned up some of the traces, and even re flowed some solder. I don't how stable it is, but its running dead silent with no heat issues. I will continue to run some test tomorrow before putting the cover back. Once it all test out, I will then put int in my system to see how she runs. I wouldn't mind coming across a known good card to keep on hand. Well I guess time will tell.

Thanks for everybody's help.

Edited by duder1982
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What tests are you to perform?

 

Most basic is %THD v. power output for a range of fixed frequencies (20, 200, 2000, 20kHz) for each channel between, say 250mV to full output voltage, across a non-inductive 8Ohm load, with the load on both channels.

 

For vintage McIntosh spec is 0.2% max. typical  My experience is that if these traces look good, the unit will sound about as good as it will get.  On a properly functioning unit, %THD will drop to insignificant values as output power increases. 

 

I'll provide some plots for you to give some sense of the magnitudes of what's possible. 

Edited by John Warren
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What tests are you to perform

 

I don't have any real test equipment. But I need to put the covers back on, I find doing things one at time helps catch problems.  Once I put the covers all on and continue to run it through out the week. Then I feel safe putting it in my system,  I will.  

 

Most basic is %THD v. power output for a range of fixed frequencies (20, 200, 2000, 20kHz) for each channel between, say 250mV to full output voltage, across a non-inductive 8Ohm load, with the load on both channels
 

 

Could this be measured with a voltmeter, cause that I have.  Once thing I intend to get is a scope, once I find a decent one for the right price. 

 

I'll provide some plots for you to give some sense of the magnitudes of what's possible

Thank you. 

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Most basic is %THD v. power output for a range of fixed frequencies (20, 200, 2000, 20kHz) for each channel between, say 250mV to full output voltage, across a non-inductive 8Ohm load, with the load on both channels
 

 

Could this be measured with a voltmeter, cause that I have.  

 

You cannot measure harmonic distortion with a VOM(?).   But with two VOMs you can do a linearity check (Volts input/Volts output = constant to rated voltage output across a non-inductive load).  

 

There's dedicated hardware (audio analyzers) or separates (signal generator + spectrum analyzer).  Like everything else, when you have the tools, much more can be known.  

 

Once thing I intend to get is a scope, once I find a decent one for the right price. 

 

You get EXACTLY what you pay for with test instrumentation.  STAY AWAY from ANYTHING on EBAY!  ESPECIALLY "vintage" analog Tek.  Just assume it's ALL GARBAGE sold by liars and cheats and you'll be fine.  Look at new Rigol scopes, low cost. 

 

 

I'll provide some plots for you to give some sense of the magnitudes of what's possible

Thank you. 

 

Edited by John Warren
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A common problem with these vintage amps is line hum, 60Hz.  Though not always audible, it can be easily found and isolated by running a continuous FFT.

 

I did a complete overhaul on an MA6100 integrated and traced the hum back to a mechanical rivet that chassis grounded the center-tap of the transformer secondary that provided power to the output section of the amplifier.  By far, the single biggest change I could measure that was in the direction on goodness.   That, plus a new bridge rectifier, new PS caps, new output transistors, new caps, clamping diodes, etc...yielded additional gains in both %THD and %IM.  Reworking one channel power amp output board and comparing to the other, as-is board, provides insights to the swaps that are most effective in distortion and noise floor reduction.  

Edited by John Warren
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I had a Mac amp that I shut off the bias for one channel and it was making the output transformer get hot on one channel.

 

The diff pair was out-of-balance and the amp was putting a small amount of DC into the autoformer.

 

Just another something to check for.

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Djk, thanks for the input, thanks for the input from everyone. The diff input transistors took care of the problem. Well so it seems, I ran it for about 2 hours and no issues. I will run more tomorrow with it in my system even longer to confirm.

Next mc2100 I purchase with a issue will get a complete overhaul. Next on the list is replacing all the caps on the input along with the multi section can. Now that I know everything seems to be in working order.

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