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Author Topic: Slipping VFO tuning on the Heathkit SB-401  (Read 2902 times)
K2OWK
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Posts: 1252




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« on: February 05, 2010, 09:54:49 PM »

I have a Heathkit SB-401 transmitter. The cicular tuning dial slips as does the dial pointer. I heard this is a common problem with this transmitter. Does anyone have a fix for this?
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K4TLJ
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« Reply #1 on: February 06, 2010, 02:47:05 AM »

Try tapping on the top of the plastic bezel. That may drop it enough to make the friction drive get a better grip.
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W5RKL
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Posts: 1031




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« Reply #2 on: February 06, 2010, 05:40:07 AM »

K4TLJ said:
"Try tapping on the top of the plastic bezel. That may drop it enough to make the friction drive get a better grip."


That will not fix the problem. The dark green escutcheon is attached to the front panel with "lips". The escutcheon have very little, if any, up and down movement and it is not the source of the problem and tapping on the top of the escutcheon will not fix the problem.

The problem is caused by the main tuning knob shaft rear disc do not have sufficient downward pressure on the mail dial's inner silver ring.

To fix the slippage, remove the main LMO knob and loosen the shaft nut. Apply some downward pressure on the shaft, not too much, and tighten the nut. Rotate the shaft and the dial should rotate smoothly. If it is tight and difficult to turn you have applied too much downward pressure. Excessive downward pressure is the cause of the "stress cracks" seen on the main dial.

You should be able to apply just the right amount of downward pressure that the dial does not slip yet tunes as smooth as oil on glass. That is assuming a previous owner did not damage (bend) the disc assembly at the rear of the main LMO tuning knob shaft. It also assumes there is no excessive wear in the brass LMO tuning shaft bearing. Many people tend to apply too much torque to the knob set screws which causes the aluminum shaft to produce "burrs". Pulling the shaft out of the brass bearing causes scaring of the bearing inner surface which leads to wear and 'wobble' in the shaft that affects tuning.

I have an SB-303 that tunes as smooth as oil on glass. It takes practice and quite often a number of tries to get the tuning right. Do NOT loosen the main dial set screw! This will cause the dial alignment to be off.

73s
Mike W5RKL
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K2OWK
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Posts: 1252




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« Reply #3 on: February 06, 2010, 03:31:38 PM »

Thank you Mike. The information you gave me was dead on. The tuning is operating smoothly. I have just a few more questions about this unit. While the tuning is smooth now it seems slighty out of calibration at least as far as the assembly manual. It is very close but, the full left on the dial is less then 90 degrees at the 2.5 scale marking. I would guess this is not to important as I will use a calibrated receiver as my tuning source. Also the tuning from full clockwise to counterclockwise does not move the full scale of the main dial. The last problem I hope is the zero set dial does not move the the zero set. It looks like the friction type pully does not contact the zero set plastic adjustment face.
Just for information I have not been a ham radio operator for more then 50 years. I am a retired electrical engineer and picked up the sb-401 to work on so when I take my test in April I will be ready to go. I have a Yaesu FRG7000 that I have been playing with that I will use as my receiver. Any help out there on the sb-401 will be greatly appreciated.
Regards
Barry
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WB2WIK
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Posts: 21737




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« Reply #4 on: February 06, 2010, 07:08:26 PM »

Sounds like the SB-401's in "rough" shape.  I've gone through many of these, very recently, that all align and work perfectly.  I suspect yours is "beat up" for some reason (no idea why).

However, if you can "spot" your receiver frequency and trust that to be accurate, then whether the dial on the 401 reads correctly or not doesn't really matter.

You need a way to "spot," and mute the receiver so when you transmit you don't experience acoustic feedback.  Simply keeping the RX volume low and using headphones might be good enough to accomplish that, but you still need a T-R relay or switch of some sort.
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W5RKL
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Posts: 1031




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« Reply #5 on: February 07, 2010, 06:41:22 AM »

Receiver muting and antenna change over are readily available in the SB-401 transmitter.

Antenna change over is done through the SB-401's RCVR ANT RCA socket. Simply connect the FRG-7K's antenna SO-239 to the SB-401's RCVR ANT RCA socket. In transmit, the antenna connected to the SB-401's SO-239 is switched from the RCVR ANT socket to the SB-401 PI Network output. The RCVR ANT socket is NOT grounded in transmit, simply disconnected from the antenna through the SB-401's internal relay, just like an external T/R relay would do.

After reviewing the FRG-7K manual's MUTE requirement, it appears the FRG-7K mute terminal is grounded to mute the receiver. Unfortunately, the SB-401's MUTE socket is grounded in standby and not grounded in transmit, the opposite of what the FRG-7K requires. Therefore, you have 2 options to choose from for receiver muting:

1. Use the Linear Relay socket to mute the FRG-7K if you do not intend on using a linear amplifier

or

2. Use an external 120VAC T/R muting relay circuit through the rear panel 2 prong socket (120VAC relay coil required) on the SB-401. In transmit, 120VAC is applied to the 2 prong socket on the rear of the SB-401 through the SB-401's internal relay terminals, pins 1 and 5. In standby, the internal relay is de-energized, removing the 120VAC from the 2 prong socket. A simple SPST 120VAC coil relay will do the muting job just fine.

Spotting with the SB-401 is done through the 401's Function switch Spot setting.

73s
Mike W5RKL
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K2OWK
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Posts: 1252




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« Reply #6 on: February 07, 2010, 03:03:49 PM »

Thanks again Mike. I am not going to use a amp, so I will be able to use the socket to groung and mute the FRG 7K receiver. I have to do some work on the tuning system, but it does not look like a big problem. Your fix for the slipping circular dial worked. Now I just have to align the tuning dials and pointer. Then try to descover why the zero plate adjust is not working. I do have the original manual for this transmiter, so I can follow the alignment proceedure. Again thank you for the excellent advice on this unit.

Regards, Barry (Magazoid)
PS: I hope I will be able to add a ham call to my post after I take my test in April here in Mississippi. I was a general operator in 1955 when you needed the code and the test looked more difficult. I was K2OWK. I see that call sign is still not used in New York.
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K8AC
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Posts: 1739




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« Reply #7 on: February 09, 2010, 03:16:30 AM »

Barry - just in case you're not aware of this since you've been away so long - you could probably get your old call back via the vanity call program.  These days, use of a K2 call isn't restricted to NY and NJ. 
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