Looking at the schematic, the AVC is coming off the plate of the diode. The cathode is connected to a divider off the VR tube but it will not be strongly positive. On a strong signal the AVC should swing negative as it is going to the control grids. I would suspect a grid to screen grid short somewhere which could have started after a power cycle. A less likely cause would be a leak or short on the capacitor feeding the plate of the AVC diode.
Excellent, good things to look over. YES! You were correct. V4 6SK7, I found low resistance, removed the tube, gone. Replaced the tube, receiver is working correctly. Tested that tube, fine on my B&K 747. Intermittent, tossed it! Listening to the RX now.
@@d-labelectronics Arghh .. a short in a tube was also crossing my mind, but for some reason I discarded that idea. Still happy that you found it. But still fits to my explanation why bands 1&2 were low sensitivity. And I suspected the correct stages .. ;)
I really think things will make more sense when the plate voltages are halved. By the way you have taught me way more than this little little give back. Blessings and 73, Albert KR3HAB
i appreciate this problem solving in real time d-lab,,these are questions ive had on my nc183d about grid voltage and cutoff point or overload,, im watching
The S meter is driven by the plate current of the RF amplifier which is controlled by the AVC voltage on the control grid of this amplifier. The negative AVC voltage also goes to the control grids of the first and second if amplifiers. The cathode of the left diode in the 6H6 is slightly positively biased by the voltage divider and is regulated by the VR150 regulator tube. This voltage can be calculated by using the resistor division ratio. The left diode plate of the 6H6 is rf coupled to the plate of the third if amplifier and the shunt rectifier action generates a negative AGC voltage proportional to the rf voltage on the third if amplifier plate. This dc voltage may be as high as a few volts on strong signals. Either the 6H6 has failed or some component in the AGC DC voltage going to the RF amplifier control grid has failed.
Terry I worked on 3 of these SX-71 and 2 of them had a problem in the 2nd converter unit , there is tuning slugs in that unit and they have problems with them coming off the brass tuning rods, and I had had one that had a bad pico farad capacitor, hopefully this helps out, your friend Jim Gauthier KD8FHC
I have this receiver. The "S" meter was missing. The 2 wires for the meter were tied back and the ends taped. The pot adj for the S meter barely moved maybe a quarter of an inch.
Found on the antique radio lab forum from 2012 ......................... This is from the manual S meter adjustments mechanical adjustment- Turn the receiver off. Directly below the S-meter on the front panel is a round metal medallion which is pivoted at the bottom so that it may be rotated to one side to provide access to the meter adjustment screw. Carefully rotate this adjustment screw until the pointer is in line with the right hand index mark on the "S" meter. ---------------------------- If this doesn't work then you are going to have to investigate the following parts first, then voltages. Before plugging in the set do the following check to make sure the values are correct. C10A cap 4000mmf from ground may be defective and reducing voltage. R9 330,000 ohms R42 should be 3,300 ohms and c108 4000mmf R43 ("S" meter adjust) can be either 200 ohms or 500 ohms R 44 82 ohms 1/2 watt yellow lead from meter For voltage checks set front panel control to the following: Electrical adjustment- 1. Short the antenna terminals to the chassis 2. Set the receive/standby switch at, RECEIVE, B.F.O. switch at OFF, SENSITIVITY control fully clockwise and the NOISE LIMITER switch at OFF. 3. Turn the receiver on. 4. The electrical adjustment control for the "S" meter is located on the rear apron of the chassis, Slowly rotate this control until the "S" meter pointer indicates zero. BFO off Reception normal I.F. Noise limiter on Main tuning - low freq end of dial Band-spread - low freq end of dial Band selector band 1 tone CCW Volume CCW (switch on) Receive/ Standby Receive Sensitivity CW Check for the following voltages. V2 (6AU6-1st Converter) Measured between socket terminals and ground with zero signal input (K) cathode 5v (SG) suppressor grid 75v (P) plate 250v (G) (PIN 6)Grid -12 (or 0v varies depending on setting of main tuning gang and switch)
If this does not reveal anything, my best diagnostic tool was run resistance checks (unpowered) either to ground or to the B+ supply as follows for V1: Pin 1: BFO off - 1 Meg BFO on - 2.2 Meg Pin 2 & 7 - 8K Pin 3 GND Pin 4 (filament) Pin 5 (to Pin 2 of V13) - 4K Pin 6 (to Pin 2 of V13) - 11K. If there are significant or nonsense results check for resistor or miss-wired problems.
I have the info for the other tubes but I think this will be enough for you to find the problem. Suspect everything. You don't have bandspread markings for the 15 meter band which indicates you probably have a Run 1 or Run 2, and there is a service bulletin #1950-109 December 1 1950, which applies to the early Runs where they found leakage between B+ and AVC across the terminal boards of IF transformers T-7 and T-8 which causes the RF and/or the 1st IF tube to fail after 1/2 hour to 6 hours of use. This may be the reason for over 10 times the expected AVC voltage, You should check tubes V1 and V2 again. If they now have a problem, then the only solution Hallicrafters had was to replace the IF transformers.
