Palomar 225 amp with a smoking resistor

[AronDouglas]

I picked up this amp form a friend who never used the thing (it was passed onto him form someone else). He had no idea if it worked or not, so I gladly took it off his hands for free.

I wired it up to a power source to see if it was "ok", but a resistor starts smoking. One side of the solder joint is getting super hot and its melting the solder and smoking. I am not electrical smart, I can work on electronics but I dont have a mind set for it... I'm more of a mechanic. I just want to know why the thing is smoking, I can easily replace it.

 

[gewecke]

What you saw smoking was a diode, not a resistor. Someone may have applied reverse polarity ( backwards)
Advice would be to sell this to a ham operator, since they would be licensed to use this.

73,
n9zas

 

[AronDouglas]

The picture posted is not my amp. It was late last night so I didn't want to find my camera. But mine is identical.

So is that diode shot, should I replace it? I'd rather sell a working amp rather than sell it for cheap with broken parts.

 

[AronDouglas]

Its a 1N4004 diode. I'll just replace it. Do you think anything else may have been toasted? Knowing a little how diode work, I'd assume it did its job so everything else should be ok.

 

[AronDouglas]

One trip to radioshack later and the diode is replaced. Now when I turn on the amp, the high/low light comes on as well as the amp light, even thought the high/low switch is on the 'off' position. The light/low is a 3 position switch (on/off/on).

 

[AronDouglas]

Ok, more trouble shooting has resulted to these confusing results.

When power is applied:

All lights 'OFF' and switches 'OFF'

Amp switch 'ON', amp light 'ON', high/low switch 'OFF', high/low switch light 'OFF'

Amp switch 'ON', amp light 'ON', high/low switch 'ON high' or 'ON low' results in no high/low light

Amp switch 'OFF', amp light 'OFF', pre-amp switch 'ON', no light

Amp switch 'OFF', amp light 'OFF', pre-amp switch 'ON' with high/low switch 'ON high' or 'ON low' no pre-amp light or high/low light

Amp switch 'ON', amp light 'ON' pre-amp switch 'ON' results in high/low light 'ON'

If anyone wants, I can upload pictures and explain whats going on.

 

[AronDouglas]

Ok, pictures. I have done some work on it. I just replaced the switches and LEDs and made a new face plate to match.




AMP on, high/low power off



AMP on, high/low power Low



AMP on, high/low power High



Pre-amp on, high/low power off (notice no light)



Pre-amp on, high/low power Low (still no light)



Pre-amp on, high/low power High (still no light)



AMP and Pre-amp on (no high/low power), notice the lights.

 

[JayMojave]

Hello AD: Wow you did a little work there on that amp.

The LED Light is just a simple resistor and a LED light wired, so that when 12 volts is applied to the switch, the 12 volts goes to the resistor and LED turning on the LED.

Question: When the Amp is on, and the Pre-Amp switch turned on does the receive get stronger or noisier indicating the pre-amp is working? This may indicate a help in trouble shooting.

The Pre-Amp is usually just a small signal transistor like a 2N2222 or close, so that when the Pre-Amp Switch is turned on it applies 12 volts to the transistor, and the resistor and LED.

I looked over at CBTricks.com and didn't see a callout for the Palomar 225 amp, but this is a common simple type amp made in many a Garage and is a copy from a Motorola Application Note that came out in the 70's.

I am sure you can probe the net to find a schematic and such, or even something close that will show how the LED's are wired.

These amps were a good boost in output power with say 10 times the output power as compared to the input power, that's a 10dB increase in output power, that a good increase. And it doesn't require large welding cable wire to supply the 12 volt power.

Look over at the CCI web site they have the Motorola Application Notes, and parts and pieces for transistor amps and such.

Application Notes

Good luck, let us know what you find.

Jay in the Mojave

 

[AronDouglas]

I do not have a radio to test it out on as of yet. My friend has a 959 coming in the mail some day and hes going to get this amp. I'd rather not start testing this thing on his new radio (he just put about $300 into it in parts and tuning). So I cant say what exactly its doing when its hooked up to a radio.

I put a meter on the switches and LEDs today. The amp switch had 11 volts ish in the 'off' position (test battery is only pushing 11.4 or something). I flip the switch 'on' and it reads 0 volts. I put the meter on pre-amp and it reads 0 volts whether its on or off. And the LED reads 0 as well. Same goes for the high/low power.

What bothers me is when the amp and pre-amp are on, the high/low power LED comes on. I can safely say its not bad wiring, switches or LEDs (all are brand new and tested).

I had a brain fart this morning and decided to check for a short. I put a meter lead on one side of the amp switch and the other on case ground (amp switch is the only thing I can get a reading off of, everything else reads 0v). Sure enough, when I flipped the switch I read 11.4v. Now I get to spend the day looking for a short or a burned out part (already spend about an hour looking for bad parts in the past and everything looked good.

 

[gewecke]

I like your choice of switches...

73,
n9zas

 

[AronDouglas]

The only thing I could find that looked wrong was this resistor (yes, I got my terms right this time ) But it didn't look bad, it just looks like some solder flux residue.




