I don't Believe I did this, Reverse power connections

pete01 · Post by **pete01** » Fri Apr 28, 2006 10:15 pm

I was using a color-coded leads, but the "boot" connector that attachs to the radio was backwards! And I Know better (that will teach me not to do things this late at night, with not enought sleep, that involve electricty).

The Radio is a GM300 M44GMC20D3AA, PA Board, still smelling of that "You Fried Me" smell is HLE3772A.

the "Sacrificial" part, CR2770 Moto PN 48-80236E07 needs replacement.

New To this forum and learning about these radioes (the hard way)

Question: If I replace this Diode will all be fine just put the radio back together and talk away, or are adjustments required? (via RSS)

Have RSS, legal (and checkbook proves it), but am I ready for the Service Monitor yet? (the jury's-out)

I have 1/2 decent Pwr Mtr and Dummy Load, (not me, but tonite I think could pass for one))

I got a few GM300's, could cannibalize (one that needs a $30.00 interboard connector replacement, drifts a bit like me so I don't notice it much)

CR2770, PN 48-80236E07 "part not found on Moto site" Oh well, Help, Should I guess how much they would charge?

How can I order this?

But you know, it still fun for me, and I like it.

Thanks for taking the time to read this post.
Pete T.

Al · Post by Al » Sat Apr 29, 2006 3:53 am

First, you need to drop the "-" from the part number when searching on the Moto site, so the p/n would begin "488xxxx". I just ordered some of these a few weeks ago, so they should be available.
Whether or not there was other damage done by the reversed power connections depends on whether there was a 15A or less fuse inline that opened when you applied power. If there was no fuse, there's a chance that something else was damaged. If the A+ line was fused and the fuse opened, chances are that you can replace the diode on the PA board and be on your way. However, if you have a service monitor available it would be a good idea to quickly check tx power, frequency, deviation, and rx sensitivity before putting the GM300 back in service.

pete01 · Post by **pete01** » Sat Apr 29, 2006 7:03 am

Thanks for your quick response.

No fuses opened.
No inline, but the ouput of the PS is fused @ 10 Amp., (bench set-up).
This could be bad.

Over-night, the "smell" has filled the room, should I bury it ?

No major trouble-shooting was required to determine that CR2770 was bad.
It literally fell out of the PA unit when I opened it, Reverse-Current flow had unsoldered the diode's ends, (leads remained intact on the board).
Mentioned above just to indicate there was major reverse-current flow.

Just a crazy thought,

What should the resistance be on a good, intact CR2770 ? ( I got 1.53 MEG Ohms, forward and wide-open, reverse, if my test leads aren't reversed!)

No service monitor, don't know anyone in this area with one.

Pete T.

kcbooboo · Post by **kcbooboo** » Sat Apr 29, 2006 7:32 am

Usually, the diode will short-circuit itself when it blows up, due to a large amount of reverse current. Because yours melted the connections and fell out, I wonder if maybe the reverse voltage actually made it through to the RF and logic boards and fried just about every semiconductor in the radio. If the PS fuse didn't blow, and reverse voltage was applied for some time (considerably more than a few seconds), chances are that the entire radio is toast.

Since the diode de-soldered itself, I question the 10A power supply protection device. Something was capable of providing an awful lot of current for quite some time for the solder to have melted. Usually you need heat in excess of 600 degrees to melt solder, although once the connection opens, the heat dissipates. A 10A fuse should have blown. A 10A power supply should have given up after a second or so.

Almost any semiconductor junction (diode or transistor) will measure around 10 ohms in the forward direction, and very high (hundreds of thousands of ohms or more) in the reverse direction. Not getting the low reading in one direction usually means it's open. Getting a low reading in both directions usually means it's shorted, but a shorted diode should read lower than one ohm. By then, it's no longer a diode.

Bob M.

pete01 · Post by **pete01** » Sat Apr 29, 2006 4:40 pm

What if I sub another PA unit, just to determine if the radio is DOA, or would that run the risk of harming the "Donor" PA unit ?

If it fails, which I think after reading your last post it most likely will,
then I would not bother even to order a relacement diode
and look into getting a better protected PS (and more sleep) to avoid a repeat situation.
Thank-you for your input,
Pete T.

kcbooboo · Post by **kcbooboo** » Sat Apr 29, 2006 5:52 pm

I doubt the radio can do anything to harm a good PA, so go ahead and substitute it. If the radio powers up and actually works, I'd be very surprised.

