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Author Topic: Heathkit SB 230 Amp. Help please???  (Read 1736 times)
KC9AWN
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Posts: 22




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« on: December 24, 2002, 11:58:26 AM »

Hello All,

I am new to this, and this is my first Amplifier, so be gentle with me (HI-HI).

I have pieced together a Heathkit sb 230 HF Amplifier and am having proplems.  I did not know the history of these parts, so I was very detail oriented (you might say anal retentive) about checking all the parts and taking several days to go through the complete assembly manual to ensure all was assembled correctly.

Here's what happened:
I am working the initial checks here, with no coax, radio, antenna, just the stand alone amp.
I am on page 88 of the assembly manual.  Meter zero - GOOD.  Check #1 - all GOOD.  Check #2 pulled wire from pin 3, cover back on, this part goes good, hi temp light works.  replaced wire on pin 3, replaced cover.  Plugged in line cord. Turned amp. on.  I got the delay light, and it went out within the 60-90 second window.

Turned the meter switch to exciter only. The "exciter" light works only in the exciter position, as it is supposed to.

Now is when things go wrong.  I switched the meter to plate, they tell me to find a piece of "hook-up wire" to short out the relay phono socket. So, I found a wire and I am just starting to lean over the cabinet, to reach the relay socket ( I never actually shorted the relay socket out), it is now 15-20 seconds after I swithed to plate, and I see a spark, through the cabinet top in the area of the relay (RY1), I blow a house circuit breaker, also the fuse in the back of the amp blew.

I must state at this point, the fuses I bought were .75 amp fuses, but not slow blow.  I hope that explains my following actions.

I removed the cover, did not smell anything burnt or see any visual damage after a detailed inspection to both sides of the amp.

O.K. perhaps the fuse was faulty, I tell myself, or the fact that they call out for slow blow, and I could only find fast blow fuses in the correct amperage. Maybe the flash I saw was the relay contacts???...

I talked myself in to trying another fuse.  This time It took about 5 seconds (it did not happen instantly), I had what sounded like fire crackers, and could see several sparks dancing in the rf enclosure. (I am sure it was arcing, Going on, I could see it through the top at C15, in the rf enclosure.) Quickly, I shut off the main power switch. This time I did not blow the house circuit breaker.

Of course the fuse blew again.  This time after removing the cover, I removed the RF cover to find no arcing spots on the capacitor plates (guess I am lucky).  I did however, find R17, 33ohm, 1 Watt resistor blown to bits. This resistor is located just off of the grid of v1.  

About the 8873 ceramic tube.  The filament pins 5 and 6 show continuity.  and the grid pins (4,7,11) are not shorted to the cathode pins (1,2,3,8,9,10).

Does the fact that that resister blew, and the fuses blew, mean I have high grid current?  Did the arcing in the rf enclosure happen because the tube was energized, and no dummy load was connected to the amp?? Is this amp trying to amplify power, when it is not supposed to?  Does the "fireworks display" in the rf enclosure at least prove the tube is good or amplifying like it is supposed to?

I have no idea what to do next.  Anyone who could point me in a direction, I would be greatful.

Thank you for taking the time to read through this!!!

73,

Scott KC9AWN

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N8FVJ
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Posts: 692




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« Reply #1 on: December 25, 2002, 10:09:53 AM »

I am not familar enough to troubleshoot this amplifier. Two rules I always follow. I energize unknown amps without the tubes first. I always have a dummy load connected when a tube(s) are in place. Almost sounds like your amp went in to oscillation.
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WA9SVD
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Posts: 2198




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« Reply #2 on: December 25, 2002, 03:02:54 PM »

First of all, attach a dummy load that can handle the power output of the amp.  It is almost certain you have a parasitic oscillation that is causing the problem, and no load will only make the matters worse.
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KC9AWN
Member

Posts: 22




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« Reply #3 on: December 25, 2002, 06:18:14 PM »

O.K., Does anyone want to explain to a "Newbe" what a parasitic oscillation is, and how do I get rid of it???

