Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
Stew Based on your description my expectation is that you had a voltage breakdown of the bleeder resistor. While we normally think of resistors in terms of the power they can dissipate, there is a voltage limit that can be sustained across the resistor as well as from the body to ground. I learned this the hard way when I had a transformer fail and ended up replacing it with a P. Dahl transformer with a 10% higher voltage. (3100 V no load, 2950 V load). In that case removed the 2 50K bleeders and installed 10 20k 20W resistors mounted on standoffs, lost a couple of those due to voltage punch through from resistor to chassis before understanding the issue and properly insulating the resistor bodies from the chassis. These particular resistors had about a 2KV breakdown limit through the ceramic insulator to the resistor surface. I don't remember what the actual voltage limit across the resistor was but is the reason that I used the 10 devices in order to keep the drop in the 300V range per resistor as well as keeping the power dissipation in the 5W range, I think around 1K volts was the voltage breakdown limit for the devices I had on hand, but not sure (voltage limit due to power is 630V) . The original Drake 50K 50W bleeders obviously could sustain in the 1500V range across the device, not sure what their rating to ground was, but the power limit is at 1.581KV which I think is pushing them more than needed. The L4PS chassis is not at ground until you hook to the amplifier. So if you have a resistor body that can flash over to ground it will not be apparent until you hook up to the amp. My initial mounting arrangement had the resistors heat sunk to the chassis which turned out to be a dumb idea and was corrected by mounting the resistors on standoffs. I would test the supply out on the bench, worked great then hook up to the amp and experienced quite a light show as the resistors arced to chassis. The above transformer change also required adjusting the bias level due to operating point shift which was done by placing 2 5.6 V zeners in series with filament center tap. This has been working fine now for 8 years. 73 de K5MWR Date: Mon, 20 Feb 2012 07:26:18 -0800 (PST) From: GALE STEWARDk...@yahoo.com To: Dino Papask...@cox.net, Mail List - Drake Gear Drake Gear drakelist@zerobeat.net Subject: Re: [Drakelist] L-4B/L7 Power Supply Rebuilds Message-ID: 1329751578.83763.yahoomail...@web110514.mail.gq1.yahoo.com Content-Type: text/plain; charset=iso-8859-1 A timely question about the L4/L7 bleeder resistors. I recently had an issue with one of my L4PS supplies. It turned out to be a bad cap which I replaced. I'm going to install a Heathkit Shop board eventually but I wanted to get the amp back on line for use in the ARRL CW DX contest. While the supply was open I checked the 50W bleeder resistors and found them both open. I replaced both with new Ohmite resistors. Sunday morning at about 1230Z, one of the new bleeders failed in dramatic fashion by lighting up like a 100W bulb. I quickly swapped in another L4PS and continued on. Last night after the contest I opened up the failed supply to find that the resistors had failed but all other components were all OK (no bad caps, etc.). I replaced the failed bleeder (again) and all was again normal. I'm still quite puzzled as to why this resistor failed in the first place. GL with the project. 73, Stew K3ND ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
As a side note, a properly wired L4, L4B, or L7 power supply chassis is ALWAYS at ground potential for safety reasons. The safety ground from the outlet goes directly to the chassis of the power supply, whether or not the RF deck is plugged into it or not. A number of years ago, there was a production flaw in the Dale resistors (pitting, gaps, etc. ) in the ceramic that led to flashover on the ends of the resistor. Nit: the L4 power supply used two 100K 50 watt resistors while L4B/L7 supplies typically used 50K 50 watt resistors. -Original Message- From: David Box david_...@comcast.net To: k3nd k...