Tristar 777 no rx/tx

[SiriusHardware]

Looks like a superb collection of old information, thanks. The only similar thing I have is two collected volumes (paper copies) of the 'Midnight Express' newsletter which has a lot of similarly useful stuff in it.

[Techman]

Originally Posted by SiriusHardware

On the similar 121 chassis it was not uncommon for one of the final tunable coils to get cooked and all the enamel to be burnt off the windings causing shorted turns on that coil, but I never saw that happen on a 125 chassis like this one.

Yes, very common on the 059 boarded radios - Jumbo1, Multimode 2, Major 588 etc., etc., when the carrier power had been wound up to match the SSB output. A single shorted turn takes the RF output down to virtually nothing, so you'd know about it if that was the case. The FM carrier power is already at maximum by design on the 777, but strangely they don't seem to be susceptible to the burnt coil - unless I just haven't seen any.

My 777 is plumbed in inside the cupboard and is the radio I use to monitor and natter to the locals on at the moment, as it has the EPROM for the UK40 plus all the others and strangely it's a particularly nice toned radio to listen to compared to some others. I just checked the output and it's around 15 watts on FM as standard on the lower bands, dropping off a bit at the top end of UK40, but I did fit a power reducer to bring it down to around 4 watts on FM, switchable via the old local/DX switch, which I thought at the time was a logical switch to use.

Someone brought a Jumbo round for me to look at yesterday. They'd just bought it with a described fault of something to the effect that after three hours it went in and out of transmit. He said it arrived on Friday and he connected it up and it worked and he actually spoke to someone on it, but a bit over three hours later he was sitting down having is tea when the thing suddenly went bananas. He switched it off and left it until the next day (yesterday) when he switched it back on, but it went straight into the fault condition without any delay. It was pulsing in and out of transmit at a rate of at least twice a second with the relay clicking in and out. I thought at first it was power rail instability due to a low electrolytic, but measurements proved otherwise and I found the fault was due to the 'bleep' board, so as he said he wasn't bothered about it, I just electrically isolated it. I know these original Ham International bleep boards can cause problems, but I've never seen that particular fault before. After replacing the blown meter bulbs (I was a heathen and fitted LEDs from an old board from some scrap gear I had lying around with 560 ohm resistors in series with each one - he was happy with that, as my stock of original filament bulbs had depleted) I replaced a slightly dodgy capacitor and one or two other little bits and bobs with dicky switches needing 'working' (you can't get into them with switch cleaner), the radio was sorted. It was actually a very nice example that was in virtually original condition and looked like it had never been twiddled or adjusted by 'others'. I was pleased that it didn't have to hang around and went back out of the door the same day, as I'm too busy at the moment to have to spend time on other folks radios - unless my arm is really twisted!

[SiriusHardware]

In my experience failure of those Ham International Roger Beep modules was always down to failure, low resistance or short-circuit from c-e, of the SMD PTT output transistor. It sounds like your example was hovering on the threshold of the resistance (down to 0V) needed to put the radio into PTT.

Back in the day we used to desolder the SMD transistor and solder a short-legged conventional NPN transistor to the vacated pads as SMD devices were hardly even a 'thing' then, but nowadays general purpose SMD transistors are very easy to obtain.

We never did get to the bottom of why some radios which were designed to put out high power on FM (e.g Tristar 777) never took that coil out but radios with 121 chassis often did. It never happened on the Concorde II which had the same FM power as AM power due to the RF PA in that model running on half-supply from the modulator darlington even in FM mode, but other models using the same chassis with direct supply to the PA in FM mode frequently did cook that coil.

[SiriusHardware]

For dirty switches which you can't get into but you can get to, I have found that putting a bit of heat with a temperature controlled iron on each terminal of the switch in turn while exercising it can often clear intermittent problems, the theory being that it softens the dirt / grease on the contacts and allows the moving contacts to plough a clear track through it.

I've had particular success with record-playback switches in cassette recorders using this method, the main thing is not to heat the terminals so much that the insulated parts of the switch (or the insulated wires going to it) deform or melt.

[John M1JWR]

ive found a led bulb for the hi light, fits into the white holster bit that fits into the rubber front panel holster, white wire goes to the hi band switch and the red goes to a resistor on the mainboard at the front which says 2700j on it, its very bright is there a way i can dull it a bit, i suppose adding another resistor maybe ?

[SiriusHardware]

That's a 270R resistor, you will be exceeding the safe current through the LED running it on 12V through a 270R. You don't want to damage it and end up having to fit another one.

Normally if running a LED from 12V ish I use a series resistor of about 1K to 2.2K, adjust for preferred brightness.

You can either replace the original resistor or just insert the new resistor in series with the red wire. Doing it the second way will leave the option of removing the extra resistor and fitting the original type of bulb again, should the opportunity ever arise.

[John M1JWR]

Funny *** moment, i installed a 2.2k resistor, then measured voltage on 3 points of 270k furtherest back from front edge of board is 0v, the end at edge of board is 1.34v and the join between wire and 2.2k resistor is 12.3v, thought this cant be right, so i tried the resistor on the other wire coming from the band switch, which is 14.2v and tested again, and now the resistor cuts the voltage down to 3.23v where it joins the white wire, still no dimmer though, managed to find one of those genuine cybernet sleeves to go over it aswell.

[Techman]

Originally Posted by SiriusHardware

Normally if running a LED from 12V ish I use a series resistor of about 1K to 2.2K, adjust for preferred brightness.

Interesting. I would have said that was way too high.

It needs to ideally be between 5 - 600 ohms. I've just checked on what I usually use in series with LEDs on 12 - 14 volts and it's always 560 ohms, as that's the nearest value that I keep in stock.

