Sailor T128 Marine Transmitter.
 

Trying to get a Sailor T128 Tx running... and after a bit of help. The T128 manual is a bit sparse on Tx circuit descriptions; and I am floundering a bit with the driver stage which is based on a 12HG7 pentode.
There is a 330R 1.5W resistor R515 which I think is supposed to provide the driver with a bit of negative feedback from the main PA valves (???)

When I first tried a Tx, the set did emit a small amount of RF but then this resistor R515 started arcing and eventually failed open circuit.
I tried a spare resistor in place and it immediately failed also with a puff of smoke.

Could anyone please explain in a bit more detail the theory of operation of this Tx?

Many Thanks

Re: SP / Sailor T128 problems - valve help please
 

Cresta, the linear amplifier driver valve on the left is probably faulty, or else a short circuit capacitor associated. The feed back is for neutralisation of the 3 linear amplifier valves. I see there is a comprehensive PDF manual found via Google which describes setting up, operation, and technical details. The RF output circuit needs critical adjustment to match your dummy load or possible aerial.

I suppose you are licenced and may want to use it on LSB on 160 or 80m. The text makes is clear how to select an appropriate crystal frequency.
73, Andrew

Re: SP / Sailor T128 problems - valve help please
 

Smoke from that resistor sugests either that the PA is oscillating, or that there is a problem (leakage) in the 2 small capacitors that connect one end of the resistor to the PA anodes.

The resistor is indeed part of the PA neutralising circuit: a RF potential-divider is formed by the 2 series capacitors to the PA anodes and the capacitor from one end of the smoking resistor to earth. The smoking resistor provides a way to 'lift' the end of the PA grid-coils slightly to RF voltages, so the divided-down RF from the PA anodes can be fed to the end of the grid-coils in the correct phase to give neutralisation.

It's relatively easy to re-draw it as a bridge neutralising setup; the original was developed by a guy called Bruene who worked for Collins Radio in the 'states.

Re: SP / Sailor T128 problems - valve help please
 

Thanks, I now have something to chew on.
My "dummy load" is/was simply a length of wire which is not really ideal, postponing further investigation pending arrival of a decent dummy load and a bit of reading-up on valve PA's.
I have some familiarity with valve Rx but none with power Tx circuits
can anyone recomend any particular books on the area?

Indeed the vague idea was to get it running on top band, though it is not entirely suitable for ham operation with channelised freqs only and of course no LSB. it is a fairly good aesthetic example, ex coastal MFV, with very little corrosion & all mechanicals working perfectly. I have its partner R105 operational separately.

Excuse my ignorance, but why is it still critical to tune the aerial cct if I have a good dummy connected? I thought the purpose of a dummy load was that it will quietly absorb anything that the output produces (I do appreciate that a dummy load will not of itself stop a wonky amplifier from oscillating)

Reading through the tuning procedure for adding channels is enough to make me sweat. It certainly makes one appreciate the VFO and ATU.

Re: SP / Sailor T128 problems - valve help please
 

No, you have a valve power amp with a resonant impedance transforming circuit on its utput - probably a pi-tank but it's off the page.

You have to tune it to resonance for the frequency you are using, even into a dummy load. And you also have to adjust the transformation ratio so the resistance of your dummy load gets transformed to present the right amount of loading to the anodes to take the amount of power you want. Oh, and all the controls for this area interact. Look up tuning a pi-tank for more info.

Just working into a length of wire will radiate signals on whatever frequencies you tune to, and that is quite a powerful transmitter. You do need a proper screened dummy load.

David

Re: SP / Sailor T128 problems - valve help please
 

Cresta, the RSGB or ARRL handbooks, especially older editions, will educate you about valve transmitter technology. Only attempt tuning on low power. I would expect you to damage the set if you transmitted without first properly tuning the PA into the dummy load. I would remove the PA valves and make sure the driver was giving output first.

You can put it on LSB by using a crystal 600 KHz below the transmit frequency. Modern miniature crystals will work in the circuit. I'm an ex marine R/O and ship service engineer.
What's your callsign?
73, Andrew GW3OQK

Re: SP / Sailor T128 problems - valve help please
 

Be careful without a correct dummy load as mine went o/circuit while testing a Yaseau FL2000B and took out the PA valves.

Re: SP / Sailor T128 problems - valve help please
 

The valve heaters are in a series-parallel arrangement for 24v working, so the driver won't work in that case. Take case
Andrew

Re: SP / Sailor T128 problems - valve help please
 

Hmm, ok. Thanks very much for this help.
call is MM0TJR.

Yes I wondered about the general idea of disabling the PA. My gut feeling was that the RF I detected was only coming from the driver, so it may well be that the PA has never been entirely operational.
I think I need to gear up with an ESR meter and a good dummy load really before I tackle this one properly.

Would it be valid to simply disconnect the PA valves' anode voltage at source (pin 3 of the multiplug) in order to safely disable the precious PA amplifier, whilst I check out the driver?

the manual is here, with the full schematic at the bottom of the PDF.

http://peel.dk/SP/pdf/Sailor%20T128%...20(Manual).pdf

With reference to the aerial tuning system, am I right in thinking that in a worst case scenario (ie, most dangerous case) it could only take C714 (the cap immediately to the left of the coarse/fine coils to fail short-circuit, in order for anode voltage to become present onto the antenna leadin horn?

