Looking for an RF coil for Galaxy DX 2527 (RCI 2950)

[PoDuck]

Are the finals from a known good source? 2312 transistors are quite rare.

They are obvious knock offs from ebay, but as I said, the original ones went out once I placed them back in the radio too. I didn't replace them until I wasn't getting signal from the old ones. That could be a problem though, I suspect. I have purchased other finals from the same seller, and they worked just fine, although I'm not sure if they were knock offs or not. These ones are obviously not actual Mitsubishi chips.

 

[PoDuck]

Ok, there should be the same voltage readings "small difference" across both 255 and 257 they are essentially what provides the bias voltage to the finals. Another word if you see 8.61 volts on 255 you should also see 8.61 on 257 that would be the feed side so to speak and should see the same level whatever it may be on the other side of either resistor.

That is what I see.

 

[PoDuck]

That is what I see.

Starting to wonder if I got chips that don't work. They all tested okay on my cheap tester, but that isn't a fully comprehensive test.

 

[Low_Boy]

2sc1969 is a good replacement for the 2sc2312. There is no 2sc2321. I have never saw a knock off that worked correctly. Even the NTE I tried for grins exploded. I WOULD be wondering if the finals are bad and throwing everything off. VT or Andy could better answer that. Try RF parts for finals. Pardon misprints and spelling errors using my phone again

 

[PoDuck]

2sc1969 is a good replacement for the 2sc2312. There is no 2sc2321. I have never saw a knock off that worked correctly. Even the NTE I tried for grins exploded. I WOULD be wondering if the finals are bad and throwing everything off. VT or Andy could better answer that. Try RF parts for finals. Pardon misprints and spelling errors using my phone again

Yeah, Andy wrote it out 2321, so I copied it. They are supposedly 2312's.

 

[kaos513]

Eleflow1969 finals are good as well

 

[Handy Andy]

Writing this faster that my finger work, works...sigh. Sorry PoDuck. - ok Yes, 2312 - but, no - nothing - hmm, what does the afterwards look like?

Ok, why I'm suggesting the resistor change...

This deals with modes, mods and catastrophic failures...

IF you ever run into a problem with bias, it is usually due to TOO MUCH voltage on the part - caused by too much current - that blew the very device used to protect the very device it was to keep in Class of operation.

Think about it this way...not to throw a lot at you but this deals with ohms law and voltage dividers and how they split power amongst the two (in this case) the 150 ohm and the 18 ohm - do some quick math and you'd see the 18 goes into the 150 about 8 times and then a little left over. Then just look at the ratio of voltage it would share - so the 150 ohm resistor would drop about 80% of the voltage - being 8 volts from TX leave you about 1 volt left for bias - but an RF Transistor turns on at 0.7 volts of so - still too many volts is 1 volt too many, but at least you have limited it's current. It's why you get these readings - then the diode drops off the rest of it.

Well, if life was all rainbows unicorns and sunny skies, we would be fine, but no, it's full of spikes, lightning, rain, thunder, pot holes, shakes and stuff that can affect radio equipment in ways - well - you already know...another might not but that's beside the point...

The 18 ohm and 180 ohm is a good balance - 200 ohm is too large of a value and it's drop would be too great - you'll need 180 ohm - so we'll work with what we got.

The above is why - for when the radio heats up - it will change the voltage drop across the resistors, diodes and transistors - they all are supposed to stay close enough together to track but that doesn't always work well as they get older.

If the didoes blew due to too much forward current - which is possible when you run hot mics thru Class AB radios - the voltage sags and rises and the falls of current -stress all the parts involved. Some are designed to absorb much more than they usually would be thrown at them is these radios but then there is an age and factor called MEAN TIME BETWEEN FAILURES (MTBF) - the radio may only be a decade or two old at the most, but the parts may have been subject to high SWR and poor maintenance.

So the 180 ohms gives me the assurance that had any diodes (bias ones) failed - I was not going to supply the Driver and Final with too much current thru that voltage divider the Bias circuit works thru - and blow parts I can't get anymore.

200 ohms would work, if I was an AM'er only - but the SSB side - the Driver and Final need just enough voltage pressure to allow the RF signal (a weak one at the start in SSB) to make the transistor turn on at the start of a word not in mid-sentence.

 

[PoDuck]

I was thinking that the 1 watt thing had something to do with it also, as the original resistors are 1/2 watt.

I have probably 50 values of 1 watt resistors, and 180 ohm is not one of them. Some day I may acquire all the values, but for now, I would have to special order those resistors.

Anyway, I'm going to be ordering some new finals, again. Hopefully they will work, as it seems to me that everything is working up to that point. I might order some 180 ohm resistors as well. Then we will return to this saga at a later date.

 

[Handy Andy]

Glad to know you're willing to give it a try.

Been kinda hectic around here lately - just to sit and answer a post or two - not to say I do sloppy typing - but it is a chore proofreading when you only have 10 minutes to answer - take it or leave it - and all those typos...SIGH

The 180 ohm versus 150 - again it is an issue of hard driven radios and their heat factors.

You could put in a 15 ohm resistor - but that's pushing the lower limits of higher power dissipation - for the 15 ohm will develop a heating issue and hot spots. Go too low, the resistor burns up and then the diodes and the transistor go - usually faster than you would notice, or could react and switch off the power.

But while I have a minute - I wanted to commend you on the job you did of cleaning up the parts - the diodes and all the dirt and goo everywhere. You really did a nice job in cleaning that up.

