Copper Talk » Modifications » Amplifiers » M-800 « Previous Next »

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Saturday, January 25, 2003 - 1:32 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

I have a messenger m-800, and i had some problems with it. There are a couple of resistors that are cooked, but i'm not sure if the pills are bad. It still does about 300-400 watts, and has a bad swr on the input side. On med and high, it hangs up on transmit really badly.

Do you think that replacing the bad resistors could solve the problem?
Top of pagePrevious messageNext messageBottom of page Link to this message

Tech671
Posted on Saturday, January 25, 2003 - 4:28 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Kind of doubt it :(
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Saturday, January 25, 2003 - 6:34 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Do you know anyone that could fix it? I would love to get this unit going!
Top of pagePrevious messageNext messageBottom of page Link to this message

Fewb
Posted on Saturday, January 25, 2003 - 9:01 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

yes it will.
Top of pagePrevious messageNext messageBottom of page Link to this message

2600
Posted on Sunday, January 26, 2003 - 12:20 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

The resistors aren't the problem, they are a SYMPTOM of the real problem, INSIDE the RF power transistors (pills). The problem with those pesky pills, they never LOOK bad once they suffer damage. The damage is all inside that tough, white ceramic, where you can't see it. The internal short-circuit that takes place inside the pill is what smokes the resistors you describe. You will find them in two generic places.

1)The small, half-watt part that goes from the base (input) leg to ground is usually ten or fifteen ohms resistance. When the pill is good, this part doesn't get much power thrown at it. Remember, it's on the INPUT side. Some folks use quarter-watt instead of half-watt resistors there.
When the pill breaks down inside, the 13.8 Volt (or whatever) power supply now feeds directly into that little resistor. This now causes 15 or 20 Watts of power to be thrown to this little part. Doesn't take it for long. "Snap" and smoke.

2)The other resistors you will see go down in a box with 4 or more pills are stretched across the toroid "doughnut" combiner coils. These are the larger ones that join pairs of pills leading to the output side. When all the pills are running, delivering equal power those resistors will throw a half-watt or watt of power each. They routinely use a two or three watt part there. When one or more pills stops delivering power, that resistor now gets slapped with about half the power delivered by the remaining pills that still work. Not for long, though.

Shorted (or open) defective pills cause one other nasty habit. They create unwanted POSITIVE feedback, from the output side back to the input circuit inside the linear. Kinda like holding a hand mike flat against a talkback speaker. That trick will get you an audio "howl" feedback. The internal feedback caused by one or more blown pills causes the RF equivalent. The RF "howl" inside the amplifier fools the keying circuit. It can't tell that you unkeyed the radio, if the amplifier is creating its own carrier, causing the relay to stay pulled in.

There really is no cheap, easy fix for this breakdown, unless you get your RF transistors for free.

73
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Sunday, January 26, 2003 - 4:56 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

There are two resistors that have heated up, and i can't tell their value. All of the other resistors like it are 100 ohm, so i'm guessing that's it. They are the resistors under 2 of the combiner coils. Those are the only 2 things that have visual damage.
I'm not exactly sure how to test the RF transistors, especially since they are in-circuit. Since there are 8 transistors and a driver transistor, would i have to buy a matched set of 8 to repair it?
Top of pagePrevious messageNext messageBottom of page Link to this message

Crafter
Posted on Monday, February 10, 2003 - 4:00 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Pinger I sent you some e-mails hope it helps. I got a m4000, wonder if their close to the same. let me know if it works out for ya.
John
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Sunday, February 16, 2003 - 3:31 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Crafter, i haven't gotten anything from you yet. Drop me an email.
Top of pagePrevious messageNext messageBottom of page Link to this message

Crafter
Posted on Sunday, February 16, 2003 - 8:55 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

Hmm..I'll re-send them again. i wonder were it went I didnt get it back.
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Saturday, February 22, 2003 - 10:09 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

How many watts should i be getting out of this amp? I'm only getting about 300 watts :(
Top of pagePrevious messageNext messageBottom of page Link to this message

Tech671
Posted on Saturday, February 22, 2003 - 11:35 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

What are the transistors and quantity thereof, what wire are you feeding it with hot and ground, what drive is it getting, what alternator do you have, what is the voltage at the amp under load, what antenna, what swr, what meter.
Top of pagePrevious messageNext messageBottom of page Link to this message

Pinger
Posted on Sunday, February 23, 2003 - 9:00 pm:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

SWR is 1.1-1.2. It has Eight MRF454s, 4ga wire (about 5 feet) to the battery (+ and -). I haven't tested the voltage at the amp under load, but i only have a stock toyota alternator. I'm using a Diamond SX-100 meter for output, but for setting the swr i used an MFJ swr analyzer. I'm driving it with a 2.5 watt deadkey and 12-15watt swing(AVG).
Top of pagePrevious messageNext messageBottom of page Link to this message

Jpryorx2
Posted on Monday, October 27, 2003 - 12:53 am:   Edit Post Delete Post    Move Post (Moderator/Admin Only)

if you can send me a pic of where the burned ones are i'll let you know what they are i'm running a m800 that the driver has been changed to a 2290 and with 1.5 watts drive i get around 600-650 watts out of it

jason
jpryorx2@yahoo.com

Add Your Message Here
Post:
Username: Posting Information:
This is a private posting area. Only registered users and moderators may post messages here.
Password:
Options: Enable HTML code in message
Automatically activate URLs in message
Action: