Easy fix for EPT3600 9-Volt regulator faults

[nomadradio]

Had a persistent "flutter, flutter, clop, click" coming from a General Lee radio on receive and transmit.

Sounds like PLL trouble. Sure enough, a quick listen to the 16 MHz PLL output reveals the VCO is the source of the noise. Another quick listen to the 10.25 and 14.01 crystals took them off the list of possible causes. Both are steady as a rock.

Changed the PLL chip.

No change.

Changed the 7310 PLL mixer/offset oscillator chip.

No change.

Then my idiot light began to glow dimly. Put the 'scope onto the power-supply pin of the PLL chip. Sure enough, the 9-Volt supply is flicking up and down a tenth of a Volt or so in step with the receiver noises it's making.

Now my idiot light is on bright enough to light up the room.

Which gets me to the real point of this. When the 2-transistor regulator in this kind of radio works like it should, it works just fine. Seldom any need to mess with it.

When it screws up, I have no desire to find out why. The fix is easier than finding the exact component that's the culprit.

Just remove the PNP power transistor and replace it with a 3-terminal regulator. If you have a 7809T, that will do the job just fine. I only stock the 7808T, so it gets a 7/10 of a Volt boost by placing a diode between its ground pin and the pc board ground. Close enough to 9 Volts.

I had a hard enough time finding the all-plastic version of this chip. Finding the 9-Volt version looked like even more work, so I just stock this one. You can use the metal-tab version if you also come up with a mica insulator, shoulder washer and a bolt that's long enough. The factory screw is only long enough for the plastic-tab part. The all-plastic part saves labor.

The left-most leg of the new regulator is the input. The on on the right is output, and the center leg is ground. Almost opposite of the original part. The output pin is the easiest. Just cock the new part towards the rear, and lay it along the trace where the old transistors's center leg was attached.

The output pin needs a wire to extend it. It's the pin towards the rear of the radio, and the solder pad towards the front is where it needs to connect. The short red wire does this, with a sleeve to insulate the lap splice.

The lighting sucks on these two shots, but it shows the anode (not banded) end of the 1N4001 diode lap-soldered to the regulator's center lead.

The sleeve protects the diode from touching anything it should not. The cathode (banded) end goes to ground near the big filter cap.

Sure enough, the radio's PLL quieted down just like it should with a new regulator.

And if I had 'scoped the PLL's power supply first, it would have saved some money and trouble.

Some days are like that.

73

 

[Handy Andy]

Good job Nomad, I just got done talking in PM to someone else about the lasting after-effects of this.

Wanted to caution you though, the 4558 chip uses Zeners that can "Clamp" if you set your 7808 closer to 9 votls than to 8.



The D78 and D76 may be the same Zener - D78 is a 7.5V but D76 can be 7.5 INSTEAD OF 5.1 - and if you "overvolt" the board the 4558 chip will think powers good and the zeners will "clamp" and try to drop RX and TX votlages because they'll sink the current the 4558 produces and toggle or attempt to toggle lines.

So the 7808 is great - and as long as you keep the radio below 9V the survivability of the toggle section that looks at PLL and Power good - to select TX and RX - won't start to play flip-flop on you.

Especially if someone "volts up" these radios.

 

[711]

Good job Nomad, I just got done talking in PM to someone else about the lasting after-effects of this.

Wanted to caution you though, the 4558 chip uses Zeners that can "Clamp" if you set your 7808 closer to 9 votls than to 8.

View attachment 28143

The D78 and D76 may be the same Zener - D78 is a 7.5V but D76 can be 7.5 INSTEAD OF 5.1 - and if you "overvolt" the board the 4558 chip will think powers good and the zeners will "clamp" and try to drop RX and TX votlages because they'll sink the current the 4558 produces and toggle or attempt to toggle lines.

So the 7808 is great - and as long as you keep the radio below 9V the survivability of the toggle section that looks at PLL and Power good - to select TX and RX - won't start to play flip-flop on you.

Especially if someone "volts up" these radios.

Wouldn’t the 7808 regulate the input and maintain the 8v no matter what? Maybe I’m missing something , please help me.

 

[sp5it]

Interesting reading. After 29 years of repairing CB never saw that failure on my bench.
Mike

 

[Handy Andy]

The original - being discrete - is the problem.

When you change over to a 7808 - you're fine - ADDERS and all - but when you go up in voltage to say a 7809 - then problems arise from the voltage level that the 4558 TX/RX switching uses. It starts to rise and can toggle the RX and TX lines - meaning they will accelerate the regulators failure. The 4558 uses a simple voltage divider and looks to the regulator then to the PLL - but if the PLL is not ready, the extra voltage present can cause a rise in the divider sense and toggle anyways...this can hurt the PLL chip trying to "seek" a frequency. The main one being the OOL condition that exists and the chips power is flowing BACK into the chip thru that pin which can lead to self-destruct.

R 200 that 1K "swamps" that 7.5 volt zener - so you'll have some voltage swing that can hold on input variance of 11 to 13 volts - but apply too much voltage to the power supply inputs - say 16 volts - that 8 volts you expect to see will climb to about 9.5 volts and things start going bad quickly...go try it yourself if you don't believe me - and do not say I did not warn you. The problem here is their requirement of amperage - R200 supplies or keeps the Regulator "working" even when it's not - regulating - when the power supply drops below 12 volts - the Voltage Drop across the Regulation Transistor (TR41) itself (P-N Junction) is the problem R200 tries to avoid.

Came across radios that had Zener diodes that after some age and this symptom - they conduct easily and turn on when they shouldn't. It is an age and abuse issue - when people overvolt or try to run more power or ground the wrong places - the chassis voltage can rise and "Spike" these diodes - although they can take a spike or two on their own with little problems - continuously spiking them with overvolt - read this as more than 13.8-14VDC - the 4558 and the built in regulator can fail - and do a lot of damage due to the low-power CMOS PLL and ADDER's that are used in their channel selector matrix.

You can wind up replacing a lot of parts if you go higher than 8 volts.

What Nomad suggested is to raise the voltage - that is your risk IF YOU MUST NEED TO. Else, I'd just use the 8 V 7808 and be done with it - the radio will live to see another day.