Another 55v no xmit

[Hawkeye351]

Got another Galaxy DX 55V on the desk.
Been tracking this issue down for about 3 weeks now.

New driver
New final
New pre-driver
New Passthrough regulator
New AF regulator
New diode at D77
New 2.2uf 50v bias cap
Both 22uh inductors new
When keying, radio will switch from receive to transmit fine, and change the receive/xmit light as it should.

No output on meter whatsoever, not even a hair movement.

Monitoring on nearby radio (FT-950) 1ft away with antenna connected, I get a signal, although not as high as it should be, and can't hear any modulation when I speak.

Radio receives great, VCO, Buffers, oscillators all aligned perfectly with no spikes or issues.

Pre-driver doesn't show right voltage on emitter, it shows the same voltage as the base (around 1.3v). Collector shows right (tad over 8v). In transmit mode.

I get 0.00v on the base of driver and final in transmit, but collector shows right voltage. In transmit.

All other votages (Passthrough, AF, etc..) shows fine.

 

[Cable Guy]

I get 0.00v on the base of driver and final in transmit, but collector shows right voltage. In transmit.

Had one doing almost the same today but with bias voltage expected, turns out the predriver wasn't receiving 27 MHz from the tx mixer because of a shorted capacitor shorting the TX mixer out. Wouldn't have noticed if I hadn't felt a resistor giving off too much heat. A quick look at the thermal showed the current limiting resistor barbequing the PCB.

Is r220 reading 4.7 ohms? If open, could maybe explain the same reading as the base.

Regarding the base voltages on the final and driver, looking at the schematic for the 55v, seems like another "class c" bias hurdle. I'd say if the collectors are right, the transistors test fine and the coupling caps between stafes are fine, id look back toward the mixer. What do the signals at IC9 look like?

This is just off the top of my head, forgive me if I missed something. I had dental work and feeling off. Good luck Hawkeye. Lol I'm actually watching M.A.S.H right now.

 

[Hawkeye351]

R220 in circuit tests as 465.3 ohms. Might need to yank it out to test it.

 

[nomadradio]

I get 0.00v on the base of driver and final in transmit,

Check bias diodes on the driver and finals. And the bias trimpots.

73

 

[Hawkeye351]

No bias diodes or bias trim pots in a 55v. Has a 22uh inductor in each trim pot location for driver and final.

Got another 55v from the same guy for a parts radio. It hasn't had any parts removed yet so I figured I would see if I could get it going instead of the no xmit 55v. Well this parts radio shows 24.??? mhz on the internal frequency counter. Checked VCO voltage and it was below 1v. Adjusted it slowly up and down. All the way up flush with can equals 22mhz, all the way down flush with bottom equals 27.??? mhz. Guessing the VCO transistor or the 2.2uf tantalum at the pll is shot.

I was at first thinking the adders were shot, but when I found I could get 27mhz back by turning VCO voltage all the way down (3.10v), then I figured it must be the VCO circuit.

Gonna play with both at the same time to see which one I can get done first.

I've had more problems with 55v's than any other radios.

 

[Hawkeye351]

Well...

Removed R220, tested in tester shows 465 ohms (same as in circuit). Double checked with two ohm meters, bout the same value of 46? ohms. Replaced it:
Then the emitter of the pre-driver went up to 2.21v, base and collector remain unchanged. Still no output, still no voltage on the base of driver or final (collector remains unchanged).

Probed around for a shorted disc capacitor. C182 showed as a short in circuit. Removed it and tested, showed as "Bad or unknown part". Tested two more times, same thing.

Strange thing is, after I removed C182, that circuit still showed a short.

Got some running around to do, catch y'all back in a bit.

 

[Hawkeye351]

Oh, I see why the circuit still shows as a short with C182 removed. That cap goes to two coils inside two different IFT's. The other end of those coils go to circuit board ground.

 

[Hawkeye351]

The transmit mixer shows around the right voltages in transmit.

Found C175 shorted.
Found C182 shorted.
Found R220 was open.
After replacing those, then the collector voltage on the driver and final went down to 0.41v, they were the right voltages before I changed the above parts. Checked the collector of the Passthrough regulator, af regulator, both showed 0.41v. replaced both.
Checked TR49, shows 0.41v on one outside lead and 0.06v on the other lead. Yanked it out, tested it and it shows as great with a good hfe.

