Antique "Acoustic" amp needs attention ASAP

[Faradave]

Ok, this is a long one. Antiques are my side hobby. A buddy recently gave me this amp that causes my hobby ven diagram to have overlapping areas.

It is clearly old. It had obviously spent some time outside. It had leaves and sand in it but still pumps alot of noise. I need help figuring out its proper usage and fixing the issues, if yall wouldn't mind. I know I should hit up a musical forum for this, but I trust yall more and I wanna run it by you 1st.

AudioGuru, I know you got this.
1st, is this a regular amp? Or is this just an equalizer and I need to plug an amp into it? Notice it has regular and -10db inputs on the front, "power amp input" and "pre-amp output" on the back. I can plug my phone or PC sound output into either of those back ports and I get great sound, just the volume knobs and equalizer on the front does not affect it.

When I plug into the front, I have volume and equilizer control. That "BRT" button on the front doesn't seem to do anything. Plugging straight into the front without power amp yields low quality sound and low volume. Plugging into the back via "pre amp output" or "power amp input" yields great sound and volume. It seems odd to me that pre amp output works as an input...

Anyways here's the issue the amp has: After about 15-30 minutes of music (using front or back as input), the sound slowly turns into static until all you hear is static. I feel like that huge monster cap (7800 uF, 100VDC) is bad since it slowly dies out. Then when I can no longer hear music, I turn off the amp and for a few seconds until power runs out, the sound quality is loud and good again, then the amp powers off.

So how can I fix this badboy? Not sure I have that same monster cap, but I'll buy one if need be. This is a killer amp. Enclosed, you will find tons of pictures and a youtube vid showing various angles incase you need it. If you need more angles, lemme know. Yalls help keeps me looking like a god to the yokels around my neighborhood!

Thanks!

 

[Ylli]

May be "Vintage", but not "Antique". It has transistors......
I see a "Model Number" tag, but can't see if there is a model number listed.

 

[Faradave]

Well.... Find me a 100 year old guitar amp..
for guitars, not a PA... as far as amps go, it's antique... sorry, yea, the model is 126. Found a bunch of info once I had that lol

n

 

[Faradave]

For the purposes of this convo, antique is redefined as something 40 years old or older. *claps once*

 

[Ylli]

"Antique" - old with tubes
"Vintage" - old with transistors
Whatever....

The mystery of why you can plug in to either the pre-out or power-in can be seen here in the 220 manual.

The normal and -10 dB inputs can be seen here with different series resistors on the input:

The 'BRT' switch is probably "Brightness", as it looks like a high boost (S101).

As for the fade to static, if the large filter cap was bad, you would likely gt a blown fuse. If the issue exists when you are feeding the signal into the power amp-in jack, then the probem lies in the power amp.

 

[Faradave]

Yeah, I actually found more than a few bad capacitors. Capacitors, that is, that do not increase in resistance to Infinity. Many of them have a set resistance and do not move.

 

[Faradave]

And yeah, I'm jealous cuz I don't have any tube amps.

 

[Faradave]

Okay, guys, here's what I got.

Every diode on this board allowed electricity to flow in either direction. Can I replace these with similar glass ones? , or do they have to be the exact diode?

Three capacitors were bad on this board. I recently figured out that those items are inductors And that I really don't want them to go bad because they are almost impossible to replace.

Now That's what I call a sticky situation! on the underside of this board, it was a real gooey mess. I assume that's grease? I only found one bed capacitor here. Although there's some other items Here that I cannot identify And so I cannot test their functionality.

I'm about to replace The capacitors and diodes, unless anyone warns me not to. We'll see what we got afterwards.

 

[Nigel Goodwin]

And yeah, I'm jealous cuz I don't have any tube amps.

You're obviously young - people from back in the valve days couldn't care less about valve amps, and were glad to see the back of them.

However, having said that, they do have uses in particular guitar styles, where you're wanting low quality and distortion.

Interesting schematic though, it's unusual to have a long tailed pair in such an old amp, and even more unusual to be running the long tailed pair at a lower voltage, and dividing the DC feedback from the output to it.

 

[Nigel Goodwin]

No need to replace the diodes, they won't be faulty - you can't test them in circuit - you're more likely to cause damage than help anything.

Likewise the capacitors, you can't test them in circuit - so you've no idea if they are faulty or not (although capacitors DO age, and only have a finite life span).

The inductors are part of the tone controls, leave them alone, they don't go faulty (unless messed with!) and as you say would all but be impossible to replace.

The 'goo' is most likely glue used to hold components in place (to help prevent dry joints etc.), however various types of glue used over the years have begun to conduct and can cause troubles.

Really, instead of randomly trying to check components (which you're not even doing properly in-circuit) you need to fault find. First job, find out WHERE the issue is - if it's preamp or power amp, then you only have one section to deal with. You've got sockets for power amp in, and preamp out, so it's easy to check - but also be aware that one of the sockets (probably power amp in) will most likely have switch contacts on it, and these often give trouble and require cleaning.

