Quite an attention getter!! Obviously there is an issue yet undiscovered since the original R506 failed and now this one. Thoughts or ideas?
Yeah; I have had that experience with R506!! sucker will also give you blisters for about 5 minutes after it explodes! AMHIK....THe last one I had that popped I could see it arcing inside the ceramic..
Does your Vishay replacement have the correct leads for the board? The ones I have ben getting need a little trimming and bending to fit. I bought about 5 just in case...
I have though a bit about that circuit, and the operation is as follows:
Referring to the schematic below, when the power switch is closed, AC current flows flows into J505, through the switch and in J507. From there it goes to the open contacts of the relay, where is stops. The only viable path is through R506, which goes out to the transformer via the little extra tab on top of the relay (Point 6 in the schematic). This current goes through the transformer,and get rectified by D501. This rectified current then goes throughout he RC circuit formed by R502 and C501. This takes a few seconds to charge up the capacitor, at which time the voltage is sufficient to trigger the relay. The relay then shorts out R506, dropping the current through it to basically zero.
So, if the relay doesn't open, then R506 will sit there with most of the amplifier current being drawn through it, and it will cook.
So it is important to figure out if the diode D501R502 and the relay are working. I did this with a power supply hooked across the AC end of the diode, and ground. As I ran the current up, I could hear the relay finally click on.
I think what happens when these amps get old is that the relay gets balky, and if it fails to close then the resistor cooks.
Good luck!
What you need is a variac and a shorting clip lead. That will help you run partial power to the amplifier, and with the resistor shorted it doesn't cook. You will see excessive current draw. It's technician time now.
-Chris
-Chris
The only reason for the excessive current draw through the resistor, however, is that the relay isn't closing..so find out why that is, and the problem is solved.
Variacs are pretty hard to come by!
Having owned a pair of GFA-565's for about thirty years now I am very familiar with the distinct audible "click" sound when the relay pulls in. I definitely did not hear that when this unit was powered up. Yes, a variac would be a nice tool to have but hard to justify for a hobbyist. Looks like $75 - $150 on eBay versus $.90 for these resistors. I suppose I could substitute a known good AC input board assembly to eliminate all components external to it and if that passes concentrate on the items as suggested by cogeniac.
The relay isn't pulling in due to excessive current draw. Short the resistor out so you can blow even less expensive fuses then.
All I can do is attempt to guide you. I was authorized warranty for Adcom and have fixed countless amplifiers over many years, the first of these when they first came out. That kinda trumps your experience I think.
A variac can be had for less than you state on Ebay. You quoted full list for a new one. You only need one rated for a couple amperes. You currently need this tool. There is a reason why technicians use them. This would be it. I could have suggested you use a variable dual tracking power supply to bring up the amplifiers alone (I have one), but that would not have been a helpful suggestion owing to the cost.
So either short the resistor and blow fuses while getting no closer to the fix, or spend some time on your browser and buy a damn variac. Once you have it, you'll use it for all kinds of things. They aren't that much money off Ebay.
-Chris
All I can do is attempt to guide you. I was authorized warranty for Adcom and have fixed countless amplifiers over many years, the first of these when they first came out. That kinda trumps your experience I think.
A variac can be had for less than you state on Ebay. You quoted full list for a new one. You only need one rated for a couple amperes. You currently need this tool. There is a reason why technicians use them. This would be it. I could have suggested you use a variable dual tracking power supply to bring up the amplifiers alone (I have one), but that would not have been a helpful suggestion owing to the cost.
So either short the resistor and blow fuses while getting no closer to the fix, or spend some time on your browser and buy a damn variac. Once you have it, you'll use it for all kinds of things. They aren't that much money off Ebay.
-Chris
Probably sage advice.. 🙂
I suppose another approach to finding the fault is to isolate the current draws. I would never short that resistor out. that's madness!
The relay may not be pulling in for two reasons. One is that the diode or the resistor feeding the relay are open. That would cause no current to the relay coil, so it would not close. The other, as Chris states, is because there is an excessive current draw from elsewhere in the system. I have seen this with a shorted output transistor.
