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Author Topic: SB-220  (Read 12908 times)
K9RR
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Posts: 10




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« on: January 18, 2011, 12:20:42 PM »

I recently bought a SB-220. It has several of the Harbach kits installed including the RM-220 Metering Board. I decided to add some parasitic suppression upgrades from AG6K. After partial installation of the upgrades I decided to check for normal operation before finishing. I forgot to replace the perforated top panel before applying power. With the HV shorted I got a cloud of smoke from around the filament transformer/metering board area before quickly removing the ac. After replacing the top panel everthing seemed to be working except no plate idle current in either cw or ssb. Jeff, W8CQ told me I probably took out the 1N4005 diode string that provides the bias voltage for the 3-500Z's and subsequently the idle current and/or the three panel meter resistors R1 - R3. I checked the diodes and resistors and they seem to be ok. What else can go wrong or did I do wrong in my evaluation? Thanks much!
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W1QJ
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« Reply #1 on: January 18, 2011, 03:16:30 PM »

You created a dead short from B+ to ground by not putting the top screen on and the interlock grounded the B+.  That should have thrown the circuit breaker.  Did it?  Most of the time you will blow a rectifier diode in that case.  Did you check the rectrifier diodes?  Are you seeing HV on the meter now?  If one of the bais diodes shorted you will still see idle current, it one blew open then you won't.  Just to be sure, do you see HV on the meter now?  Also, "if" you have HV, when you key the amp up, do you see a small drop in plate voltage?  You should.  If not check also the small RF choke that comes off the interlock.  Check with an ohm meter to see if it is open.  Report back.
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K9RR
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« Reply #2 on: January 18, 2011, 08:05:15 PM »

The ckt breaker did not trip. There is hv reading on the multimeter. When I key the amp I see abt 200v drop in the plate voltage. I did ohm out the sml choke and the plate choke - both ok. I ohm'd the 8 bias diodes and they all read abt 80ohms in the fwd mode so I assume they are ok.
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W1QJ
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« Reply #3 on: January 19, 2011, 04:45:46 AM »

OK, It seems like the CB saved you.  I would check the .82 ohm resistor on the upper right hand side of the rectifier board.  Usually when that goes you will see negative grid current.  Are you sure the reading on the grid meter is not negative?  To properly check the .82 ohm resistor you should lift one leg.  That resistor usually blows when high amounts of grid current or an arc or short occurs.  You should be seeing idle current on the plate meter.  Do you?  The fact that the HV drops 200v means the tubes are drawing idle current so you should see it on the plate meter.  You won't see any grid current until yoou apply RF to the tubes and drive them.  have you tried driving the amp with RF?  Lou
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K9RR
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« Reply #4 on: January 21, 2011, 09:29:02 AM »

Hello again Lou: Tnx for your help. I applied RF and everything appears OK, for 30w in I get abt 400 out on 20m. There is still no idle current on the plate current meter (in either cw or ssb), but current does go up and down with drive applied. I double checked the 3 meter resistors agn, R1, 2, & 3 by taking them out, they are all ok. And agn I just keyed the rig, no rf and the plate voltage drops abt 100-150 volts . I have no idea what to do at this point, can you help further? Tnx agn, Bill K9RR
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KF7CG
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« Reply #5 on: January 21, 2011, 10:16:04 AM »

I know nothing about that particular Amp or the Mods. So just a blind question. I have recently seen where many amplifier companies are touting that their amps now completely shut down the tubes between characters and words (RF bias or something like that for a trade name). Could it be that one of the mods applied to the SB220 is of that type and it is now "designed" to draw zero idle current?

KF7CG
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W8JI
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« Reply #6 on: January 21, 2011, 11:13:53 AM »


As someone pointed out, you applied HV with the B+ line shorted to the chassis. That puts high current through the meter shunt, the multimeter (if in the plate current position), and the grid shunt and grid meter.

Current from the interlock being closed does NOT flow though the bias system, so it should be untouched.

If you added diodes across the meters, maybe you blew a diode that protects the meter.
It is also possible the meter is damaged.

