Galaxy DX-55V stuck in transmit mode...

[Hawkeye351]

Got this Galaxy DX-55V on the desk. This is one of two 55v's I been working on lately. The other had no driver bias, thanks to another user (Cable Guy I think) it's back to working.

However, this other one is a boo boo on my part. As usual, I yanked out the junk regulator swing mod, replaced all three limiters (yea I know it makes no sense to remove them all, lol...) and replaced R174 and R187 with 5.1k Ohms each. Those spots come factory with 10k ohm each, but some "watt meter" techs like to drop those values down as low as they can just for watt meter pride. These locations should only be changed out IF, and only IF you require a little more drive to the mic amp. To low of a value in those two spots will overdrive that mic amp and could cause issues. Why stress out a good radio for only a couple of watts difference? If I feel the radio needs more drive, then I'll swap those two resistors out with 5.1k ohm in each spot. This radio had 2.2k ohm in each spot, way too low for my liking.

Well, after I replaced those two resistors with 5.1k Ohms each, then is when the radio got stuck in transmit mode. Every time I turn the radio on, it's keyed up by itself. Even with no mic hooked up. If I hook the mic up and press the PTT, then it makes no change, still stuck in transmit mode.

After checking over my work I noticed I had shorted out the R174 (TR32 collector side) with ground. I also noticed I shorted out the R187 lead (farthest lead from chassis) to the other two closest solder joints beside it, one went to ground (emitter of most of the transistors in that section).

After completely removing those resistors and cleaning up the spot, it is still stuck in transmit.

The mic amp (OP Amp) was showing odd numbers so I replaced it with a new JRC-4558D, same results and numbers.

Checked the following transistors:
TR32, in circuit it showed 8v on collector. Out of circuit it tests fine on tester and multimeter diode test.

C128, C129, tests as good caps, tolerance well within limits and ESR is good.

TR33, TR36, TR37, TR38 all test good out of circuit in tester and multimeter diode test.

C134 tests as a good cap.

Checked D77 in and out of circuit, tests as good.

Check all diodes around TR36-TR38 section, all test good.

Beginning to think C199, C131, main 8v regulator or the 7222AP IC.

Any suggestions guys/gals?

Lifted collector of Passthrough and AF regulator for safe measures to prevent blowing up the Max mod or finals. Still stuck in transmit.

 

[Hawkeye351]

Here is a pic from of where the shorts were after I replaced those two resistors.

Shorted R174 in circled area.
Shorted R187 in circled area.
Solder ball between positive and negative of C194.

Something from above caused the radio to be stuck in transmit mode at all times now. This issue was not present before I made the above three boo boo's.

 

[Cable Guy]

Check D81, D82, R167, R168, R170, R171. What are the readings on 5,6,7 rx/tx of 4558?

 

[Cable Guy]

R174 and R187 are in the mic audio line, can't imagine they're hurt.

 

[Hawkeye351]

By the way, I did check the radio each time I put those parts back in one by one.

I replaced the Mic OP amp after I realized it was stuck in transmit after realizing I made some boo boo's and the mic amp was showing odd numbers. Even with a brand new one, it still shows the odd numbers, so it has to be somewhere else.

I then went the following route:

Example:
After I put D77 back in, I checked it, no transmit when powering on.

After I put TR36 back in, I checked it, no transmit when powering on.

After I put TR37 back in, I checked it, no transmit when powering on.

After I put TR38 back in, I checked it, transmits when powering on. Although TR38 tests great in the tester and checks out on the multimeter in diode mode.

I then checked the diodes in the TR36-TR38 section, all checked out as good.

I still have TR32 and C128 (I think, I could be C129) tantalum out of circuit until I get it straightened out. They both checked out great, just waiting to put them back in after I get the stuck transmit issue fixed.

I also pulled TR33 (Roger beep oscillator) and the Roger beep timing capacitor (C134 I think) and tested both, radio still stuck in transmit mode with these two parts removed.

 

[Hawkeye351]

Sorry Cable, was typing as you sent your messages. I'll check those, I was looking over the schematic and block diagram (99v) and noticed D81 and D82 could possibly be an issue since they're in the transmit line. Thank you Cable.

 

[Hawkeye351]

Kinda confusing on checking for voltages in the xmit/rcvr circuit since it's stuck in transmit all the time, lol...

