SB-101 Refurbishment

I have owned this SB-101 for a long time and already modified it to include Top Band so it was worth spending some more time replacing any components that had aged out of their specification limits. The following tasks were undertaken to fix a number of operational problems:

1. Transmit Audio Quality and Gain

Measurements showed that the screen voltage of the microphone preamplifier V1A was significantly higher than the anode voltage and the value of resistors R1 and R2 were within specification. Changing valve V1 made no real difference so I changed the value of R1 to 220K and R2 to 1Meg. This resulted in the anode voltage being higher than the screen voltage, an increase in voltage gain and a significant improvement in the distortion of the high level audio signal at the anode of V1.

2. Component Ageing

A significant number (30%+) of the fixed carbon resistors had aged well outside their tolerance and all of those have been replaced with new half watt 5% carbon film types.

3. Thermal Dissipation

The screen feed resistor for V3 has been changed from 22K to 33K 2W as the valve was running much too hot and the total dissipation greater than that specified for the 6AU6. The S meter zero works correctly although the 100K S meter zero bias resistor had previously been changed to 82K and the zero potentiometer replaced. The valve is still hot but the total dissipation is now within the specified maximum figure.

4. Receiver Sensitivity

Changing the RF stage to a 6HS6 and the first mixer to a 6HS6 or 6AH6 will improve the sensitivity and gain whislt retaining the correct heater rail voltages. No component changes were necessary and both valves worked within their specified ratings. This mod may have a negative impact on the dynamic range of the receiver as a result of increasing the front end gain.

5. VFO(LMO) Frequency Instability

Tuning around any one band displayed certain points at which the VFO would suddenly become unstable or jump frequency and this has already been shown to be due to the lubricating grease having dried up. The ideal solution is to remove the VFO from the chassis, open it up and clean and re-lubricate all of the moving parts. However, a visual inspection by removing the top plate showed that the inside parts were clean so all of the moving parts were carefully sprayed with a little lubricated switch cleaner using a long flexible tube and a dental mirror. Any excess was removed and the VFO retested - frequency instability gone. The overturn stop mechanism was also treated and the VFO closed up. If this is shown not to last then the entire assembly will be removed and thoroughly cleaned.

6. Crystal Filter Ripple

5dB ripple was apparent with the SSB filter and slightly less with the CW filter. Exchanging the SSB filter with another made no difference so matching networks were tried with the overall ripple then down to less than 2dB. Tests showed that corrective networks (adjustable parallel tuned circuits at 3.395Mhz) on both the input and output of the filters helped but the best results were obtained with just a 75uH RF choke (two 150uH chokes in parallel) from the switch side of C506 to ground. The resulting ripple with both the CW and SSB filters was below 2dB. Other SB-101 units may of course be different so owners will have to see what works best for them. This mod suggests that stray circuit capacitance of 29.5pF has been tuned out with the additional inductor.

7. SB Series Frequency Dial

These are becoming brittle with age and can also be damaged by over-tightening the friction drive connected to the back of the main tuning knob shaft. They can be repaired if they have not degraded too far:

a) The cracks can be sealed with a thin solvent based acrylic adhesive but you have to take a risk by gently pressing on the dial to open the cracks a little in order to get the adhesive in place and then pressing in the opposite direction to close them up. The advantage of this adhesive is that when dry it has a very small volume so any that gets trapped in the spiral groove does not impede the pointer from moving. The adhesive that I use comes in a plastic case with a one inch long thin metal applicator tube and it appeared to dissolve the dial plastic a little which helps to strengthen the finished joint.

Excessive amounts can be scraped out before completely dry or removed when dry with a small rat tail file or modellers scalpel. You may need a small jig to apply pressure to keep the dial flat until the adhesive has dried.

b) The black paint is on the back surface so as long as the white lettering is undamaged then the black paint can be over-painted. The white lettering can be replaced with white paint and a small ink drawing pen or bows before doing the black paint.

c) When re-assembling the dial, avoid excessive stresses on the plastic - the friction drive only needs sufficient pressure to avoid slippage.

8. General Checks

Inspect all of the solder joints, switches, valve-holders and capacitors to ensure that they are securely soldered, making good contact and are of the correct type and value. The rotary wafer switches had oxidised and were cleaned with very small amounts of Electrolube switch cleaner and lubricant. Do not use excessive amounts as it can be absorbed by the paxolin wafer material.

A previous owner may have made some changes that are less than obvious.