Hi
This amp had its power output module blown. I have a LH1000 amp which is working OK. I realised from a number of websites that the LH500 and LH1000 power module is identical. So I connected this LH1000 module on to the LH500 amp. I realise that the relay clicks ON but there is very faint sound from the amp. I took the signal output from the preamp and the signal is fine. Looking at the schematic , I realised that the point marked J1 between R30 and R323 and the point marked J2 between R39 and R332 should be open for LH1000. But this module I took from the LH1000 has this points soldered together and the LH500 has this points open. (which is the opposite as shown on the schematic). Can anyone please explain what is the function of these two junctions J1 and J2 and can it be because of this junction connections that I am getting only a faint sound from the amp. Please assist
Thanks
Carlos
This amp had its power output module blown. I have a LH1000 amp which is working OK. I realised from a number of websites that the LH500 and LH1000 power module is identical. So I connected this LH1000 module on to the LH500 amp. I realise that the relay clicks ON but there is very faint sound from the amp. I took the signal output from the preamp and the signal is fine. Looking at the schematic , I realised that the point marked J1 between R30 and R323 and the point marked J2 between R39 and R332 should be open for LH1000. But this module I took from the LH1000 has this points soldered together and the LH500 has this points open. (which is the opposite as shown on the schematic). Can anyone please explain what is the function of these two junctions J1 and J2 and can it be because of this junction connections that I am getting only a faint sound from the amp. Please assist
Thanks
Carlos
Q311 and Q312 are part of the short-circuit protection circuit for the amp.
The output current of the Q313 develops a voltage drop across R324. R322 and R323 feed this voltage to the base of Q311, and if the voltage is sufficient, Q311 turns on, and robs base current from Q313; this has the effect of not allowing the current through Q313 to increase any further, so it provides a current-limiting action to save Q313 in case of excess output current (due to an output short circuit, for example).
If the "X" is not open, then R30 is in parallel with R323, and slightly lowers its value. This will have the effect of lowering the current-limit threshold; it will protect the amp at a lower output current. Presumably this is for the smaller, lower-power version of the power amp, which cannot safely cope with as much current as its big brother.
The circuitry around the other "X" works in exactly the same way, for the PNP output transistor Q314.
No, I don't think so. This area only has any effect if the amp is already playing very loud (too loud) or the output is short-circuited.
You found out that there is a signal at the preamp output - now, is there a signal at the power amp input (check both ends of R350)? Or did the signal disappear between preamp output and power amp input? (Maybe a bad connector, loose contact, etc.)
Beware of testing with the loudspeaker connected - when you solve the problem, things may suddenly get very, very loud, and could damage your hearing or cause ancillary damage if you reflexively jerk away in response.
-Gnobuddy
Well answered Gnobuddy They adjust how much current the one power transistor produces before the anchors are put on the current drive. Pity they only sample one of the set each side.
The favourite issue with these amplifiers is the Effects in/out or pre out/power in sockets.
They are rarely used and get tarnishes intermittent contacts causing sudden or gradual loss of signal to the power amp.
The favourite issue with these amplifiers is the Effects in/out or pre out/power in sockets.
They are rarely used and get tarnishes intermittent contacts causing sudden or gradual loss of signal to the power amp.
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