As a quick check, you can disconnect the power lines to the output stages. Be sure to tape up the terminal ends!! If doing that, the relay turns on, then the problem is somewhere in the output stages, and the SS board should be OK (assuming you changed the resistor before trying this).
So I did manage to take a functioning AC input board and swap into my amp now affectionately known as "Lightning" and presto it too instantly opened the R506 resistor. Not in such dramatic fashion but it is open nonetheless so the root problem is definitely elsewhere in the amp. Should your suggestion of disconnecting the output stages be my next move?
Hi cogeniac,
All that resistor does is control the current surge at turn-on. You can short the resistor with a clip lead. That would create the higher inrush current, but that resistor is normally shorted by the relay shortly after turn-on. It's not like you would run the amplifier that way! This is just for testing purposes.
Hi maceoc3,
If you do use a variac (the only safe way to power something up gradually), you are advised to short that resistor with a clip lead. Many service manuals are clear on that point.
The series light bulb trick does limit the energy into a device in series with it, but the exact current is somewhat uncertain, and you can't easily increase the incoming power up to full power. Not without going to a lot of trouble with multiple sockets, light bulbs and switches. The light bulb thingy is for people in denial. It's just safer and easier to use the right equipment in the first place. A little patience and active searching on Ebay will probably net you a nice one. I got a B&K 1655 AC Power Supply for about $60. It's good for about 4 amperes, does voltage and current readings as well as AC leakage to the case of equipment too. That was darned cheap for what it does. Go look!
-Chris
How do you think we power one up with an open resistor?
All that resistor does is control the current surge at turn-on. You can short the resistor with a clip lead. That would create the higher inrush current, but that resistor is normally shorted by the relay shortly after turn-on. It's not like you would run the amplifier that way! This is just for testing purposes.
Hi maceoc3,
If you do use a variac (the only safe way to power something up gradually), you are advised to short that resistor with a clip lead. Many service manuals are clear on that point.
The series light bulb trick does limit the energy into a device in series with it, but the exact current is somewhat uncertain, and you can't easily increase the incoming power up to full power. Not without going to a lot of trouble with multiple sockets, light bulbs and switches. The light bulb thingy is for people in denial. It's just safer and easier to use the right equipment in the first place. A little patience and active searching on Ebay will probably net you a nice one. I got a B&K 1655 AC Power Supply for about $60. It's good for about 4 amperes, does voltage and current readings as well as AC leakage to the case of equipment too. That was darned cheap for what it does. Go look!
-Chris
Hi maceoc3,
Absolutely not! You have found exactly what I knew you were going to find.
Short the dead resistor - I don't care how you do it. If you must because you do not have a variac, set up a lamp socket in series with an outlet and stick a low wattage lamp in it. Plug the amp in and turn it on. The lamp will be near full brilliance I'm guessing. Measure from the speaker output terminal (Red) to each power supply rail in turn. One channel will show zero voltage difference between the speaker output and one of the supply voltages.
Alternate test. Power off and unplugged - discharged completely. Most DVMs will also check this using diode check (higher output current than resistance testing). Without knowing your meter I don't want to make any assumptions about it. Look for a very low to short reading between the speaker output and each power supply. Do make sure the speaker fuses are okay before trying this.
-Chris
Absolutely not! You have found exactly what I knew you were going to find.
Short the dead resistor - I don't care how you do it. If you must because you do not have a variac, set up a lamp socket in series with an outlet and stick a low wattage lamp in it. Plug the amp in and turn it on. The lamp will be near full brilliance I'm guessing. Measure from the speaker output terminal (Red) to each power supply rail in turn. One channel will show zero voltage difference between the speaker output and one of the supply voltages.
Alternate test. Power off and unplugged - discharged completely. Most DVMs will also check this using diode check (higher output current than resistance testing). Without knowing your meter I don't want to make any assumptions about it. Look for a very low to short reading between the speaker output and each power supply. Do make sure the speaker fuses are okay before trying this.
-Chris
I will do a more diligent search for a power supply. So as we're bringing the power up slowly are you thinking it would be able to get up to full power? Or are we just trying to provide enough power to perform some more diagnostics?
