Perry what do you think went first on this last blowout?Did the outputs go first and harm some of the power supply transistors?Or was it the opposite?Is it possible that maybe I'm seeing lower resistances in my power supply transistors because the outputs are bad?Or is the issue with the power supply unrelated?I'd hate to replace all of the parts for nothing but I know it may be necessary and definitely worth it for this amp.
Something in the audio section failed and caused the power supply to fail (if it has failed).
One thing that concerns me is the fault that you had that was causing the single transistor and resistor to overheat. It could be that it's an intermittant fault and is now causing the power supply transistors to read differently.
It's virtually impossible for the defective outputs to affect the readings on the power supply transistors.
Did you try to determine if only one output transistor failed in the group of four?
One thing that concerns me is the fault that you had that was causing the single transistor and resistor to overheat. It could be that it's an intermittant fault and is now causing the power supply transistors to read differently.
It's virtually impossible for the defective outputs to affect the readings on the power supply transistors.
Did you try to determine if only one output transistor failed in the group of four?
spooney said:
Now in car amps, the power supply fets are in banks of parallel devices. And because of this measuring one device is like measuring all in that bank of devices. This is where things get abit unique.
Anytime you have paralleled devices (like power supplies, and output stages) You just can't take a reading across any two leads as those leads are connected to other devices directly, so your reading everything all at once.
You should probably lift one of the leads out of circuit, and then measure off that center lead as a common for the other two leads of the device under test.
Happy New Year !
This morning I removed the 6491's and the 6488s from the output channel that had low resistance readings.None of these devices are shorted and the 6491's and the 6488's both have approximately the same resistance readings from pin 1 to pin 2 and from pin 1 to pin 3 .I had an unused 6491 laying around also that i measured resistance on for a comparison and it matched the ones I pulled from the amp.I also removed all of the transistors(6488's also) from the power supply section and had no shorted transistors.They all read about the same resistance wise (roughly 430 ohms) from pin 1 to pin 2 and from pin 1 to pin 3 just as the output transistors did except for two of them.Those two were not much lower at about 415 ohms.This leads me to believe they are either all dead (which is pretty unlikely I think) or that they are ok and the problem is somewhere else.
450 ohms is extremely low but if you tested an unused part and got the same reading, it must be your meter. For most meters, you'll read something in the mega-ohm range.
If the transistors read the same regardless of the meter lead position, they are defective. Placing the meter leads on the transistor one way should give continuity. Reversing the leads should show open for this type of transistor.
For most meters, placing the red lead on the first leg of the 6488 and the black lead on either of the other legs should show continuity. With the black lead on the first leg of the 6488, you should read an open circuit when the red lead is touched to either of the other legs.
For the 6491, the test is the same but you'll get continuity when the black lead is on the first leg.
Does the meter have a diode-check function?
Do you have a diode that you can test to see if it reads the same low value?
If the transistors read the same regardless of the meter lead position, they are defective. Placing the meter leads on the transistor one way should give continuity. Reversing the leads should show open for this type of transistor.
For most meters, placing the red lead on the first leg of the 6488 and the black lead on either of the other legs should show continuity. With the black lead on the first leg of the 6488, you should read an open circuit when the red lead is touched to either of the other legs.
For the 6491, the test is the same but you'll get continuity when the black lead is on the first leg.
Does the meter have a diode-check function?
Do you have a diode that you can test to see if it reads the same low value?
I really need a new meter.All I have is a cheap one from radioshack.I checked the unused 6491 as you stated and it checks out just like the ones I pulled from the amp.So maybe my meter is a little screwy. After removing all the output transistors I noticed I still had very different resistance readings from one channel to the next.The pads where the outer legs of the 6491s would sit gave me a reading of 60 ohms on one channel and 270 on the other.I did a little investigating down the circuit and I found two small shorted transistors.I removed the two transistors and it brought my resistance from the low channel right up with the other channel.
It could be that one (or both) of those transistors were defective. Many times, you'll have a transistor that's not completely shorted. They are simply 'leaky'. They act as if there is a resistor in parallel with the transistor. If they are not found, they can cause very serious damage when the amplifier is powered up.
Leaky transistors can cause the output to be distorted or cause the amp to draw excessive current. Sometimes, you'll have to load the amp and drive it relatively hard to see the distortion (using a scope). Other times, the distortion is so severe that you can easily hear it.
If you're going to buy a new meter, look at the Fluke true RMS meters. The low end meters are relatively inexpensive.
Leaky transistors can cause the output to be distorted or cause the amp to draw excessive current. Sometimes, you'll have to load the amp and drive it relatively hard to see the distortion (using a scope). Other times, the distortion is so severe that you can easily hear it.
If you're going to buy a new meter, look at the Fluke true RMS meters. The low end meters are relatively inexpensive.