73 and thank you,

Scott KC9AWN
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OBSERVER
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Posts: 130




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« Reply #4 on: December 26, 2002, 04:56:06 PM »

Under the right conditions, an amplifier makes an excellent high power oscillator. Parasitic Oscillations can cause serious damage to tank circuit components including tube destruction. Parasitic Oscillations typically appear as, Low-Frequency (LFPO), VHF (VHFPO) and High-Frequency (HFPO). LFPOs occur when a resonant circuit is created between the tube’s inter-electrode capacitance and input or plate RF choke inductance. Typical fix is to change the filament or plate choke configuration/inductance. VHFPOs (my personal favorite) occur when a resonant circuit is created between the tube’s inter-electrode capacitance and interconnect lead inductance (grid/plate). Typical fix is adding or changing the parasitic choke configuration between the plate and plate choke/plate tank circuit. Last but not least is HFPOs, typically caused by a grossly off resonant input tank circuit. With POs, the plate and grid currents are limited by the tubes gain (mu), lead resistance, and power supply's maximum output capacity. So if the power supply were capable of providing large currents, your oscillator's (amplifier's) output would continue to build until something failed.

Although POs are possible, if equipped I would check the zener diode in your biasing circuit. If this were internally shorted or shorted to ground it could cause the tube to conduct a very large amount of current. Considering that the problem occurs when you switch on the biasing circuit (key the amplifier), I would suspect a problem with this circuit. Phenomenon would be like flipping a switch across the + and – terminals of a power supply.

Your problem is complex to say the least, I can think of several things that would cause what you have experienced. Without schematics or knowledge of what components are not original equipment, this problem would be very difficult to troubleshoot in this forum.
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WA9SVD
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Posts: 2198




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« Reply #5 on: January 12, 2003, 07:40:46 PM »

Hi Scott,
     I haven't checked to see if I could find a schematic for the SB230, but you DO have a problem.  First of all, since you havbe the assembly manual, you shouls also have the schematic.  Just where is the 0.75 Amp fused used in the circuit?  With a power amplifier, that doesn't seem the correct rating.  That would only allow about a 90 W. rating for the entire amplifier!
    Before tou replace the resistor and turn on the rig again, you need to do some serious trouble shooting.  Again, follow a previous post, and remove the tube and go through the preliminary checks.  Then, if all goes well, connect an adequate dummy load before replacing the tube and proceeding.  Without the dummy load, you could possibly get enough stray feedback to cause a "parasitic oscillation" to wreak havoc.  (That's probably what took out the resistor in the first place.)  BTW, where in the circuit is that resistor located?  I don't remember if you told us that.
    Also, is the amplifier neutralized?  If the neutralization is misadjusted, that could easily cause the amp to go into oscillation, causing your problem.
    (Remember an oscillator is an amplifier with sufficient feedback.  An amplifier is an oscillator LOOKING for sufficient feedback...)
Good luck, and keep us informed as to your progress.
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WA9SVD
Member

Posts: 2198




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« Reply #6 on: January 12, 2003, 10:45:21 PM »

Hi Scott,
    I finally found the schematic for your amp.  The fuse IS in the grid circuit.  And 750 mA (0.75 A) is WAY too much grid current.  The first thing I'd check is the Zener diode in the grid circuit; it's a 1N2806GA.  The circuit indicates it's a 18.6 V, 1.5A diode, but that doesn't correspond to a zener diode spec.  I would think they meant a 18.6 V, 1.5 WATT Zener diode.  I'd replace the diode before anything else, perhaps with a 5W zener diode of the same voltage rating.  If the diode was bad, it could explain the high grid current; perhaps the original design was marginal.  Nonetheless, try to identify and eliminate any source of unwanted feedback that could result in oscillation of the circuit, which could also lead to excess grid current.
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