I have a Fluke 87 and a Fluke 12B... There is one B&K 1655 on eBay and it is in my state so I pulled the trigger on it. (that's rational, right?) If wife gets upset- it's anatech's fault!
Hi maceoc3,
Hey, I'll take the heat for that all day long! I'm so glad that you scored a good variac.
Added bonus I didn't mention. The 1655 also is an isolation transformer on top of being a variac. You bought yourself a lot of safety and convenience. How much did you spend on it?
The Fluke meters are great. The 87 is a fine meter. I used to be a calibration technician as well.
Okay, right now the 1655 will allow you to apply partial voltage to the Adcom. You can then do diagnostics without any stress or added smoke. The 1655 uses a breaker located on the rear panel.
My 1655 had an intermittent meter issue, which I fixed. It was a while ago, but if you run into that I can help you through that. It turned out to be a resistor.
-Chris
Hey, I'll take the heat for that all day long! I'm so glad that you scored a good variac.
Added bonus I didn't mention. The 1655 also is an isolation transformer on top of being a variac. You bought yourself a lot of safety and convenience. How much did you spend on it?
The Fluke meters are great. The 87 is a fine meter. I used to be a calibration technician as well.
Okay, right now the 1655 will allow you to apply partial voltage to the Adcom. You can then do diagnostics without any stress or added smoke. The 1655 uses a breaker located on the rear panel.
My 1655 had an intermittent meter issue, which I fixed. It was a while ago, but if you run into that I can help you through that. It turned out to be a resistor.
-Chris
$262 with shipping "new condition still in original factory box. Been reading the B&K manual, I'm kinda excited now! I have access to a couple of oscilloscopes but I am also bidding on a Rigol DS1052E so I can have one of my own.
I have a B&K 1655 and a Sencore PR57, both had meter problems, the germanium diodes are usually the problem for out of tolerance meter readings. The current meter let's you see what's going on without the use of DBT. I'd be lost without my Variacs! After everything checks out on one of those the DUT goes on the monster 20A Variac.
Craig
Craig
So, if the resistor is frying with a good board, then, as Anatech said, the amp is drawing too much current.
I have found this to be cause by one of two overall things:
A) The control board outputs are so messed up that they are causing both sides of the output circuits to be on at the same time. This is a Class AB amp, so there is a bit of this class A operation to be expected, but if it is improperly biased kit can cause the outputs to basically conduct through from one rail to the other.
B) One or both of the outputs has a shorted transistor.
One simple way to test this is to disconnect the plus and minus rails from the output (these are screwed on to metal straps on the output boards) stages and measure the resistance to ground. It should be fairly high (megohms, IIRC). If it is not, then you will know which side is shorted. You can then disconnect the wire that connects the two board on that side and narrow down where the short is.
As Anatech noted, if one transistor is shorted they should all be changed, although that's a tough call, given how difficult it is to find the transistors...
Another way to test things is to disconnect the wires from the control board to the output stages. If the amp powers up OK with the drive to the outputs disconnected, then this means the contra board is biasing the output in something resembling Class A.
I have found this to be cause by one of two overall things:
A) The control board outputs are so messed up that they are causing both sides of the output circuits to be on at the same time. This is a Class AB amp, so there is a bit of this class A operation to be expected, but if it is improperly biased kit can cause the outputs to basically conduct through from one rail to the other.
B) One or both of the outputs has a shorted transistor.
One simple way to test this is to disconnect the plus and minus rails from the output (these are screwed on to metal straps on the output boards) stages and measure the resistance to ground. It should be fairly high (megohms, IIRC). If it is not, then you will know which side is shorted. You can then disconnect the wire that connects the two board on that side and narrow down where the short is.
As Anatech noted, if one transistor is shorted they should all be changed, although that's a tough call, given how difficult it is to find the transistors...
Another way to test things is to disconnect the wires from the control board to the output stages. If the amp powers up OK with the drive to the outputs disconnected, then this means the contra board is biasing the output in something resembling Class A.