The SX-71s are notorious for having undocumented changes put into them by former owners. I am guilty also as I experimented with the optimum cathode bias on V5, for example. I will give a diagnosis prescription in steps, one comment at a time. First the plate voltage by inference from the S-meter voltage is double anything that is used on any plate in an SX-71. The voltage readouts for V1 (6BA6) should be with BFO off: Pin 1 - no value (GND) Pins 2 & 7 - 1.8 Pin 3 - 0 Pin 4 (filament) Pin 5 (plate) - 210 Pin 6 - 120. Likely there is a miss-wire in the power supply to the Dc Power socket and Standby switch or somewhere in there that a former ham owner messed up or could T22 the power transformer have an open center tap that is supposed to go to ground?
Don't really know the radio, just kinda good at troubleshooting random electronics .. but regarding the low sensitivity in band 1+2: As far as I see on the schematic, it works as a single conversion receiver in bands 1+2 and dual conversion starting from band 3. This is necessary since the higher IF (2075kHz) would be in the middle of band 2. It just skips the second converter V3 (6BE6) by connecting the IF filter on its output to the plate of the first mixer. Just an idea .. what if you have significantly lost gain somewhere, preventing the receiver from going into AGC (it does have delayed AGC, cathode of 6H6 AGC rectifier is biased), but stronger signals still coming through. Band 1+2 skip a stage, further reducing the gain, making it more noticeable there. I suspect the RF amp to be working, since it can still drive the S-Meter as an DC amplifier. The two mixers should be working fine .. the would stop oscillating if there is something seriously wrong there. So the 3 IF stages are suspicious in my eyes. But maybe it's a known fault by the experts.
Looking at the schematic, the AVC is coming off the plate of the diode. The cathode is connected to a divider off the VR tube but it will not be strongly positive. On a strong signal the AVC should swing negative as it is going to the control grids. I would suspect a grid to screen grid short somewhere which could have started after a power cycle. A less likely cause would be a leak or short on the capacitor feeding the plate of the AVC diode.
Excellent, good things to look over. YES! You were correct. V4 6SK7, I found low resistance, removed the tube, gone. Replaced the tube, receiver is working correctly. Tested that tube, fine on my B&K 747. Intermittent, tossed it! Listening to the RX now.
@@d-labelectronics You have provided so much good advice I feel honored to have helped in any way.
@@d-labelectronics Arghh .. a short in a tube was also crossing my mind, but for some reason I discarded that idea. Still happy that you found it.
But still fits to my explanation why bands 1&2 were low sensitivity. And I suspected the correct stages .. ;)
I was chasing my tail on a sx99. I read these posts, and checked my tubes again. Both 6sg7 had grid shorts!
My father's boss gave me a beautiful Halicrafters Radio when I was a child, too bad I don't even remember what happened to it. I wish I had it now.
This video is great because of all of the positive comments from viewers. I am glad it is working so well now.
I really think things will make more sense when the plate voltages are halved. By the way you have taught me way more than this little little give back. Blessings and 73, Albert KR3HAB
i appreciate this problem solving in real time d-lab,,these are questions ive had on my nc183d about grid voltage and cutoff point or overload,, im watching
The S meter is driven by the plate current of the RF amplifier which is controlled by the AVC voltage on the control grid of this amplifier. The negative AVC voltage also goes to the control grids of the first and second if amplifiers. The cathode of the left diode in the 6H6 is slightly positively biased by the voltage divider and is regulated by the VR150 regulator tube. This voltage can be calculated by using the resistor division ratio. The left diode plate of the 6H6 is rf coupled to the plate of the third if amplifier and the shunt rectifier action generates a negative AGC voltage proportional to the rf voltage on the third if amplifier plate. This dc voltage may be as high as a few volts on strong signals. Either the 6H6 has failed or some component in the AGC DC voltage going to the RF amplifier control grid has failed.
Maybe try simply swapping the AVC tube and see if the behavior changes? Definitely sounds related to the AVC circuit.
Terry I worked on 3 of these SX-71 and 2 of them had a problem in the 2nd converter unit , there is tuning slugs in that unit and they have problems with them coming off the brass tuning rods, and I had had one that had a bad pico farad capacitor, hopefully this helps out, your friend Jim Gauthier KD8FHC
I have this receiver. The "S" meter was missing. The 2 wires for the meter were tied back and the ends taped. The pot adj for the S meter barely moved maybe a quarter of an inch.
Beautiful piece of equipment!
Found on the antique radio lab forum from 2012 .........................
This is from the manual
S meter adjustments
mechanical adjustment-
Turn the receiver off. Directly below the S-meter on the front panel is a round metal medallion which is pivoted at the bottom so that it may be rotated to one side to provide access to the meter adjustment screw. Carefully rotate this adjustment screw until the pointer is in line with the right hand index mark on the "S" meter.
