total noob+dying amp questions

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Hello, I have an old Yamaha B100 II bass guitar amplifier.

So, the symptoms are:
First came the hard crackle and pop upon switching on some time ago.. then came slight volume fluctuations while in use.. now the volume fluctuations have rendered the amp useless.
I am good with solder (built the Paia tube preamp with success, rewired guitars, microphones etc) and so removed the chassis from the case. It looks pretty simple.. nothing LOOKS wrong ie leaking, bulging, obviously disconnected.

Based on the symptoms, are there "most likely culprits" with which I could start.
I have other amps but this is a nice size for quick practices and is not valuable so I could leave it at friends' houses without worry. So I don't mind learning a bit on this one too. Any ideas?
 
Do you have a schematic for it?

Tubes?

Do they all light up?

Supply voltages?...observe safe measuring techniques.

OR when powered down for an hour or so (and unplugged from power) measure winding resistances.

Resistors all measure to marked values?

Speaker work with a different amp?
 
Schematic

I'd start with the usual ailments of old gear - check switches in signal path (incl. those in jacks), check power switch (burnt contacts do happen), clean fuseholders, turn tone and volume pots to find intermittency there. Finally, check the +30V supply, maybe the regulator transistor is low on beta (that happens after a few decades if they run hot). The odd electrolytic capacitor is also likely to be begging for replacement, explaining the turn-on noise (if it's not a flaky power switch).

Power amp supply should be +120-ish V or so (it's a single-rail design, so there's just one). The amp itself looks like a pretty standard PA circuit of the day, with a fairly rugged quasicomp output stage. Not sure about the biasing arrangement though... I guess it holds up if the pot doesn't decide to crumble away.
 
I thought I would continue to post my progress/issues with this amp.

So last night I sprayed the pots with contact cleaner, twisted the knobs as I sprayed.. and let it "dry" overnight.
I plugged it in this A.M., and the treble and bass knobs seem to act as volume knobs now.. ie. if treble is turned all the down, there is NO sound.. bass knob is similar but with less impact. hmmm.. tapping the knobs has no effect.
The pop and crackle at switch on did nor happen with so much force HOWEVER the volume was affected by the tone knobs.

Next I need to learn how to discharge the caps.. I probably should have done that already.

Cheers!
 
Do you have a voltmeter? You can't measure the caps as "safe" without one. The caps may be discharged after turnoff already by design, but you don't know. Neither can you measure the supply voltages without one. I use the Sears 82140 for everything but AC music measurements. I find autoranging meters annoy me by always switching scales up from the .01 V one instead of showing me a number I can use. Everytime your probe slips an autoranging meter starts the whole process again.
On tube sections, usually cold tubes will allow you to take valid readings on resistors, except on the power supply cap. On transistor sections usually you have to lift one end of a resistor to get a valid reading.
I measure electrolytic capacitors with a calender. If the amp has problems, and they are over 20 years old, they all go. Potentiometers I measure roughly by number of times adjusted. Volume pots I replace pretty freely after 20 years. Trimpots are often cheap and wiggling or spraying off can bring them back. Transistors usually work or they don't, but if you end up with low volume you can check with AC voltage in versus voltage out. Not on a DVM, most won't measure music accurately. For that I use a VTVM, 200 kohm/volt VOM with a 2 VAC scale, or if I were rich, a scope. (I have 3 broken scopes now, the latest lasted a month before e-caps took out the sweep).
 
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