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Old 15th Jul 2017, 10:32 am   #41
crestavega
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Default Re: Sailor T128 Marine Transmitter.

Upon looking at the diagram with a clearer head, I can see that the -70V is likely "coming from" pin 10 of the power connector, which is stated -84V by design. I guess this is the grid bias, and must be kept fairly negative in order to check the electrons acceleration through the tube. (have I got this part of valve theory right?)

However, it seems odd that the test meter (which is a center zero dial) should be pegging hard left on the position 5 (Final Drive) setting. this at least doesnt seem right for a design. The meter is pegging negative because there is -70V across it in this position.

please help!
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Old 15th Jul 2017, 10:39 am   #42
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Default Re: Sailor T128 Marine Transmitter.

I've also been looking at the diagram and have come to much the same conclusion.

With the meter switch set to position 2 it should be possible to measure the negative bias voltage directly.

Regarding the meter "pegging" in position 5, it appear that there are diodes D603, D604 and D607 which will only conduct if the grid voltage goes positive. This should prevent the meter from pegging when the grid voltage is in its normal negative state. I haven't looked at this in much detail though.
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Old 15th Jul 2017, 11:00 am   #43
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Default Re: Sailor T128 Marine Transmitter.

At the risk of looking a bit like I am having an argument with myself here!!

I think I may have developed a theory - see attached diagram
there are diodes D603,D604 & D607 (highlighted orange) in the measuring circuit for S1002-position5.

I think the purpose of these diodes is to "block" the large negative grid bias from the meter, so that in the quiescent mode the meter will see 0V.

if one or more of these diodes has gone bad and shorty, the meter will see grid bias.

by extension, if this happens, there could be a low resistance path to 0V for the relevant grid(s), which case would presumably result in abnormally high anode current for the affected valves.
I am still seeing this *occasionally* on the middle valve (anode metal starts to get red hot) This only happens on keydown, which is I suppose because HT is only supplied to the caps on keydown.

please could more experienced eyes comment on the above leap of faith?
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Old 15th Jul 2017, 11:14 am   #44
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Default Re: Sailor T128 Marine Transmitter.

You're definitely going to have -84V or W.H.Y on the anodes of those diodes. If you've got -84V on the cathodes you have a problem.

The cathodes are all connected together, so you'll need to test them using the diode function of a DMM. or just disconnect one end of all of them and reconnect them one at a time.
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Old 15th Jul 2017, 1:49 pm   #45
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Default Re: Sailor T128 Marine Transmitter.

Thanks, yes indeed I will lift the diodes. In fact I think I will just replace them all. This set came off quite a small MFV and the PA heat must have convected damp salt air up through it. Quite a bit of corrosion around this area

Thanks very much for looking at my problem.
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Old 17th Jul 2017, 7:57 am   #46
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Default Re: Sailor T128 Marine Transmitter.

Predictably, D603 was the culprit. strange, because I had actually replaced that diode a few weeks back - purely because it looked bad.
This has fixed the problem of the final drive dial pegging hard left, and the problem of the anode starting to glow after a few seconds of tx.

However still not much RF coming out of the PA. Hmmm.
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Old 17th Jul 2017, 8:02 am   #47
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Default Re: Sailor T128 Marine Transmitter.

I assume you've selected AM, have the power level switch set to max and have a proper dummy load attached to the aerial connection?
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Old 18th Jul 2017, 9:57 pm   #48
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Default Re: Sailor T128 Marine Transmitter.

Yes. I am using the TUNE function for Tx which sounds like just a full carrier. I have a dummy connected indeed. power level does not seem to make a difference.

May be a problem with drive. there is a test meter position (pre-drive - position 1) which cuts PA function and shows the RF Drive voltage, to enable the peaking of the drive signal:
I dont seem to see any needle twitch at all on this, so maybe there is not enough drive to make the PA work.
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Old 19th Jul 2017, 8:12 am   #49
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Default Re: Sailor T128 Marine Transmitter.

Being primarily an SSB transmitter, there's a lot of circuitry involved in generating the drive.

A check on the DC voltages around V501 is probably in order. Then it's time to get the scope out and check the two tone oscillator, SSB generator, double balanced mixer etc.
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Old 19th Jul 2017, 9:11 am   #50
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Default Re: Sailor T128 Marine Transmitter.

Yes, I do appreciate that the "brains" of this radio is in the small signal department. However, I am pretty sure that is all working. I can hear decent SSB, carrier, and the two-tone signal (not all at the same time!) on a nearby receiver, pretty well on the advertised frequencies. so I think I can safely say that it is working all the way through to the balanced modulator and probably beyond that.

as I understand it: the built-in test meter, in pre-drive position, sniffs the half-wave recitifed (D602) voltage from the QRP signal being applied to the PA valves.

P.8 of the original manual (Tunin-up main channels procedure), step 10, says to adjust the drive level for a peak on the pre-drive meter. Now I cannot get the meter to even twitch in this position so either there is no signal at all getting to the PA grids or there is something amiss with the sniffing line leading to the pre-drive meter.

I have put a small dc voltage on the anode of D602 and the meter moves so it's ok to there. Beyond that is 8.2pF C605. to test that properly I need to get out the sig gen and scope etc as you say.

I think it probably is pointing in the direction of a problem around the driver tube V501 as you hint, which I have not even investigated up to now (other than replacing a toasted resistor R515 I think it was.)
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Old 19th Jul 2017, 9:53 am   #51
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Default Re: Sailor T128 Marine Transmitter.

You say there is not much RF coming out of the PA. Have you actually measured this into a dummy load or are you going by the measurement of aerial current?

The drive is indeed sniffed by diodes, so a check of these would be a good idea. You've had problems with diodes before.
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Old Yesterday, 12:27 pm   #52
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Default Re: Sailor T128 Marine Transmitter.

I was just going by the aerial current actually.

I'll pop a bird on it but I am pretty certain just by feel that it's not "working". You get a feel for when equipment is actually pulling amps don't you?
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