Heathkit SB-303 amateur band receiver. - UK Vintage Radio Repair and Restoration Discussion Forum

Scimitar · 28th Apr 2016, 12:24 pm

I bought an SB-303 as untested. You can guess that when it arrived, it was not only faulty, but had signs of being got at.

Initial tests showed no power supply. That was traced to a duff circuit breaker on the back panel. The screw had stripped and the switch was floating inside. That has been removed for now.

Next was the low Voltage board. Here the voltages were all over the place. A new set of capacitors cleared that up. This caused the IF/Audio board to start smoking. As I don't have any extender boards, I couldn't see what was getting hot. Replacing all the electrolytics again helped. Final issue with that board was fitting a mute plug to get the audio back!

So now the IF and audio was functioning. Touching the IF input gave a rise in noise, although not as much as I expected.

Moving to the mixer board, there were lots of crackles so it was removed. Brushing the dust off the board moved the dual gate Mosfet, so a close look revealed that it had been replaced with a 3N201 and dry jointed on two of its legs. I repaired that and it is now much more alive than it was. The preselector peaks RF noise and removing the long wire aerial drops the noise level, so the front end is largely working. However, the radio is deaf. Tuning up and down all the bands (directly on the shaft because the knob slips) have a few odd heterodynes from the computer and at one point a very weak SSB station on 40m. I have cleaned all the switch wafers and reseated all the boards. Also cleaned the ground contacts on the plug in boards.

I no longer have a signal generator so now at a bit of a loss about where to look to sort it. Trawling the web doesn't give much help and I have only the circuit diagram to work from.

Anyone with ideas please? The receiver cost me £100 so if I sell it on, I will lose on it, plus I wanted to use it anyway!

Station X · 28th Apr 2016, 2:38 pm

Without an SG you're a bit stuck.

Listen for the Xtal oscillators on another receiver?

Check that voltages match those on the diagram?

Circuit here, but I suspect it's the one you have:-

http://www.vintage-radio.info/heathkit

28th Apr 2016, 12:24 pm	#1
Scimitar Heptode Join Date: Oct 2015 Location: Gloucestershire, UK. Posts: 719	Heathkit SB-303 amateur band receiver. I bought an SB-303 as untested. You can guess that when it arrived, it was not only faulty, but had signs of being got at. Initial tests showed no power supply. That was traced to a duff circuit breaker on the back panel. The screw had stripped and the switch was floating inside. That has been removed for now. Next was the low Voltage board. Here the voltages were all over the place. A new set of capacitors cleared that up. This caused the IF/Audio board to start smoking. As I don't have any extender boards, I couldn't see what was getting hot. Replacing all the electrolytics again helped. Final issue with that board was fitting a mute plug to get the audio back! So now the IF and audio was functioning. Touching the IF input gave a rise in noise, although not as much as I expected. Moving to the mixer board, there were lots of crackles so it was removed. Brushing the dust off the board moved the dual gate Mosfet, so a close look revealed that it had been replaced with a 3N201 and dry jointed on two of its legs. I repaired that and it is now much more alive than it was. The preselector peaks RF noise and removing the long wire aerial drops the noise level, so the front end is largely working. However, the radio is deaf. Tuning up and down all the bands (directly on the shaft because the knob slips) have a few odd heterodynes from the computer and at one point a very weak SSB station on 40m. I have cleaned all the switch wafers and reseated all the boards. Also cleaned the ground contacts on the plug in boards. I no longer have a signal generator so now at a bit of a loss about where to look to sort it. Trawling the web doesn't give much help and I have only the circuit diagram to work from. Anyone with ideas please? The receiver cost me £100 so if I sell it on, I will lose on it, plus I wanted to use it anyway!

28th Apr 2016, 2:38 pm	#2
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,286	Re: Heathkit SB-303 amateur band receiver. Without an SG you're a bit stuck. Listen for the Xtal oscillators on another receiver? Check that voltages match those on the diagram? Circuit here, but I suspect it's the one you have:- http://www.vintage-radio.info/heathkit __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.