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K2QI
06-26-2009, 03:14 AM
Not sure what happened tonight, but everything was working fine on 40m and 20m until I decided to try and tune up on 10m.

When tuning on 10, I noticed sparks coming from the left parasitic suppressor's resistor. I shut the amp off. The left suppressor resistor has scorch marks on its body, and will need replacement.

I also checked everything that I've replaced so far including the mica caps plus the resistors and diodes on the Harbach metering board, and they all seem fine upon initial inspection. However, whenever I attempt to key the amp now, the plate meter pegs at full scale on key down, the load knob doesn't work as it should (turning clockwise will increase grid voltage past a shallow dip), and SWR is high.

Not sure what went wrong, but any suggestions on where I should start looking would be greatly appreciated.

K2QI
06-26-2009, 03:27 AM
BTW with one tube in, key down with no drive shows .22A lp, and SWR > 100 on output. SWR reading into the linear from the K3 shows 1.2. HV reads fine on CW/SSB modes at 2.4kW and 3kW respectively on idle.

K2QI
06-26-2009, 01:20 PM
Any suggestions? I'm thinking perhaps something with the bandswitch went awry, but any other ideas would help.

AF6LJ
06-26-2009, 02:04 PM
Idling current is high for starters.


BTW with one tube in, key down with no drive shows .22A lp, and SWR > 100 on output. SWR reading into the linear from the K3 shows 1.2. HV reads fine on CW/SSB modes at 2.4kW and 3kW respectively on idle.

Do you mean 2.4KV , 3KV??

Have you tried moving that remaining tube to the other socket just to see if you obtain the same result?

At this point you know that tube is not shorted.
leave the other one aside for now.

In your first post you talk about grid voltage, what do you mean?

Which meter are you reading?
If you are reading the multimeter what position is the meter switch in?

K2QI
06-26-2009, 02:55 PM
Idling current is high for starters.

Yes, noticed that right away also. I checked the metering board which contains the zener diode replacement for any blown resistors, and all check ok with a multimeter.


Do you mean 2.4KV , 3KV??

Oops, sorry for the typo. I need to sleep more.


Have you tried moving that remaining tube to the other socket just to see if you obtain the same result?

Yes, I've moved the other tube over and the result is similar.


At this point you know that tube is not shorted.
leave the other one aside for now.

In your first post you talk about grid voltage, what do you mean?

Prior to this problem, turning the load knob clockwise would produce an apparent drop in lg. Now, turning the knob clockwise will produce a small dip in lg, after which the grid current begins to rise again excessively the further clockwise I rotate the knob.


Which meter are you reading?
If you are reading the multimeter what position is the meter switch in?

I'm looking at the built-in plate and grid meters on the SB-220, and at an MFJ watt meter. I do not have a multimeter capable of HV at the moment.

AF6LJ
06-26-2009, 03:37 PM
Yes, noticed that right away also. I checked the metering board which contains the zener diode replacement for any blown resistors, and all check ok with a multimeter.

Have you checked the ziner I think it's bad.
That is a symptom of the larger problem.

When you moved the tube to the other socket you should have identical readings, if they are not your problem may be with one or more of the components in the grid circuits.

At this point I would replace both parasitic suppressors (the resistors) and re run the test. the one resistor could very well be open.



Prior to this problem, turning the load knob clockwise would produce an apparent drop in lg. Now, turning the knob clockwise will produce a small dip in lg, after which the grid current begins to rise again excessively the further clockwise I rotate the knob.
The behavior of the grid current is normal.
The pi network was set up for operation for a given plate load impedance. This doesn't surprise me....


I'm looking at the built-in plate and grid meters on the SB-220, and at an MFJ watt meter. I do not have a multimeter capable of HV at the moment.[/quote]
often tunes that meter that measures grid current, plate voltage and relative output is called a multimeter short for multiple-function meter.

The band switch can be visually inspected if it is indeed bad you would see signs of it, these could be arcing physical damage caused by mishandling cracked wafers etc....

by the way has there ever been any arching on last few plates of the tune capacitor?



Off to the post office see y'all when I get back.

K2QI
06-26-2009, 05:08 PM
Zener diode is long gone; replaced by a string of resistors on the metering board.

As for the behavior of the grid current being normal; it certainly didn't act that way prior to this incident.

