1.5 Volts of offset is not a minor problem. When the unit goes in to that offset, other transistors start heating up which can cause a boom as well. It is just a matter of time to other things start going wrong. If it would be normal I would live with it. Since it is not I'll try to fix it.
Thanks guys.
Ricardo.
Thanks guys.
Ricardo.
Hi Ricardo,
That's the spirit! I am fairly sure you can fix it. Your other equipment is pretty good too.
EchoWars,
Don't worry, I didn't think you were suggesting that. I did get the feeling you hadn't heard one. I agree with you 100% that DC offset should be less than 50mV. You can calculate the offset expected by looking at the circuit. Some designs will settle as high as 250mV when the diff pair is balanced. Just poor design.
-Chris
That's the spirit! I am fairly sure you can fix it. Your other equipment is pretty good too.
EchoWars,
Don't worry, I didn't think you were suggesting that. I did get the feeling you hadn't heard one. I agree with you 100% that DC offset should be less than 50mV. You can calculate the offset expected by looking at the circuit. Some designs will settle as high as 250mV when the diff pair is balanced. Just poor design.
-Chris
Quote:
Some designs will settle as high as 250mV when the diff pair is balanced. Just poor design.
Hummm, I have serviced hundreds of different amps and never run across one yet with DC offset over 50mv as a factory setting.
If I did I would never let one out the door that way. I would have corrected the problem/design.
Some designs will settle as high as 250mV when the diff pair is balanced. Just poor design.
Hummm, I have serviced hundreds of different amps and never run across one yet with DC offset over 50mv as a factory setting.
If I did I would never let one out the door that way. I would have corrected the problem/design.
Re: Yes, but fixing something that is working is also dangerous.
What i'd say is use a plastic screwdriver (i've made up a few myself) for adjusting anything with the amp on, for prodding, adjusting, pushing, etc, NOTHING beats plastic.... I've used metal things before and put them through my hand (i know, i know, should have done it on the bench!) but one slip and you are looking at a holy-hand, or a black one if you hit the right wire!!!
The offset is a worry and i'd also like to hear how it's calculated... Although i do doubt even 0.5v is anything to worry about if it is a good amp and the other components are overspecced, if not, you'll run into troubles!
It sounds like cayo71 know's what he's doing, and if he doesn't, he can send it here and i'll keep it for him
Aaron
destroyer X said:
What i'd say is use a plastic screwdriver (i've made up a few myself) for adjusting anything with the amp on, for prodding, adjusting, pushing, etc, NOTHING beats plastic.... I've used metal things before and put them through my hand (i know, i know, should have done it on the bench!) but one slip and you are looking at a holy-hand, or a black one if you hit the right wire!!!
The offset is a worry and i'd also like to hear how it's calculated... Although i do doubt even 0.5v is anything to worry about if it is a good amp and the other components are overspecced, if not, you'll run into troubles!
It sounds like cayo71 know's what he's doing, and if he doesn't, he can send it here and i'll keep it for him
Aaron
Hi EchoWars,
Calculating the DC offset can be easy. If both bases return to ground and the feedback path with equal resistances. Figure out the tail current, divide that in two (we will assume the gains are equal, ie. matched). Divide that by the gain of one transistor and mutiply that by the base to ground resistance. This is the design offset voltage, positive or negative depending on whether the diff. pair is NPN or PNP. If the circuit has gain at DC, multiply that figure by the DC gain figure. This does not take into account the effect of a DC offset pot or DC servo. If the amp has a servo, measure the output pin to see how much correction is required. Normally, the lower this voltage is, the better balanced the amp is naturally.
burnedfingers,
I too will correct DC offset as a normal course. Two situations where I won't. 1. Correction of the design fault is a major undertaking (and sometimes not possible or worthwhile (given the cost of the amp) or 2. The repair is a warranty job from the manufacturer and the unit is within spec. (modifying the circuit will null the warranty, and get me in trouble).
Of course, if the customer is a cheap S*B I won't fix a design error, but I will note it. BTW, I have serviced well over a thousand amps. No kidding! I've used over three Weller stations to death in the process.
-Chris
Calculating the DC offset can be easy. If both bases return to ground and the feedback path with equal resistances. Figure out the tail current, divide that in two (we will assume the gains are equal, ie. matched). Divide that by the gain of one transistor and mutiply that by the base to ground resistance. This is the design offset voltage, positive or negative depending on whether the diff. pair is NPN or PNP. If the circuit has gain at DC, multiply that figure by the DC gain figure. This does not take into account the effect of a DC offset pot or DC servo. If the amp has a servo, measure the output pin to see how much correction is required. Normally, the lower this voltage is, the better balanced the amp is naturally.
burnedfingers,
I too will correct DC offset as a normal course. Two situations where I won't. 1. Correction of the design fault is a major undertaking (and sometimes not possible or worthwhile (given the cost of the amp) or 2. The repair is a warranty job from the manufacturer and the unit is within spec. (modifying the circuit will null the warranty, and get me in trouble).
Of course, if the customer is a cheap S*B I won't fix a design error, but I will note it. BTW, I have serviced well over a thousand amps. No kidding! I've used over three Weller stations to death in the process.
-Chris
I just found the forum today. You should check the varistors. They produce the bias to the transistors. I have the 300DC. Great amplifier. It developed a thermal problem on one side. The varistor (sorry I don't have the schematic in front of me) went bad and the output transistors got super hot! The thermal protection kicked in. Check the bias at the top and bottom pins that stick out on the top of the amp board. One should read .6V and the other -.6V
Ron
Ron
Chris,
As you may know, there are only about 7 electrolytis on the amp board. How did your repair go? I recaped mine when the thermal run away problem happened and it never sounded better. Also recap the power supply and peak indicator board.
Check all solder joints on each board. The amp can get quite hot and over time the solders dry out.
You'll be set for another 25 years.
As you may know, there are only about 7 electrolytis on the amp board. How did your repair go? I recaped mine when the thermal run away problem happened and it never sounded better. Also recap the power supply and peak indicator board.
Check all solder joints on each board. The amp can get quite hot and over time the solders dry out.
You'll be set for another 25 years.
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