The SB220 was a good amp, and had very few things that actually need changed or that really makes it better. This is the problem adding mods that really do nothing to help. There is always a risk associated with changes, so keeping changes to a minimum is best. :-)

The single most beneficial change in the SB220 would be getting rid of the grid chokes and mica caps on the grids, and directly grounding the grids. Also a keying voltage modification and meter protection is a good idea. The rest of the changes really don't help anything, and some even make things worse.

73 Tom





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W1QJ
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« Reply #7 on: January 21, 2011, 08:53:48 PM »

Bill, you claim that the plate current meter does respond to the RF drive showing plate current.  That means the meter is OK and not blown.  Tom may be on to something with the fact that if the amp has the dual back to back diodes on might be shorted. Or maybe one is.  If your plate meter has those back to back diodes, cut them free and check to see if you have idle current.  If so then replace the diodes with new ones.  If you do not have those B to B diodes then we need to look elsewhere.  I asked if you are getting a negative grid current reading, don't know if you answered that.  But check for those diodes across the meter.
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K9RR
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« Reply #8 on: January 22, 2011, 06:10:22 PM »

Lou / Tom Tnx for your help. I removed the diodes from across the meters.
There is no movement in the plate current meter when the ac is applied via the front panel switch.
So, we know the meter works, as it registers when you apply drive, and the three meter resistors are good as I've taken them out and measured them twice. We know the plate voltage drops over 100 volts when the amplifier is keyed w/o drive, which indicates that there is idle current flowing through the tubes.
So we are left with the same issue we started. Why isn't the idle current showing on the plate current meter?
I can only leave this question to you knowledge folks and hope you may have some more ideas on what the problem might be.
Tnx agn for your help to this point. Bill K9RR
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W8JI
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« Reply #9 on: January 23, 2011, 01:12:58 AM »

Lou / Tom Tnx for your help. I removed the diodes from across the meters.
There is no movement in the plate current meter when the ac is applied via the front panel switch.
So, we know the meter works, as it registers when you apply drive, and the three meter resistors are good as I've taken them out and measured them twice. We know the plate voltage drops over 100 volts when the amplifier is keyed w/o drive, which indicates that there is idle current flowing through the tubes.
So we are left with the same issue we started. Why isn't the idle current showing on the plate current meter?
I can only leave this question to you knowledge folks and hope you may have some more ideas on what the problem might be.
Tnx agn for your help to this point. Bill K9RR

Bill,

The best I can offer is it is either wiring or a component in the cathode system up to the negative of the power supply.  It is impossible to say what is the problem.

If it is NOT related to the HV interlock, it could be you now have excessive bias. With bias set above 30 volts there will be no idle current. You don't have a backward or defective bias diode do you?

I'm not trying to pick at you at all, but all the modifications for an SB220 are not always good changes. The amp should work with the changes, but with every change comes a risk. It also complicates trouble shooting. Most of the mod advice is OK, some does nothing, and some is even harmful. I just can't help because so much has changed.

73 Tom
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W1QJ
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« Reply #10 on: January 23, 2011, 05:20:31 AM »