Check the power connector. The exposed terminalcoming from the supply or battery is negative; the covered terminal is positive. On the radio, they're the other way around: the exposed terminal is positive, the covered terminal is negative.

When you put this radio up on eBay, please let people know that it's "as-is", doesn't turn on, and hasn't been tested !

Bob M.

Max-trac · Post by **Max-trac** » Sat Apr 29, 2006 5:54 pm

The diode is just for protection, no?
You don't even need it in there.

Are you sure the 10 fuse on the PS is in the DC line and not the AC?

Put a 2 amp fuse inline for RX testing, then bigger if all looks OK for TX testing.
Then power it up (correct polarity this time) and see what happens.
My guess is more got fried after the diode de-soldered.

kb0nly · Post by **kb0nly** » Sat Apr 29, 2006 6:30 pm

I was just going to suggest the same thing, the fuse on the back of the power supply is most likely to protect the AC feed supplying it rather than the DC output of the supply. Not many supplies out there have a fuse on the output since it's the customers responsibility to fuse connected equipment.

Yes, the diode is just protection, but it's not meant to protect the radio unless an appropriate sized fuse is used. When the polarity of the supply voltage is connected in reverse the diode conducts and pops the fuse in the power cable. Therefore if you pumped enough amps through there to desolder the diode then you couldn't possibly have had the DC input to the radio fused and the power supply was just pumping it's guts out into the short across the diode. The diode got hot from conducting that much current and desoldered.

The diode is only intended to conduct enough current, and only long enough, to pop the fuse. In reality if you clip out the now damaged diode to prevent a short on the DC input and connect a fused supply to it and the radio doesn't work then it's toast.

My guess, it's toasty.

And i agree with Bob, just get rid of it on ebay and don't forget to put:

"Unable to Test"

LOL

pete01 · Post by **pete01** » Sat Apr 29, 2006 9:16 pm

Update: checked-out PS fusing (why did I do this!) I was swaping Power Supplies,around prior to powering-up the GM300, and forgot to make the inline fuse connection) Have corrected that situation.
Put together a "ALTOIDS" box inline fuse box, works fine.

Swapped PA units with another radio, no indication of life when powered-up radio. was beginning to disassemble, realized I had not replaced the 16 pin accessory plug for the test, went back and tried again.

That last over-site caused me to think about something that I had read in the manual. "remove fuse F801, the 2A fuse for ignition control...."

powered-up the radio, took voltage reading before..Yes voltage, after the fuse, no voltage.

Test leads jumped together bybassed F801, radio came to life.

Had daughter come-in and change channels, and PTT, (while I held test leads in place, shorting F801 , and tuned another radio to listen for audio, it works!

Some distortion one mode 1, fine audio on mode 8 (GMRS 15-22) (alignment thing-hopefully)

I should order the fuse and diode, and determine if the orginal PA will be OK?

Thanks for helping me through lesson 1 people.

Titled: "Fuse and Power your radio correctly"

kcbooboo · Post by **kcbooboo** » Sat Apr 29, 2006 10:57 pm

You may have to buy a small quantity of fuses for F801, depending on who you get them from. I believe it's just a PicoFuse, about the size of a 1/4 watt resistor, probably 4 amps. Mouser and Digikey should have them. 30 years ago I could buy them locally in packs of 5; who knows how they come now.

The bigger diode might be more problematic. You can buy something that works the same but has solder-on leads. You can install the new one by soldering it's leads to the existing contacts somehow. The right way would be to pull the entire PA circuit board off, but that entails removal of the DC power connector. However, if you do that, you can also look at both sides of the circuit board and make sure nothing else got burnt up in the process.

Good thing it wasn't a selenium rectifier - when those things pop, you don't want to be in the same house !

Bob M.

jackhackett · Post by **jackhackett** » Sun Apr 30, 2006 9:06 am

I believe you'll find that the picofuse is a 2A. I get mine from Mouser.

As for the diode, well.. it's not really just a diode, the term they use is "transient voltage suppressor" and it's more like an 18V zener. Diode action in one direction protects against reverse polarity, zener action in other direction protects against voltage spikes.

You can usually replace those without removing the board, but in your case I would suggest taking the board out to do it, mainly because I'm thinking you're going to find some burnt traces or components on the bottom of the board. Overheating that diode doesn't usually make a lot of stink, if it's smelling up the room as you said there's probably something else burnt.

pete01 · Post by **pete01** » Sun Apr 30, 2006 12:37 pm

jackhackett wrote:I believe you'll find that the picofuse is a 2A. I get mine from Mouser.

