Galaxy Saturn Turbo (old 2950 board) catastrophic failure

[loosecannon]

sigh...

This is typical of the kind of radios i get to repair quite often, and it always makes me yearn for the easy repairs LOL. (no really, i've heard they exist!)

got an old school saturn turbo in the other day with the complaint that when the mic was keyed and unkeyed, the radio would "just sit there and click".

sure enough, checked it out and found no TX output, and upon unkeying the mic, the radio just sat there and clicked LOL.

thought about checking the RX TX switching circuit, and just went for it before even looking at the voltage chart.
voltages seemed weird (can't remember what they were), so i went back to the voltage regs, and found 25 volts on the input of IC15 (7808 reg).

backed up again to the power supply itself with already a pretty good idea what failed, and sure enough found a shorted TO-3 transistor on the back heatsink.

replaced it, and its partner in crime, and was back at 13.8 volts to the board. Damn that section sucks to work on as you have to basically take the whole damn thing apart to replace those TO-3's.

now for the fun part, assessing the chain of damage caused by the 25 volts.

at first, it seemed as though everyone made it through ok, but RX/TX switching was only working about 80% of the time.
after a few more minutes of testing, the radio lost the ability to receive signals (radio would still TX), and i found that IC15 had finally given up the ghost and showed no 8 volt output.

replaced it, but still no receive, which led me to touch TR40, and found it hot as the dickens.
pulled it and found it shorted C to E. makes sense that it was so hot!

had to leave for work, and didn't have time to replace TR40, but i will later tonight and the saga will continue.
(yes, i will check peripheral components)

so, just to keep this from being a pointless thread, what would you check next?

anyone had this happen on this chassis before?
if so, how far down the rabbit hole did you get?

IC6 (8 volt reg for PLL section etc.) and IC4 (7805) seem to be fine, but that's just from quick voltage tests.

I do hope that TR40 and TR37 (forgot to mention that it checked ok with DMM testing) are the only suspects but i really don't know what else im going to find.

thanks for any and all input and/or horror stories,
LC

 

[nomadradio]

That power supply was suicidal from the factory. We routinely found two mica insulators stacked under each of the two regulator power transistors, the rectifiers and (sometimes) the AM-modulator transistor. Learned to remove the extra insulator and make sure both faces of the single mica had compound on them. The "twin" insulating washers usually had one side dry, with no compound on it.

Got quizzical reactions from owners of a brand-new radio who thought they only needed to have the radio 'converted' from 10 meters to 11. We offered to skip the "suicide intervention" if asked, but recommended against that.

We worked out some mitigation strategies for that power supply, like matching the two regulator transistors, adding a 3-ohm ballast resistor in line with the base leg of each regulator power transistor and putting a fan on the rear of the radio. Still saw breakdowns way too soon after repairs.

When the cheap 12-Volt 30-Amp switchmode "brick" power supplies became common on Ebay we adopted that solution, removing the original transformer and linear regulator altogether.

I won't repair the original supply in that kind of radio now that we can install a switchmode brick for the same price or less.

I'm gonna have to figure out what to do with the stack of the heavy original power transformers we have pulled out of them. Don't have enough doors in the building to use them as doorstops.

One advantage of the switchmode brick. You no longer need a fan on the rear heat sink. Once the power-supply heat is removed from it, this heat sink is plenty big enough for the radio's linear alone. Won't overheat unless you lock it down and broadcast for hours.

73

 

[loosecannon]

thanks for the reply nomad.

yes, i have seen the doubled mica insulators on pretty much every one of these i've worked on, and with paste on one side.

I did try talking the guy into going with a switchmode supply, but i would have to order it, and i had the TO-3's here already.

he wants the radio back this week if possible, so i had to go with the repair.

I will recommend the switchmode supply for next time i see this radio. (i will see it again i guarantee it!)

i should try to convince him to do it now based on the weight difference!
LC

 

[Tallman]

sigh...