What do you think about the high output SWR? When I try to key up, the amp bypass alarm cuts in due to output SWR. However, taking the amp out of line, there is no problem into the dummy load. Input SWR into the amp is still low at ~1.2

BTW, do you know what those parasitic resistors are rated at? I don't have the amp in front of me, but they looked like 2A, 33ohm carbon resistors.

AF6LJ
06-26-2009, 07:31 PM
Zener diode is long gone; replaced by a string of resistors on the metering board.

You need to replace that zener, you can probibly get one from Mouser. A string of resistors isn't going to cut it as a matter of fact it can lead to the situation you are current in.



As for the behavior of the grid current being normal; it certainly didn't act that way prior to this incident.

What do you think about the high output SWR? When I try to key up, the amp bypass alarm cuts in due to output SWR. However, taking the amp out of line, there is no problem into the dummy load. Input SWR into the amp is still low at ~1.2

BTW, do you know what those parasitic resistors are rated at? I don't have the amp in front of me, but they looked like 2A, 33ohm carbon resistors.

Read me the color code of the resistors.
They are two watt carbon resistors.

In regards to the SWR

Is this between the radio and the amp

Or
Between the amp and a dummy load

I am not too clear on that.

K2QI
06-26-2009, 08:09 PM
The high SWR appears to be between the amp and the dummy load. The dummy load was tested prior too and afterwards, and the reading is 1.2:1 nominal. This is what baffles me.

The measured input SWR between the exciter and the amp is ~1.2-1.3, which is normal.

I'll get the resistor codes when I get home, but if memory serves they are 33ohm carbon units.

I'm thinking at this point that either one or all of the following have failed:
1. bias control
2. band switch
3. shunt resistor



You need to replace that zener, you can probibly get one from Mouser. A string of resistors isn't going to cut it as a matter of fact it can lead to the situation you are current in.



Read me the color code of the resistors.
They are two watt carbon resistors.

In regards to the SWR

Is this between the radio and the amp

Or
Between the amp and a dummy load

I am not too clear on that.

AF6LJ
06-26-2009, 08:41 PM
The high SWR appears to be between the amp and the dummy load. The dummy load was tested prior too and afterwards, and the reading is 1.2:1 nominal. This is what baffles me.

This could one of the following;
Dummy load when it warms up open circuits.
Depending on the dummy load this could be likely.
If the DL was a Cantanna (Heathkit dummy load) with transformer oil I would be less likely to think this was the problem. Air cooled dummy loads have an inherent problem of what to do with all that heat that appears at once.

Could be a bad patch cable..
This is a lot more likely.

You have a bad connection there someplace after the amplifier, that is apearent.



The measured input SWR between the exciter and the amp is ~1.2-1.3, which is normal.

I'll get the resistor codes when I get home, but if memory serves they are 33ohm carbon units.

I can't remember if the amplifier (the one I fixed) had 33 or 47 ohm resistors in the parasitic suppressors.



I'm thinking at this point that either one or all of the following have failed:
1. bias control
2. band switch
3. shunt resistor

Yes fix the bias, and the meter shunt.
While you are at it do a careful inspection of the bandswitch.
I have had that bracket that the bandswitch is attached to out of the amplifier in order to make repairs to the output tank curcuit.
Only do this if YOU HAVE TO DO IT. It takes a lot of dissembley.

WB2WIK
06-26-2009, 09:17 PM
As Sue implied, it is impossible that "high SWR" between the amplifier and your dummy load is the fault of the amplifier. You've got a different problem, and Sue's guess about that being a patch cable is pretty likely.

The parasitic resistor values are not critical. 33 Ohms or 120 Ohms is all the same, they're not there to drop voltage, just act as little blocks of carbon to absorb VHF transients if any exist. They must be proper construction but the actual value isn't important.

There's no way the original cathode zener has been replaced by a "string of resistors," that won't work at all. A string of rectifiers, perhaps.

WB2WIK/6

K2QI
06-26-2009, 09:24 PM
As Sue implied, it is impossible that "high SWR" between the amplifier and your dummy load is the fault of the amplifier. You've got a different problem, and Sue's guess about that being a patch cable is pretty likely.

The parasitic resistor values are not critical. 33 Ohms or 120 Ohms is all the same, they're not there to drop voltage, just act as little blocks of carbon to absorb VHF transients if any exist. They must be proper construction but the actual value isn't important.

There's no way the original cathode zener has been replaced by a "string of resistors," that won't work at all. A string of rectifiers, perhaps.