R.Wrangler, thanks/ regarding radiating signals, the shack is conveniently a faraday cage so, whilst I admit it was a bit rash to tx into a wire, I was reasonably sure that no unlicensed signals were getting out

I dont think that I have damaged the PA valves - no arcing around them, no excess heat. I never tx'd for any longer than about a second really. I think that resistor going off pop was anode volts coming past those capacitors. ESR meter needed to positively confirm.
is it considered generally safe to use a Fluke 87 to check this level of anode volts? (ie around 1kV) I mean, I did use my Fluke and I didnt feel anything, but is this daft? test probes are 1kV rated.
Thx vy much for your help and patience

Re: SP / Sailor T128 problems - valve help please
 

ESR? surely SWR meter. Effective Series Resistance meters is for checking capacitors.

Re: SP / Sailor T128 problems - valve help please
 

Yes I did mean ESR, for checking the old caps that will be under strain. I have an SWR meter. Seems to me that certain of the capacitors in the output section are somewhat safety critical!

Re: SP / Sailor T128 problems - valve help please
 

ESR meters are for checking electrolytic capacitors, not low value micas or ceramics. 1 mfd is generally the lower limit for these.

If they are shorted, a simple VOM will tell you. If they are breaking down under high voltage, you need a capacitor tester that applies rated voltage, such as an Eico 950 or Sprague Tel-Ohm-Mike, not sure what the equivalent in the UK would be. Or just change the capacitors and move on with testing.

Re: SP / Sailor T128 problems - valve help please
 

Did not know that. Thanks.

Could anyone encourage / blow me out the water regarding my above idea of disconnecting anode voltage whilst checking driver stage? I am definitely lacking in swathes of practical experience in this general area.

Re: SP / Sailor T128 problems - valve help please
 

TJR, you need to disconnect anode and screen together. Disconnecting anodes only could damage the valves if the take excessive screen current. There's probably nothing wrong with the PA valves anyway.

The resistor in the driver burnt out, and burnt out again when you replaced it so it could be the driver valve faulty. Faulty capacitors are easy to check as pointed out already. If in doubt disconnect one end of them. I would do this if I was working on the set. Replace the resistor and then remove the driver valve before trying again. If the resistor stays ok disconnect the capacitor from the driver that feeds the PA and replace the driver valve. Try again. If the resistor burns out the driver is taking too much current.

At least that's my thought. I am sure someone will say do not touch it as you admit are not qualified. Should you carry on you do it entirely at your own risk of course.

Andrew

Re: SP / Sailor T128 problems - valve help please
 

Correct, and mains derived EHT is lethal on this equipment I afraid there won't be a second chance.

Re: SP / Sailor T128 problems - valve help please
 

Appreciated re the EHT, I actually do have a bit of experience with "handling" that side of things in dealing with receivers (though admittedly those voltages are rarely above 500V) It was more the transmitter-specific circuit arrangements & techniques that I am unfamiliar with
btw, the eht I am dealing with here is derived from 24VDC by the radio's own PSU. The 24V is provided by battery though so is relatively unlimited in its cranking power.

Re: SP / Sailor T128 problems - valve help please
 

It is still a low source impedance, and therefore very capable of killing.

If any of the capacitors that block the PA HT from getting back to that resistor have gone leaky (a strong possibility) this will certainly cause that resistor to burn up.

Sailor T128 Marine Transmitter. Replacing polyester capacitors.
 

Once again, I apologise for ignorance
Is it ok to replace polyester capacitors with ceramics? the application is HF rf, per attached schematic.

in the schematic, C801, 1000pF to ground is a polyester axial rated to 500V. I thought that these capacitors were "dry" so not much prone to age decay, but light meggering shows that it is low-ohms at about 100V of pressure, and short at 500V.

I think it would explain why R515 was overloaded

I have been around the PA board and found a couple of other polyesters that exhibit similar character.
is it likely that polyesters have gone bad?
the packaging of the caps does not appear to be particularly tight, looks like gaps around where the leads enter the body, so it is possible that damp has got in to the caps....
if so, is it ok to replace with ceramic/mylar etc of appropriate rating?

the pa board in this radio is most at risk, due to where it is; very vulnerable to a damp atmosphere and crud falling down onto it. the rest of the boards in the radio are very well protected from the elements.

thanks for help.

Re: Sailor T128 Marine Transmitter.
 

The cap is part of a tuned circuit. I wouldn't use ceramic as their capacitance is not precise and varies with temperature. Best replace it, and all the caps you suspect, with new silver mica or polyester caps. To get the voltage rating required (1kv +) I've always had to use several in series, even a string of 6 in my T1154.
73, Andrew

Re: Sailor T128 Marine Transmitter.
 

Thank you very much

should then, 2 of these:

http://cpc.farnell.com/epcos/b32529c...00v/dp/CA05599

strung up in series, give me a more stable 1100pF at nominally 800V?

that bit doesnt require to handle kV, it is only the grid of the driver circuit