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Old 17th Jun 2022, 3:46 pm   #1
seb5347
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Default FT-102 Very distorted modulation in SSB and AM

Hello
I m restoring a ft-102, lot of issue, had 24v to 12v regulation bad 2sb705 and 2sa733, i have repair the power supply. 12v ok now but 24v arrived on many boards instead of 12...
In the previous message I also repaired the display which displayed the frequency incorrectly.
There remain two problems, one of which is very serious: the modulation in SSB and AM is very distorted, almost incomprehensible. Quiescent current is good on the PA at 75ma and the oscillators are well tuned. Logically it could come from the balanced modulator but in AM it shouldn't.
I note that in tune position I go out 180w so everything is good level valves. In 50/60w modulation speaking in SSB but it is not understandable
I note that I changed the 2SC1971 on RF-UNIT, it is an amplification stage before the 12BY7 (it had burned out because of the 24v)
I also point out that in FM the modulation is correct. Do you have any idea because this is the first time I have encountered this phenomenon. 73's SEB
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Old 17th Jun 2022, 4:34 pm   #2
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Default Re: FT-102 Very distorded modulation in SSB and AM

A good start would be to check what the modulation is like at the transverter drive socket with the heaters of the valves switched off.

It comes from the 2SC1971 you have replaced. If it is no good there the driver and PA will only make it worse.
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Old 17th Jun 2022, 4:46 pm   #3
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Default Re: FT-102 Very distorted modulation in SSB and AM

In what way did the original 2SC1971 fail, short-circuit? If so check the condition and value of the emitter resistor R37.

There is also quite a widespread problem with the faking of RF power transistors from this period, so I wonder where your replacement one came from?
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Old 17th Jun 2022, 6:13 pm   #4
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Default Re: FT-102 Very distorted modulation in SSB and AM

Hello again
Thank you for all these very clever suggestions.
so:
-I actually replaced R37 which was cooked
-The 2sc1971 that I put is a good one that I had. (an old). I've had a lot of trouble with Chinese knockoffs, I know the problem (usually low gain and reverse emitter and collector!)
-Result after replacing R37 1.5ohms: I had hope but same same problem.
-on the transverter output, same problem too.
in addition now the modulation compressor no longer gives any output when you put it even at the bottom. It used to work but I doubt it's related.

73's
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Old 17th Jun 2022, 6:42 pm   #5
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Default Re: FT-102 Very distorted modulation in SSB and AM

That compressor observation could be a good one. Fried Op-amps in mod chain come to mind. In FM they probably put a separate board in parallel across the mic input.

None of this confirmed by schematic so far. Will look later.
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Old 17th Jun 2022, 7:06 pm   #6
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Default Re: FT-102 Very distorted modulation in SSB and AM

Yes ok good idea. Just before replacing resistor R37, the compressor was working (as described with very poor modulation) and suddenly nothing!
73's
SEB
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Old 17th Jun 2022, 7:26 pm   #7
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Default Re: FT-102 Very distorted modulation in SSB and AM

Mic Out (for FM Modulation) is taken off the Mic Amplifier just before the balanced modulator for SSB. That says if you have FM mod that sounds good then the SSB should sound good too unless it is being overdriven or there is too little carrier drive (455kHz).

Various lines on the AF unit do say CW12V, SSB12V, FM12V, AM12V.

Depending which mode was selected when that 12V regulator event occurred, one or more of them could have sustained damage.
Maybe all of them if a switch twiddler was trying to get some life out of it afterwards.
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Old 17th Jun 2022, 7:48 pm   #8
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Default Re: FT-102 Very distorted modulation in SSB and AM

hey,
yes I immediately thought of the balanced modulator but how to explain that the modulation in AM is about the same as in SSB, with the same deformations?
73's
SEB
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Old 17th Jun 2022, 8:00 pm   #9
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Default Re: FT-102 Very distorted modulation in SSB and AM

Typically, Japanese transceivers of this era unbalance the balanced modulator to let the right amount of carrier through, and switch out the SSB filter so the carrier isn't attenuated and both sidebands make it to the PA strip.

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Old 17th Jun 2022, 8:37 pm   #10
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Default Re: FT-102 Very distorted modulation in SSB and AM

The FT102 doesn't appear to do that. The AM and FM TX lines seem to do some DC VXO on the CW TX crytal (8.2109 MHz). There is a pin called FM AM Car out driven by that VXO.

I think it's more like the FT221 in that respect. AM was done by a series pass arrangement on the Driver/PA with constant IF drive level.
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Old 17th Jun 2022, 9:18 pm   #11
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Default Re: FT-102 Very distorted modulation in SSB and AM

Quote:
how to explain that the modulation in AM is about the same as in SSB
Is the distortion possibly happening at a late stage in the microphone amplifier / compressor, then? What does the audio look like at the input to the modulator when the set is in AM and SSB modes?