WB2WIK/6

Yes, string of rectifiers is what it is; sorry about that. I really do need to sleep more. :p

Are you by any chance familiar with the Harbach RM-220 metering board? I'm trying to determine which is/are the shunt resistor(s). It was R3 on the old Heathkit board, but I'm not sure what they are now on the RM-220 board.

From this diagram, it looks like it's also R3 at .82ohm, 3W but I just want to be sure.

http://www.harbachelectronics.com/_mgxftp/pdffiles/RM-220.PDF

BTW, regarding SWR... if I turn the amplifier off while leaving it connected to the dummy load, SWR reading is fine. Only when it's on does it cause the SWR meter between the dummy load and amplifier to go bonkers, and as a result it trips the amplifier cut-off relay. Also, the dummy load is an oil-filled MFJ unit.

WB2WIK
06-26-2009, 09:32 PM
Yes, string of rectifiers is what it is; sorry about that. I really do need to sleep more. :p

Are you by any chance familiar with the Harbach RM-220 metering board? I'm trying to determine which is/are the shunt resistor(s). It was R3 on the old Heathkit board, but I'm not sure what they are now on the RM-220 board.

From this diagram, it looks like it's also R3 at .82ohm, 3W but I just want to be sure.

http://www.harbachelectronics.com/_mgxftp/pdffiles/RM-220.PDF

::Yes, it's the 0.82 Ohm. I've never used the Harbach board, but then I've never had a single failure with an SB-220! I built one in 1972 and used it for almost 30 years. Other than blowing the dust out and oiling the fan bearing, I never serviced it in any way and it still works fine...

WB2WIK/6

AF6LJ
06-26-2009, 09:43 PM
As Sue implied, it is impossible that "high SWR" between the amplifier and your dummy load is the fault of the amplifier. You've got a different problem, and Sue's guess about that being a patch cable is pretty likely.
{/quote]
A newbee gotcha.

[quote]
The parasitic resistor values are not critical. 33 Ohms or 120 Ohms is all the same, they're not there to drop voltage, just act as little blocks of carbon to absorb VHF transients if any exist. They must be proper construction but the actual value isn't important.
to elaborate;
a parasitic suppressor is a self resonant circuit (inductance plus shunt capacitance) with a load resistor to absorb the parasitic energy near the self resonant frequency.



There's no way the original cathode zener has been replaced by a "string of resistors," that won't work at all. A string of rectifiers, perhaps.

WB2WIK/6In a word Exactly :)

AF6LJ
06-26-2009, 09:54 PM
::Yes, it's the 0.82 Ohm. I've never used the Harbach board, but then I've never had a single failure with an SB-220! I built one in 1972 and used it for almost 30 years. Other than blowing the dust out and oiling the fan bearing, I never serviced it in any way and it still works fine...

WB2WIK/6
I might add..
The SB-220 I fixed had the parasitic oscillation problem that Richard Measures had documented.
It wasn't easy to fix. I managed to make it stable without burning up ether the filament transformer or loosing a tube. After the second time I saw the effects of that problem I fused the cathode circuit (rather tightly) until the problem was solved.
That SB-220 has no degenerative feedback in the grid circuit.
That seemed to tame it down, there is a lot of stray lead inductance in the grid circuit, that's bad for grounded grid amplifiers.
/sue likes grid driven amps :)

K2QI
06-26-2009, 10:04 PM
::Yes, it's the 0.82 Ohm. I've never used the Harbach board, but then I've never had a single failure with an SB-220! I built one in 1972 and used it for almost 30 years. Other than blowing the dust out and oiling the fan bearing, I never serviced it in any way and it still works fine...

WB2WIK/6

I'm glad yours was working well... and that you built it yourself. In my case, I'm having a bit of difficulty determining where the old Heathkit ends and where the modifications begin.

WB2WIK
06-26-2009, 10:16 PM
I'm glad yours was working well... and that you built it yourself. In my case, I'm having a bit of difficulty determining where the old Heathkit ends and where the modifications begin.

::Understand. However the SB-220 is really a very simple amplifier with absolutely no complex circuitry. The Harbach and other mods complicate it quite a lot compared with the original design, which was actually brilliant in many ways. There are probably more SB-220s on the air than any other amplifier ever built except maybe some more recent Ameritron models.