...... there's probably something else burnt.

PICO II 250 Volt..Series 263 Catalog No. 0263.002, 2Amps @ 250 VAC or
0263.004, 4Amps @ 250VAC (does it make a difference here that I applying it to a DC circuit, I think I remember something about DC Current Would be higher that AC.. Does it matter here ?

I suspect L2771, but I am looking at a schematic for the HLE8271A PA Board.
(Do not have one for the HLE3772A) Much difference between to two?
Parts list for the HLE8271A indicates it is a .23 uH , MOTO PN 24-84346a02

Should I continue to trouble-shoot this PA Board and rebuild it, (good for pratice) or just sub another? (Already ordered some parts for the PA board), should I "cut my losses" here ?

Thanks Pete T.

Bob W · Post by **Bob W** » Sun Apr 30, 2006 3:43 pm

kcbooboo wrote:Since the diode de-soldered itself, I question the 10A power supply protection device. Something was capable of providing an awful lot of current for quite some time for the solder to have melted......
Bob M.

One of the problems with a current limited power supply (Astron, etc.) is that it will reduce the output voltage while still providing whatever current it's limited to.

If he was powering the radio from a 35 amp hour gel-cell when the leads were reversed and had a 10 amp fuse in line, there may have been a quick burst of maybe 200 or more amps across that diode until the fuse blows. Diode may or may not short. If the fuse blows in something like a 1/2 second, and the drop across the diode is ~1 volt (normal drop at low current is ~.3 to ~.7 volts) during this time, the total power dissipated in the diode is on the order of 100 watt seconds.

Now take the output of the current limited supply. If it was limted to 10 amps, and it took about 10 seconds to figure out the mistake, the power disspated in the diode (we don't know what else passed reverse current after the diode cleared, so I can't calculate that) was about 100 watts over a period of 10 seconds. Still 100 watt seconds . joules, but the time involved gives the heat time to make it past the local junction, and also allowed the junction to open.

Sometimes high available current is a good thing - especially if it will open a fuse safely.

jackhackett · Post by **jackhackett** » Sun Apr 30, 2006 4:58 pm

PICO II 250 Volt..Series 263 Catalog No. 0263.002, 2Amps @ 250 VAC or
0263.004, 4Amps @ 250VAC (does it make a difference here that I applying it to a DC circuit, I think I remember something about DC Current Would be higher that AC.. Does it matter here ?

The voltage rating doesn't matter, as long as it's higher than the circuit you're using it in. It doesn't really have anything to do with the current, it's about what voltage it's safe to use at, depends on stuff like insulation properties and electrode spacing, for instance a shorter fuse will arc over at lower voltage.

I'll post the part number for the ones I get when I have a chance to look at work monday.

pete01 · Post by **pete01** » Sun Apr 30, 2006 5:05 pm

If this helps with the discussion:

The PS suffered no ill effects, still functional.(not even a blow fuse) (internal).

Note(s) from manual:

The Power Supply is a Samlex America, Inc., Switching Mode Power Supply Model 1212.
Capable of 12 Amps Continuous
Intermittent 15 Amps @ 50 duty cycle (5 mins).
Current limiting 15.5 Amps.
Overload protection-16VDC
Short Circuit Protection- Yes

" If the unit has been overloaded or has experienced a dead short across the output terminals, the SEC 1212 is designed to re-start automatically after the condition has been remedied.

Crowbar circuit protects any device from excessive voltage.

Comment: the last quote from the manual, I guess my problem was not excessive voltage, but current. There is something for protection there, it's called a fuse.

jackhackett · Post by **jackhackett** » Mon May 01, 2006 4:45 am

Looks like they've changed the part numbers since the last time I ordered some of those fuses, but they appear to be the 251L series, 2Amp.

wa2zdy · Post by **wa2zdy** » Mon May 01, 2006 9:08 am

That explains the power supply. When the diode opened up, the power supply resumed full output. This is why you need a fuse on the power line to the radio. The diode shorting would have taken out the fuse and there would have been no further voltage to the radio. Radio saved. Instead that didn't happen.

Lesson learned, yes?

pete01 · Post by **pete01** » Mon May 01, 2006 3:55 pm

To all.

Thanks for help and suggestions.

parts are ordered.

I'll left you know what happens when suspected bad components are replaced, and if any other problems are encountered.