Bill, I assume the idle current showed up OK before the B+ was shorted to ground, is that correct?  If so, obviously something went wrong to cause the problem as a result.  Thinking outloud, You have the Harbach rectifier board which has a string of diodes in reverse to develop bias voltage.  Normally, with the old method using the zener you would see about 90ma idle in CW and about 180ma in SSB.  Usually when an event happens the zener shorts and you have about 220ma of bais showing on the meter, beacause of the lack of bais voltage supplied by the zener.  Now you have the Harbach board which has the string of diodes for bais.  Did you check each diode on the board to see if they were all good in the bias string?  I think you said you had installed the Harbach board and when you went to test it that is when the short occured.  So you really don't know if you would have showed idle current with the board.  On the other hand you mentioned you tested the 3 metering resistors, which the Harbach board does not have.  Those resistors whould not effect that anyway.  They are only for metering HV not for any current reading on the plate meter.  Next question, If you go ahead and load up the amp with 100 watts of drive you do see normal output power?  What does the plate current read at that point?  Now I have seen this before on RARE occasions, the plate meter will work and show some plate current on the meter, but the readings will be low.  When I measured the meter internal resistance it was way low by more than half.  I did not know how that happened but when I replaced the meter all was fine.  I am wondering if instead of the meter blowing out it arced or something to cause this problem I have seen before.  Lets assume that is the case with yours due to the B+ grounding.  Now you have the situation I have seen.  A small amount of current through the meter now will not cause the same deflection and you may not read a small amount of current that would indicate 180ma of idle.  Also there is a 3600 ohm resistor on the rectifier board that could have changed value with high current passing throug.  Usually they open, but sometimes other thigs happen.  Can you remove all wires from the plate meter and measure the meter resistance on a high scale on your vom?  it should be 1400 ohms.  If it is much less, the meter is shot.  Also check the 3600 5 watt resistor on the board as well as the 1 ohm 5 watt resistor on the board.  They both play a part.  you should have to lift one leg off the board to get the correct reading.  Once this is all done, we have covered every base.  Until then NOT.  Lou
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K9RR
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« Reply #11 on: January 23, 2011, 09:15:23 PM »

Lou I had the Harbach kits installed and everything was normal. There was around 70-80 idle in cw and something around 150ma. in ssb. I then went to put in the Rick Measures fixes. The first was to install diodes across the meters, which I did. I then decided to do a power up check to make sure everything was still good and that is when I forgot to replace the top cover before pushing the ac switch.
I checked the meter. It measures 1382 ohms. I have checked the three meter resistors twice by taking them out and measuring them. They measure right on.
I ohm'd the 8 1n4005's and they all measure the same, about 80 ohms forward and very high in reverse.
I did a low power Rf test. With 30 watts in I get 480 out. The meters reads 2350 volts and 370ma. or about 870 watts input or about 55%.
Don't know what efficiency to expect. The tubes must be original with a 1971 date code stamped on them.
So, don't know if we are any further along solving the problem of no bias current showing on the plate current meter. I guess one thing I haven't done is switch between cw and ssb and watch the voltmeter to see if it drops even more when I key the radio, indicating higher bias current in ssb then in cw.
Even if it does, not sure that tells us anything new.
Thanks Lou, Bill K9RR
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K7ZRZ
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Posts: 279




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« Reply #12 on: January 24, 2011, 07:15:04 AM »

Bill,

What does the bias voltage actually measure, both in keyed and unkeyed condition? Kinda sounds like the amp is not actually getting out of the un-keyed plate idle current cutoff condition when you key the amp.
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Brian K7ZRZ
W1QJ
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« Reply #13 on: January 24, 2011, 01:18:21 PM »

Brian, Bill says when he keys down with no drive he sees a reduction in plate voltage, that indicates the tubes are drawing current and therefore there should be idle current seen on the meter.  If the tubes were staying cut off they would not be drawing any current and the HV would not drop.  He said he did an RF test and everything seems right.  Nice output with the drive he indicated with seeming correct plate current.  He measured the meter internal resistance and it is correct so the meter is fine.  It seems that everything is OK.  Tell you the truth, I have been working almost daily for 20 years with the SB-220 amp and I have never seen this particular problem.  I just can't imagine what would keep the idle current from showing up on the meter, but yet when he drives the amp the plate current is being shown.  He has the Harbach rectifier board in this amp which uses a string of reverse polarity diodes to set the idle current, he says he checked them and they are all OK.  If everything was normal, he should see about 90ma idle current on CW and 180ma on SSB.  He sees 0 ma on either.  I wonder for about $2.00 if it would be wise to change out the 8 diodes in the string, and make them 1N5408's if they aren't already.  Maybe one diode is funky when it passes current?  I am at a loss at this moment. Lou
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VK4TUX
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« Reply #14 on: November 13, 2011, 02:51:25 AM »

I would be checking the built in meter protection diodes on the Harbach board.

Adrian ... vk4tux
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