AF6LJ
06-26-2009, 10:19 PM
I'm glad yours was working well... and that you built it yourself. In my case, I'm having a bit of difficulty determining where the old Heathkit ends and where the modifications begin.
You have a Greaf Kit
That's okay my ex boyfriend had one, and He built it.
It wasn't entirely his fault, he had higher than average gain tubes. Your amplifier can be put right it's going to take time.
It's a shame you don't have one of us living close by to Elmer you in person.
People who want to learn deserve to have help.
:)

Steve is right the SB220 is about as simple as you can get and when it was in production it was the best value out there. The only amplifier that supersedes that value in my opinion is the SB200.
/Sue likes the old stuff as apposed to the new stuff with Chinese parts in them. :)

K2QI
06-26-2009, 10:36 PM
Thanks Sue and Steven; both of you are doing a fine job as it is helping me. I do have a strong desire to learn and figure this stuff out, but I'm probably starting a bit later in life than most, and as such don't have a lot of electronics knowledge to depend on when problems like this arise. So, it'll probably take me a bit longer to figure this stuff out than others, but I'm determined to get this working well.

Anyway, I've been pondering the mystery of the high output SWR readings. This only happened after the parasitic suppressor resistor began arcing. If I shut the amp off, but leave it in-line (K3 -> SB-220 -> SB-610 -> MFJ-828 -> Dummy Load), SWR reads 1.2 to 1.3 on the MFJ meter when I key the exciter. When I turn the amplifier on and key down, SWR goes crazy and can range from 20:1 to > 100:1. The MFJ at that point will kill the amplifier relay and an alarm will sound as it's gone beyond the 2.0:1 threshold I set. I should mention that this occurs even with no drive from the K3.

If it were an issue with a patch cable, wouldn't I be able to replicate the problem with the linear off? I'm using high quality connectors and LMR-400 cabling between the linear, SB-610 monitor scope, watt meter, and dummy load.

AF6LJ
06-26-2009, 11:51 PM
Thanks Sue and Steven; both of you are doing a fine job as it is helping me. I do have a strong desire to learn and figure this stuff out, but I'm probably starting a bit later in life than most, and as such don't have a lot of electronics knowledge to depend on when problems like this arise. So, it'll probably take me a bit longer to figure this stuff out than others, but I'm determined to get this working well.

Anyway, I've been pondering the mystery of the high output SWR readings. This only happened after the parasitic suppressor resistor began arcing. If I shut the amp off, but leave it in-line (K3 -> SB-220 -> SB-610 -> MFJ-828 -> Dummy Load), SWR reads 1.2 to 1.3 on the MFJ meter when I key the exciter. When I turn the amplifier on and key down, SWR goes crazy and can range from 20:1 to > 100:1. The MFJ at that point will kill the amplifier relay and an alarm will sound as it's gone beyond the 2.0:1 threshold I set. I should mention that this occurs even with no drive from the K3.

If it were an issue with a patch cable, wouldn't I be able to replicate the problem with the linear off? I'm using high quality connectors and LMR-400 cabling between the linear, SB-610 monitor scope, watt meter, and dummy load.
Not necessarily;
There is a large current difference which could make a marginal connection into an open circuit.

Here is what I would do.
Got some spare coax?
If not ten feet won't cost much.
Get some RG-8 or RG-214 four new PL-259s and make up two new patch cords (or buy two new ones)
Ether way the cost isn't going to be too high and you can start with something that should be known good.

First take one of the patch cores and connect the dummy load to the amplifier. (no watt meter)
Use the relative output meter position to tune the amplifier.
Take note of the tune and load control settings.

put other new jumper in place of the first one and verify the tune and load settings by tuning the amplifier for maximum output on the relative output meter on the amplifier.
the load and tune control positions and the output indicated on the meter should be exactly the same.

IF they are the same; place the watt meter in the circuit now using both new jumpers.
Tune the amplifier using the relitive output meter on the amplifier.
The tune and load settings should be the same.

If they are not and you have allowed sufficient time for the dummy load to cool (assuming it is an air cooled dummy load and is capable of handling the power output of the amplifier)
Then the problem lies in your power meter.

One other thing you can do;
Check your dummy load while it is hot with an ohmmeter.
It should read 50 Ohms plus or minus ten percent. (45-55 ohms)

Problems in the test set up are rookey errors nothing to worry about everyone has had their test setup bite them in the butt before. By the time I was working at Loral Corp I was solving rookey test set ups in two minutes or less.
HF is onthing compared to UHF and SHF. :)
Everybody starts out making these kinds of errors.
Not a big deal :)

WB2WIK
06-27-2009, 12:20 AM
Sue's advice, as usual, is good.

