Heatkit SB-100 Need Help!! no grid or relative power showing on meter.
I'm a new ham and I really need some help. I've been checking every forum I can find but I can't find this exact problem...
I've been working on an SB-100 I picked up on Ebay for a few weeks now and I'm just stuck. It receives fine but it's not transmitting. I am not getting any grid current or relative power out at the meter. I have swapped out the 6146 finals with no change. I have replaced every resistor on the RF driver board and about half of the resisters on the audio, IF, and bandpass boards, the ones that were out of spec, with no change. I have tightened every board down and cleaned all the tube pins. I don't have an O-scope to test with. I can pick up RF on my shortwave sitting right next to it on the right frequency and sideband but I'm not making any real power. I have also checked the meter circuits and they seem to be in spec, and they read Plate, HV, and ALC ok. If I read the across the meter with my VOM I do see VERY tiny currents but not enough to move the needle, even with the relative power control all the way up.
I have not yet rebuilt the power supply, and I'm only getting about 780 volts which is about 20 volt shy of spec, but would that cause a problem like this? Any ideas? Any help would be greatly appreciated.
Check the screen voltage on the 6146 tubes. This voltage is removed when neutralizing the final amplifier and also when the SB-100 is used with the Heath SB-500 2-meter transverter.
The transceiver may have been modified to use with the SB-500 or the connection for the screen voltage may not have been replaced (or the connection was mis-wired).
My SB-102 had the same problem, the T/R relay was bad. Check it out or replace it, good luck sir.
"Patriotism is the last refuge of a scoundrel" Samuel Johnson
I do not have an SB-100 but I have an HW-100, and I expect to dig into it some more when I get a chance.
First, have you replaced the driver tube?
If you can hear the signal on a receiver then the TR circuit must be working, more or less but as suggested you should check it.
Did you run through the resistance measurements in the manual? By the way, the HW-100 manual has some serious errors in that egard; I don't know about the SB-100
These rigs have a problem where the two layer CW level/Mic gain control pot gets separated and when you are turning one to max the other one is actually at min. Make sure those are lined up properly.
Well I checked out the CW/Mike pots, and they seem to be moving together, but are they supposed to be different value pots? I get about 8k ohms on one and about 800k ohms on the other.
Also, I noticed that when I am in Tune or CW mode, the ALC meter stays steady, but in LSB or USB the needle falls to zero, is this normal? I'm wondering if maybe its a problem with the ALC circuit...
And, thanks for answering, I really really appreciate it.
I checked the 6146's and I'm reading -50 volts on pin-5/Grid receive or transmit.
I've got -03/+240 (rec/trans) ON PIN-3/screen?.
I'm reading about +0.8 on the plate/pin-4 on tune or CW, which drops to about zilch on usb/lsb.
I've got +800 up top.
Last edited by KD2CGG; 05-31-2012 at 03:20 AM.
If your plate voltage measurements are correct, .8VDC in TUNE/CW then dropping to zero in USB/LSB then the final tube's plate voltage circuit has a problem. I recommend you check the voltage on both side of the final amplifier's
Originally Posted by KD2CGG
RF choke, L901. You said you have +800VDC which I assume you are reading from the plate meter. Don't relay solely on the plate meter to determine if HV is present. Measure the plate voltage on both side of the RF choke L901.
If HV is present on one side of the choke but not the other than L901 is open or has a very bad solder connection. You also can measure the plate voltage at the tube's plate caps but use caution when doing so. HV is "always"
present whether in receive or transmit.
No, I read 800 VDC from both caps using my multimeter. The 0.8 volts I got directly from pin-4 which the manual says goes to "Plate" on the meter. I have been trying to do an alignment by the manual, and I have the bias adjusted so the plate reads 50ma but I can't get any farther since the next step is adjusting relative power and I don't seem to have any.
The reading you're getting on pin 4 of the 6146 PA is okay. That's connected directly to the cathode of the 6146.
There can be a number of things that can go wrong.
Place the unit in tune and with the driver properly tuned bring the carrier control up while watching the plate current meter. As you turn the carrier control up the plate current should start to raise. If it does that then the driver maybe working properly, if there is never an increase in plate current then your problem is probably the driver or a stage before it.
Try these things and let us know what happened.
Check the 6CL6 driver tube.
Bill - K5MIL