This is typical of the kind of radios i get to repair quite often, and it always makes me yearn for the easy repairs LOL. (no really, i've heard they exist!)

got an old school saturn turbo in the other day with the complaint that when the mic was keyed and unkeyed, the radio would "just sit there and click".

sure enough, checked it out and found no TX output, and upon unkeying the mic, the radio just sat there and clicked LOL.

thought about checking the RX TX switching circuit, and just went for it before even looking at the voltage chart.
voltages seemed weird (can't remember what they were), so i went back to the voltage regs, and found 25 volts on the input of IC15 (7808 reg).

backed up again to the power supply itself with already a pretty good idea what failed, and sure enough found a shorted TO-3 transistor on the back heatsink.

replaced it, and its partner in crime, and was back at 13.8 volts to the board. Damn that section sucks to work on as you have to basically take the whole damn thing apart to replace those TO-3's.

now for the fun part, assessing the chain of damage caused by the 25 volts.

at first, it seemed as though everyone made it through ok, but RX/TX switching was only working about 80% of the time.
after a few more minutes of testing, the radio lost the ability to receive signals (radio would still TX), and i found that IC15 had finally given up the ghost and showed no 8 volt output.

replaced it, but still no receive, which led me to touch TR40, and found it hot as the dickens.
pulled it and found it shorted C to E. makes sense that it was so hot!

had to leave for work, and didn't have time to replace TR40, but i will later tonight and the saga will continue.
(yes, i will check peripheral components)

so, just to keep this from being a pointless thread, what would you check next?

anyone had this happen on this chassis before?
if so, how far down the rabbit hole did you get?

IC6 (8 volt reg for PLL section etc.) and IC4 (7805) seem to be fine, but that's just from quick voltage tests.

I do hope that TR40 and TR37 (forgot to mention that it checked ok with DMM testing) are the only suspects but i really don't know what else im going to find.

thanks for any and all input and/or horror stories,
LC

I would first of check with the owner and see if he is willing to pay big bucks for the repair job. A circuit board that has been hit with twice the normal voltage will never be reliable. Changing the circuit boards would be the most expedient but cost prohibitive.

 

[Tallman]

thanks for the reply nomad.

yes, i have seen the doubled mica insulators on pretty much every one of these i've worked on, and with paste on one side.

I did try talking the guy into going with a switchmode supply, but i would have to order it, and i had the TO-3's here already.

he wants the radio back this week if possible, so i had to go with the repair.

I will recommend the switchmode supply for next time i see this radio. (i will see it again i guarantee it!)

i should try to convince him to do it now based on the weight difference!
LC

I have one of those radios in the shop. The linear is fried but all else looks good. I would sell the whole radio for $35.00 and you pay shipping. It has the Switch mode power supply in it.

 

[ExitThirteen]

There's going to be some capacitors you're going to likely have to replace, as there will be several 10V and 16V caps that have failed from the over-voltage. Likely, those will all have to be pulled and replaced before sending it back to the owner.


~Cheers~

 

[nomadradio]

Hey Tallman!

If he doesn't want the radio with the blown amplifier, I'll take it. Looking for a guinea-pig to try the MRF101 as a replacement for the IRF520 firestarter linear board.

Sounds perfect.

73

 

[Tallman]

Hey Tallman!

If he doesn't want the radio with the blown amplifier, I'll take it. Looking for a guinea-pig to try the MRF101 as a replacement for the IRF520 firestarter linear board.

Sounds perfect.

73

I will even deliver it to your shop and then go over to HopCat for lunch.

 

[nomadradio]

Holy Moley!

Can't turn down that deal. I'll be here all day Thursday after about 11 AM.

73

 

[loosecannon]

yes tallman, please give it to nomad.

thanks for the offer though!

I had re-capped and aligned this radio for him about two years ago, and all still seems well with that part of it, although i did notice that i was warbling on TX just a bit, and it turned out that VR21 was wonky so i just unlocked the clarifier for him.

The radio works now, but power out on AM is low.
voltage going to the test point in AM shows only about 2 to 3 volts.

looks like the AM reg took the hit too.

on down the rabbit hole we go!
LC

 

[Tallman]

yes tallman, please give it to nomad.

thanks for the offer though!

I had re-capped and aligned this radio for him about two years ago, and all still seems well with that part of it, although i did notice that i was warbling on TX just a bit, and it turned out that VR21 was wonky so i just unlocked the clarifier for him.

The radio works now, but power out on AM is low.
voltage going to the test point in AM shows only about 2 to 3 volts.

looks like the AM reg took the hit too.

on down the rabbit hole we go!
LC

Wish you the best of luck fixing the damage by over voltage. That is almost as bad as a lightning hit.

 

[Tallman]

Holy Moley!

Can't turn down that deal. I'll be here all day Thursday after about 11 AM.

73

You have it in your capable hands!
The TALLMAN delivers.