[John M1JWR]

by the looks of the circuit diagram one wire goes to the band switch and the other goes to the 270r, as i see there is 14.2v coming from the band switch and there is 1.34v at the 270r, so at the moment i have the 2.2k resistor on the wire coming from the band switch which lowers the voltage at the light to 3.23v, i think i got that right, its working well but rather bright.

[Techman]

I think modern LEDs are rather brighter than they were years ago when these radios were made.

When I checked the power of the 777 on maximum carrier FM earlier, it was just into the antenna with the meter in-line at the time. I just did what I should have done earlier and put the Bird dummy load on the meter and it was a level 13 watts from 1 low band to 40 high band, it dropped off slightly to about 11 watts on 40 UK, which is just out of band for the original radio channels and is just below 28 meg, so very level output - I don't think it's ever been 'twiddled' with.

[John M1JWR]

i read that 12 to 14w on fm is spec in the cybernet manual, couldent get another soss out of this one, it appears to be all flat out as in a previous post

[Techman]

It sounds like you're pretty much getting there with it.

I'm tempted to get mine out from the cupboard and have a look inside for reference. it's been a while - we'll see.

[John M1JWR]

should have said post no 35 techman, its been awhile with this one too

[SiriusHardware]

at the moment i have the 2.2k resistor on the wire coming from the band switch which lowers the voltage at the light to 3.23v, i think i got that right, its working well but rather bright.

If you have a Vfd of 3.2V over the LED it might be a high-bright one so you may have to starve it even more to get it to dim down, maybe try 4.7K or 10K. If those values still don't make it any dimmer you may have to reappraise your assessment of the wiring.

Techman, my suggestion of 1K-2.2K for a LED running from 12V has absolutely no basis in technical fact, it is simply the case that for LEDs running from 12V I've always used ~1.2K in series, 470R for 5V supplies, and 330R for 3V / 3V3 supplies.

I've never had any problems with those values for those voltages. Obviously what you're meant to do is know the forward voltage drop for the LED under consideration, subtract that Vfd from the intended supply and then use Ohm's law to calculate the series resistor value for the ideal forward current through the LED.

Life is too short.

[SiriusHardware]

I read that 12 to 14w on fm is spec in the cybernet manual

I owned one from new and I can confirm that it was 14W on FM out of the box. I never touched the adjustments on it myself because I knew it was fresh from the factory and it was never likely to be better aligned than it already was.

I have to say I found the receiver awful (wide open to off channel interference) compared to the numerous radios using 121 chassis, which I always liked. Techman and I have batted this one back and forth before, he quite likes the 059 'Mufflemode' chassis which I really never took a shine to.

[John M1JWR]

I think nowadays as there isent many people on, the rx on the 125 board will come into its own, its not much behind the 706 with its pre amp on, and thats saying something, i know the rx on it is a bit more sensitive than the 121 radio, havent had a 059 since i had a multimode 2 in 1983, the cobraman preferred those aswell and hated the tristar 747 and its relatives, is what it is.
i have a 10k resistor to hand, any other value i will have to rob the scrapper, genuine cybernet item of course !!

[John M1JWR]

that is now installed, voltage at the 270r has dropped from 1.3v to 0.24v, and the voltage at the bulb end of the resistor has also dropped from 3.3v to 2.08v, i dont know if we are barking up the wrong tree cause it hasent made hardly any difference to brightness, maybe a little, looking like techman might be right in this case, they are brighter to start with, i read that they work happily on around 8v, looks like this one is going to have a long life then.
one other thing regarding output power, i dont know if this makes any sense, i think i mentioned it, if i touched the top of the rv3 trimmer with say a metal screwdriver, the output power went up by 3w, maybe as the cobraman used to say, might be a clue ladies and gentlemen.

[SiriusHardware]

Normally when you tickle or touch a preset and that has a significant effect that is a fair indication that the preset is intermittent, which they are sometimes known to be on these old radios. I've looked back but I didn't see anywhere where you said you had tried replacing it, sorry if you did and I missed it.

In this particular case the preset's sensitivity to being touched may have something to do with the fact that there is RF in the immediate circuit area so you need to try touching it with a completely non metallic (plastic?) tool and see if it behaves the same way when you do that. If it does it would suggest that the preset is physically intermittent.

Final try with the LED resistor, put the Hi band lamp wiring back exactly as it was originally, desolder and lift one end only of the 270R resistor to take it out of circuit. Turn the radio on briefly and check that disconnecting the 270R resistor has stopped the Hi band lamp from working, to prove that the 270R resistor is actually the one associated with the Hi band lamp.

Then, tack-solder one of your alternative series resistors to the 270R resistor's pads on the underside of the PCB and experiment with the values to see if you can get an acceptable LED brightness.

If the LED is still too bright with a much higher value substituted for the 270R that must be one super sensitive LED.

[John M1JWR]

ive done one test, actually fm has gone up half a watt on output, back to the test, i touched it with a plastic part of a pen, no change, and the metal screwdiver test it goes up 1 watt now each band, i will do the other experiment tomorrow night, if i remove the 270r what do you suggest i put in its place, i dont have a resistor in series with the 270r, as it stands the only resistor is the one cutting voltage from the mode switch, i take it i should leave the 10k one inline with the mode switch alone.

[SiriusHardware]

I'm suggesting that you first of all restore the wiring exactly as it was to begin with, then remove or lift one end of the 270R and verify that doing so stops the Hi band LED from working: Then, try a variety of alternative values temporarily as before, but this time in place of / instead of the 270R.