Help with Ameritron AL-82

[townsend chris]

I could use some help troubleshooting my al-82. When switched on, tubes light, fan starts, but shows no high voltage, meter lights are not working either.
Amp is about 5 years old, and had started to act up, " kick in and out" a few times before it went out completely. After it sat about a month, it worked a few hours and then stopped again. No pops or bangs, or funny smells. Any advice , or ideas as to what to check first? I do have a little experience repairing my
SB-200, so I'm not completely green. Thanks in advance, any advice appreciated.

 

[SuperLid]

I could use some help troubleshooting my al-82. When switched on, tubes light, fan starts, but shows no high voltage, meter lights are not working either.
Amp is about 5 years old, and had started to act up, " kick in and out" a few times before it went out completely. After it sat about a month, it worked a few hours and then stopped again. No pops or bangs, or funny smells. Any advice , or ideas as to what to check first? I do have a little experience repairing my
SB-200, so I'm not completely green. Thanks in advance, any advice appreciated.

First you need to see high voltage and if you don't you need to figure out why. Do your troubleshooting with the tubes removed. I would start by checking opr/standby switch.

 

[townsend chris]

First you need to see high voltage and if you don't you need to figure out why. Do your troubleshooting with the tubes removed. I would start by checking opr/standby switch.

Thanks for the quick response. I have checked the opr/standby switch for continuity, and it checks out good. Also removed tubes. Any advice for tracing the path for high voltage. Can it be checked with a Fluke digital volt meter?

 

[SuperLid]

Thanks for the quick response. I have checked the opr/standby switch for continuity, and it checks out good. Also removed tubes. Any advice for tracing the path for high voltage. Can it be checked with a Fluke digital volt meter?

Did you bypass the interlock switch? You will need a high voltage probe to check the HV. I don't want to seem condescending, but this voltage you are working with, will kill you. Your questions leave me wondering, if this is something you should be trying to work on.

 

[townsend chris]

Did you bypass the interlock switch? You will need a high voltage probe to check the HV. I don't want to seem condescending, but this voltage you are working with, will kill you. Your questions leave me wondering, if this is something you should be trying to work on.

Yes, I have temporarily bypassed the safety interlock switch. I'm aware of the high voltage dangers.

 

[linearone]

Yes, I have temporarily bypassed the safety interlock switch. I'm aware of the high voltage dangers.

Dude. You can't measure 3kv with a fluke. If you have to ask how to do that you shouldn't be inside that box. No offense meant, but if you knew how to do this kind of work safely. You wouldn't ask these questions, you'd know what tools you need.
The hv circuit is fairly simple, their board is confusing but make sense when you look at a schematic. Do you have the manual? There is a full schematic in it.

 

[linearone]

Also, tbh, you can test most everything "cold", you really only need to have the box hot for final operation. Testing these components is best done cold.

 

[nomadradio]

There are two enclosed relays at the rear. One with two contact poles, at the outboard-rear corner of the circuit board. The other one plastered to the inside rear panel. It usually has just one visible contact pole.

Do both of them "click"? Can you see the armature move from off to on for both?

Both of them must close to power the big HV transformer. The small transformer that runs the filament and lights can power up even if the big HV transformer is shut down.

73

 

[townsend chris]

There are two enclosed relays at the rear. One with two contact poles, at the outboard-rear corner of the circuit board. The other one plastered to the inside rear panel. It usually has just one visible contact pole.

Do both of them "click"? Can you see the armature move from off to on for both?

Both of them must close to power the big HV transformer. The small transformer that runs the filament and lights can power up even if the big HV transformer is shut down.

73

No, both relays don't "click" . I found that the large 10 ohm 12 watt resistor attached to one of those relays is cracked and split. (R3) on the schematic. And for what ever reason mine is fit with a 20 ohm 10 watt resistor at that location. Someone mentioned it works as a soft start? I need to get one and will proceed from there.