However I might point out one thing: If the amplifier has a VHF parasitic and your dummy load is for "HF only" and no good on VHF, it's possible the SWR into the dummy load will be very bad with the amplifier on, because it's oscillating at a frequency much higher than the dummy load can handle.

I use Bird dummy loads that are good to about 3000 MHz or higher, so I don't see this effect. But if I use a crappy load, like an MFJ or something that's only good for HF, I could see it.

Based on your description, it sounds like you have developed some sort of parasitic or oscillation problem because there's no way the amplifier should have ANY output without any drive!!

That is a "grief kit" indeed.:p

But the SB-220 is such a simple amp, it can always be fixed. If you bought it second-hand with "modifications" already installed, the first thing I'd do is take out all the mods and revert back to the original circuit, which was damned good to begin with. There aren't many parts in an original SB-220, which is one of the reasons it was so easy to use and to work on.

The only times I've seen an SB-220 become "unstable" were when I was working on amps that weren't put together very well originally, and there was always a bad solder joint or loose ground connection somewhere. When soldering the parasitic suppressors and also the filament chokes, I always used silver solder which has a higher reflow temperature than regular 60/40 "radio" solder, and keeps those connections made even at very high operating temperatures routinely encountered in the amplifier.

60/40 solder reflows at about 230C, and that's very close to the actual temperatures encountered at the anode caps and filament pins.

WB2WIK/6

K2QI
06-27-2009, 12:36 AM
I'll give that a shot Sue.

By the way, I got home and desoldered R3 on the metering board. It's supposed to be .82 ohm, but reads between .8 and 1; probably my meter. It seems fine though.

I also took a closer look a the resistors used for the parasitic suppressors. They are discolored and the bodies are brittle; pieces are flaking off when poked and the band markings are faded to the point that I can no longer tell what they were. I unsoldered the one resistor that had scorched, and I get a 18 megaohm reading. The other resistor (which I also unsoldered) doesn't read anything at all. Interesting.

K2QI
06-27-2009, 12:41 AM
Steve, the only mods that I can tell were put in by the previous owner include the Harbach metering board and filter capacitor bank. There is also a Harbach softkey. I myself recently replaced all of the mica capacitors with 200pF 1000v units from Mouser. There also seem to be some newer capacitors near the bandswitch, but I cannot tell what they are unless I take the face plate off. Apart from that, it seems like everything else is pretty much original down to the circa 73 3-500's (which might I add are very strong).

WB2WIK
06-27-2009, 12:55 AM
Steve, the only mods that I can tell were put in by the previous owner include the Harbach metering board and filter capacitor bank. There is also a Harbach softkey. I myself recently replaced all of the mica capacitors with 200pF 1000v units from Mouser. There also seem to be some newer capacitors near the bandswitch, but I cannot tell what they are unless I take the face plate off. Apart from that, it seems like everything else is pretty much original down to the circa 73 3-500's (which might I add are very strong).

::Why'd you replace the mica capacitors? Those are not known for going bad, as far as I know. At least, mine never did. Frankly, my filter cap bank never went bad, either... I always suspect when I see this stuff changed it's because stuff overheated due to cockpit error. The SB-220 normally runs cool as a cucumber in normal SSB-CW service.

K2QI
06-27-2009, 01:32 AM
::Why'd you replace the mica capacitors? Those are not known for going bad, as far as I know. At least, mine never did. Frankly, my filter cap bank never went bad, either... I always suspect when I see this stuff changed it's because stuff overheated due to cockpit error. The SB-220 normally runs cool as a cucumber in normal SSB-CW service.

Well, truth be told the day I got the amp, I plugged it in and turned it on. I heard a loud bang. I shut everything off and took the unit apart. Found one of the original mica caps blew apart. I decided to replace everything right there and then.

That was probably a good sign something else was amiss, but after I replaced the caps everything worked fine until last night when I tried to tune up on 10m. (10m did tune up fine before as well)

I probably should have just bought brand new, but the price was good and the seller seemed to be trustworthy as he's done quite a few deals here on the Zed and in other places, all with good feedback.

