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Amp blows resistor between output transformers.

M

mr_fx

Sr. Member
Oct 8, 2011
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Kansas City
I have been trying to bring an RM Italy KL-500 back to life. Originally it had 4 x MS1051 transistors. When they blew Audioshockwave offered me some MRF453 (large case MRF455 transistors) the original KL-500 came with 4 x MRF455, then 4 x SD1446, then MS1051, then back to SD1446.

So I figured people swap SD1446 in place of MRF455 all the time, and the MS1051 was being used in place of the SD1446, which was being used in place of the MRF455, so by that logic the MRF453 (or MRF455 for that matter) should drop right in.

And it is did, I hit it with around 15 PEP from a Roadpro Titan II 485, which has the single final, 2sc1969.

Talked good for a few minutes, got good reports, THEN it started putting out about 1/4 to 1/3 of the power it used to put out. I shut it down and disconnected everything

Upon inspection I found that resistor R28, was burned, and had a resistance of around 0.58 ohm Also it is a 1 watt part, the parts list from RM Italy calls for a 2 watt 100 ohm resistor...

This resistor sets between the output transformers. So it is for balancing I would guess. (I am still new to amps)

Part of the problem I see here is that RM Italy says to use a 2 watt part, but the part being used is clearly a 1 watt part... maybe this is the issue? But I would think it would have blown with the MS1051 transistors.

Oh well fun fun fun right.

http://rmitaly.com/download/manuals/KL500-manual_rel_500.pdf
 

That resistor goes from the base of the transistor to ground and burns when the transistor is over driven or in this case most likely a serious imbalance between the 2 stages due to mismatched transistors I suspect.

Look at the resistor R26 for signs over heating which is there as an indicator of an imbalance also at the combiner for that stage unless it's one of the smokeless resistors which is blue or gray in color and it may show signs or not due to it's high heat tolerances.

Bet is that transistor TR-9 is toast.
 
desolder and lift the base and collector tabs of that transistor to test, attach the negative lead to the collector tab and the positive lead to the base tab which should show continuity and not in the other direction and from the collector to the emitters should show continuity and across the emitters.
 
Yes, As I said in the PM, that resistor is on the output combiner were the 2, 2 transistor sections are combined into one for the output.
If it burns, it tells you that you have an imbalance across the combiner.
One or the other of the 2 sections is not putting out the same amount of power as the other.
The Transistors are all of the same date code, and all 4 are red dot
Motorola uses two methods to designate d.c. current gain of RF Power Transistors, color dot or letter code.
You can find the table here
All 4 should be very close in gain and output with each other.

Very carefully go over your work and make sure that everything was put back the same as it was before you swapped the transistors.
If anything was not correct, it could have caused the 2 stages to be un-balanced ( one side making more/less power than the other)
Also check the Bias Mods you were doing to make sure it is all correct.
Did you do any testing of the Bias voltage after doing the modification?
Also, ( as I had already said in PM`s) the 455/453 have higher gain than the 1446.
With the 1446`s RM says that on AM not to use the #6 setting with input of over 12 watts, as the 455/453 has higher gain, it will take even less drive .
I would also Float the Tabs on the transistors and check them again, to be sure they did not get damaged.
Um, did you get time to take a picture of the board in the Amp?

73
Jeff
 
mackmobile43 said:
That resistor goes from the base of the transistor to ground and burns when the transistor is over driven or in this case most likely a serious imbalance between the 2 stages due to mismatched transistors I suspect.

Look at the resistor R26 for signs over heating which is there as an indicator of an imbalance also at the combiner for that stage unless it's one of the smokeless resistors which is blue or gray in color and it may show signs or not due to it's high heat tolerances.

Bet is that transistor TR-9 is toast.
Click to expand...

They are all Red dot transistors

r28 should be a 68 ohm 2 watt but is a 100 ohm 2 watt(101 ohm), either way it is working, r25 , r27, r24, r31, r32 all check out in spec.

