Tristar 747 (PTBM121D4X) Intermittent Fault

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Phuture
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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby Phuture » 17 Aug 2017, 22:47

Hi Mitch

Yes, I have a frequency counter and I was close to completing the alignment on this radio when the fault began. I did begin with checking the reference frequencies and got nothing at TP3 on any band or mode, which unfortunately didn't narrow things enough for me to progress. :?

Hi ibcomms,

Many thanks for your advice thus far and persevering with me. :)

Purple, pink1 and pink2 connections on the squelch solder tags all reading about the same between, but have since dropped to 4.64V in the failed state. I took the measurements again at the terminal posts the wires are connected to and I get the same voltage. Turning the squelch fully clockwise the purple wire connection (connects to terminal #14 on PCB) drops to 2.62V.

I'm not sure which mode switch connection tag to test but the voltages I got were around the supply voltage, 4.88 and 3 point something. Is there a specific switch tag or PCB terminal you could direct me to for this test?

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby lbcomms » 18 Aug 2017, 12:22

First of all, don't worry yet about frequencies or waveforms. None of that will be right until you get the supply rails working properly.

A supply rail dropping to 4.64 volts indicates a break somewhere, given that you have the correct 9.3 volts at the emitter of the main regulator (the D325). You just need to find it.

#6 is the output of the 9V regulator. Have you got 9V and a bit volts there?

This rail will go all over the set, including the mode switch and squelch controls. You'll need to trace them through.

i.e. if you have 9V at the emitter and only 4.64 at #6, you'll have a cracked track or a dry jointed solder post.
But if you have 9V at #6 but only 4.64 at the mode switch, you are more likely to have a broken or high resistance / corroded wire.

Turn the squelch half way and measure the resistance from the wires on the squelch pot to the wires on the mode switch.

One of the squelch wires will be zero ohms or close to that in all positions of the mode switch. Once you've found that, you've found the 9V feed wires to the mode and squelch controls. You should also have close to zero ohms to the emitter of the regulator and post #6 and those two wires.

Anything over 1 ohm: trace the connections through to find out the cause of the high resistance.

PS: The post number is from memory and I haven't seen one of those radios in a few months, so check the schematic if you can't find it or you get unexpected readings.

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby Phuture » 22 Aug 2017, 21:16

Hi ibcomms,

Apols for the delayed response - was away for a few days.

The radio came back to life for a short time this evening and I was able to measure 9.16V on the squelch control and mode rotary switch. Tried to find terminal #6 but could only find #6 on the sub-board (PTOS011A0X), which measures 8V when the mode switch is on HI. Probably not relevant, but thought I'd mention it. Anyways, as you say it's a case of tracing back from the mode switch to the D325 in the hope of spotting the break. Interestingly no vibration or flexing the board anywhere makes any difference. With it being intermittent, if it's a track or wire issue I'd expect a reaction but there's nothing. Wondering if it's a flaky diode or perhaps a leaky cap. Just thought I'd update to let you know I'm still looking at it. Thanks for you help again. Hopefully I can isolate the problem shortly.

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby lbcomms » 24 Aug 2017, 13:14

When the radio is in a working state:
1) Find the wire on the mode switch (which may go to more than one terminal) that always has 9V on it (i.e. in all modes and Tx and Rx)
2) Find the wire on the squelch pot that always has 9V on it (i.e. no mater what position the control is turned to)

Turn the radio off, points 1 to 2 above should measure close to zero ohms in all modes and in all positions of the squelch control.
This will double confirm it's the "BAS" rail, which is sourced from the emitter of the 2SD325 (marked "D235) regulator pass transistor.

With the radio good (in a working state), turn it off and measure the resistance from 1/2 above to the D325 emitter. It should be close to zero.

Wait for the radio to fail. Turn it off and repeat the same measurement. If it's significantly higher then you have a break somewhere.

If that is the case, connect the points 1 or 2 to the above to the D325 emitter through something to limit the current (such as a 1 ohm 1/4W resistor or a 1A fuse). If that "fixes" it, use the low ohms range of your multimeter to do hunting for the break in the circuit.

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby Phuture » 25 Aug 2017, 23:52

Hi ibcomms,

With the radio off 1 and 2 measure pretty much 0V in all modes and in all positions of the squelch control.

I get 1.5R and 28R respectively when the radio is in the working and failed state (I've observed it jumping and staying stable at these values between the two states).

I've yet to do the bypass link as it's late and I'd rather be super alert for that one. :) Unfortunately I may not have time to carry this out until Sunday evening but I'll be sure to reply then. Thanks so much for guiding me through this - very much appreciated. :)

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby timbob58 » 26 Aug 2017, 18:55

Hi i have been following these posts with interest , because i have a radio here Tristar 777 , with the 125 board and have the same problem by the sound of it , a mild white noise with crackling noise , i discovered a break in one of the holes that go through the board . So i soldered a link wire , as seen in photo , now all up and running , so hopefully it will give you some idea where to look .
Regards Timbob .
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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby lbcomms » 27 Aug 2017, 00:19

The proper name for those through hole connections is a "via", all modern boards use them and they are reliable. In the early 80's though, that technology was in its infancy and was not so good. There was no was to inspect them to see if they were good as the holes were filled in, the connection could have been hanging on by a whisker and would have still measured OK and worked - for a while. Modern boards don't fill the hole in in most cases, making them inspectable by person or machine for plating quality before the components are loaded.

One of the early users of the filled in type vias were the later Cybernet multimode and LC7131 PLL (single conversion) radios such as the "C" model GE. We still see the odd one with via faults now, 35 years on. Your fix is the cure, find the failed via, locate both ends of the component side track, and solder a wire link between those points on the track side.

Just in case the fail was caused by overcurrent (i.e. a failed cap somewhere) or you make a pad location mistake, I always limit the current for the initial test of the link. A one ohm 1/4W resistor used as a fuse or setting the power supply current limit to about 500mA could save smoking up a track on an unobtainable (customers) radio. If the radio then comes good when powered up, make the wire link permanent.

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby timbob58 » 27 Aug 2017, 09:18

hi ,ibcomms ,as you stated .I connect the points 1 or 2 to the above to the D325 emitter through something to limit the current (such as...A 1A fuse). i used the low ohms range on my multimeter to do hunting for the break in the circuit,That is what i did i followed what you said,and it worked,
regards Tim,
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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby Phuture » 27 Aug 2017, 22:03

Hi all,

Hi Ibcomms, just did this test and indeed it brings the everything (RX audio/TX) back to life! :clap: :thumbup: So, I definitely do have a break somewhere, I presume, most likely, as you suggested, via a "via". :) So I'll bypass the fault with a wire link but anything else I need to do or keep in mind before solder the link in?

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby lbcomms » 28 Aug 2017, 06:16

No, just make sure it's secure and can't short out to anything, i.e. if it's soldered to the mode switch, secure the new wire to the loom with a cable tie near the switch. That way, if the wire flexes or gets pulled, the joint on the end will be safe.

Glad to see you got it working. Enjoy...

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Re: Tristar 747 (PTBM121D4X) Intermittent Fault

Postby Phuture » 28 Aug 2017, 19:57

Excellent! Many thanks and much appreciate your time and advice on this fault. 73s!


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