Anyway, lesson learned. Now it's time to learn more about the amp and how to fix it. :D

BTW, can I re-use the coiled filament leads that the resistors were soldered too, or should I replace those as well? I looked at the kit offered on the Harbach site, and it appears they're out of stock.

WB2WIK
06-27-2009, 01:36 AM
BTW, can I re-use the coiled filament leads that the resistors were soldered too, or should I replace those as well?

::Those are the filament chokes. They're just made of wire, they can't go bad.

K2QI
06-27-2009, 01:53 AM
Took some pictures so you know what I'm dealing with a little better

http://jamespaulsarte.com/temp/ham/sb2200002.JPG
http://jamespaulsarte.com/temp/ham/sb2200003.JPG
http://jamespaulsarte.com/temp/ham/sb2200004.JPG
http://jamespaulsarte.com/temp/ham/sb2200005.JPG

http://jamespaulsarte.com/temp/ham/sb2200007.JPG
http://jamespaulsarte.com/temp/ham/sb2200008.JPG
http://jamespaulsarte.com/temp/ham/sb2200009.JPG
http://jamespaulsarte.com/temp/ham/sb2200010.JPG

K2QI
06-27-2009, 02:36 AM
::Those are the filament chokes. They're just made of wire, they can't go bad.

I was more concerned about length and size of the coiled form. :confused:

K2QI
06-27-2009, 03:13 AM
BTW, I'm having trouble finding 2w resistors. Would 2 1-watt 100 ohm resistors in parallel per tube suffice, or must I look for 2w resistors?

AF6LJ
06-27-2009, 04:46 AM
Steve is right about the dummy load, (dah) a bad assumption on my part I have a dummy load that is good from DC to a GHZ. His diagnosis of what is happening has been my suspicion.

As a last resort for the resistors you can get them from RF Parts.
They have a minimum order policy that's why I said as a last resort. http://rfparts.com/resistor.html

Mouser has metal film 2 watt resistors, What I am not so sure about is if they are non inductive. Some are, some are not.
You can parallel But I would try and find another solution if I could.

Those pictures bring back memories. :)

K2QI
06-27-2009, 11:48 AM
The dummy load is an MFJ-250. This is what MFJ states regarding SWR and frequency range: The MFJ-250 Dummy Load has low VSWR to 400MHz. Under 1.2:1 to 30 Mhz. 1.5:1 for 30 - 300Mhz. 2:1 for 300-400MHz.

At 29 Mhz, the SWR according to my analyzer is 1.2:1.

I'll try and look for the proper 2w resistor, but would carbon film 1w resistors in parallel work in a pinch, or is it necessary to get 2w resistors? In parallel, two 100ohm resistors would be equal to 50 ohm, but would still only be rated to 1w current handling.

AF6LJ
06-27-2009, 02:02 PM
The dummy load is an MFJ-250. This is what MFJ states regarding SWR and frequency range: The MFJ-250 Dummy Load has low VSWR to 400MHz. Under 1.2:1 to 30 Mhz. 1.5:1 for 30 - 300Mhz. 2:1 for 300-400MHz.

At 29 Mhz, the SWR according to my analyzer is 1.2:1.

I'll try and look for the proper 2w resistor, but would carbon film 1w resistors in parallel work in a pinch, or is it necessary to get 2w resistors? In parallel, two 100ohm resistors would be equal to 50 ohm, but would still only be rated to 1w current handling.

Baring anything wrong with the dummy load were back to my previous post regarding the patch cables.

When these amplifiers oscillate it is usually around the lower third of the VHF spectrum. (80-120 MHZ) Your load should be fine there, unless something is wrong.

You can parallel 1 watt resistors to get the required power dissipation. The fact that one parasitic suppressor had arcing was a sign that there was a lot of RF energy that was near the self resonant frequency of that parasitic suppressor. When there are carbon composition resistors they usually smoke when there is that much energy at or near the self resonant frequency of the suppressor.

/sue goes and gets her second cup of coffee
Good Morning :)

N4CR
06-27-2009, 03:41 PM
Those look like metal film resistors, not carbon. It's difficult sometimes to tell wire wound from metal film, but those certainly do not look like carbon.

AF6LJ
06-27-2009, 03:50 PM
Those look like metal film resistors, not carbon. It's difficult sometimes to tell wire wound from metal film, but those certainly do not look like carbon.
I agree;
Metal film would be ideal, finding non inductive ones is not easy.

