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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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17th Oct 2016, 1:57 pm | #21 |
Pentode
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Re: Sailor T128 Marine Transmitter.
That will be OK. I can't see what the HT on the driver is, but presumably less than 800v.
Andrew |
14th Feb 2017, 2:02 am | #22 |
Pentode
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Sailor T128 confusion
Please could someone help
regarding my Sailor T128 (attempted) restoration, and with reference to its manual http://peel.dk/SP/pdf/Sailor%20T128%...20(Manual).pdf there is a "test switch", shown on p.52 of the PDF. in sections C (iv) and C(v), (p.10 of the PDF), there are instructions to set the test switch to position"TRANSMIT". I cannnot find a position marked TRANSMIT. this test switch and its functions are summarised in Section D (p10-11) of the PDF. None of these functions are specifically labelled as "TRANSMIT" however, the final paragraph of this section states "leave switch in position 10 Ik3 before reassembling tx" Is it a safe assumption that position 10 is the TRANSMIT position? |
14th Feb 2017, 9:24 am | #23 |
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Re: Sailor T128 Marine Transmitter.
The test switch merely sets what the meter measures. There is no TRANSMIT position as such, so it's safe to assume that the document should read position 10.
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
14th Feb 2017, 10:43 pm | #24 |
Pentode
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Re: Sailor T128 Marine Transmitter.
indeed, closer inspection of PDF p10-11 (section D) shows the exact function of the test sw. the Pre-Drive position "automatically cuts screen grid voltage" which presumably (?) effectively stops any effective PA work.
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15th Feb 2017, 9:52 pm | #25 |
Pentode
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Re: Sailor T128 Marine Transmitter.
I set up dummy load and had a tentative transmit on my T128 this afternoon. seems to sort of "work", with a pleasant sounding clear audio on a nearby rx.
The unit has not been tuned in a long time so clearly requires setting up as per the manual, which I will get around to. meatime, I wonder if any more experienced hands can advise me about this - the middle TT22 valve of the three in the PA, upon key-down, starts to glow noticably hotter than its neighbours, and there is a hint of blue glow around the base. does it mean that the bottle has gone "soft" or is this a normal symptom of an un-neutralised pa? also was somewhat perturbed that the aerial amps needle was not moving at all. although I have not actually checked the movement of the meter. Last edited by crestavega; 15th Feb 2017 at 10:09 pm. |
16th Feb 2017, 11:06 am | #26 |
Pentode
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Re: Sailor T128 Marine Transmitter.
Cresta, I don't worry about blue glow at all. If the anode glows red it would be a different matter. You are not going to get any aerial current until you get round to doing the setting up. Good news you can pick up some signal from it.
Andrew |
16th Feb 2017, 11:31 am | #27 |
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Re: Sailor T128 Marine Transmitter.
The test switch enables the cathode current of each of the three PA valves to be checked individually. Any problems are likely to show up as large differences in cathode currents.
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16th Feb 2017, 4:08 pm | #28 |
Nonode
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Re: Sailor T128 Marine Transmitter.
Have you tried swapping the valves around? Does the glow stay with the middle position, or move with the valve?
73's Aub
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16th Feb 2017, 11:18 pm | #29 |
Pentode
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Re: Sailor T128 Marine Transmitter.
in swapping the valves around, indeed the problem stays in the middle position.
Andrew - I think yes, the anode is glowing red hot shortly after keydown (at least, it is the outer plate(s) of metal inside the bottle so I presume that would be the anode).also, the test meter shows the nr.2 kathode current pegging hard on tx. (Ik1 and Ik3 are similar at half scale deflection) So hopefully that indicates just that the PA settings need balancing up.... thanks very much for helpful commentary. |
5th Jun 2017, 11:09 pm | #30 |
Pentode
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Sailor T218
back on this somewhat recalcitrant T128 transmitter.
I seem to have a problem with the cathode current of the middle (nr.2) tube. I was trying to neutralise (?) the PA tubes, per the manual. Believe that I am supposed to adjust kathode kurrent for 35mA. Initially, Ik2 was pegging hard over 100mA on Tx, and I discovered that the adjustment VR P605 was failed opencircuit, as was the 22 ohm R807 between kathode & ground. I replaced these and got a more sensible Ik2 = 55mA but adjusting P605 did not change the current. The problem is defninitely in the position, not the bottle. occasionally, the valve in this position also starts to glow alarmingly red on Tx. I am convinced that the PA stage is just not "working"; there is no aerial current, and I can just hear & see that the tubes are not loading up with Tx drive. I am working straight into a big oil filled dummyload. I can hear the signal on frequency, with convincing sounding audio mod on a nearby Rx. |
5th Jun 2017, 11:14 pm | #31 |
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Re: Sailor T128 Marine Transmitter.
