in biasing a 250ml, I am confused by the service manual instructions.
It says" adjust r114 bias pot for 5 millivolts across r106 and r123.
is that across both of them, as in series? or 5 millivolts across r106
and 5 millivolts across r123? This should be done without a speaker load??
Sorry guys most of my repairs have been on vintage fender tube
amps. I would appreciate the help,
I have included a pdf of the service manual .
You uploaded the wrong one :o, that´s the user manual, so please upload the right one.
That said, I *suppose* R106 and R123 are the emitter/ballast resistors for the ouput power transistors and are in series.
And they should be low resistance wirewound power resistors, typically around 0.33 ohms 5 or 7W, although value might range from 0.22 ohms to 0.47 ohms.
IF so, 5mV across each one should be fine.
One doubt is that that amp is a *stereo* one, meaning you have two equal power amps, yet your instructions seem to refer to just one , unless R106 is on one of them and R123 is in the other.
Sorry for all the IFs and shoulds but I´m guessing 3/4 of what you are asking, without the proper schematic and full instructions ;)
Here's the service manual:
J.M. Fahey,
Thanks for your reply. I did post the correct pdf of the service manual.
Those emitter resistors are in series, one going to a tip33c and the other a 34c.
thanks again
Tom
Um... all I get from your link in post #1 is a user manual, the only diagram is a block diagram. I see no individual parts or schematic. g1 posted the full manual.
Yes, this is a stereo amp, so there are two amp channels to adjust.
What they are inferring by across R106 & R123 is exactly that.
Across the legs of two resistors that are attached to the emitters.
The middle point of the two resistors is the output.
They could have stated more clearly "Measure the voltage from Q13 Emitter to Q17 Emitter."
Thanks Jazz p,
I did set the 5 millivolts across the two but I am still blowing output transistors.
It only works for a short time. Diodes and other transistors check fine??
perplexed Tom
Are you using a lamp limiter or a Variac? if not you will struggle to fix the unit.
You need to establish the fault while the amp is running but current limited so you don;t keep blowing Op transistors.
Simple Current limiter here;
http://www.ssguitar.com/index.php?board=2.0
*first post*
Phil.
Please work with NO load on the output until we are sure. Is there any DC offset on the output?