Heathkit SB 101 Transceiver

[VU2CDW]

I got a Heathkit SB 101, assembeled in 1967,Was in use uo to Year 2000 as told by previous owner, after that was put in a shelf and was practically not in use.

I powered it up with all precautions suggested in other threads, initially there was only hissing sound for couple of days- I made a habit to switch it ON every day for at least half an hour, one day somehow got USB 20 Mtr Band working for receiving (40 Mtr also started working for receiving but now LSB switch is not putting the rig in LSB Mode, in fact, when I switch to LSB there is no sound !! Not even hissing sound !!! few weeks back it was USB which was not working now it is the turn of LSB it seems !!!!!!!)

I tried to tune the rig as the manual says but meter does not show any plate current - sometimes when meter is on ALC position, it shows some reading. When I press the PTT, I hear a click sound as if the relay is getting energised- but immediately without going in to the transmit mode rig comes to receiving.

I wish I could use this rig- no matter even if it is working on 20 mtr band.

How to fix this trnsmit problem- I need your suggestions and much sought help.

73

VU2CDW

[K9STH]

You need to clean all of the switches in the unit. Then tighten all of the machine screws that go through the chassis including those which hold the circuit boards in place. These become loose and/or build up corrosion over the years. Just by tightening the screws I fix about 90 percent of the problems in all of the "boat anchor" equipment that I repair for others.

Glen, K9STH

[W5RKL]

I agree with Glen, clean the switches.

The MODE switch does 2 things

1. Switches plate voltage between V1A and V1B

2. Selects either LSB or CW carrier crystal when V1B is the selected carrier oscillator.

USB carrier crystal is hardwired to V1A and only comes into play when V1A is used as the oscillator (USB and CW receive). The CW carrier oscillator crystal is used ONLY during TUNE and CW transmit. CW receive uses the USB carrier crystal. This is achieved through relay RL1 pins 1, 9, and 5 and MODE switch wafer 1R and 2R.

It's quite possible plate voltage applied through MODE switch wafer 1R and relay RL1 pin 1 and 9 to either carrier oscillators V16A and/or V16B and/or carrier oscillator crystal selection through switch wafer 2R may not be working due to dirty MODE switch wafers and relay terminals.

In receive, if the either V16A and/or V16B carrier oscillators are not working there will be no carrier oscillator RF signal present at the Product Detector V13C's cathode, pin 9. If this happens then there will be no recovered audio at the output of the Product Detector resulting in no audio applied to V14A and V14B audio amplifiers and nothing heard from the speaker. If V16A or V16B are not working, this also will result in no RF output in transmit mode, regardless of the MODE switch setting and the MIC/CW LEVEL control setting in TUNE and CW modes.

Final tube bias voltage level is controlled through relay RL2 pins 3, 11, and 7. In receive, bias voltage from the bias pot is applied through R916 and RFC902 directly to the grids of the final tubes cutting both final tubes OFF. In transmit, relay RL2 energizes and feeds a positive voltage from regulator tube V18, through RL2 pins 11 and 7, through R916 and RFC902, to the grids of both final tubes, reducing the level of bias voltage applied to the final tube grids, turning both final tubes ON, which results in a no signal idling current flow in the final tube (50ma plate current). It's quite possible the meter switch is very dirty/corroded or a wire has broken off of the switch, if the PLATE meter position, in transmit, is showing zero. Check the switch wiring and if the wiring is okay, clean the meter switch!

Relays RL1 and RL2 are controlled by relay amplifier V12B. The PTT microphone connector, pin 2 on the microphone socket, is wired to the cathode of V12B and the MODE switch wafer 2F. When the microphone socket pin 2 is grounded OR the MODE switch is placed in TUNE, the cathode of V12B is grounded, turning V12B ON and energizing both relays, RL1 and RL2.

If pressing the PTT button does not keep the relays energized but placing the MODE switch to TUNE energizes and keeps the relays energized, the problem is either in the microphone's PTT switch, wiring from the microphone's PTT switch to the microphone plug, pin 2 wiring from the microphone socket to the cathode of V12B, or the microphone plug pin 2 is not making good connection with pin 2 on the microphone socket.

