FT-102 Very distorted modulation in SSB and AM - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

seb5347 · 18th Jun 2022, 2:01 pm

The SN16913 has been changed it is new, it is not him. Hard to know if I have a standing bias without the correct board schematic. For the 455khz it is too weak to be sniffed with a probe and listened to on another receiver.
For modulation, it looks like a non linear SSB, very dry as a final bias problem

seb5347 · 18th Jun 2022, 2:05 pm

I also thought: my 2sc1971 that I changed or could the front floor be at fault? (when the heater is in use I manage to get 180w out so I thought it was ok) (the 2sc1971 that I put in has already been used) Is this circuit functional when listening on the transverter output?

Jon_G4MDC · 18th Jun 2022, 3:04 pm

Don't assume all is well with the balanced modulator just because the IC has been changed and it made no difference. It can have other problems. I really do advise to try the 455kHz DSB signal for distortion first.

Yes the 2SC1971 is the last stage before transverter. If that is to blame then just because it amplifies, it doesn't mean it is linear.

You can try this very crudely with CW. With key down the CW drive control should increase output power proportionally to rotation.
If you have jumps or hysteresis in the action of CW drive it's a sign that the amplifier is non linear.

seb5347 · 18th Jun 2022, 10:03 pm

Yes you are right, I failed to do the DSB 455 kHz test for distortion because I can't hear the 455 with my probe on another receiver.

OK for 2SC1971 which is the last stage before transverter. Admittedly it amplifies but that does not prove that it is linear, it is perhaps by there that there can be a cause.

Otherwise in CW the drive control increases the output power proportionally to the rotation. This may not be enough to conclude that 2sc1971 is good.

New observation this evening: when I am plugged into the transverter output, the compressor works. When I'm with the heater and therefore with the power of the tubes, the modulation compressor does not work at all even at the bottom (no modulation when I switch it on regardless of the level)

73's
SEB

Jon_G4MDC · 19th Jun 2022, 10:09 am

I'm beginning to wonder if these two problems are connected.

The speech processor clipping is done on the 455kHz filtered SSB before it is mixed up to 8.2 MHz, filtered again to remove clipping IMD, then sent to the RF Board and the 2SC1971 and the rest.

Do you have a second receiver that could look at the 8.2 MHz output from the IF Unit at J15 Pin 6? Here you should see whether SSB is distorted and whether the processor is working or not.

seb5347 · 19th Jun 2022, 5:29 pm

Hello Jon,
Good fault found!!
It was very simple but I searched for a long time. I started by disconnecting the 2sc1971 and putting a wire on the collector of the 2sc1589 just before. Transverter output test, perfect modulation
Looking at the schematic of the RF-unit I suspected a lack of polarization on 2sc1971. I noticed that R35 (39 ohms on 2sc1971 base) was burned. After replacing R35 modulation ok.
Base voltage changed from 0.5v to 1.2v. (moreover they give 1.2v on service manual, that seemed to me too much and an error because often the polarization is done from 0.665v in class AB. But it is certainly this transistor which is different. So it works flawlessly now and the modulation compressor is working.

Otherwise I had another question.
Otherwise I had another question: at first I changed the PNP 2SB705 transistor on the chassis (12v regulation) to a BD304B (which is slightly less powerful peak 12A 60v). Do you think I can leave it or not?
73's
SEB F5SSL

Jon_G4MDC · 20th Jun 2022, 9:34 am

Good result Seb! Yes the 2SC1971 having no bias would put it in Class C operation - very nice to mangle up SSB modulation.

On the transistor it's not dropping much Voltage only 15V to 12V so dissipation shouldn't be much of an issue. Does it run warm in use? I might leave it until I came across something better.

seb5347 · 20th Jun 2022, 9:47 pm

Hello Jon,
Ok for your opinion. The original transistor would like -140v 10a continuous 15a peak and the one I put -60v 8a continuous 12a peak. I too found the original transistor oversized. I ordered some from China but it will take time. The one I put heats normally. 73's SEB

18th Jun 2022, 2:01 pm	#21
seb5347 Triode Join Date: Jun 2022 Location: Alençon, Normandy, France. Posts: 15	Re: FT-102 Very distorted modulation in SSB and AM The SN16913 has been changed it is new, it is not him. Hard to know if I have a standing bias without the correct board schematic. For the 455khz it is too weak to be sniffed with a probe and listened to on another receiver. For modulation, it looks like a non linear SSB, very dry as a final bias problem

18th Jun 2022, 2:05 pm	#22
seb5347 Triode Join Date: Jun 2022 Location: Alençon, Normandy, France. Posts: 15	Re: FT-102 Very distorted modulation in SSB and AM I also thought: my 2sc1971 that I changed or could the front floor be at fault? (when the heater is in use I manage to get 180w out so I thought it was ok) (the 2sc1971 that I put in has already been used) Is this circuit functional when listening on the transverter output?

