Hello Everyone, Newbie here ! Need help repairing a amplifier.

Hi Larry,
These appear to have a second name....Flame 400's!!
Have you checked for burnt resistors? That certainly seems to be a known fault with these amps.
Where are you measuring 56VDC from the right channel? These amps also have no output protection either.
Perhaps some nice clear close photos of the board would help.
Also what board is it? pl?

Martin

 

Usually the first thing to fail on these amps are the big electrolytic capacitors. When they leak (electronically), circuits start failing.
My personal experience with single channel failures are the selector switches themselves. They get dirty or just plain wear out.
First thing I check is to make sure the speaker wires are tight and not crossed (shorting each other out), speaker fuses if so equipped, then I check the selection switches, then I open it up and look around.
Good luck.

 

You must be more specific. Where is the 56 volts coming from? "Was" or "is" coming from? Give us a link to the schematic and point out where the 56 is coming from. Is the negative 56 vdc good? Is this a stereo amp? Is it bridgeable? Ed.

 

Since you are referring to this problem as a "right" channel problem, you must have 2 pcb 14A boards. Am I right? If not. then you must be referring to one half of a single channel amp; one half being the positive fed side and the other being the negative fed side.
Make sure you measure the dc volts with negative lead on the negative side of electrolytic C20. Not sure if the common point shown is chassis ground or dc common (neg side of C20 and pos side of C21). With no speaker load, the supply dc should be a + and - 80 volts at the 5 amp fuses. With no power, Q13 thru 18 should read .5 ohm from emitter to base when neg lead of meter is connected to emitter with transistors removed. You should also get .5 ohm between C and B with neg lead on E, pos lead on B. All other readings should be high. The emitter resistors should all be .22 with one lead disconnected and transistors removed. Q11 and 12 emitter resistors should be 10 ohms.
Q11 and 12 are drivers and Q13, 15 and 17 are paralleled and the positive half of the output with Q14, 16 and 18 the paralleled negative half of the output. Check all electrolytic caps and diodes. Before powering up, replace each fuse with a 115 v/100 w lamp and watch lamps closely during turn-on. If a lamp turns on bright and stays bright, you have a short in that circuit. If not, check voltages at collector of Q8 and Q9 (- and + 1.4) and get back. Ed.

 

Some more questions! I have the PCB 14A board schematic and layout diagrams. It shows only one channel so I am assuming both channels are identical. This is good because you can 'bump' the readings of the good channel against the readings of the bad channel. All output transistors are NPN (Q13 thru Q18) and so are Q11 and Q12. These are all heat sinked I believe. Using a diode checker and since they are NPN, they must be checked with the neg lead on emitter with pos lead on base and then neg lead on collector and pos lead on base. Only these 2 positions should show approx. .5 ohms if transistor junction is good. The .5 refers to the forward voltage drop (E to B), not ohms, but the ohms scale is being used. The .5 means the junction is silicon. A germanium would read .3. Don't see too many germanium types these days.
Your numbering system above doesn't mean much to me so you may be able to convert these to "Q" numbers such as Q13 etc. This way I will know exactly which transistor you are referring to since I have the schematic. Of course, all transistors must be disconnected (removed) so they don't interact which one another during checking.
The number 6 and number 7 you call out as reading approx. 5.5 and 5.1. This is not right! On NPN transistors, you should get a .5 reading when you are connecting neg lead to emitter/pos lead to base or when you are connecting pos lead to base/neg lead to collector. All other lead connections when checking will give a reading of open or inf. I am also assuming that you are using an ohm meter with a diode check function. Here is what you should read on each disconnected or removed NPN transistor.
Black on emitter/red on base= .5
Black on collector/red on base= .5
Black on base/red on emitter= open
Red on emitter/black on collector= open
Red on collector/black on base= open
Red on collector/black on emitter= open

Make sure that mounting transistors to heat sink doesn't short out any of the transistors in any way. Ed.

 

Larry...... Did you check R20? It's a 5K bias pot. It could be open, which will likely cause your problems. Compare the readings with the one from the other (good) channel.
check Q6 too, which seems to be mounted on the chassis, somewhere.
Scary amp..... brrrr.

Oh, don't discharge the power supply caps with a screwdriver. It's + and - 80V. That'll make some sparks and a bang.
Use a 5 or 10 watt 100 ohms or 1K resistor and hold it with insulated pliers.

 

Q6 is heat sinked but not necessarily on the chassis. R20 will probably be good unless Q6 has a collector to base short. D14 (1n4004) is suspect since you have dc at the speaker output, assuming the output transistors are good. Check all diodes in fact. Another good idea is to finger test all semiconductors for excessive heat with power on. Be careful with this.