@yahoo.com Cc: drakelist drakelist@zerobeat.net Sent: Mon, Feb 20, 2012 12:30 pm Subject: Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds Stew Based on your description my expectation is that you had a voltage breakdown of the bleeder resistor. While we normally think of resistors in terms of the power they can dissipate, there is a voltage limit that can be sustained across the resistor as well as from the body to ground. I learned this the hard way when I had a transformer fail and ended up replacing it with a P. Dahl transformer with a 10% higher voltage. (3100 V no load, 2950 V load). In that case removed the 2 50K bleeders and installed 10 20k 20W resistors mounted on standoffs, lost a couple of those due to voltage punch through from resistor to chassis before understanding the issue and properly insulating the resistor bodies from the chassis. These particular resistors had about a 2KV breakdown limit through the ceramic insulator to the resistor surface. I don't remember what the actual voltage limit across the resistor was but is the reason that I used the 10 devices in order to keep the drop in the 300V range per resistor as well as keeping the power dissipation in the 5W range, I think around 1K volts was the voltage breakdown limit for the devices I had on hand, but not sure (voltage limit due to power is 630V) . The original Drake 50K 50W bleeders obviously could sustain in the 1500V range across the device, not sure what their rating to ground was, but the power limit is at 1.581KV which I think is pushing them more than needed. The L4PS chassis is not at ground until you hook to the amplifier. So if you have a resistor body that can flash over to ground it will not be apparent until you hook up to the amp. My initial mounting arrangement had the resistors heat sunk to the chassis which turned out to be a dumb idea and was corrected by mounting the resistors on standoffs. I would test the supply out on the bench, worked great then hook up to the amp and experienced quite a light show as the resistors arced to chassis. The above transformer change also required adjusting the bias level due to operating point shift which was done by placing 2 5.6 V zeners in series with filament center tap. This has been working fine now for 8 years. 73 de K5MWR Date: Mon, 20 Feb 2012 07:26:18 -0800 (PST) From: GALE STEWARDk...@yahoo.com To: Dino Papask...@cox.net, Mail List - Drake Gear Drake Gear drakelist@zerobeat.net Subject: Re: [Drakelist] L-4B/L7 Power Supply Rebuilds Message-ID: 1329751578.83763.yahoomail...@web110514.mail.gq1.yahoo.com Content-Type: text/plain; charset=iso-8859-1 A timely question about the L4/L7 bleeder resistors. I recently had an issue with one of my L4PS supplies. It turned out to be a bad cap which I replaced. I'm going to install a Heathkit Shop board eventually but I wanted to get the amp back on line for use in the ARRL CW DX contest. While the supply was open I checked the 50W bleeder resistors and found them both open. I replaced both with new Ohmite resistors. Sunday morning at about 1230Z, one of the new bleeders failed in dramatic fashion by lighting up like a 100W bulb. I quickly swapped in another L4PS and continued on. Last night after the contest I opened up the failed supply to find that the resistors had failed but all other components were all OK (no bad caps, etc.). I replaced the failed bleeder (again) and all was again normal. I'm still quite puzzled as to why this resistor failed in the first place. GL with the project. 73, Stew K3ND ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
Would agree with you about the input AC power, but if you look at the high voltage the high voltage return side is not tied to the PS chassis but runs to Pin 6 of the power connector and then in the amplifier where it is only tied directly to chassis when the interlock switch is engaged. The interlock switch also shorts the HV to amplifier chassis ground. Under normal operation the high voltage return feeds through the plate current meter and to the center-tap of the filament transformer with a 1ohm 10W resistor to ground, this 1 ohm resistor forms the voltage necessary for the grid current measurement as the grids are tied to chassis and the voltage across this resistor is only due to grid current. This metering arrangement for both plate current and grid require that the HV return not be tied to chassis in either the PS or amplifier. Pin 7 of the power connector is used to tie the PS and Amplifier chassis together. So when you are using just the supply the chassis is at AC power ground but the high voltage return is floating with respect to chassis. With the cable connected to the amplifier there is the meter resistance in series with 1 ohm resistor to ground which is the change that does result in the PS chassis referenced to HV and therefore you can arc the bleeders if you do something stupid like I did of not providing a good HV isolation mount for the bleeders. Playing with these units is not something to be done without a lot of prior thought and making sure you really understand the entire circuit and safety precautions needed. I was probably lucky in not destroying the meter and other parts. 