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Messages - g1

Can you verifiy that R97 is still 100 ohms?
Also measure voltage across R98 when you have the 5V across R97.
And also voltage across R41.
Quote from: saturated on May 04, 2024, 03:18:47 PMPretty funny the book noted "although Vo can be directly measured across A and B it is good practice to make all measurements with respect to ground"
Does Vo mean V offset?  Where measuring direct from A to B should give zero when they are balanced?
Quote from: markorock37 on May 02, 2024, 07:49:32 PMT3 pins 1,2,3=C,B,E
T5 pins 1,2,3=E,B,C
As far as I know, there is no such thing as CBE pinout for these.  But, apparently there are 2 versions of each, and could be either EBC or ECB.
Can you post a link to the exact ones that you bought?
The other thread ended with Q8 catching on fire and no resolution.  It's better if you just tell us what is going on with your amp.

You can monitor the voltage across R97 (one probe at each end).  If you see more than 5VDC, stop.  As you move the variac voltages may jump around so give it a couple seconds to stabilize.
Also keep checking for DC at amp output.
If you are seeing low current on the variac meter, you should be ok to raise the voltage.
see attached
Again, voltages that are impossible for e,b,c locations.
Is it another mistake, or is it possible you have some transistors in backwards?
If you do not have a load connected, continue increasing voltage from variac and monitoring DC at output.
It would help if you could monitor AC current from the variac as well, does the variac have a current meter?  Do you have a dim bulb tester?
Yes, pictures should help.  Especially of any trace repairs that have been done.
Also, e,b,c voltages for T3 and T5.
I think you have your AC source there hooked up to the output pins.
The error when installed backwards would have reversed the + and - output pins.  The AC would have still been correct as it is non-polar.
Amplifier Discussion / Re: Jazz Chorus 60 repair
April 14, 2024, 02:44:32 PM
The service manuals usually go by serial number, what is it?
Follow the trace from the bridge + lead to filter cap.  Is it going to + or - end of cap?
Quote from: markorock37 on April 13, 2024, 02:57:58 PMI don't see 2SA872 on Mouser, is KSA992 a good sub?
Yes, those should work.
Quote from: saturated on April 12, 2024, 07:53:18 PMSo I'm going out on a limb here and proposing that the 8k ohm 10w resistor is shorted to ground.  It measures 170v on one end and zero on the other.
A quick look at the schematic (a forensically enhanced version kindly provided by the Mighty Mick) to me places cross hairs directly on the zener diode.

You cannot view this attachment.

So I'm guessing that the zener diode is shorted...idk.
That's not going out on a limb, that is good solid logic. (assuming you verified the resistor is ok)  :)
Amplifier Discussion / Re: Switching Jacks Question
April 11, 2024, 12:24:42 PM
Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 
It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.
Yes it would seem T1 is bad as you verified R5 and D1.  But you also need to check their solder connections.
As T1 and T2 form a differential amplifier, it is best to replace them as a pair.