I have very little experience with electronics repair, about a year now of getting a few vintage stereos back to working.   I have the basic tools of the trade (solder station, DMM, O-scope), nothing pro or too fancy.  Has been a LOT of fun making things work that were trash.   

I recently picked up a really cool B15T, which was "working" in that it makes sound.  But at full volume, doesnt make enough output to be uncomfortable, its more like watching TV.   The amp will also cut in and out with a slap.    I dont SEE any solder joints or anything obvious (to my novice eyes). Can anyone advise on a good place to start?  It's too cool of an amp to be broken

Ok easy stuff first,
I don't know the amp but from pictures the amp has 2 effects loops, one front the other on the back.
Just bridge them both with a couple of spare guitar cords.

If that fixes the issue then one of those FX loops has a failing contact switch.

FX loop failure is a VERY common issue as the sockets are often cheap crap.

If you want to save yourself a whole lot of work just get a couple of stumpy jumper leads and leave them plugged in.

If it's still not working then you look deeper.
HTH, Phil.

If you have a signal generator, use the CRO to check the signals to the power amp.
But first, check the speaker output for the DC volts with no input, if you have a DC voltage of even a few volts then do not leave the speaker connected.You cannot view this attachment.

An awesome start! Did not know about the effect loop in back I will start.

Forgot to mention the output is distorted and kind of farty.


Will report back with results.  I will post a pic of the scope.  It input a sine wave to input but the output was far from clean, but never worked on a guitar amp so didn't know if that was normal

I input a nice clean 1k sine wave, and here is the result from both the preamp out and from the power amp out... both appear to be wonky. 

I guess this tells me at least that the distortion is starting in the preamp section at least?  But it still seems that the power amp should be louder.

I guess I also need to understand the function of Gain and Master...Gain is there for crunch, but doesnt control volume?

First time using this scope, it is really cool to use such a powerful tool for diagnosis, cant wait to use to greater potential and understanding.

Quote from: GeezerB on August 23, 2023, 10:28:32 PMI input a nice clean 1k sine wave, and here is the result from both the preamp out and from the power amp out... both appear to be wonky. 

I guess this tells me at least that the distortion is starting in the preamp section at least?  But it still seems that the power amp should be louder.

I guess I also need to understand the function of Gain and Master...Gain is there for crunch, but doesnt control volume?

First time using this scope, it is really cool to use such a powerful tool for diagnosis, cant wait to use to greater potential and understanding.

Gain is really preamp volume.  The higher you turn up the gain, the more it will saturate into distortion (depending on the amp, this could be setting the gain on 3 or 4, or maybe 5 or 6 before serious distortion kicks in).  The master is the volume control between the preamp and power amp.  This controls the overall volume at the speaker level.

To be clear, turning down the gain will decrease volume as well but it does this in the preamp section and as it is turned down it will decrease volume as well as distortion.

You gotta love good advice! 

So, my troubleshooting job just got 50% easier thanks to the advice on this forum. 

1K sine wave direct into POWER amp plug, results in a perfect waveform on my scope... perfect. 

So now I know it is the preamp.   Im new to reading schematics, any good general advice on finding the likely problem on this preamp board?  Can it be traced in such a way that I work through the signal path to find where it gets ugly?

Yes, tracing through the preamp with scope should enable you to locate the fault.
You need to highlight the signal path on the schematic and check points along the way with the scope.  But you also need to have an idea of what to expect, as the signal level may normally go up and down along the way.

awesome

im looking forward to learning how to do this

I would inject a signal and then test all the testpoints with the scope, if you leave one channel on the input signal and then probe around with the scope using the second channel you should soon see where the signal distorts.

I would start with the limiter off and inject 0.22 volts on the input like the chart says to do.
Please note that there is an AC signal shown and a DC voltage as well, when it shows a DC voltage it means the AC signal is centered at that voltage so do not earth the probe you are using to test the checkpoints.

Earth the probe on the input signal to the chassis or the ground tab of the input jack, but again, DO NOT EARTH THE SECOND PROBE.

There should be a way to bring the trace from probe 2 up to the center of the screen even when the center of the vawe-form is at -16v or -11v, otherwise it will be at the bottom of the screen.

Good luck and keep us updated, we love to hear of a win.
(Have you tried to get a wave-form from your tongue yet?)

I'm about 99% sure that I am there... I found that I'm supposed to have +45 and -45 going into preamp... then 16V at the test points.. but I was not getting -16 after the zener diode, pulled diode and it tested as a short on the tester... new Zener diode on order and we'll see.  Exciting. 

TassieViking, thank you for the that advice I will try that.   I saw that on the SM, but I could not figure out how to do that, it is beyond my understanding.   

I do have a scope.  How would I input .22 volts?  With a bench type power supply and then read it with the scope?  I am trying hard to learn here but have a LONG way to go.

The 0.22v is the 1kHz test signal you are putting in, if you have a signal generator then you should be able to turn the voltage (volume) up and down on the signal.
 
In the instructions it says to put the tone pots on 5, volume and gain pots on 10, balanced line out pot on 10, and then I can't make out the test signal clearly.(with 600ohm load. 1kHz input?)
On the chart RHS it says to leave the limiter off and inject a ac signal at 1kHz with a voltage of 0.22v peak to peak.

Peak to peak (vp-p)means that the signal should be that voltage from the top of the sine wave to the bottom of the sine wave. That is best seen on the scope as the multimeter will be measuring volts rms which is a lot lower then p-p.
When you see peak to peak (p-p) it will always be an AC voltage that is being described, all audio signals are some form of AC but the wave shape can differ.

Good luck

All of that was in the same SM we were both reading and had no idea what it meant.   Leveled up today! Thanks for the help. Will report back on if the diode does the trick

There are lots of good lessons on the internet that explains all about it.
It can be vary daunting to try to learn when you are new to it but it gets easier the more you do.
If you have stompboxes etc you could inject a small signal and measure the output with the scope so you can see what they do, it can give you a good insight into how they work.(and it can be a good fun way to get used to the gear)

https://www.learningaboutelectronics.com/Articles/What-is-peak-to-peak-voltage.php
https://www.allaboutcircuits.com/tools/peak-to-peak-voltage-calculator/