I grabbed another meter I had laying around the house and re-checked all the transistors.I'm getting resistance readings more in-line with what you were talking about a few post ago Perry.The new 6491s read 4.8 mega ohms from pin one to pin two and read open with the leads reversed.The old 6491s that were in the amp all read 3.8 mega ohms from pin one to pin two.Would that big of a difference in resistance between the old and new 6491s cause a problem?I rechecked all the 6488s as well.Out of 25 6488's only eight are getting readings from pin one to pin 2 in the 4 mega ohm range.The rest all read in the 2.2 K ohm range.The other transistors I found are totally shorted.These might be hard to find as I have located one of them online yet.One of them is a MPSU07 and the other is a MPSU57.If anybody knows where I can find some suitable replacements for these I would appreciate it.
The 2.2k ohm transistors are defective.
The difference in the 4.8 meg and 3.8 meg is insignificant. On the resistance setting, the meter generates a voltage across the leads. When you connect it across it across a resistor, the voltage across the leads is pulled down. It calculates the resistance by using the voltage across the leads when the resistor is connected. When you check semiconductors on the resistance setting, the voltage across the leads is pulled down to the breakdown voltage of the semiconductor. The meter calculates the displayed resistance from that breakdown voltage. A slight difference in the breakdown voltage can result in a large difference in the resistance displayed. On 'diode check' it displays the voltage across the leads directly (no conversion to resistance).
Try http://www.universal-semi.com/ for the 07/57 transistors. If someone else has another source, please post it.
For testing (so you can check for other faults), you can use an MPSA06 in place of the 07 and an MPSA56 in place of the 57. They won't hold up under heavy loads but using them may prevent having to place multiple parts orders.
The difference in the 4.8 meg and 3.8 meg is insignificant. On the resistance setting, the meter generates a voltage across the leads. When you connect it across it across a resistor, the voltage across the leads is pulled down. It calculates the resistance by using the voltage across the leads when the resistor is connected. When you check semiconductors on the resistance setting, the voltage across the leads is pulled down to the breakdown voltage of the semiconductor. The meter calculates the displayed resistance from that breakdown voltage. A slight difference in the breakdown voltage can result in a large difference in the resistance displayed. On 'diode check' it displays the voltage across the leads directly (no conversion to resistance).
Try http://www.universal-semi.com/ for the 07/57 transistors. If someone else has another source, please post it.
For testing (so you can check for other faults), you can use an MPSA06 in place of the 07 and an MPSA56 in place of the 57. They won't hold up under heavy loads but using them may prevent having to place multiple parts orders.
Perry, when you say the mpsa06 and mpsa56 won't hold up under heavy loads are you saying they are only good for testing purposes?If I use those and I get to the point where I might see some output from the amp will I have to swap them all out before i can even try hooking up speaker to the amp?I'm not talking about dropping a two ohm load on it or anything just maybe running an 8 ohm speaker to verify that the amp does play and isn't distorted.
They will be OK into an 8 ohm load. They will likely be OK when driving a lower impedance load for short periods of time.
Remember that you need to have the amp mounted into the heatsink with the transistors clamped if you're going to run it into a speaker load for more than a few seconds. Even without a load, you should have it clamped to the sink when powered up.
When installing the clamping screws, be very careful to find the original threads. When you find the original threads, the screw will go in very easily. If it's not easy to screw in by hand, you're likely cutting new threads. This will eventually strip the threads from the mounting hole.
Remember that you need to have the amp mounted into the heatsink with the transistors clamped if you're going to run it into a speaker load for more than a few seconds. Even without a load, you should have it clamped to the sink when powered up.
When installing the clamping screws, be very careful to find the original threads. When you find the original threads, the screw will go in very easily. If it's not easy to screw in by hand, you're likely cutting new threads. This will eventually strip the threads from the mounting hole.
is there anything else that can replace the original parts?I mean like maybe something that isn't necessarily listed as a replacement but would do the same job as the other parts?As much as the original replacements cost its starting to get to the point where I don't know if I want to drop that much money on the amp to "maybe" fix it. At what point/dollar amount would you consider an amp unsalvageable?
Except for the 07/57, the original parts are all inexpensive. You can use the 06/56 for testing to determine if you've found all of the defective components. When you're confident it's back in good working order, then you can buy the more expensive 07/57.
Depending on the condition of the heatsink, a collector may pay good money for the amp so it may be worth repairing. If you post a good quality photo of the outside of the sink, I'll send it to a friend who collects old orions to see what he thinks it's worth.
Depending on the condition of the heatsink, a collector may pay good money for the amp so it may be worth repairing. If you post a good quality photo of the outside of the sink, I'll send it to a friend who collects old orions to see what he thinks it's worth.
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