@ thanks gewecke. My friend who wants this amp picked out the switches. He also is using dual antennas and wanted two antenna hook ups on the amp, so I made a new back plate as well. The power plug was my idea.

 

[gewecke]

Fyi, Those resistors look like they *may* have been replaced previously.
Suggestion would be to check value on them and possibly replace again, as they may have gotten cooked.

73,
n9zas

 

[prcguy]

You can run dual antennas from the extra connector you added but unless you feed each one with a critical length of 75ohm coax to give a combined impedance around 50ohms you will cause lots of problems.
prcguy

 

[prcguy]

Also, what is a XTMR? The common acronym for transmitter is XMTR.
prcguy

The only thing I could find that looked wrong was this resistor (yes, I got my terms right this time ) But it didn't look bad, it just looks like some solder flux residue.




@ thanks gewecke. My friend who wants this amp picked out the switches. He also is using dual antennas and wanted two antenna hook ups on the amp, so I made a new back plate as well. The power plug was my idea.

 

[AronDouglas]

I actually have no idea what XTMR means, but it was stamped onto the old back plate. I knew it was suppose to be XMTR, but oh well.

My friend knew about the external connectors for dual antennas, but he didn't like those. Both sort pieces of wire (RG6 wire, about 3/4 inch long) inside the amp are the same length though. One wire goes to the board and one connects both connectors together (SO239 connectors).

 

[prcguy]

Ok, just be aware if you run two separate coax cables from the connectors on the back of the amp to two separate antennas, the type of cable and length becomes very critical because it needs to cause an impedance transformation. 75 ohm cable is normally used and the length is calculated from the frequency range of CB and the velocity factor of the coax, or the lengths are cut and tested with an antenna analyzer or similar.

Then you would probably want to connect both antenna cables to a coax T adapter and test both antennas as a system before connecting to the new connectors on the amp.
prcguy

I actually have no idea what XTMR means, but it was stamped onto the old back plate. I knew it was suppose to be XMTR, but oh well.

My friend knew about the external connectors for dual antennas, but he didn't like those. Both sort pieces of wire (RG6 wire, about 3/4 inch long) inside the amp are the same length though. One wire goes to the board and one connects both connectors together (SO239 connectors).

 

[KNY2ADQ]

Also, what is a XTMR? The common acronym for transmitter is XMTR.
prcguy

It's a CB thing, who have to be a brother of a the fraternity and know the secret handshake for the answer

 

[AronDouglas]

Ok, so after more testing I have come to the conclusion to change out the 2 relays and the 2 transistors. While slowly flipping the amp and pre-amp switches I heard an extra clicking sound. It was coming from (one or both) of the relays...these>>> DS2Y-S-DC12V Panasonic Electric Works | Mouser <<<

And a guy also recommended I change out the transistors as well. They are cheap, so what the heck.

Only problem is, I'm not 1000%sure what transistor to get. Both have 2N 3904 on them, but one has -741 and the other has -738. I can find 2N 3904 all day long, but the extra numbers have me sort of confused.

 

[AronDouglas]

Ok, I cant decipher these transistors. One of the 3 lines of numbers is to be ignored while the other two will tell me what part it is. I have 2 transistors and they read as follows,

2N
3904
-741


and the other reads,

2N
3904
-738

I've read some sites that say to read the first and second string of numbers, and other sites say use the first and third string of numbers on the transistor to find what part I need. So am I looking for a 2N3904 or a 2N741/2N738??? I can find all 3 transistors, but all three are different from each other.

2N3904 data sheet. 2N3904 Datasheet & Equivalent - 2N3904 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog

2N738 data sheet 2N738 Datasheet & Equivalent - 2N738 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog

2N741 data sheet 2N741 Datasheet & Equivalent - 2N741 Transistor DataSheet PDF. Parameters and Characteristics. Electronic Component Catalog

I've also read that in some cases you can have transistors that have an 'A or B, ect' after them. This just means that they are enhanced versions. Could I use those or should I stick with whats on the amp already?

This is out of my understanding of electronics guys, so you'll have to help me out. I can find and install the parts, I just need to know what to get.

Thanks for your help thus far.

 

[prcguy]

Why do you think the relays are bad? Did you test the coil and contacts? Why did someone recommend you change out the transistors? Did you test them?

The numbers after the P/N are usually the date code. 738 would be the 38th week of 2007, etc.
prcguy

Ok, so after more testing I have come to the conclusion to change out the 2 relays and the 2 transistors. While slowly flipping the amp and pre-amp switches I heard an extra clicking sound. It was coming from (one or both) of the relays...these>>> DS2Y-S-DC12V Panasonic Electric Works | Mouser <<<

And a guy also recommended I change out the transistors as well. They are cheap, so what the heck.

Only problem is, I'm not 1000%sure what transistor to get. Both have 2N 3904 on them, but one has -741 and the other has -738. I can find 2N 3904 all day long, but the extra numbers have me sort of confused.