This will be interesting.

wa2zdy, Yes, lesson learned, (and schools not out).

Pete T
N1ZRL

pete01 · Post by **pete01** » Fri May 05, 2006 7:05 pm

Thanks for the information and assistance above.

Replace parts L2771,CR2770,F801.

Radio works, but the tx audio distorts after about 10 sec.
BOARD INTERCONNECT is a 2804637J01

Just like 2 other radioes I have, and they got the 2804637J01 also.

Ref OSC adjustment in order ? Freq tag (on this repaired radio) is difficult to read, odd looking figures, will have to work on making-out the numbers)

Read some about the Ref OSC adjustment, will read some more before trying it,
(and await suggestions)

No service monitor.

Got a DVM, scope, and a freq counter (good to 100 MHz).

Thanks Everyone,
Pete T.

kcbooboo · Post by **kcbooboo** » Sat May 06, 2006 4:20 am

The GM300 is electrically almost identical to the MaxTrac. The entire transmit audio chain is DC-coupled, and most of the op-amp outputs are around 4.8VDC. There's a circuit that generates that "bias" voltage; check that. Also there's a 9.6V regulated supply; check that too. Look at the various op-amp outputs with a DC voltmeter and see which one is moving after 10 seconds, then look for a leaky capacitor connected to that point or to the input of that op-amp. Also, you should look at the modulation audio with a scope to see how it's distorting. If the DC voltages move around, you'll probably see positive or negative peaks getting flattened.

Bob M.

jackhackett · Post by **jackhackett** » Sat May 06, 2006 9:36 am

You shouldn't need to mess with the reference oscillator unless you've changed the board or crystal, those numbers get entered in during the board replacement procedure and tell the micro how to compensate for temperature drift. What you might need to do is adjust the warp setting, you'll need a service monitor for that anyway.

The warp setting on a GM300 is usually at 103 from the factory, if it needs to be adjusted much off of that to get it on freq then something is wrong, such as someone could have turned the oscillator coil (never do that!).

I'd find someone with a service monitor to check the freq, it usually won't sound noticably distorted until they're at least 1KHz off, so if it's only a couple hundred Hz or less off, check the audio circuits.

pete01 · Post by **pete01** » Sun May 07, 2006 10:30 am

jackhackett,

Thanks for the heads-up about the oscillator coil (VCO right?). I knew there was something I should not do, just was trying to remember what when I read your post.

I warp is set at 104 presently.

I'll hold-off on the Ref Osc. adj.

kcbooboo,

Started voltage measurements.

Logic Board HLN 8074E

U651A output, pin 7, before transmitting at 4.85 VDC,
changes to 4.34 VDC on transmit.

U652A output, pin 1, before transmitting at 4.97 VDC,
changes to 4.45 VDC on transmit.

When I heard the distortion start, (on monitor radio), voltages remain unchanged, at the lower level.

U651A pin 5 (Looking for 4.8 VDC), measured 4.57 VDC, before transmitting.
Changes to 4.07 VDC on transmit.

U651A pin 8 (Looking for 9.6 VDC), measured 8.99 VDC, before transmitting.
Changes to 7.94 VDC on transmit.

Again, When I heard the distortion start, (on monitor radio), voltages remain unchanged, at the lower level.

This last measurement, U651A pin 8, Fluctuates +/-1.0 VDC
(the others are Fluctuating apx +/- 0.5 VDC).

CR651, the diode feeding, U651A pin 8, measures 9.66 VDC before diode, (and, of course, same as 651A pin

measured 8.99 VDC, before transmitting.
Changes to 7.94 VDC on transmit.

(not sure when DVM was last Calibrated, but I think it is right-on)

Question: are these Fluctuates normal ?

I am bit concerned about the CR651 diode voltage measurements.

I am going to take a known "working" radio, do some comparison voltage readings, before continuing to the RF board.

Pete T.

DaBigBR · Post by **DaBigBR** » Sun May 07, 2006 12:30 pm

I didn't have time to read all of the other replies, but I have found that the el-cheapo power cords from Radio Shack and other places frequently have polarity reversed from /\/\'s plugs. I have gotten smoky results from a couple of radios that I have tried using them on, but never burned one up.

Good luck to you.

kcbooboo · Post by **kcbooboo** » Sun May 07, 2006 3:51 pm

The fact that almost all of the voltages are dropping in transmit makes me think the following:

1. your meter is susceptible to RF coming out of the radio, or
2. your power supply isn't maintaining +13.8VDC on transmit.