TR9 test out perfectly
 
Thought I should mention that this is the amp that the wife laid a coat on in the middle of the night, then I not knowing talked on it for 2 hours or so... it got the heatsink hot enough to burn my hand.
 
mr_fx said:
They are all Red dot transistors

r28 should be a 68 ohm 2 watt but is a 100 ohm 2 watt(101 ohm), either way it is working, r25 , r27, r24, r31, r32 all check out in spec.

TR9 test out perfectly
Click to expand...

How does it test after this issue happened? that is what I explained about.
 
If R-28 were rated at half the amps output this would not occur but can you imagine a 250 watt resistor, oh wait that's a small dummy load but it's not in paralleled with the output combiners so the smaller one acts as an "INDICATOR" of the imbalance only and that's why it goes nuclear in a nanosecond.

Problems also occur if the transistors are not seated properly to the heat sink such as being over tightened which causes the center portion of the transistor to actually lift from the heat sink surface and begin to heat rapidly and things change fast and imbalances occur at that time also.

Tell tale signs of over tightening of the transistor is marring in the screw holes and this can be checked by placing the transistors on a perfectly flat surface, there should be no clearance between the transistor surface.
 
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Something had to be a miss for it to take out 2 that fast, I am sure you know to make sure the Load had good SWR.
Is the heat sink distorted under the transistors...It would have to get damned hot to do that but....
What power setting were you running it at?
I know the transistors were good, all 4 were tested before shipping, and I am sure you tested them again Correct?


73
Jeff
 
mackmobile43 said:
Problems also occur if the transistors are not seated properly to the heat sink such as being over tightened which causes the center portion of the transistor to actually lift from the heat sink surface and begin to heat rapidly and things change fast and imbalances occur at that time also.

Tell tale signs of over tightening of the transistor is marring in the screw holes and this can be checked by placing the transistors on a perfectly flat surface, there should be no clearance between the transistor surface.
Click to expand...

they where seated correctly, I wish it was that simple
 
AudioShockwav said:
Something had to be a miss for it to take out 2 that fast, I am sure you know to make sure the Load had good SWR.
Is the heat sink distorted under the transistors...It would have to get damned hot to do that but....
What power setting were you running it at?
I know the transistors were good, all 4 were tested before shipping, and I am sure you tested them again Correct?


73
Jeff
Click to expand...

Oh yes I tested them before they went in, lol, I know you are not to blame here. I think something is up with the amp.

oh lord there is no way it got that hot

As I understand it running it at '6' is like the switch is not even there.

I have an old Palomar TX-150, 2 x MRF455, I run it on the same radio all day long, sounds great, no complaints.
So if that radio is not overdriving a 2 x MRF455, then there is not way it will overdrive a 4 x MRF453, we are only talking about 15 PEP on SSB, I was planning to remove that switch and make room for something else

unless tuning it up to '6' is somehow multiplying the power coming out of the radio before it hits the pills then I can't see how it would effect the pills, it's not like we are hitting it with 50 watts in
 
Poor Fella!.. Your combiner balancing should always use a 100 ohm/ 2 watt flameproof resistor... It "smoked" because the amp blew-up, again... Something may be wrong with the amp besides the outputs and now the resistor... I've seen shorted caps, open resistors, bad bias circuit components or high bias current... You gotta use your print and check everything!..The common misperception among amp users is that when the devices "pop", you can just replace 'em with similar devices that fit the hole... I rebuilt an old, ugly Varmint mobibile amp from the 70's, I believe it was the first one by four ever commercially made... The ancient low gain devices were no longer available, and I had a new quad of 1446's laying around, so I used them...Simple right ?.. Four Hours Later(sigh!) I discovered I had to retune the transformers, rebuild the biasing circuitry, and I had to strap across the driver, they were too drive sensitive!..There's a Huge Difference between MRF453's and the SD1446's...Even if you restore it with original parts, but can't determine the original problem, the results will be catastrophic failure, again... The best option you have is to find a tech that Knows His Amps to repair it, if you can't figure it out... I know you don't want to hear this, but I'd hate to see you blow up another Quad of Devices at $175 Bucks a Pop!.. These things aren't just light bulbs to change out... Sorry about that, brother... I see you've posted on the CBRT forum as well, I hope out of all these guys you get the help you ask for... Good Luck... CBSUPERTECH
 

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