Galaxy DX-55V & Saturn

[Hawkeye351]

Got a Saturn (EPT-360014B) and a 55V (EPT-360014B) with the same issue:

Both receive great.
All functions work great.
New caps throughout.

Both have no transmit due to no base voltage on neither the driver nor final. Collector voltage is present, but not base.

Checked pre-driver, shows over 1.23v on the emitter. All parts in circuit test as good. Looked for hairline cracks, loose solder joints, cold solder joints, even touched up a few. Still no base voltage on driver nor final.

Drivers and finals all test good in tester and on multimeter testing.

Regulators in both are working as they should, collector on all drivers/finals show around 4.78v, bout the same as the collector of the Passthrough Regulator.

What would cause no base voltage on neither driver nor finals when collector voltages are correct on all?

 

[TM86]

May be time to break out the schematic and trace the voltage path. When it disappears you know where to start looking for broken connections (yes, again, sorry) and/or shorted components.

But to actually answer your question, shorted MV1Y diodes could do this. Be strange to have two (or more) each in two different radios fail in the same way at the same time, but the odds are slightly better than winning the lottery.

Then again, I'm looking at a schematic and guessing. So feel free to disregard.

 

[Cable Guy]

I have a dx66v on the bench RN with the same symptoms, was thinking of posting then saw this. Watching....

 

[Cable Guy]

I checked the 22uh inductors from base to gnd, and the 680pf caps feeding the base from previous stages. All tested fine. There is rf coming to the bases as touching a probe to either causes enough rf to radiate that it's picked up on the bench radio. I could see if the B-E junction was shot, no rf getting rectified above the inductors and no base voltage. Nope, driver, final and pre driver are fine. Just nothing at the base of either driver or final.

 

[Hawkeye351]

Yea, my two 22uh chokes were testing as .03mh, so I replaced those with new 22uh. I get voltage up to the 560pf disc capacitor in front of the driver (around 7v), but no voltage or anything at the point where the 560pf cap and the 22uh choke meet at the base. Replaced that 560pf cap with a known one, still no voltage to base of driver. On one side of that cap I get around 7v to 8v, but on the other side that's tied into the 22uh choke on the base lead I get nothing.

 

[Cable Guy]

Yea, my two 22uh chokes were testing as .03mh, so I replaced those with new 22uh. I get voltage up to the 560pf disc capacitor in front of the driver (around 7v), but no voltage or anything at the point where the 560pf cap and the 22uh choke meet at the base.

Same. We can learn together as soon as experience walks in and schools us.

 

[brandon7861]

Those AM/FM only radios are usually self-biased class C things and do not have DC voltage on the bases. Lou Franklin's CB repair book briefly mentions it on page 152. The choke merely keeps the RF from going to ground. The Choke also holds the base at 0v DC while the signal (across the internal base resistance) develops a small negative bias internal to the transistor which further turns it off. Only when the RF signal is almost to its peak is the transistor being turned on, and only for a brief pulse.

 

[Cable Guy]

I figured the choke would provide some resistance, although small ~.8 ohm, and would allow a small voltage to develop on the base, being rectified thru the base of the bipolar. In my mind, it would be a negative voltage... I need to grab my book and study. So Brandon, are you saying an insufficient signal from the pre-driver would cause a lack of turn on for the driver? Or that no voltage present on the base is to be expected?

 

[brandon7861]

I figured the choke would provide some resistance, although small ~.8 ohm, and would allow a small voltage to develop on the base, being rectified thru the base of the bipolar. In my mind, it would be a negative voltage... I need to grab my book and study. So Brandon, are you saying an insufficient signal from the pre-driver would cause a lack of turn on for the driver? Or that no voltage present on the base is to be expected?

My understanding of it is that the transistor is only supposed to turn on for roughly 15-20% of the waveform. To only conduct for that short of time, a little negative bias is needed.

You are correct in reasoning the base voltage would be driven down below 0v, Whether from the resistance of the inductor or the internal base connection, a negative bias develops. In some radios, a resistor is added to the choke to have better control over the amount of voltage developed.That negative bias is what brings the duty cycle down to the small numbers common to class C amplifiers.

 

[brandon7861]

Lets look at 1MHz for a minute, its what the arb sig gen in my scope goes up to. This will explain the reason for negative bias.

Assume the DC bias is controlled so that only 20% of the waveform is above the turn on voltage of the transistor in both of these cases.