Hi cogeniac,
Use MJ2119x parts and you'll be absolutely fine. You can also use MJ1502(2/3/4/5) as well. Use an oscilloscope in case you have to adjust the compensation capacitor. Look for oscillation or bursts of oscillation on negative or positive peaks.
You can run into these problems even using the original transistors, that is one reason why an oscilloscope is considered to be a basic instrument that you must have.
When I got my 1655, it had two faults that were intermittent. One was an intermittent solder connection, the other was an out of tolerance resistor (way high). Harder to find than fix.
-Chris
Use MJ2119x parts and you'll be absolutely fine. You can also use MJ1502(2/3/4/5) as well. Use an oscilloscope in case you have to adjust the compensation capacitor. Look for oscillation or bursts of oscillation on negative or positive peaks.
You can run into these problems even using the original transistors, that is one reason why an oscilloscope is considered to be a basic instrument that you must have.
When I got my 1655, it had two faults that were intermittent. One was an intermittent solder connection, the other was an out of tolerance resistor (way high). Harder to find than fix.
-Chris
I have good Tektronix 60 MHz scope. I have used that a lot in troubleshooting these amps.
Do you need to match the output transistors?
Scott
Hi Scott,
I do. They tend to be much closer than transistors were even 15 years ago. They have their processes under much better control these days and I can only imagine that their yields are much better than at any time before now.
On Semi actually measured the differences between matched output transistors and not matched. They came up with a finding that vindicates what I have been doing for over 30 years now. Matching the output transistors can reduce the distortion in the output stage by a factor of 10! That's 20 dB less distortion that feedback doesn't have to deal with. That will for sure show up in THD testing and listening. Of course this assumes that you have also matched the input diff pair transistors so that the diff pair can do it's job.
Remember too, simulators assume perfect matches between transistors (and more perfect transistors than we can buy). Reducing distortion throughout the amplifier circuit always ends up with lower distortion readings. They may be small changes (due to feedback doing it's job), but they are there to be seen.
Best, Chris
I do. They tend to be much closer than transistors were even 15 years ago. They have their processes under much better control these days and I can only imagine that their yields are much better than at any time before now.
On Semi actually measured the differences between matched output transistors and not matched. They came up with a finding that vindicates what I have been doing for over 30 years now. Matching the output transistors can reduce the distortion in the output stage by a factor of 10! That's 20 dB less distortion that feedback doesn't have to deal with. That will for sure show up in THD testing and listening. Of course this assumes that you have also matched the input diff pair transistors so that the diff pair can do it's job.
Remember too, simulators assume perfect matches between transistors (and more perfect transistors than we can buy). Reducing distortion throughout the amplifier circuit always ends up with lower distortion readings. They may be small changes (due to feedback doing it's job), but they are there to be seen.
Best, Chris
So, do you set up a current source with a base current to set the collector current at about midpoint? I assume with 10 transistors, this would be about 500 mA each (to get around 175 watts into 8 ohms). SO, assuming a bet of about 75, then this would need to be about a 6 -7 mA base current...
I assume I would use the same circuit I used for the darlingtons, except changed to provide this higher base current.
I assume I would use the same circuit I used for the darlingtons, except changed to provide this higher base current.
Hi Scott,
I generally use a jig I made that holds 4 transistors at a time. I measure across a resistor in the collector circuit for current. The base current is supplied by a DC power supply through a resistor on each base. The jig allows me to get the transistors warm and at about the same temperature so that comparisons between transistors are valid. The power supply for collector voltage is heavier duty and can supply amperes up to 60 VDC. You set this voltage depending on how hot you want to run the transistors, and possible at the same collector voltages they will see in actual use if you want.
-Chris
I generally use a jig I made that holds 4 transistors at a time. I measure across a resistor in the collector circuit for current. The base current is supplied by a DC power supply through a resistor on each base. The jig allows me to get the transistors warm and at about the same temperature so that comparisons between transistors are valid. The power supply for collector voltage is heavier duty and can supply amperes up to 60 VDC. You set this voltage depending on how hot you want to run the transistors, and possible at the same collector voltages they will see in actual use if you want.
-Chris