Make sure you have a dummy load attached to the RF output of the radio, and measure the raw +13.8VDC coming into the radio. It should be rock steady. If not, investigate and fix that first.

The +9.6V regulator should also be rock steady. The +4.8V source just cuts the +9.6V in half, so if 9.6V varies, so will 4.8V on U401 pin 7.

Check the 9.6V regulator Q401, Q402, and U401 pins 1, 2, and 3.

You may need an oscilloscope to check for distortion further. Look at U651 pins 1 and 7, and U652 pin 7, which should also be around 4.8VDC.

I don't know if you're using a microphone or a good audio signal generator to provide modulation. I'd recommend a known good, low distortion audio signal of around 50-100 millivolts that can be fed through a 10uF capacitor (+ lead towards the MIC jack) directly into the radio. There will be DC voltage on the MIC lead, hence the need for the cap.

The two signals that leave the logic board and go to the RF board called VCO MODulation and REF MODulation should be very clean and not distorted. You will see a lot more PL/DPL on the REF MOD pin. You should shut that off while troubleshooting the distortion, unless you can see distortion of a PL signal in addition to the regular transmit audio. Set the PL for 100.0 Hz, as it's very scope-friendly. It should be a very nice sine wave when it comes out of the VCO MOD or REF MOD pins, and it should remain so.

Comparison with another radio (MaxTrac, Radius, GM300) would be useful, but I don't think all those voltages should be fluctuating as much as you're seeing. In particular the +9.6V should hold be very constant, within a few millivolts. Try another meter or even a DC-coupled scope.

Bob M.

pete01 · Post by **pete01** » Mon May 08, 2006 5:33 pm

DaBigBR- Thanks for the wish of good luck, it helps !

Bob M.,

I am glad you suggested checking the raw 13.8 VDC. The supply would drop to apx 10 VDC. Swapped Power supplies, tried the Samlex America, Inc., Switching Mode Power Supply, it held a solid 14.2 VDC. And just what? No audio distortion on the GM300’s transmit signal! Maybe that has been the source of the problem for the other 2 radios.
(I’ll run some checks).

Checked the 9.6 VDC, steady.
Put the scope on REF MOD & VCO MOD, just to see what you were talking about. Normal voice, no distortion, loud voice I see the cut-offs of the high & low peaks, cool.

Need to fab a RJ-45 box with a input (BNC) to inject an tone.(and accept the the RIB cable)

Have an old tone generator, 100 or 1000 KHZ, from a place I use to work, (unit was headed for the trash bin., maybe 25 years ago,) dirty signal form, maybe try to repair it for that use. Single tube “CBS HYTRON” “117N7 GT”. Electrolytic capacitor, 2-3/4” tall by 1-1/2” DIA, orange cardboard wrapper, no indication of manufacturer or values.
( one of the capacitors in this unit has the old military color code, (six colored dots, I have to dig out a reference book to read that one.)

Side thought-Seems I read something on this board, when I was reading about Service Monitors, it was mentioned, ( in one of the equipment related forums) about maybe these “Orange cardboard wrapped” capacitors.

Maybe it’s time for a new/used signal generator.(never owned a really good quality one, just made use of what was on hand)

I am rambling now, tired.

I will run those checks on the 2 other radios, and let you know if the power supply was the problem.

Thanks for the suggestions, I have learn some important stuff here.

Best regards

Pete T.
N1ZRL

kcbooboo · Post by **kcbooboo** » Mon May 08, 2006 5:55 pm

I've been fixing stuff for quite a long time, and I will admit that power supplies are the last thing I check, but more often than not, they end up being the problem. They really should be the first things to check.

Meters on power supplies are a great innovation - every supply should have them. They do tell you a lot.

That tube is probably a dual triode which would make for a nice oscillator, but it's better destined for the trash can or maybe Dayton next week.

Glad things are back to normal after your near meltdown.

Bob M.

jackhackett · Post by **jackhackett** » Mon May 08, 2006 6:45 pm

kcbooboo wrote: That tube is probably a dual triode which would make for a nice oscillator, but it's better destined for the trash can or maybe Dayton next week.

It's a combination rectifier & beam power tube, 117volt filament, generally used as a half wave rectifier and output amplifier in ac/dc receivers, good for a bit over a watt of audio out (according to the 1947 RCA receiving tube manual).

I don't Believe I did this, Reverse power connections

I don't Believe I did this, Reverse power connections

Functional Again