18th Jun 2022, 3:04 pm	#23
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: FT-102 Very distorted modulation in SSB and AM Don't assume all is well with the balanced modulator just because the IC has been changed and it made no difference. It can have other problems. I really do advise to try the 455kHz DSB signal for distortion first. Yes the 2SC1971 is the last stage before transverter. If that is to blame then just because it amplifies, it doesn't mean it is linear. You can try this very crudely with CW. With key down the CW drive control should increase output power proportionally to rotation. If you have jumps or hysteresis in the action of CW drive it's a sign that the amplifier is non linear. Last edited by Jon_G4MDC; 18th Jun 2022 at 3:22 pm.

18th Jun 2022, 10:03 pm	#24
seb5347 Triode Join Date: Jun 2022 Location: Alençon, Normandy, France. Posts: 15	Re: FT-102 Very distorted modulation in SSB and AM Yes you are right, I failed to do the DSB 455 kHz test for distortion because I can't hear the 455 with my probe on another receiver. OK for 2SC1971 which is the last stage before transverter. Admittedly it amplifies but that does not prove that it is linear, it is perhaps by there that there can be a cause. Otherwise in CW the drive control increases the output power proportionally to the rotation. This may not be enough to conclude that 2sc1971 is good. New observation this evening: when I am plugged into the transverter output, the compressor works. When I'm with the heater and therefore with the power of the tubes, the modulation compressor does not work at all even at the bottom (no modulation when I switch it on regardless of the level) 73's SEB

19th Jun 2022, 10:09 am	#25
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: FT-102 Very distorted modulation in SSB and AM I'm beginning to wonder if these two problems are connected. The speech processor clipping is done on the 455kHz filtered SSB before it is mixed up to 8.2 MHz, filtered again to remove clipping IMD, then sent to the RF Board and the 2SC1971 and the rest. Do you have a second receiver that could look at the 8.2 MHz output from the IF Unit at J15 Pin 6? Here you should see whether SSB is distorted and whether the processor is working or not.

19th Jun 2022, 5:29 pm	#26
seb5347 Triode Join Date: Jun 2022 Location: Alençon, Normandy, France. Posts: 15	Re: FT-102 Very distorted modulation in SSB and AM Hello Jon, Good fault found!! It was very simple but I searched for a long time. I started by disconnecting the 2sc1971 and putting a wire on the collector of the 2sc1589 just before. Transverter output test, perfect modulation Looking at the schematic of the RF-unit I suspected a lack of polarization on 2sc1971. I noticed that R35 (39 ohms on 2sc1971 base) was burned. After replacing R35 modulation ok. Base voltage changed from 0.5v to 1.2v. (moreover they give 1.2v on service manual, that seemed to me too much and an error because often the polarization is done from 0.665v in class AB. But it is certainly this transistor which is different. So it works flawlessly now and the modulation compressor is working. Otherwise I had another question. Otherwise I had another question: at first I changed the PNP 2SB705 transistor on the chassis (12v regulation) to a BD304B (which is slightly less powerful peak 12A 60v). Do you think I can leave it or not? 73's SEB F5SSL

20th Jun 2022, 9:34 am	#27
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: FT-102 Very distorted modulation in SSB and AM Good result Seb! Yes the 2SC1971 having no bias would put it in Class C operation - very nice to mangle up SSB modulation. On the transistor it's not dropping much Voltage only 15V to 12V so dissipation shouldn't be much of an issue. Does it run warm in use? I might leave it until I came across something better.

20th Jun 2022, 9:47 pm	#28
seb5347 Triode Join Date: Jun 2022 Location: Alençon, Normandy, France. Posts: 15	Re: FT-102 Very distorted modulation in SSB and AM Hello Jon, Ok for your opinion. The original transistor would like -140v 10a continuous 15a peak and the one I put -60v 8a continuous 12a peak. I too found the original transistor oversized. I ordered some from China but it will take time. The one I put heats normally. 73's SEB