(To make this easier to follow, here is another link to the service manual)

http://www.iw2nmx.it/Yaesu/FT-102/FT...e%20Manual.pdf
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Old 17th Jun 2022, 9:59 pm   #12
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Default Re: FT-102 Very distorted modulation in SSB and AM

in the description ssb mode transmit:
"... a portion of the output from Q3004 is diverted to the optionnal AM/FM unit, when installed. while the remainder is applied to the balanced ring modulator composed of schottky diodes D3002-D3005"

So am is not in relation with balanced ring modulator like other japanaise tranceivers.
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Old 17th Jun 2022, 10:51 pm   #13
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Default Re: FT-102 Very distorted modulation in SSB and AM

They don't make it easy but I think D3002-3005 are in a 'special' part Q31, see the attached diagram. All parts on the Af board are prefixed 3xxx so this is where they should be an I can see discrete diodes D(3)001 and D(3)006 but not 002, 003, 004 or 005, therefore I think they are part of Q31 which is described by its datasheet as a 'Mixer diode'.

The attached is just to show you where I'm looking, refer to the AF board diagram in the PDF linked to earlier for better detail.

So the first question is, is the audio being input to that mixer (pin 1 of Q31, or test point TP05 on AF board), clean and undistorted?
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Old 18th Jun 2022, 9:16 am   #14
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Default Re: FT-102 Very distorted modulation in SSB and AM

It has to be since FM is fine. Stated above.

I think what is more odd is that AM is not good when it appears that is done by the AM/FM Board too. How it produces AM needs more study. I haven't got that yet.
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Old 18th Jun 2022, 9:31 am   #15
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Default Re: FT-102 Very distorted modulation in SSB and AM

Hi there,
Well actually my balanced modulator consists of an SN16913P. It doesn't match anything in the service manual. I think that the AF unit plate exists in several versions and I don't have the plan for mine.
The microphone signal arrives on pin 5.
The 455khz arrives on pin 2
The output is in pin1
Seen on the oscilloscope, the signals seem correct, as on TP05 (where the amplitude is still higher). I feel like it's good, but I'm not sure either.

Finally and contrary to previous tests, the modulation in AM is good. The problem is only in ssb.

I had an SN16913P I changed it and same result.
73's
SEB
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Old 18th Jun 2022, 10:08 am   #16
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Default Re: FT-102 Very distorted modulation in SSB and AM

Thanks Seb. That point about AM is inportant!
Now we know the problem is in the SSB chain and the circuits we are looking at are not right.

Do you have the serial number of this unit please? I see some changes which say above or below Ser# 030000.
Maybe that was a big design change?

Last edited by Jon_G4MDC; 18th Jun 2022 at 10:30 am.
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Old 18th Jun 2022, 10:34 am   #17
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Default Re: FT-102 Very distorted modulation in SSB and AM

yes thinnk big change on AF unit
serial 3k090811
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Old 18th Jun 2022, 10:40 am   #18
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Default Re: FT-102 Very distorted modulation in SSB and AM

Now I do all the tests without the tubes on the transverter output. My previous finding of bad AM modulation was probably due to bad levels but this is good in AM.
I go back to Q2005 on the IF UNIT now.
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Old 18th Jun 2022, 11:07 am   #19
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Default Re: FT-102 Very distorted modulation in SSB and AM

OK & thanks for serial number.

I have found a few different versions of the service manual but they all have the same diode based balanced modulator in the AF unit.

On the FM and AM Board, I think I understand it now.

The TX IF enters on P60 Pin3 and it is either AM modulated by Q01 a TC7069 (can find no data on it) and sent out again on P60 Pin 5 or otherwise for FM the TX IF is divided down by Q03 as a reference to a PLL which locks Q06, a varicap tuned oscillator, and applies MIC audio.

The division used is 1/256 so that is ~8MHz down to about 30kHz for the loop reference frequency.

Will keep looking for a different version AF Board schematic.
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Old 18th Jun 2022, 12:38 pm   #20
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Default Re: FT-102 Very distorted modulation in SSB and AM

Interesting that the FT102 generates SSB at 455kHz then mixes it up to the first IF ~8.2MHz. It means that SSB is double filtered once by a ceramic filter at 455kHz and again at 8.2MHz.

If the up conversion oscillator is off frequency then these two filters will not overlap correctly and the passband would become narrow. That is not introducing distortion but it won't sound right.

I think I would try to sniff the 455kHz with another receiver to see what the DSB sounds like before the filters. If it's distorted suspect the balanced modulator stage SN16913. Maybe there's a leaky capacitor and it's got a standing bias that should not be there?
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