Heath SB-220 Squeals on AM

[mitch169]

I just bought an SB220, which squeals on AM 10 and 11 Meters. It has the Harbach Electronics SK-220 Soft-Key Keying Interface and Replacement Filter Capacitor Block.
SSB is nice and clear both 11 and 10 meters.
110v
Does anyone know what it could be?

 

[linearone]

I just bought an SB220, which squeals on AM 10 and 11 Meters. It has the Harbach Electronics SK-220 Soft-Key Keying Interface and Replacement Filter Capacitor Block.
SSB is nice and clear both 11 and 10 meters.
110v
Does anyone know what it could be?

Squeal or buzz?

 

[mitch169]

It's coming from the tubes. I did more testing if I move the mike away it stops. so I'm thinking it must be my voice or room noise reverberating from the tubes and causing feedback or it's maybe RF?

 

[linearone]

It's coming from the tubes. I did more testing if I move the mike away it stops. so I'm thinking it must be my voice or room noise reverberating from the tubes and causing feedback or it's maybe RF?

Squeals are always rf getting into the mic audio

 

[mitch169]

I just ordered Star Quad Balanced Male to Female Microphone XLR Cables with Amphenols

 

[KneeBiter]

I just bought an SB220, which squeals on AM 10 and 11 Meters. It has the Harbach Electronics SK-220 Soft-Key Keying Interface and Replacement Filter Capacitor Block.
SSB is nice and clear both 11 and 10 meters.
110v
Does anyone know what it could be?

Check the anode plate cap on top of the tubes. Make sure both the screws are tight. Also look at the parasitic resistors coming off the top of tube and see if they are burnt or cracked. BUT BE SURE ITS UNPLUGGED AND DISCHARGED BEFORE DOING SO.

 

[sp5it]

Test for squeal on dummy load and come back with result.

 

[linearone]

You didnt say you were an outboard audio guy.enjoy it

 

[mitch169]

Check the anode plate cap on top of the tubes. Make sure both the screws are tight. Also look at the parasitic resistors coming off the top of tube and see if they are burnt or cracked. BUT BE SURE ITS UNPLUGGED AND DISCHARGED BEFORE DOING SO.

Yes, One of the resistors is burnt and the other looks like it's getting there.
The person that I bought it from said he would send me them.

 

[KneeBiter]

Yes, One of the resistors is burnt and the other looks like it's getting there.
The person that I bought it from said he would send me them.

You are probably hearing your voice resonating through them. Replace them and report back.

 

[AudioShockwav]

And no real radio person uses AM anyway. lol

Your not correct with that statement, I have had lots contacts on AM on many bands over the years. There is often stations running AM on 29 Mhz.

73
Jeff

 

[nomadradio]

Just scab another resistor across the existing suppressor. No need to remove the one with the 'scar'. If I'm right, you're hearing an arc across the gap in the resistor's film layer. Scabbing a new resistor across it will stop this if that's the cause.

The other common cause we see is loose screws, especially the ones bolting the Tune/Load to the chassis. Next would be path from tube cap to the red blocking cap on the triangular bracket at the rear of the Tune control. Steel screws will cause this. We adopted the habit of using brass fasteners in high-current RF circuits.

73

 

[Captain Kilowatt]

The other common cause we see is loose screws, especially the ones bolting the Tune/Load to the chassis. Next would be path from tube cap to the red blocking cap on the triangular bracket at the rear of the Tune control. Steel screws will cause this. We adopted the habit of using brass fasteners in high-current RF circuits.

73

Good move. Steel hardware should never be used in a high RF path. In the broadcast business everything was stainless steel or brass. Even the transmitters were put together without a single regular steel screw/bolt/nut. They must be non-ferromagnetic or they can actually heat up considerably. In the ATU circuits for the AM sites, the ATU (antenna tuning unit) would be subjected to extremely high RF fields and the coils/capacitors were always bolted down to a large aluminum or stainless steel plate and all hardware was to be stainless. A few were brass if used to bolt copper feed lines etc. I offer this as a piece of advice to home brewers so they may avoid potential seemingly impossible to find problems down the road.