----------------------------
If this doesn't work then you are going to have to investigate the following parts first, then voltages.
Before plugging in the set do the following check to make sure the values are correct.
C10A cap 4000mmf from ground may be defective and reducing voltage.
R9 330,000 ohms
R42 should be 3,300 ohms
and c108 4000mmf
R43 ("S" meter adjust) can be either 200 ohms or 500 ohms
R 44 82 ohms 1/2 watt yellow lead from meter
For voltage checks set front panel control to the following:
Electrical adjustment-
1. Short the antenna terminals to the chassis
2. Set the receive/standby switch at, RECEIVE, B.F.O. switch at OFF, SENSITIVITY control fully clockwise and the NOISE LIMITER switch at OFF.
3. Turn the receiver on.
4. The electrical adjustment control for the "S" meter is located on the rear apron of the chassis, Slowly rotate this control until the "S" meter pointer indicates zero.
BFO off
Reception normal I.F.
Noise limiter on
Main tuning - low freq end of dial
Band-spread - low freq end of dial
Band selector band 1
tone CCW
Volume CCW (switch on)
Receive/ Standby Receive
Sensitivity CW
Check for the following voltages.
V2 (6AU6-1st Converter)
Measured between socket terminals and ground with zero signal input
(K) cathode 5v
(SG) suppressor grid 75v
(P) plate 250v
(G) (PIN 6)Grid -12 (or 0v varies depending on setting of main tuning gang and switch)
Nice to see that went you are stumped that you can edmite it thank you terry Mr. D lab
Yep, I always know there is a wealth of knowledge out there. Plus its good to get others involved
@@d-labelectronics I hope it helps
If this does not reveal anything, my best diagnostic tool was run resistance checks (unpowered) either to ground or to the B+ supply as follows for V1: Pin 1: BFO off - 1 Meg BFO on - 2.2 Meg Pin 2 & 7 - 8K Pin 3 GND Pin 4 (filament) Pin 5 (to Pin 2 of V13) - 4K Pin 6 (to Pin 2 of V13) - 11K. If there are significant or nonsense results check for resistor or miss-wired problems.
Sir solid model nice receiver and thanks for sharing
I have the info for the other tubes but I think this will be enough for you to find the problem. Suspect everything. You don't have bandspread markings for the 15 meter band which indicates you probably have a Run 1 or Run 2, and there is a service bulletin #1950-109 December 1 1950, which applies to the early Runs where they found leakage between B+ and AVC across the terminal boards of IF transformers T-7 and T-8 which causes the RF and/or the 1st IF tube to fail after 1/2 hour to 6 hours of use. This may be the reason for over 10 times the expected AVC voltage, You should check tubes V1 and V2 again. If they now have a problem, then the only solution Hallicrafters had was to replace the IF transformers.
The SX-71s are notorious for having undocumented changes put into them by former owners. I am guilty also as I experimented with the optimum cathode bias on V5, for example. I will give a diagnosis prescription in steps, one comment at a time. First the plate voltage by inference from the S-meter voltage is double anything that is used on any plate in an SX-71. The voltage readouts for V1 (6BA6) should be with BFO off: Pin 1 - no value (GND) Pins 2 & 7 - 1.8 Pin 3 - 0 Pin 4 (filament) Pin 5 (plate) - 210 Pin 6 - 120. Likely there is a miss-wire in the power supply to the Dc Power socket and Standby switch or somewhere in there that a former ham owner messed up or could T22 the power transformer have an open center tap that is supposed to go to ground?
👍👍Nice receiver 73s QRO
APPRECIATED.
Terri, where does the meter sit with the power off the sx 71 is a little weird the meter sits all the way on the right at rest.
I missed the bottom part of your pinned comment, however, those plate voltages are way too high, Albert
this looks like a question for Mr. Carlson!
👍👍
Don't really know the radio, just kinda good at troubleshooting random electronics .. but regarding the low sensitivity in band 1+2:
As far as I see on the schematic, it works as a single conversion receiver in bands 1+2 and dual conversion starting from band 3. This is necessary since the higher IF (2075kHz) would be in the middle of band 2. It just skips the second converter V3 (6BE6) by connecting the IF filter on its output to the plate of the first mixer.
Just an idea .. what if you have significantly lost gain somewhere, preventing the receiver from going into AGC (it does have delayed AGC, cathode of 6H6 AGC rectifier is biased), but stronger signals still coming through. Band 1+2 skip a stage, further reducing the gain, making it more noticeable there.
I suspect the RF amp to be working, since it can still drive the S-Meter as an DC amplifier. The two mixers should be working fine .. the would stop oscillating if there is something seriously wrong there. So the 3 IF stages are suspicious in my eyes.
But maybe it's a known fault by the experts.
Thank you for your input. I will investigate. There will be a follow up video