 

[nomadradio]

The double-pole relay that goes "click" serves as the actual power switch. The switch on the front panel only turns on this relay.

The 10-Watt resistor is in series with the primary of the HV transformer. At first, the HV filter caps are empty and draw a surge of current as they charge up. The resistor serves to limit this surge as the caps charge up. A second relay with a 120-Volt AC coil is wired across the primary of the HV transformer. As the caps charge up, the voltage across this primary winding rises until it's high enough to close the second relay. The contact on this relay shorts across the start resistor and now delivers full voltage to the primary of the HV transformer.

Until something slows this sequence down. The surge resistor is only large enough to survive a brief surge of startup current. If you have any kind of overload in the HV circuit, like a shorted filter cap, this sequence will get slowed down. Slow it down enough and the resistor gets smoked.

A 20-ohm resistor serves to limit that pulse of inrush current down to about 12 Amps. If there were a dead short on the HV, the entire 240 Volts remains across the resistor. 12 Amps times 12 Amps times 20 ohms is 2880 Watts. More than enough to shatter a 10-Watt resistor.

Another way to smoke this resistor is to power up the amplifier slowly with a variable-voltage AC power supply (variac). Always good policy to check for this feature before you attempt the "safe" way to power up an amplifier.

Then again, if the contact points on the second relay are bad, it will fail to short across the resistor, and it will smoke only a little more slowly.

73

 

[suffolk]

Hello Guys....... I just purchased an AL82 when i plugged it in, it blew the fuses, i pulled the cover and compared wiring on terminal block to the owners manual, it was incorrect. I simply pulled the spade connectors off and rewired to what the owners manual says....... Now the amp powers on, the meters light up, But I have no High Voltage, the meters light up but show nothing, also just discovered the amp powers up with the cover removed !!! I have looked at timer relay and big ceramic resistor , nothing appears burned........ I cant help but think its something simple.

Can any of you point me in the right direction ??? This is an older 1990 AL82 with Peter Dahl Transformer Thank you in advance Will

 

[nomadradio]

big ceramic resistor , nothing appears burned........

I suggest pulling the plug from the wall and then checking that resistor for continuity. Ceramic-body resistors can become an open circuit while still looking perfectly okay. That resistor is the soft-start component. A short like the one you had can blow it out. If the soft-start resistor becomes an open circuit, the high voltage just won't start up.

73

 

[BJ radionut]

I am also curious to know how the plug and power transformer "were wrong"(?) and how the transformer and cord are wired now. Reason said: easy to have one wire/(jumper) in wrong location and be "dropping" a leg?
Don't ask me how I know!!!!

 

[linearone]

Hello Guys....... I just purchased an AL82 when i plugged it in, it blew the fuses, i pulled the cover and compared wiring on terminal block to the owners manual, it was incorrect. I simply pulled the spade connectors off and rewired to what the owners manual says....... Now the amp powers on, the meters light up, But I have no High Voltage, the meters light up but show nothing, also just discovered the amp powers up with the cover removed !!! I have looked at timer relay and big ceramic resistor , nothing appears burned........ I cant help but think its something simple.

Can any of you point me in the right direction ??? This is an older 1990 AL82 with Peter Dahl Transformer Thank you in advance Will

I think there is a singular diode all by itself on the metering board by the front. Usually they always break open when there are problems like this, or shorted tubes etc. check to see if its still there.

 

[suffolk]

I suggest pulling the plug from the wall and then checking that resistor for continuity. Ceramic-body resistors can become an open circuit while still looking perfectly okay. That resistor is the soft-start component. A short like the one you had can blow it out. If the soft-start resistor becomes an open circuit, the high voltage just won't start up.

73

Thank you for the input sir, I will be testing that resistor tomorrow for sure. I just acquired an AL1200 with the same transformer. If resistor tests good, i`m going to swap transformers, oh and yes i get high voltage on this AL1200 but only makes 300 to 500 watts, I think the tube is shot.