Hi, I'm a proud owner of a Randall collection. My Century 200 II seems to have some problems. There's some missing component in the PCB. The amp lacks power and two of the four power transistors are cold when working (they are NOT working).
Her's a pic of the whole circuit, and a pic of the missing (seem to be cut) component. My electronic repairman says is should be some kind of resistance. Some help? I need to fix this a.s.a.p... :(
It would be of great help the schematics of the amp: Randall Century 200 II. Impossible to find, even at Randall :(
It is/was a resistor.
It looks like it relates to the output stage bias setting.
It could be a range select that is a "clip during manufacture (if required)".
It's impossible to read the value properly in the pic, but common sense and intuition suggests it will be a low value, perhaps 15-47 ohms.
It's going to require some circuit tracing by someone. A good close up on the copper side could help.
Type numbers of output devices?
Have they been replaced since the amp was new?
If this amp used to work, then that resistor is not the problem. As Roly said, it was probably a clip-out resistor to set the range of the trim pot. It doesn;t seem so likely that someone took the amp apart, clipped out the resistor and put it back together.
Quote from: Roly on October 15, 2013, 08:07:41 AM
It is/was a resistor.
It looks like it relates to the output stage bias setting.
It could be a range select that is a "clip during manufacture (if required)".
It's impossible to read the value properly in the pic, but common sense and intuition suggests it will be a low value, perhaps 15-47 ohms.
Is this common? I've not heard of it, but am still just a hobbyist in this field really.
Yes it is.
Normal would be to use a bias trimmer and adjust it, but it's "expensive" :loco (that's the, literally, "cheap" thinking) plus the real dangers of the contact getting dirty (it's a pot after all, and usually open frame) or the owner messing with it :trouble , so common practice is to use fixed resistors which are *almost* right, and add one or 2 in parallel, much higher value, which can be clipped out of the circuit.
Say, if the "adjustable resistor" is 4K7, it usually has 47K and 150K in parallel, which allow for 10% and 3% adjustment.
Quote from: joecool85Is this common?
"Common" depends on what you are used to staggering in through the workshop door. ;)
Moreso with smaller scale production gear, and you find similar in industrial stuff, like a resistor marked "(15k) S.O.T", nominally 15k but
Select On Test.
Quote from: Roly on October 19, 2013, 09:17:40 AM
Quote from: joecool85Is this common?
"Common" depends on what you are used to staggering in through the workshop door. ;)
Moreso with smaller scale production gear, and you find similar in industrial stuff, like a resistor marked "(15k) S.O.T", nominally 15k but Select On Test.
Learn something new every day. Thanks for the explanation Juan and Roly.
This pic shows the transistors. Two of them work, I mean they are hot when working, the other two are inactive and totally cold.
The resistor is missing.
It seems the amp lacks power, it sounds ok but seems to have a low output.
Help?
You are showing only 2 transistors but mention 4.
Also, cold *when*? , at idle or after 1 hour loud playing?
Always. 2 transistor get hot, 2 remain cold all the time.
Still see only 2 .
Wellll... still no circuit, but I've been having a close scrute at the internal pix and I may have an answer as to why two power transistors are warm and two are not.
It appears that two of the four TO-3 packages (all MJ15003's) are actually power drivers, their Emitters going to the respective Bases on the only two that have local Emitter power resistors (the others, the power driver Emitter resistors are apparently back on the PCB). From the power supply end we appear to have <booster - driver - booster - driver>; they aren't parallel pairs as I was expecting.
Looking at the circuit for the Randall RG100 they seem to have a kind of cascade output stage where the drivers deliver some of the power output, and have secondary transistors as "boosters" for higher power delivery, and it would be these boosters that are cold when the amp is undriven, no matter how long it's switched on for, since they are biased off at idle.
Quote from: soundroiIt seems the amp lacks power, it sounds ok but seems to have a low output.
This could be consistent with the power drivers working but one or both of the "boosters" not.
I'd carefully mark all four so you know which came from which position, and because they are socketed remove them and test out of circuit. Since silicon transistors normally fail short circuit and the the amp is still working my guess is that they
may all turn out to be okay, so while they are still out check the emitter resistors, in particular the two right near the transistor sockets for the boosters, and I'll take a punt that one or both are open circuit.
Since we don't have the actual circuit I'm attaching the RG100 circuit which I suspect will be very similar.
Also checking out this site and messaging this poster might be a way to get the right schematic. Maybe I will try just as I am curious. It is an old thread so who knows we might have missed the train... :afro:
http://www.mtsforum.grailtone.com/viewtopic.php?t=1629