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SB-220 Problem on 80M

WA8ZYT

WA8ZYT

North Florida
Feb 21, 2012
201
37
38
Near Gainesville, Florida
dmfarms.com
My amp has about 70% reflected power on 80M. When just running Kenwood tranceiver, 2%. That can only mean one thing. The darn amp is putting out harmonics "somewhere". A check with a grid dip meter shows the tank circuit is resonant on the 2nd harmonic, more so than on the intended 80M frequency.

Most likely culprit is the bandswitch went FUBAR. Some CBer converted this to 11M after he burned out the 10M position. I may have no other choice but to make this a single band amp, as I am not really inclined to replace the bandswitch.
 

IT may or may not be the band switch.

Could be a few other things wrong with it. Cold solder joint finally showing up after all the years.

Input matching circuit caps could have went out of tolerance.

Or the cap in the tank used to add more capacatiance for 80 meter could have failed.

Band switch is easy enough to check with a visual inspection and an ohm meter.

I am not that far from you, I am in Clay county FL. So if you have any questions drop me a PM.
 
If the previous owner burnt up the 10 meter contact on the bandswitch, you may be having a problem switching in the 500 pf padder cap on 80 meters. The same wiper on the rotary bandswitch connects this part across the variable load cap 80. That would also cause it to tune much higher in frequency. You could jump the switch out by connecting the 500 pf cap directly across the load to test if this is the problem.

If that's not it, check the 200 pf grid bypass caps. They love to drift down in value and allow the grid to rise above RF ground. That can cause all types of oscillations. Just to check, we are talking about output VSWR here and not the VSWR the exciter is seeing? If the input VSWR is high, that's just an alignment of the input band coil for 80.
 
Honestly, the best advice I can think of is to inspect all of that amplifier (I hope you have the constructions and manuals). Especially is it's been modified for whatever reason and those modifications aren't specifically explained. It's a royal PITA, but it saves a lot of time and aggravation in the long run.
- 'Doc
 
I have directly grounded all the grids. Input circuit is fine, low swr.

Plate tuning is in the 10 m slot on the dial and loading is around 9. This makes both the capacitors barely meshed.

I get a nice dip on the GDO on the second harmonic. Around 7.5 Mhz, when bandswitch is in 80 M position.
 
WA8ZYT said:
I have directly grounded all the grids. Input circuit is fine, low swr.

Plate tuning is in the 10 m slot on the dial and loading is around 9. This makes both the capacitors barely meshed.

I get a nice dip on the GDO on the second harmonic. Around 7.5 Mhz, when bandswitch is in 80 M position.
Click to expand...

I assume you recognize these are not the correct tuning positions for 80 meters. Have you tried to tune the amp with the plate tune closer to 80 meters and more loading on the output? There are several possibilities including the fact that the way it is tuned now will cause the resonant frequency to go up.

If you can't get it resonant near the 80 meter settings, suspect that the 500 pf padder cap switched in by the bandswitch on 80 meters in not being connected through that switch. Remove the part and place it directly across the load variable to test.

Before doing this it may be helpful to post some pictures. You may have overlooked some part of the previous owners "10 meter only" mod. One example is they sometimes remove every other plate on the tune variable to increase the RF breakdown voltage and smooth out its tuning by reducing the overall capacitance.
 
Check the 500 PF and it is being switched in on 80 with ohm meter. Plate tune is between 10-15 mtrs on the dial and Load control is around 9.

On 40 both caps are at maximum mesh. Almost acts like it has not enough coil on 40 and too much on 80. Weird.
 
Played around with various things this past weekend. Got out the grid dip meter and everything dips higher in frequency than it should, by quite a bit.

It's starting to look like something is funky with the 80 through 20 coil. Nothing visual that I can see, but it's tough to really look at it, installed in amp. Acts like some of the coil turns are shorted, but I can't see anything. An extraction may be in order. Or, maybe just disconnect the bandswitch completely from the coil.
 
Been playing with this problem a little bit more:

Disconnecting 40 through 10 meters off the band switch has no effect on the problem.

Removed the 10/15 portion of the tank coil, it's 7 turns of copper tubing.

Been trying various combinations of coils to replace it.

Finally got a 35 turn on 1 1/2 coil form to resonate properly when connected to the 80/40/20 coil. It's almost as if the 80-20 coil is shorted, but I can't see anything amiss. May have to remove that coil completely from amp to do a really good inspection of coil, due to it's mounting location. Cap bank needs to come out to access screws holding coil in.

This is getting weirder and weirder.
 
The fastest way to troubleshoot this would be to get another 500pf doorknob and put it directly across the load tune cap after removing C56 from the circuit. This will bypass the bandswitch and the C56. You have to suspect C56 may have lost its value before suspecting a bad coil if the resonant frequency has doubled no matter how you tune and load it. Are you sure all of the RF output hardware is secured tightly? Anything from the bass nuts on the back of the variable caps to their grounds on the deck can mess things up quickly too.
 
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Update on problem...

I think I have finally figured this out. First I am replacing the entire tank coil bandswitch assembly, because the 10/15 coil was butchered, and the 10 meter position in the bandswitch is burned out.

Second, I have determined that half of the stator AND rotor plates on the plate tuning capacitor have been removed!

I have a new plate tuning cap on it's way, and a complete tank coil, with bandswitch assembly.

I'll let you all know how this turns out.
 

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