ept069611a single mosfet Galaxy DX 2547

[NZ8N]

He was referring to Q56 behind the 2 diodes. Not the diodes themselves.

 

[Handy Andy]

But it does raise several questions - about WHY...

To me and IMHO - they are due to a voltage drop problem in using Schottky versus regular older PN junction diode types - the newer ones only have 0.3V to maybe 0.5V of drop - which in a transistor-based system - might not be enough of a drop to accommodate the design of the circuit.

Power Regulation (8V Contant)

D72 is the one they piggy-backed another diode to,
The end result is a slightly higher voltage output from the regulator.​

Again, if it had been a PN type- the drop would have been enough between the Zener trigger and the Diode itself to make 8V plus a few tenths more.

But when you use Schottky - that drop is less.
So, you LOSE the voltage rise that can handle the trace and soldering ohmic losses in the drop across the plane of the board.​

then there's this...


They affect the Rx/TX toggling function​

It is my guess they did this to make the toggling function work correctly to prevent false triggering.

The 4558 and the power supply tie in together to make the 3-rd leg of the triad of; Power good, PLL in lock and I have enough power to turn on and off the lights - moment.

 

[Handy Andy]

But the above doesn't explain the condition of Full SSB and No AM carrier or audio.

So, does Q56 work or is it damaged?

Those diodes at location D103 are the "sense" for the envelope control the Q56 does to the AM Regulator.

So does it also affect SSB - yes, by having some control of the RF power envelope so the Regulator adjusts it's Power (voltage and current) and it's done thru the RF Power control - Q56 simply "buffers it" so if it's damaged - this circuit can wind up in two states...

So, the Diodes might play a role at D103 - but is Q56 good?

Easier way to check it just remove it, if it brings back AM power and audio - great - but there goes the RF Power adjust.

 

[Hawkeye351]

I yanked out that q56. I then stuck it into my longrunner tester. I also keyed the radio on AM with it removed and still no AM output at all, even ssb went out when that q56 is removed.

Here is the simple results from my tester for that q56:





Hfe seems awfully high. I normally see less than 200 for an hfe on any transistor I test. But, you guys tell me.

With q56 in place:
No AM carrier or modulation.
Sideband works fine (43 watts).

With q56 removed:
No AM carrier or modulation.
No Sideband power (0 watts)

I know it needs to go back in place, but I'll wait to see what you guys think due to that high hfe.

 

[Handy Andy]

Hmm...Well it answers several questions...

Let's go back to post #11

Ok, Q56 in this setup - is not only AM power Envelope control (RF Power knob)
But also, act's as the SWITCH for the SSB mode.
​

AM uses Q53 - but in SSB mode it's overridden by Q56 grounding Q55.

IT's why I asked about any RF control.

You can also try removing the transistor at Q53 and replacing it with the transistor at Q56 to see if AM (it's carrier and audio) Returns - you'll lose SSB - but we can see if AM power at VR14 will work.

So next you have to look at voltage at Q53. Hoping nothing catastrophic.

Got to see if AM Power (8V to set carrier thru VR14) is getting to Q53.

​

 

[Hawkeye351]

Here are the simple results of testing Q53:

 

[Hawkeye351]

Q53 board location keyed (AM/no audio)
E = 0.03v
C = 13.20v
B = 0.00v
This is with Q53, Q56 both removed.

Q56 board location keyed (AM/no audio)
E = 0.01v
C = 13.21v
B = 0.00v
This is with Q56, Q53 both removed.

Q53 board location unkeyed:
E = 0.73v
C = 13.20v
B = 0.00v
This is with Q53, Q56 both removed.

Q56 board location unkeyed:
E = 0.01v
C = 13.21v
B = 0.00v
This is with Q56, Q53 both removed.

 

[NZ8N]

Did you check C238 ??

 

[Hawkeye351]

NZ8N, no I didn't, but I will. I did read somewhere that those 2 electrolytic caps right next to the Q56 do fail due to the heat that q56 puts out.

I'll yank both of those in the morning and test them. I'll get back with results tomorrow. Thank y'all for hanging in there with me and being so helpful. Y'all teaching me a lot, yes I'm taking notes for future reference.

 

[Handy Andy]

​

All indications show you need output from VR14 - Each Base in the circuit is dead because there is no power from VR14...

Start doing Continuity checks for blown traces and
even blown parts - R263 is a 100Ω resistor
see if anything opened up - you have to get
VR14 sending power again!​

VR 14 sends power to Q53 thru R 264 that 5.6K resistor - ties right into the Audio Cap from the 4558 Mic Amp side (Pins 2-4) thru a jumper into Q53s' Base.

Vr14 also steers power to Q56 thru D107 - which is part of the RF Power control. - but works in a specific way thru Q56 and D108 - they are power Steering Diodes.

With Diodes like D107 and D108 that can also be made to act like logical Gates
IN this case an AND / OR situation.
​

So, if Q53 is showing good, (Yes, it is) then VR14 would send power to the BASE of Q53 all the time from the 8V Constant that is present at the tap from R263.

• You don't - where is the Power? is VR 14 even getting any?

You have Zero volts from VR14 at Base of Q53 - so that turns off Q53, which turns off Q56 - but Q56 is not going to turn on - because there is no POSITIVE presence voltage at the COLLECTOR of Q56 - because Q53 is off. So, it just sits there. - D107 and D108 work to send power from either part (VR-14's supply to Q53 or thru Q56's Collector) or BOTH parts if things are operational and in some cases - causing limiter action - the R300 that tap from ALC - also applies power to the RF Power control knob -which changes how much power is shared between D108 and D107 into the RF power knob - so one aspect can take control over the other - this sets an UPPER LIMIT to how much power the section can produce.

 

[Hawkeye351]

Just checked for the 8 volts at the 2 spots you pointed out above. The voltage is present, actually it's 8.55v at each location, and in both receive and transmit.

I ohmed out vr14 also, it goes up to around 10k and down to minimal very smoothly, although the VR seemed easier to turn than the rest, but it's a different type than the others at that, so that maybe normal for that type.

 

[Handy Andy]

It should be adjustable - not at 8.55V all the time - we need to see if it's variable.

...at the BASE of Q53...

Because if Q53 is ok, then what is its Emitter side show?

The check was done with Q53 out - now put it back in...

 

[Hawkeye351]

Just popped them back in, then checked for variable voltage at the base of q53, no voltage present on the base of q53.

The emitter of q53 is still the same, 0.03v.

I adjusted vr14 while checking voltage on the base of q53 and the emitter of q53, I also turned the RF Power external control and checked both, still 0.03v emitter of q53 and 0.00v on the base of q53, no matter where I turn the vr14 or the RF Power. The 8.55v spot on vr14 does not change neither, stays constant 8.55v no matter where vr14 is set or rf power is set.

 

[Handy Andy]

Got a Spare for Q53?

Because of the Cap is fine - you'll need to verify the Q53 mess is not the fault of Q53 OEM

 

[Hawkeye351]

I'll check and see.