73s
Mike

[VU2CDW]

Hi Mike and Glen,

Thanks for attending my problem, but before I could start working on it an another problem cropped up-

I was only receiving 20 Mtr signals using USB and SSB mode and signals were quite clearly comming in- suddenly I hear a small chirping sound - and signals were gone-

I checked completely the rig and power supply - nothing seems to be wrong- when I put the meter on grid current position, the needle banged to the extreme end of the meter- again I opened the rig from the case and observed that 4700 Ohm ( 1/2 Watt) resistor on modulator board has blackened.Can you tell me if this is the problem for making signals disappear ? If you refer the manual this is shown in the Pictorial 1-5, page 10 on the righjt hand side. This resistor is mounted close to a big Electrolytic capacitor of 20 Micro farad.

If I replace the resistor - will the signals come back and the needle will not bang the extreme end when in Grid position and there will not be any more damage to the Rig?

Please consider me a new commer to the hobby and I am really interested in bringing back the SB-101 to life.

Please help.

VU2CDW

[W5RKL]

The resistor you are referring to is R14, 4700 ohm. R14 is in series with the 300VDC that supplies the plate voltage for V1A, V1B, V16A, and V16B.

Since R14 burnt up then I suspect C12, 20ufd 350VDC, the orange round vertical electrolytic capacitor on the modulator board is shorted internally. Replace R14 and C12.

You need to fix the cause of resistor R14 burning up, which I believe is a shorted C12 (20 ufd 350VDC) first before you worry about the signals coming back. You can remove C12 and replace it with a radial lead 22ufd 350VDC electrolytic capacitor. Also replace R14 with a 4700 1/2 watt resistor.

73s
Mike

[VU2CDW]

Thanks Mike, I will try to do that today (Replacement of capacitor and resistor) - do you feel thatthis replacement would bring the transmitter also back to life??

73

VU2CDW

[W5RKL]

Thanks Mike, I will try to do that today (Replacement of capacitor and resistor) - do you feel thatthis replacement would bring the transmitter also back to life??

73

VU2CDW

I can't say one way or the other. As I said, if C12 (20ufd 350VDC) electrolytic capacitor is shorted, resistor R14 (4700 ohm 1/2 watt) will overheat and the plate voltage to the speech amplifier V1A and V1B plus plate voltage for the carrier oscillator V16A and V16B will be significantly affected, probably dropped to zero or very close to it.

If the carrier oscillator, V16A and V16B are not working when selected by the MODE switch there will be no carrier signal applied to the product detector which would lead to no recovered audio fed to the audio amplifier and ultimately to the speaker.

However, there could be other problems that need to be attended to so I can't say for sure replacing C12 and R14 will fix the SB-101. You will have to test it, once you replace C12 and R14.

Make sure when you replace C12 that you insert the "Positive" terminal of the replacement capacitor in the correct PC board hole. Pay close attention when you replace C12. The "negative" lead of the capacitor which, in this case goes to ground, will be the lead closest to the side of the capacitor with the wide black line down the side! Radial lead capacitors have both leads on one end. One lead is positive and the other is negative.

73s
Mike

[VU2CDW]

Hi Mike,

I changed the capacitor and resistor- but withing 20 seconds of switching on the power the resistor burnt out. What could be the reason?? I am worried now about this masterpiece of it's own time.

Pl.help.

73
VU2CDW

[W5RKL]

I would check for a short in the circuit where C12 and R14 is. R14 connects directly to the plate of V1A and Mode switch wafer 1R. There also could be a short in relay RL-1 wiring. There could be a short in V1's tube socket wiring, a shorted capacitor C3, etc.

Use the SB-101 schematic and trace the circuit to find the short. A pinched wire between the Mode switch and front panel, a short in relay RL-1 wiring, or a number of other sources that can be causing R14 to burn up.

I'm sorry but I can't tell you exactly what is causing R14 to burn up, you'll have to trace the circuit to find the short. I can only make suggestions where to look.

73s
Mike

[VU2CDW]

Hi Mike,

I tried to check for any short in the Modulator Board, apparently I did not find any with naked eyes on PCB tracks - however I put again the power to the rig and observed that a thin streak of smoke comes from the bottom of the mode switch. I have not yet opened the face plate - for simple reason that I have to get a small thin head screwdriver kind of thing to open the knobs. What could be the reason for the smoke from the bottom of the mode switch?? Does that mean my tubeis drawing more current??

73

VU2CDW