SWR LA 15 Weak Output

[benfransiqo]

Hello, I am repairing a SWR LA 15 amplifier, originally the amp would make a wierd noise only after being on for a long time and after it got hot it was like a bleep/bloop. I went in to repair the power amp as i figured this is where the problem was coming from especially after reading other threads on this forum talking about the power amp section over heating. So I replaced a shorted output transistor as well as all other transistors in the power amp section. The amp powers up and is stable, however the output of the amplifier is quite weak, and is also very distorted. I have tried rebiasing with the variable resistor to no avail. I have connected a pre amp from another amplifier into the power amp and while it is lest distorted, the output is still quite weak, and have tried the amp going into another speaker and it is still distorted and weak. Is there any suggestions as to what may be wrong? Capacitor C8 was smoking earlier and I have replaced that as well.

Here is a link to another thread which had problems with the power amp section and in there are schematics if any one needs a look.

http://music-electronics-forum.com/t25074/

Thank you for your help it is much appreciated

[Enzo]

First check your power supplies. SHould be two "high voltage" supplies. I forget what they are, somethiing in the 25-35 volt range I'd guess. Are both present and at about the same voltage? (opposite polarity of course.) The the preamp runs on +/-15v, make sure both of those are present and up to voltage.

If you had to replace bad power transistors, check the resistors associated with them. It is common for failed transistors to burn out resistors as they die.

[benfransiqo]

I checked the two leads at J2 that were marked +32 and - 32 they came up 58 and -58 volts apiece and then Q3 blew again

[Enzo]

Main power is +/-59v on the schematic. Then resistors take that to the preamp wher ot os further regulated down to the +/-15v.

Of output Q3 keeps blowing, I'd have to suspect Q4, R4, R9, and check R1 as well.

[benfransiqo]

I checkd all resistors on the Power Amp board R1 checked out, R4 and 5 came in at .5 ohms (my multimeter seems to be adding .4 ohms even when leads are touching together) so taking in the multimeters ofset it would be .1 ohms. R9 came in at .7 ohms and should be 100 so that would be our culprit I assume? Q4 is blown as you suspected as well as Q1 and Q5

others that I have questions on

R6 came in at 30.9 and is the matching pair for R9 and leads to the Collector of Q5/base of Q1
R13 came in at 595 instead of 1.5k (would a solder joint or something that i may have touched somewhere along the line cause this to be running in parallel with something droping it down?)
R19 was 9K instead of 18k
R24 was 32K instead of 68K
R5 shows up as 0 ohms in circuit but when taken out of circuit shows the correct amount of ohms ( is this because it is in parallel with the coil L1 so its resistance is then void as electricity will run through the coil instead?)

D3 does not show infinite resistance one way and low the other it shows 663 one way and 808 the other i don't remember which is forward biased or not ( i believe when i took it out of circuit the diode gave the correct readings)

D1 shows 100 (In diode test mode) both ways is this just showing the reisistance as it is in parallel with a 100 ohm resisitor?

To the best of your knowledge is this just a case of the two resistors being bad? How does a resistor not work correctly i thought it either reisisted or was open? Or could it be some soldering that caussed an unwanted short? Thank You much for your time

[Enzo]

Those wirewound resistors usually are either open or OK. So if you get 0.5 ohm, that tells me the 0.1 ohm resistors are probably fine.

A 100 ohm resistor is extremely unlikely to be at 0.7 ohms. But look at the schematic, a shorted Q3 would put itself and its 0.1 ohm resistor in parallel with R9. I bet if you remove Q3, then R9 will measure a lot closer to its 100 ohm value.

When Q1, Q5 short, they put tremendous stress on the transistors on the other side - in this case Q3.

R19, 24, etc. ANy time you have a resistor reading low, and I mean really low, not a few percent low, then something is in parallel with it. Maybe normal, maybe something leaky... --OR-- ther is some residual voltage in the circuit, a little charge in a cap somewher. A trick to determine that is to measure the resistance of the part, then swap your meter leads. If the reading changes substantially depending on which lead is at which end, then there si voltage in the circuit. If in doubt about any part, unsolder one end and lift it off the board. Then you can measure the part while not connected.

Yes, R5 is parallel to a coil, which is a piece of wire having almost no resistance. So R5 cannot be measured while in circuit.

ANy diode with a low resistance thing in parallel won;t test right. If you had read zero ohms, that would mean the diode was shorted, and a 100 ohm in parallel won;t make it look unshorted.

D3? first, using resistance is mostly worthless, use the diode test. See Q9 right next to it? D3 is in parallel with the B-E junction of Q9. That B-E junction acts liike a diode, and that "diode" is facing the opposite direction from D3. SO checking D3 is likley to show a diode in both directions instead of just one.

Your R13 readings make me suspect Q2, and I would be surprised if R19, R24 have anything wrong with them.

[benfransiqo]

Sorry it took so long, i had trouble with a distributer getting the correct transistors. I changed Q2 and the amp powers up and nothing blows out the sound is quite fuzzy and distorted and has a weak output. Should I move to the Pre amp section? I'll give more readings of voltages and resistance soon.