Threads merged.
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6th Jun 2017, 9:11 am | #32 |
Heptode
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Re: Sailor T128 Marine Transmitter.
Looking at the circuit, the most likely suspect would be the grid coupling capacitor for the middle PA valve. Worth checking for leakage or just replacing.
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7th Jun 2017, 1:07 am | #33 |
Pentode
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Re: Sailor T128 Marine Transmitter.
I am beginning to understand perhaps. please can you advise me if I am correct that C805 (highlighted in red on attached snippet) is the Grid Coupling cap. in question here?
this is apparently a 400V 4.7nF ceramic. am I right in thinking that the purpose of this cap is to pass RF from the previous driver stage onto the control grid of the PA tube, but to galvanically isolate this grid from the (presumably high?) dc voltage associated with the anode of the 12HG7 drive tube. if high dc bias was to get onto the grid, I can see that excessive current could flow in the PA tube. I have to confess that I cannot see where the 12HG7 driver valve gets anode volts from. presumably there is a good reason for specifying a 400V cap in that position though. |
7th Jun 2017, 8:42 am | #34 |
Heptode
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Re: Sailor T128 Marine Transmitter.
Yes, that's the one. It's the transmitter equivalent of "that" capacitor! The anode of the driver gets its HT via its tuned circuit.
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7th Jun 2017, 5:46 pm | #35 |
Dekatron
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Re: Sailor T128 Marine Transmitter.
100% agreed: plenty of otherwise-good 1980s/1990s Japanese ham-radio "hybrid" valve/transistor transceivers have suffered a PA-valve meltdown when the coupling capacitors have gone soft.
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7th Jun 2017, 11:32 pm | #36 |
Pentode
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Re: Sailor T128 Marine Transmitter.
how do ceramic disc capacitors go "soft"?
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8th Jun 2017, 7:20 pm | #37 |
Pentode
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Re: Sailor T128 Marine Transmitter.
Pulled it out and meggered it, showing 60MOhm at 500V, probably OK?
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8th Jun 2017, 8:14 pm | #38 |
Dekatron
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Re: Sailor T128 Marine Transmitter.
They go leaky and can sometimes fail S/C catastrophically (putting driver-stage-HT on the PA grid is not a recipe for happiness).
Remember that the capacitor has to handle the sum of the driver-stage anode voltage, the PA-stage negative-bias (whether derived from a separate source or by grid/cathode rectification of drive) *and* the peak RF voltage-swing. If your radio uses grid-rectification-bias for the PA, and the capacitor goes open-circuit (or loses significant capacitance) then there could be insufficient RF drive to the grid to generate the necessary grid-bias and the affected tube will then draw an excessive anode current. |
8th Jun 2017, 10:34 pm | #39 |
Heptode
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Re: Sailor T128 Marine Transmitter.
Another possiblity would be a reduction in the effective cathode resistance for that valve. You mentioned that R807 had gone o/c. Was it replaced with the correct value? Or maybe the parallel capacitor is low resistance? You also say that P605 no longer controls the Ik for the valve. I can't quite read the circuit in that area but it must be a clue.
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Keith Yates - G3XGW VMARS & BVWS member http://www.tibblestone.com/oldradios/Old_Radios.htm Last edited by Keith; 8th Jun 2017 at 10:40 pm. Reason: Further relection... |
15th Jul 2017, 2:13 am | #40 |
Pentode
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Re: Sailor T128 Marine Transmitter.
I have replaced capacitors on me T128 and now getting sensible kathode kurrent on all tubes. however I still don't seem to have any serious work being done by the PA.
I have discovered that the "Final Drive" voltage seems to sit at -70V. Im not sure this is right I discovered this on the built in test meter, which in position 5 (Final Drive) pegged hard over left in standby. no change on Tx. The manual states that in this position, the test meter will show deflection when the grid voltage of one of the tubes becomes positive wrt Earth. I think the grid is supposed to sit at slightly negative potential in order to stop excess anode current? yet sitting at -70V seems like it will choke the valve from doing anything...? I must confess I am a bit stumped as to where that -70V is coming from. One thing I did notice is that from cold, this voltage is 0V but seems to creep down to -70V as the filaments heat up. cheers Tris |