73 de K5MWR On 2/20/2012 11:08 AM, K9sqg wrote: As a side note, a properly wired L4, L4B, or L7 power supply chassis is ALWAYS at ground potential for safety reasons. The safety ground from the outlet goes directly to the chassis of the power supply, whether or not the RF deck is plugged into it or not. A number of years ago, there was a production flaw in the Dale resistors (pitting, gaps, etc. ) in the ceramic that led to flashover on the ends of the resistor. Nit: the L4 power supply used two 100K 50 watt resistors while L4B/L7 supplies typically used 50K 50 watt resistors. -Original Message- From: David Box david_...@comcast.net To: k3nd k...@yahoo.com Cc: drakelist drakelist@zerobeat.net Sent: Mon, Feb 20, 2012 12:30 pm Subject: Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds Stew Based on your description my expectation is that you had a voltage breakdown of the bleeder resistor. While we normally think of resistors in terms of the power they can dissipate, there is a voltage limit that can be sustained across the resistor as well as from the body to ground. I learned this the hard way when I had a transformer fail and ended up replacing it with a P. Dahl transformer with a 10% higher voltage. (3100 V no load, 2950 V load). In that case removed the 2 50K bleeders and installed 10 20k 20W resistors mounted on standoffs, lost a couple of those due to voltage punch through from resistor to chassis before understanding the issue and properly insulating the resistor bodies from the chassis. These particular resistors had about a 2KV breakdown limit through the ceramic insulator to the resistor surface. I don't remember what the actual voltage limit across the resistor was but is the reason that I used the 10 devices in order to keep the drop in the 300V range per resistor as well as keeping the power dissipation in the 5W range, I think around 1K volts was the voltage breakdown limit for the devices I had on hand, but not sure (voltage limit due to power is 630V) . The original Drake 50K 50W bleeders obviously could sustain in the 1500V range across the device, not sure what their rating to ground was, but the power limit is at 1.581KV which I think is pushing them more than needed. The L4PS chassis is not at ground until you hook to the amplifier. So if you have a resistor body that can flash over to ground it will not be apparent until you hook up to the amp. My initial mounting arrangement had the resistors heat sunk to the chassis which turned out to be a dumb idea and was corrected by mounting the resistors on standoffs. I would test the supply out on the bench, worked great then hook up to the amp and experienced quite a light show as the resistors arced to chassis. The above transformer change also required adjusting the bias level due to operating point
Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
To be on the safe side, bond the amplifier chassis and the PS chassis together with a suitable length of shield from a piece of coax or similar cable. It certainly makes a big difference with the Hallicrafters SR-2000 Hurricane and P-2000 power supply. It is probably not a bad idea with any two piece rig either transceiver or amplifier. 73, John, W4AWM___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
Re: [Drakelist] L4B/L7 Power Power Supply Rebuilds
John There is a ground wire in the cable that connects to pin 7 on both PS and amplifier that ties the chassis together. As I understand the metering system of the L4B/L7 you will always have the high voltage return sitting slightly above chassis due to the grid current through the 1 ohm resistor. Thats not much but, you cannot tie the HV return directly to chassis or will lose the metering based on my understanding. Where the real danger can come is if working on the PS independently, you can get in trouble if you make the assumption that the HV return is grounded. Actually when working on the PS you can use a jumper to short the HV return to the chassis which would probably be safer in that TEST configuration and allow you to catch any issues with voltage breakdown due to wire routing or component placement. Playing with 3000 volts at these current levels is a full contact sport and not something for casual or inexperienced operators. This stuff KILLS. 73 de K5MWR On 2/20/2012 7:58 PM, w4...@aol.com wrote: To be on the safe side, bond the amplifier chassis and the PS chassis together with a suitable length of shield from a piece of coax or similar cable. It certainly makes a big difference with the Hallicrafters SR-2000 Hurricaneand P-2000 power supply. It is probably not a bad idea with any two piece rig either transceiver or amplifier. 73, John, W4AWM ___ Drakelist mailing list Drakelist@zerobeat.net http://mailman.zerobeat.net/mailman/listinfo/drakelist