 

[JimB]

I'm about to replace The capacitors and diodes, unless anyone warns me not to.

I suggest that is a seriously bad idea.

I guess that you have just measured these components while they are mounted on the board, in circuit.
In which case you will see the effects of other components which are connected to the one you are trying to measure.

JimB

 

[Faradave]

Well I sure thought I knew something at all.... lol derp.... Anyways, my audio issue crops up eventually no matter where I plug my input into. Does that mean the issue is on a board common to all inputs? I obviously need my lil baby hand held every step of the way.... guide me plz...

 

[Faradave]

Full disclosure: I noticed that the LEDs installed were ugly red. I swapped them for blue LEDs, which helped the light coming out to be blue. Matches much better but I still have the same sound issue, If you can imagine!

 

[Faradave]

Look at this old cap I replaced

Look, it's prolapsed. See how that section is poking out from the middle? Is this just how they were made back then?
All the 1uF caps are like this.

 

[Nigel Goodwin]

Look at this old cap I replaced

Look, it's prolapsed. See how that section is poking out from the middle? Is this just how they were made back then?
All the 1uF caps are like this.

There's nothing visually wrong with that, to actually check it you really need an ESR meter, although for the purposes of this amp a capacitor meter and checking for leakage with a multi-meter are probably enough.

As electrolytic capacitors fail with age, it's common to replace them all - but NOT as a random event.

But you still need to locate where the fault is - as I've already said, the preamp out and power amp in sockets make this easy. Feed something else into the power amp in socket (CD player?, something like that) and see if it continues working using just the power amp. Or, take the preamp out to another power amplifier (such as your HiFi) and see if the preamp works without failing. In that way you can determine which part is at fault - it's called the 'half split method', you don't do repairs by randomly removing and replacing components.

 

[Faradave]

Ah yes yes yes. That makes sense. That's smart. I was just randomly poking my multimeter at stuff, it was so stupid looking back. It's got the whole circuit attached to it. Why would I..... There was logic to it, though, the capacitors were positioned in such a way that the reading on my multimeter made them look broken.

Okay, so I see what you're driving at after carefully studying your post. Gimme a bit. You're a genius. Thank you. Thank you. Thank you.

 

[Faradave]

Oh and I must emphasize that never again will I desolder anything so randomly, relying on serendipity and happenstance like a common hobo. If I ever choose to extract one of these components I guarantee I will have some very specific reasoning for choosing the one I do. On this you have my word.

lol just havin fun you guys rock

 

[Faradave]

Ok so, in an attempt to locate my issue, I played music until the sound became static, then attempted to plug my "pre amp output" into a seperate amp to test if my pre-amp is ok. The only other amp I have with me is a pair of decent PC speakers with built in amp. That works IF I switch my music input cable from a back input (I usually use "preamp out" or "power amp in" as input for better sound) to the front "0db" input. Then I can hear the music but at a very low volume. Almost like I'm just setting a pair of headphones on the table as speakers.

I'd say that fails the test and the pre-amp doesn't work well at this point because I can get much more sound from those speakers by plugging them into my phone or PC. I guess the question is do I get more sound with a rested, "fresh" amp.

Incidentally, when I plug my large speakers into "pre-amp out", obviously I get no sound cuz its just a pre-amp.

FYI, anytime I ever use those front "0db" and "-10db" inputs, the sound is very quiet. I have to turn up every knob to hear anything. This may be because it's expecting an input from another amp (ppl were made of amps in the 70s) with a certain amount of db to begin with.

Then I let the amp rest a bit to let the caps charge or to let the gremlin take a nap, whatever the case, so I can try again at full.... cap charge/gremlin nap. It takes a good hour min for the amp to be fully "rejuvinated", as it were. So, please join me in an hour of silence *looks at watch*

.....ok, time well spent. I got the same results with the rested amp. I get almost no sound using the front "0db" input and the rear "pre-amp output".

So.... we.... cannot rule out the pre-amp board? What now

 

[Ylli]

I believe you are dealing with two issues. Earlier you indicated that with the audio source driving the "power-in" jack, that the output eventually faded to static. That would be an issue with the power am. An addition issue seems to be low gain of the preamp/tone control sections. These issue need to be dealt with separately.

If it were me, I'd look at the power amp first. And the first thing I would do is to confirm that the +76 volts (as shown on the schematic) was there and remained there as the output faded to static.

 

[Nigel Goodwin]

Ok so, in an attempt to locate my issue, I played music until the sound became static, then attempted to plug my "pre amp output" into a seperate amp to test if my pre-amp is ok. The only other amp I have with me is a pair of decent PC speakers with built in amp. That works IF I switch my music input cable from a back input (I usually use "preamp out" or "power amp in" as input for better sound) to the front "0db" input. Then I can hear the music but at a very low volume. Almost like I'm just setting a pair of headphones on the table as speakers.

It's not very clear what you've done - have you tried the preamp out BEFORE it goes faulty?, leave it running on the PC speakers and see if it goes off after a while. By not trying it until it's gone faulty you've no idea if that's how it should sound or not.