With 1Vpk-pk, if the transistor starts conducting in that 20% window, there is 176mV of voltage swing while the transistor is turned on (which represents an ability to push more current into the base because the voltage can't, you know...)

If we wanted more power into the driver, we can't simply turn up the pre-driver without simultaneously lowering the bias voltage because the duty cycle would change. So, if the DC bias is made more negative, such that with 2V pk-pk, only 20% is turning on the base, now we have 400mV of swing pushing current into the driver.

The negative bias is necessary to to push decent power into the class C amp.

 

[BayouRadioAmplifier]

OK, I just repaired an old Galaxy Saturn days ago. (014B board)
It was golden screw driver-ed to death, and had various modifications.

Just got my new RF output bias circuit tested last night.
I ripped out the MV1Y diodes that failed and made
a more reliable bias circuit. Just some resistor changes.
Those MV1Y diodes were bad here
I will post later today when I get home from work.
Have 40 W PEP SSB output now.
in AM about 10 watts carrier, can overmodulate it to death.
(after taking that diode "NPC mod" out)

do you have the 8.3 V when keyed up?
TR38 and D77 blew out on this radio.

 

[Cable Guy]

Thanks for that reminder, I wasn't even thinking of transistor class, I was just going by the trans voltage chart for this radio and thinking the transistor didn't start conducting until it got near .7v, which is what the chart specifies. Maybe it was listed in error. I will think on it more after work.

Galaxy Radios DX66V Service Manual

Bayou, thanks for the chime in, but this radio doesn't have the mv1y diodes as that was my first suspect but they were not included on this build. See below.

 

[Hawkeye351]

Driver in this 55v I'm working on is a 2078 (hmmm), although it tests good on both the tester and by using the diode mode on my multimeter. The final has that glassy black shine to the face with light imprint of the Mitsubishi logo and 1969 stamped on it (this one could be a fake), although it does test as a good BJT in the tester and shows good using the multimeter test procedure.

 

[brandon7861]

Thanks for that reminder, I wasn't even thinking of transistor class, I was just going by the trans voltage chart for this radio and thinking the transistor didn't start conducting until it got near .7v, which is what the chart specifies

It does start conducting at 0.7v, the question is how much of the incoming waveform is above that. I don't know the exact conduction angle they were aiming for in the driver, nor do I know the pre-driver output level. With no resistor under that base inductor, very little negative bias may develop.

I just wanted to explain that the balancing act between conduction angle and drive power requires attention to the bias. It is quite possible that the pre-driver output is right for no negative bias at all.

The conduction angle sets the efficiency of the transistor, the gain and required output sets how much drive the transistor needs to see, and the bias is what balances those two worlds. If the driver needs more input, the predriver needs to be turned up. Turning it up pushes it closer to class B. Negative bias brings it back to class C.

 

[Hawkeye351]

Got another 55v waiting for me to find out what's up with it.

Symptom:
Turn power on and radio automatically keys up in transmit mode. Press the PTT and it doesn't change, it stays keyed up.

Mic op amp was showing strange numbers all the way around. Replaced with a new JRC 4558D, still same results.

TR32 was showing 8v on collector in transmit mode. Replaced tantalum cap at TR32 limiter, same results. Then replaced TR32, same results.

Collector of AF regulator and collector of Passthrough regulator shows as they should.

This radio did key up and talk good until I replaced R174 and R187. They were replaced previously with 2.2k ohm (way to low for my liking). Factory is usually 10k ohm in those two locations but a lot of people will replace those two with a lower value for more drive to the mic amp, but my opinion is 2.2k is way too much of a drop. If I need to replace those two from their factory values of 10k each, then I usually go with half of what the factory has in it, which would be 5.1k. This value seems to work best on these exports radios if the need for more mic drive comes up.

Anyway, I replaced those 2.2k resistors at R174 and R187 with 5.1k resistors. After that, then is when the automatic key up when turning power on started. After checking over a few things I noticed I had shorted out R187 to the emitter of TR33, and R174 (from collector of TR32) shorted out with the path that leads to the emitter of TR33 also.

It transmits a carrier fine, good strong carrier, just all the time is the issue. Won't unkey, even with no mic hooked up, just turn the radio on and it keys up by itself.

This thang drove me nuts, so I put it on the side for now.

Yes, it was my bad...lol...
I did this work at someone else's house, wasn't equipped and lighting wasn't as much as I am used to working with.

Even the best make mistakes at times...