K2QI
06-27-2009, 07:14 PM
I guess I lucked out today. I found an electronic supply store that happened to be open, and also happened to carry 2W carbon composite, non-inductive resistors. I picked up a few sets of the 61 and 90 ohm resistors to have spares. I was also able to get some of the more rare resistors and capacitors while I was there for spares. I'll install the new resistors today and give the amp a test.

K2QI
06-27-2009, 08:05 PM
OK, installed the new resistors and the amp is back in business! Thank you Sue and Steven for your valued assistance. :D

AF6LJ
06-27-2009, 08:18 PM
OK, installed the new resistors and the amp is back in business! Thank you Sue and Steven for your valued assistance. :D

Always Glad to help anybody who wants to learn.
I have a healthy background in RF and I am always glad to share my knowledge with others.

Have fun :)

K2QI
06-27-2009, 08:27 PM
Interesting; average power out is down a little now. Before, I could easily obtain 1100w on keydown CW with lp at .8 and lg at 220-230mA. Now, I can only get about 850-930w out. Won't make much of a difference when being heard, but interesting nonetheless.

AF6LJ
06-27-2009, 08:37 PM
Interesting; average power out is down a little now. Before, I could easily obtain 1100w on keydown CW with lp at .8 and lg at 220-230mA. Now, I can only get about 850-930w out. Won't make much of a difference when being heard, but interesting nonetheless.
All of this may have hurt one of the tubes.
There is a lot to be said for individual metering of the tubes, that however would make the cost of the amplifier considerably more.
Just take it easy and watch the grid current when you are using the amplifier Keep one eye on the SB-610 and the other on that grid current meter and you will be just fine.

K2QI
06-27-2009, 08:58 PM
All of this may have hurt one of the tubes.
There is a lot to be said for individual metering of the tubes, that however would make the cost of the amplifier considerably more.
Just take it easy and watch the grid current when you are using the amplifier Keep one eye on the SB-610 and the other on that grid current meter and you will be just fine.

Silly me. I just remembered that last night I had calibrated the MFJ-828 watt meter as it was reading a little high before compared to my K3's meter which is within 1% tolerance for accuracy. I set it to match the K3. I had taken those readings from the amp prior to calibrating the watt meter. So, I'm pretty certain that output is still the same.

I'm going to keep an eye on the tubes and lg as you said just to make sure. Hopefully this will be the last bad incident I have with the amp.

AF6LJ
06-27-2009, 09:21 PM
Silly me. I just remembered that last night I had calibrated the MFJ-828 watt meter as it was reading a little high before compared to my K3's meter which is within 1% tolerance for accuracy. I set it to match the K3. I had taken those readings from the amp prior to calibrating the watt meter. So, I'm pretty certain that output is still the same.

I'm going to keep an eye on the tubes and lg as you said just to make sure. Hopefully this will be the last bad incident I have with the amp.

The reason for watching the grid current is to make sure you are not overdriving the amplifier.
That will also tell you that the amplifier is in tune.
The grid current will rise markedly if the amplifier's load suddenly changes.
With a dedicated plate meter and the multimeter set to read grid current your covered, the SB-610 will tell you if you are flat topping and will warn you of instability if you know what to look for. (fuzzy pattern and output when there should be none. like between words)

WB2WIK
06-27-2009, 09:51 PM
Glad you got it going!

BTW the photo of your burned up parasitic suppressor resistors looks like they were the wrong type to begin with, and those are DEFINITELY not the original ones from Heath. No way. The originals were brown body Allen-Bradley carbon comp resistors. Somebody previously replaced them, and they may not have selected the correct type.

Two 1W resistors in parallel could work but they would not fit inside the parasitic suppressor coil, so that would require more "rework."

If you can get hold of a small amount of silver solder (5% silver) and use a high temperature iron, re-solder the parasitic suppressors to the lugs that attach to the 3-500Z anode caps and they will tend not to unsolder themselves from operating heat. I usually do the same with the filament choke connections to the tube socket terminals, which can also get really hot.

Don't ever solder anything while the tube is installed in the socket or the anode lug is screwed to the anode cap/dissipator on the tube!

K2QI
06-27-2009, 11:31 PM
Steve, I took the filament chokes off before soldering the new resistors in place. I was able to get the brown-body Allen Bradley carbon resistors from a local shop. They are "new old-stock". I was able to get pairs in 41, 63, and 91 ohms. I should probably go back to the store and buy a few more sets since they're getting harder to find.

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