 

[Cable Guy]

Kinda confusing on checking for voltages in the xmit/rcvr circuit since it's stuck in transmit all the time, lol...

Brain farts over here, duh lol.

 

[Hawkeye351]

Same over here

 

[TM86]

Another rabbit hole on a radio I don't have any to compare to! Yay!

I'm following the schematic and looking at the transistor voltage chart on cbtricks.org. (https://cbtricks.org/radios/galaxy/dx55v/)

TR-38 is your TX switch transistor, so it makes sense that the radio doesn't automatically jump into transmit mode when it's out of circuit. The base of TR-38 in RX (which you don't have right now) is supposed to be about 8.03V. In TX it's supposed to be about 7.75V. So it's slightly less positive than in RX. I would guess that anything at or below that voltage would key the radio. Anything above that and I would say replace the transistor.

You've already checked D77 and that tests good. Next in line is R196, which then goes straight to pin 7 of the JRC4558D chip, which you've already replaced. If R196 or D77 is open, that could cause this.

The only other components that are connected to pin 7 of the opamp chip are R168, which I think is there to set the gain of the opamp, and R195, which connects to D76, which in turn connects to the base of TR-37 and one end of R194, the other end of which goes to ground. I suppose a short somewhere in the R195, D76, R194 chain could pull the voltage going to the base of TR-38 down.

You could apply 8V to the base of TR-38 and see if that takes the radio out of TX. Might want to pop D77 or R196 out first, though, just to prevent anything feeding back to the opamp.

Anyways, that's all the guesswork I have for you. Hope it at least helps a little.

 

[brandon7861]

This is the keying circuit redrawn to make it easier to see whats going on.

There is a reason Cable Guy asked about the voltages on pins 5, 6 and 7. Let me explain how this circuit works.

R166 and R169 form a voltage divider to provide a reference voltage of 38.4% of the regulated supply and feeds it to the op amp for comparison.

R167, R171 and R168 also form a voltage divider that changes value depending on whether the PTT is pressed or not. R168, being at the output of the op amp (which goes high and low) is effectively switching between being in parallel with R167 and R171. This provides hysteresis so noise in the PTT switch doesn't cause bouncing.

When the PTT is pressed, D81 can pull the positive side of the op amp below the negative reference side, and when this happens, the output at pin 7 goes low. When the PTT is released, the + op amp input is once again higher than the - op amp input and the output at pin 7 goes high.

From there, the output of the op amp controls two circuits. RX switching and TX switching.

During RX, the high level output is enough to go through R195, the zener which drops most of it, and R194 such that enough voltage is present to turn on Q37, which in turn turns on Q36 supplying the RX rail

During TX, the low level output ensures the zener cannot conduct turning off the RX supply while simultaneously being low enough to forward bias D77 (the 1.4v diode) and the emitter-base of the PNP Q38, which turns on the TX rail.

The reason Cable Guy wanted to know about the voltages at the op amp should be evident now. He wants to know if the problem is before or after it, which narrows down where you need to search.

Edit to add cleaner schematic, attached. Also corrected minor error (Q37 has no emitter resistor).

 

[brandon7861]

At some point, was D77 (KB362 dual diode) replaced with a single junction 4148 or 4001? If so, that might keep the TX on because the op amp output cannot output rail voltages. It needs that extra voltage drop to ensure proper switching.

 

[Hawkeye351]

Haven't rechecked voltages on the 4558 again yet, but will in the morning.

As for D77, it's the original KB diode.

 

[Cable Guy]

Brandon, your schematic is well laid out and easier to understand, as you suggested. Hawkeye, also, make doubly certain you didn't miss a solder bridge somewhere, I only mention that because before, after betting the farm that I got them all, I later find a bridge.

 

[Hawkeye351]

These voltages are taken from the component side, not trace side of the board. This is immediately after turning the radio on and it immediately goes in to transmit mode.

Only components I haven't re-installed yet is TR33 (Roger beep oscillator), C128 and TR32. These components are left out during this voltage test.

Remember, it won't unkey, even with no mic hooked up. Turn it on and immediately it goes into transmit mode.

Checked the following in circuit:
R194, R195, R196, R168, D76, D61, D66, D78. They all show as their values represent (close to it, within listed tolerance). I also replaced C134 with a new one for safe measures before checking the below voltages.