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March 18, 2024, 11:48:08 PM

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#1
Remeasured T4 and voltages listed are correct. Originally had a TIP29CH replaced with TIP29C.
#2
T12 E 30.8
    B 33.8
    C -38.3

So I did have the collector wrong of T12.
#3
Amplifier Discussion / Re: Fender Stage Lead II 2-12 ...
Last post by remaker - Today at 09:02:33 PM
Thank you, Tassieviking, for the reply!

I have the head removed and on my bench. Speakers and reverb disconnected.

C->E diode tests in situ:
(Q111 and Q113 and 0.590V; Q110 and Q112 are 1.2V and climbing, never reaching a stable place.)

All of the 0.5 ohm resistors check out but the 0.1 ohm resistors as 0.0, though that could just be my cheap meter.

I pulled out the DBA100C rectifier to test. top AC to plus is 0.5, and its shows Open Circuit between one of the AC and minus. Plus to minus is open, minus to plus is 0.9. So it looks like the rectifier has failed, but I'm not sure if that will have killed more downstream components.
 
I suppose I should start with a new rectifier!
#4
Tubes and Hybrids / Re: Tube Amp Trouble
Last post by g1 - Today at 08:14:04 PM
Sometimes certain ones don't light up as much as others.  But that one picture in the dark, it doesn't look like they are lit at all.  You could feel them to confirm. (just the preamp tubes, power tubes or rectifier tube will burn you)

The red and purple wires at the sockets are the heater wires.  Looks like they are all connected.  You can swap some preamp tubes around to find out if maybe the tubes themselves are bad.  Make note of where the 12AT7 belongs, but it can be swapped around for test purposes.

Once you have read up on safety for working in tube amps, you can measure those heater voltages at the tube sockets.  Should be about 6VAC with one probe to red and other probe to purple.
#5
Tubes and Hybrids / Re: Tube Amp Trouble
Last post by saturated - Today at 07:02:40 PM
Maybe that tube being not lit up doesn't mean anything idk

But forgot to add I did have the reverb out of it idk if that matters

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 :P
#6
Tubes and Hybrids / Tube Amp Trouble
Last post by saturated - Today at 06:20:21 PM
Newly acquired Savage Rohr 15

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When I got it it seemed to work ok both channels four inputs...

I thought it didn't seem to have much gain but figured it was what it was.

I guess it had/has a problem because later it seemed to get quieter and quieter and finally one channel appearing inop and the other both volume knobs can be max and strumming a guitar 🎸 can be heard at low volume.

I worked up enough courage to take a look and perhaps see something.

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I looked at the chassis but did not touch or 🥢 chopstick anything.

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Also I reconnected the speaker before powering on.

Btw here is the tubes

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When I powered it on I noticed one 12ax7 was not glowing as well as the uncovered tube on the end which I presume to be a 12at7 (?)

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I guess I made a big mistake by not swapping a preamp tube to see if it was the tube or the chassis.

I carefully put everything back together and not too eager to further investigate until I can watch hours of reruns on YouTube about at least some basics of working on and around these units.

I also have read a bunch of the threads in this section so I'm expecting to be told check voltage ⚡ on pins xx on vx.

 :P

But alas I think this is gonna be on the back burner for a while.  I just don't yet have the stomach for poking around this amp.

Thanks

 :-X

#7
Amplifier Discussion / Re: Marshall VS100R transistor...
Last post by g1 - Today at 04:32:26 PM
Assuming that the T12 collector voltage TV mentioned is just a typo, T4 voltages do not seem right.  E-B voltage indicates it should be turned on, but collector voltage is not pulling down.
#8
I don't think my copy of the schematic is any better on the power amp stage.
Can you take a picture of the PCB so we can see the board and components ?

Disconnect speakers before any testing
The first thing to check with power off is the bridge rectifier with a multimeter set to diode checking.
If the bridge seems ok then check the power transistors for shorts from collector to emitters. (Q110-Q113)
Check the power resistors as well (0R5 5watt).(R123 - R126)
Power on and check if there is any voltage on the + or - 63v rails to ground (0V).
Check for DC voltage on the speaker output
Do this quickly so you don't have the power on too long,turn power off and check if any of the power transistors or power resistors got warm or hot.

Hopefully one of the more experienced guys will give you more/better advice.
Cheers
Mick

#9
Amplifier Discussion / Fender Stage Lead II 2-12 blow...
Last post by remaker - Today at 02:02:18 AM
I rescued a Fender Stage Lead II 2-12 blowing the main fuse. Trying to diagnose.

I have a grainy schematic.

Stage Lead II Schematics

I started with a dim bulb tester.

I have disconnected the preamp and controls, leaving only the power amp connected. Still a bright light (fuse blower). The power amp section has a fused lower-voltage section and an unfused higher voltage section (~63V, +/- ~34V). When the 63 V section is not getting power (wires pulled from molex connector), the fault goes away. When the fuses for the low voltage section are fulled, the fault persists. Voltage measures correctly on transformer leads (63VAC) when disconnected so the transformer looks OK

When in fault mode with the dim bulb tester in place, the AC voltage across the big rectifier is 2VAC if thatr is any clue.
 
Any suggestions for hunting the faulty parts? All of the electrolytics show well in range on my ESR meter, no shorts or open. Nothing obviously burnt. Not a lot of parts there!

I'm focused on the schematic on the bottom half of the second page on the linked images.


#10
The collector on T12 should not be +30.8v, it is connected to the negative rail.
Are you keeping the common (Black) lead of your meter on 0V (ground) all the time ?
It is very important to clearly indicate the positive and negative readings so the experts (not me) can work out what is wrong.
Maybe worth re-measuring the voltages ?