Also, now the collector of the Passthrough shows just 0.41v (brand new reg) and does not change when adjusting VR13 or RF power. On the bias strip (where jumpers are that you must lift to set bias) the voltage used to be around 4v, now it's 0.41v. All I did was replaced two shorted caps with original values and that open resistor with an original value new one (tested all three before installing), then is when the collector voltage of the driver and final went down to 0.41v and so did the collector of the Passthrough. Oh, and the voltage on the base (not emitter, I was mistaken) of the Pre-driver is still 2.20v, should be 0.8v. I had the pins on the pre-driver confused earlier in this thread. Base is 2.20v, collector is a tad over 8v, the emitter is 1.36v.

No shorts, no solder balls, tested new parts before installing.

Got my brain and eyes hurting. Taking a break. Anyone got any suggestions? Still receives great.

 

[Cable Guy]

Verify vr13 is good? Replacing those parts shouldn't affect the regulator unless the driver and final are getting hot from turning hard on. That would pull the regulator hard on and it would get hot too. I assume that isn't happening so check vr13 and post voltages of all included components of the regulator, in an appropriate way.

Also, to verify, pull the bases of both driver and final and see if the collector is getting pulled down. I wouldn't expect to measure much voltage on the bases.

Do you have an oscilloscope? Having a close voltage on the mixer doesn't really tell you if a signal is coming out of there or the mixing signals are arriving into it, just that it's powered up.

 

[Cable Guy]

I suspect that you have the rf pa section firing up and pulling on the regulator, and when the regulator is idling by it can keep up the voltage, but when your downstream sections start working and pulling a load, the regulator section is showing you that it still has a problem. This is assuming nothing is getting hot. I would check out the regulator area and it's feedback components.

 

[brandon7861]

Oh, and the voltage on the base (not emitter, I was mistaken) of the Pre-driver is still 2.20v, should be 0.8v. I had the pins on the pre-driver confused earlier in this thread. Base is 2.20v, collector is a tad over 8v, the emitter is 1.36v.

No shorts, no solder balls, tested new parts before installing.

Got my brain and eyes hurting. Taking a break. Anyone got any suggestions? Still receives great.

The schematic shows a predriver bias divider R221 (1.5kΩ) and R222 (330Ω), which, when powered from an 8v rail, should produce a base voltage of 1.44v. You have 2.2v there.

I can go two ways with that. The first is to ensure you don't somehow have 13v on the 8v rail because 13v would explain that. But then again, you said you had 8v at the collector (same source), so it's probably not this.

If there is only 8v there like it should be, we need to explain why the bottom resistor has twice the current as the top resistor in the bias divider. Where is the extra current in the bottom resistor coming from?

Assuming it is 8v, I would lift the 8v side of R221 and see if the base voltage disappears. If it does, you can eliminate leaky caps and a leaky collector junction and focus on the divider resistors. If there is still base voltage after lifting R221, suspect those caps and transistor instead.

 

[Cable Guy]

The schematic shows a predriver bias divider R221 (1.5kΩ) and R222 (330Ω), which, when powered from an 8v rail, should produce a base voltage of 1.44v. You have 2.2v there.

I can go two ways with that. The first is to ensure you don't somehow have 13v on the 8v rail because 13v would explain that. But then again, you said you had 8v at the collector (same source), so it's probably not this.

If there is only 8v there like it should be, we need to explain why the bottom resistor has twice the current as the top resistor in the bias divider. Where is the extra current in the bottom resistor coming from?

Assuming it is 8v, I would lift the 8v side of R221 and see if the base voltage disappears. If it does, you can eliminate leaky caps and a leaky collector junction and focus on the divider resistors. If there is still base voltage after lifting R221, suspect those caps and transistor instead.

Right, I have been talking to him in messages and didn't notice his comment, and open/changed value resistors are found along with open caps and junctions. I'm trying to figure out what happened to this rig. There are issues everywhere, but we got the regulator right now (vr13) and verified the mixer is mixing. I'm looking into a comment he left with regards to r220, he said he replaced it with a 4.7k and not a 4.7 so I'm making sure. If it is a 4.7k, wouldn't that put the emitter much higher above ground and turn that be junction off, causing the .8 volts extra on the base?

No, I don't think so, but still, need to verify he replaced the resistor with the right value.

 

[Hawkeye351]

Replaced that 4.7k ohm resistor back to a 4.7 ohm.

Checked for 27mhz at base of driver and final, present on both and holding steady at 27mhz, but, but the 4.7 ohm resistor immediately started smoking and cooked. Still nothing on meter though.

 

[Cable Guy]

Check the resistors Brandon mentioned. Check c275 while in there. I really think Brandon hit on something.

 

[Cable Guy]

And check tr45 also.