Nice to know that the "dirt wireshavings" or whatever was cleaned up. These radios looks and work great - it's when you have to work on them again does the traces begin to show heat tarnish and boils - never pretty. That is why I relocate the parts - like those caps - when it's possible. All parts accounted for and the bottom is clean so its easy enough to see where things go and nothing popped open.

So, again while I had the time after an unexpectedly busy day - I hope it works out well for you.

Good night!
:+> Andy <+:

 

[PoDuck]

Okay, I'm going to post my diagnostic procedure, and see if anyone has a problem with my thinking.

I think I may have figured out the problem, although I have yet to finally fix it. J2, which is up against Q46, appears to have been touching the anode of D90. If I am following things correctly, it appears this has brought the emitter low, instead of letting the diode shunt only any negative voltage to ground, which explains why I was getting absolutely no power after I changed the finals.

I'm still getting very low power output though. I'm only able to get a maximum of 5 watts RMS. I am also only able to get about 10 watts from SSB. When I go to bias it, I can get amperage for the driver between TP1 and TP3, which I set to be at 45mA, but I got no bias amperage reading between TP2 and TP3, so I turned it up to 55mA with the same results.

I also checked the voltage at the finals, considering that since I am not getting any amperage that there may not be voltage getting through. In RX, the collector voltages of the driver and both finals are supposed to read around 4.15 V in AM. I am getting 7.40 V on the driver and both finals. The base of the driver is supposed to read around 0.740 V on AM TX, and I am getting 0.257 V. The base of Q46 is reading a negative 0.965 V on AM TX when it is supposed to be +0.68 V, and Q48 is -1.7V when it is supposed to be +0.58 V.

Since I'm getting negative voltage readings where they should be positive, I assumed I may have a bad diode. The only two diodes close are D90 and D91, so I checked them. D90 is acting very strange. It reads as a diode sometimes, and reads shorted at other times. I've never seen that happen before, so my first assumption was that I was screwing up my connections, so I disconnected both sides and tested again, and similar results appeared. I'm going to assume it is bad. D91 doesn't conduct at all, which I am able to be confident means it is bad. Both of these diodes tested fine when I first pulled them out to change the finals, so that short must have done a number on them.

So, I guess the question is, should I locate some MV1Y diodes that are already affixed to the copper heatsink, or is there an easy way to fabricate something with a 1N4148 or 1N914 diode that will dissipate the heat? I'm assuming that's what these are. Maybe I can just mount a 1N4148 a little higher on the copper with some epoxy, and leave the old one in place and cut the leads off of it?

 

[Handy Andy]

You don't need to use an MV-1Y - although a nice flat package, you still have issues when it comes to the install.

Here is a better method -

Remove the Finals - Unbolt them - unsolder them - clean up the foil pads...
Locate the "notches" In the board - you'll see a pair of them for each final...
These Notches are for those Diode leads - they don't go DIRECTLY onto the BASE or EMITTER leads, instead they arrive to a solder pad land by it...
Install 1N4148's into those Notches from the component side - the BANDED end goes to the foil trace for FOIL GROUND...
The 1N4148's go ACROSS the 3 legs of the final ON TOP (component) - and the Finals' legs solder to the bottom...

The reasoning for that arrangement is for the HEAT the Base and Emitter leg produces, goes into the soldering pads and spreads out from that - tucking the Diode close to the legs, keeping it flat on the board by those legs, allows the heat that the part tries to dissipate - get sensed by the diode - as if it was attached to the EPOXY case (it's front) .


So you don't solder the Bias diodes to the BASE lead, instead they go to a foil pad next to the base lead - the BANDED end (Cathode) goes to FOIL Ground - the same foil ground the EMITTER of the Final solders to.

 

[PoDuck]

Why do they go through the trouble of putting the diode on that copper board right on the transistor if all you have to do is put it on the pcb by the legs, or am I misunderstanding you?

Am I correct in believing that since the resistance goes down as the heat goes up that it reduces power to the final as the final heats up? If so, would it not be best to have it as close as possible to the epoxy case of the final, as in touching it with some epoxy on it?

 

[OldTech03]

Yes, that would be correct it lowers the bias in an attempt to lower the gain and keep it from going into thermal runaway "so to speak" so yes the diode will conduct to ground and shunt the base voltage.

 

[Handy Andy]

The MV-1Y is actually a Surface Mount Diode - mounted flush on a PCB that mounts to that tab using the hardware supplied. It fits' in a little rectangular cutout and you pretty much accomplish the same thing when you use any regular diode like 1N4148 or 1N914 mounted across where the MV-1H goes.

It's why there is a "dot" to show polarity and it is epoxied cover keeps the part covered - the 1N4148 use Glass casing so it's pretty much the same idea.

When the PN junction heats up, the voltage drop across the diode changes...

You can see that when you test it with a DVM on DIODE setting and heat up the part with a soldering iron - the value in the display changes - drops when temperature goes up - the junction expands when heated - and the lattice allows more current from the testing device flow thru too - showing the result on the DVM display.

Try it, it can help you understand how Class Of Operation works and thermal tracking keeps parts that use a small bias voltage to keep the part just BARELY OFF and ready to turn on at the moment a signal crosses into it.

Too much of a good thing - usually is.

Too much heat and the part can fail.
Diodes provide the mechanism for Bias to track the thermal profile of a transistors PN junctions'. (NPN or PNP) so they don't turn on, and stay on when you don't want them to. (like SSB mode)

:+> Andy <+:

 

[PoDuck]

The MV-1Y is actually a Surface Mount Diode - mounted flush on a PCB that mounts to that tab using the hardware supplied.

If you look at the picture I shared in post number 16 in this thread, it certainly looks like a regular diode simply epoxied to the surface, not a surface mount diode.