New project arrived - Yaesu FP-12 Power Supply Unit

 I managed to salvage original accessory for my retro Yaesu FT-7 HF transciever, made back in 1978-1981?

This station accessory has DC power and 8ohm speaker built-in and the exterial condition looks good

Once unit landed on my lab desktop and put through inital tests -  it did put out volt and amps, good stuff!

BUT  8/

The DC output voltage was >14.8V, I think it is excessive for my purpose, 13.8V is the "norm" for hamgear. 

The specification of this piece states:

So - opened unit up attempting to adjust the adjustable voltage......and....well - that is not going to happen....

During the lifecycle of this unit the stock IC regulator has died? and was replaced by a 7815 fixed voltage regulator connected to the external bypass circuit, direct drive of the four NPN power transistors

The original PCB with regulator has been discarded and is nowhere to be found....

7815 is hidden under the fuseholder, with two tantalum capacitors

Clearly this unit has been opened-up before - so I started looking for other signs of "modifications"

As the original PCB has been removed, a new 230VAC fuse holder is put there, several cables show soldering iron heat damages, the isolation is compromised - some cables are joined together and insulation is ....not good

Some insulation damages, there are more....

Not sure what is going on here....semi-insulated-solderjoint?

What now?

TA7089M and TA7089P are still found on ibaj for "a price" + shipping, but as a new PCB is needed -  I'd rather look at other alternative linear regulators.

In my junkbox there are many 723 linear precision positive adjustable ICs, and in this case I will use a SMD SOIC8 version uA732CD by Texas Instr.

uA723CD and TA7089P are both 14pin, SOIC or DIL (although not pincompatible)
External output is 150mA vs. 200mA - quite similar if using external bypass transistor
Current detection is 0.65 - 0.7V, quite similar with Rsc shunt on the integrated current sense transistor B & E
Voltage setting is quite similar with resistors and potentiometer
Both uses frequency compensation while only 723 has temperature compensation

I'd say 723 is close as and electrical replacement for the original - as long as the circuit board is a new one.

So I will go for a new drop-in replacement PCB, re-using original power transistors, rectifier bridge, capacitors - adding a DC fuse and a AC fuse, adding a overvoltage VDR.

Funny thing, found a application note from 1968 where uA723 was introduced, used this note for additional adjustments of the generic circuit

Circuit

Generic uA723 design - onboarded with the integrated original parts
Resistors RA and RB are placeholders for adapting the overcurrent voltage required from the Rsc shunt resistors - not sure if these are needed.

This PCB would also cater supplies with more than 25Amp drivning many more NPN power transistors and adding Rsc resistors to match

The 100pF capacitor is being replaced with a 470pF ceramic capacitor - reading the UA723 application note from 1968 by Fairchild it states that if using an external bypass the capacitor should be increased from 100pF to 500pF

Parts used:

Ten turn 1K potentiometer - sets the output voltage

Added three RF ferrites on several DC cables, these ferrites are sourced from older TV BC power amplifier DC feed cables, suppressing RF

Below picture - design notes, voltage and amps, the Rsc shunt resistor 0.1ohm each

A VDR spec. on 13.6V and ignites at 15.7-20V, would handle 27Amp (destroying the 15A DC fuse)

NOTE: 

The calculation in below picture is not correct, according the the UA723 application note by Fairchild 1968, when the uA723 was introduced?, the current limit is set by: 0.7 (volt) / Rs (ohm)
In my case two 0.1 resistors in parallell = 0.05Ohm and provides a 0.7V voltage drop which would start the built-in transistor and thus have current limiting function at 14Ampere - which will work fine in this circuit

I should aim at 14Ampere for the current limiter Rsc value
Should do for the original specification

First step is the PCB, some CAD drawing, this layout is actually for a 20-25Ampere regulator version 

Single sided copper glassfibre laminate used

Second step is toner transfer

I put some permanent marker in the through holes

Third step is copper etching

If the etching solution is hot it will do it's job quicker - still it takes tenths of minutes to complete

Fourth step cleaning up and populate the PCB with components, with a mix of SMDs and through hole components, all components are from the junkbox

This time there was no issues to correct - just mount the parts and go

Cleaned with steelwool

Well - there was some issues - I mixed up the Rsc resistors as this pic was taken....10ohm or 0.1ohm 8)
For 15Amp two shunt resistors would do (for 20-25A in another configuration - all four resistors can be added)
On the opposite side of PCB, insulating layers of capton tape is visible under the AC fuse holder
The RA resistor is removed completely, RB is now a 1.5ohm and used as a jumper connector, as the voltage drop would be 0.7V the hope is that RA/RB are not needed.
The driver NPN TIP31C has no additional cooling - in the test made no fast heat build-up was noted in this transistor, nor in the 723 itself

The new PCB connected
Note: new cable management ;)
AC has its own conduit and a fully encapsuled fuse holder
RF ferrites on the DC terminal cable (+) & (-) and from power transistor emitters

First test showed a functional new regulator PCB

Voltage adjusted to 13.8V

DC load of ~12Amps and voltage drop to 13.4V

The added DC fuse show less than 10mV voltage drop on 12A load

Idling on 13.8V

DC Load, about 1.5ohm in total
20ohm/10ohm resistors in parallel
10W wirewound resistors

Yaesu PCB rely on a single 3A fuse on 230VAC mains, that fuse was also placed on the original PS-12 PCB

TESTING THE CURRENT LIMITER

I am not fully good with how the limit works now

Somehow the transistor on CL/CS see less than 0.7V from the Rsc shunt - which would mean that it is not triggered fully?

At full short circuit - 0.63V is measured with DMM over CS and CL pins, voltage at poles show about 1V 

Not sure what is going on, I removed RA and RB as resistors, RB is a now low loss 1.5ohm resistor functioning as a low loss circuit jumper.

Is the internal CL/CS transistor destroyed ?

Next steps would be, restoring the Yaesu FP-12 unit:

• Add 15V varistor/VDR between (+) and (-) output poles, in case excessive voltages here the VDR will (hopefully) trigger the 15A 12V fuse or the 3A mains fuse.
  The VDR has been added and seems to be "quiet" at 13.8V and under load

Finally - Unit Ready for Use

Green LED indicates +13.8V on the positive(+) pole
0.4Voltage drop on 10-12Ampere, the unit delivers also 15Amp