Help sought with Champion 600 that keeps...

TforTele

... cutting out.  Maddeningly.

The history might or might not be relevant: I bought it on this forum and it was shipped with minimal packing.  Well, it was in a cardboard box, but with absolutely zero shock absorption by way of polystyrene or the like.  In addition, the upgraded speaker had been fixed to the baffle from the wrong side so became entirely detached in transit.  It took out the power valve.  The amp had had some mods, with a gain control added, a switch for standard or 5E1 circuit, and a negative feedback switch.

I can't discern any pattern to the cutting out. Switching on and off sometimes seems to help.

Just interested to see if it might be something simple that doesn't need an amp tech.  Having some experience of simple electronics in the distant past I do know one end of a soldering iron from the other, but also have a healthy respect for electricity.

Any help / thoughts gratefully welcomed.

ICBM

When it cuts out, does it go to absolute silence or is there some residual hiss or hum audible from the speaker?

Do you have another speaker cab you could try it with?

TforTele

@ICBM - Thank you for replying!  Just had a chance to check and it is pretty deadly quiet when it goes off.  Just the faintest hum with my ear up close.  I'm afraid I don't have a separate cab to try it with.  It is intermittent and unpredictable, as I said, but I just tried physically rocking it back and that can induce it.  Just checked the speaker connectors and they're fine. 

ICBM

If the hum is definitely electrical and coming from the speaker - not a mechanical hum coming from the transformer - then it's not a fault in the speaker circuit. You can confirm that for sure by deliberately disconnecting the speaker and see if even that residual hum stops.

If rocking it back and forth (gently? I assume) make it come and go, then it's something which is making a bad connection and has enough mass to move slightly depending on the angle it's hanging at. If the speaker test above shows it's actually transformer hum you're hearing, it could still be the speaker cable or connections - not necessarily at the connectors. But if the hum is definitely coming from the speaker, then it must be something inside the amp - most likely near the output end of the circuit if it goes pretty quiet.

If so, you'll need to poke around inside with the amp running and live, so if you're not comfortable with that, you need a tech unfortunately. If you are, then there are ways to keep yourself safe when doing it.

TforTele

@icbm Just tried disconnecting the speaker and the faint hum I referred to is still there - presumably the transformer, as you suggest.

I was just rocking/tipping the amp back, not back and forth - very gentle.

So does that suggest the speaker cable/connections?

Thanks again for your patience and help.

ICBM

TforTele said:

@icbm Just tried disconnecting the speaker and the faint hum I referred to is still there - presumably the transformer, as you suggest.

I was just rocking/tipping the amp back, not back and forth - very gentle.

So does that suggest the speaker cable/connections?

Yes, probably. If the electrical sound stops entirely then the fault is most likely in the speaker circuit, since otherwise there is usually some induced hum from the output transformer which comes through the speaker.

One possibility is a damaged speaker - if it was thrown about so violently that it was ripped off the baffle, that's certainly something that could have happened. The cone braids may be damaged, either where they join the terminals or where they pass in to the cone. Try reaching under the frame and *very gently* moving them (don't pull hard on them) and see if the sound cuts out or comes back.

TforTele

@ICBM I will try that with the speaker. Actually, because the speaker was fitted to the wrong side of the baffle it wouldn't have needed any force to detach it as there was really nothing holding it in place. When attached correctly the fixings pull into the baffle from the opposite side, but they were doing nothing.

I put a meter on the little speaker cable - all good.  Opened the amp up and checked the speaker jack socket - squeezed the tip connector in lest that was the issue.  Right, off to gently do the speaker test you suggest.

TforTele

@ICBM ;It’s only a wee speaker so quite tight to insert a finger, but moving / pulling the braids(?) did not induce any cutting out. However, inducing the cutting out by tipping the amp backwards and then flipping the 5E1 circuit switch does bring it back. Tricky to do without moving the amp at all, but it seems possibly connected. Is that maybe a clue?

ICBM

TforTele said:

@ICBM ;It’s only a wee speaker so quite tight to insert a finger, but moving / pulling the braids(?) did not induce any cutting out. However, inducing the cutting out by tipping the amp backwards and then flipping the 5E1 circuit switch does bring it back. Tricky to do without moving the amp at all, but it seems possibly connected. Is that maybe a clue?

Maybe, although it could just be the vibration caused by the mechanical click of the switch doing it. Normally a '5E1 switch' just adds a cap to the first gain stage to boost gain, so that wouldn't make the amp cut out.

At this point I think you may have to open it up and poke around inside. Do you have something insulated like a wooden chopstick?

TforTele

@ICBM  Ha!  No shortage of chopsticks here - my wife is Japanese!  I thought the 5E1 mod switch was a bypass, so reverting to a simpler circuit?

ICBM

TforTele said:

@ICBM  Ha!  No shortage of chopsticks here - my wife is Japanese!  I thought the 5E1 mod switch was a bypass, so reverting to a simpler circuit?

That depends on what exactly has been done to it. All bets are off unless you know what the circuit has been modded to before the '5E1' switch was added - since the stock Champion 600 is actually a strange derivative of the BF Champ with fixed EQ. Why they did that given it would have been cheaper and better to build a proper early Champ circuit, I do not know...

TforTele

@ICBM  Ah, very interesting.  I think I had in mind the wording of the ad when I bought it, which referred to "bypassing the tone stack", or similar.  The seller kindly shared the documents about the mods, including schematics.  This is from the Mastertone "kit instructions": 

The Fender Champ 600 kit is extremely simple to install and gives the amp more usability and tones at lower volumes.
Master Volume: The original Volume control becomes the ‘Gain’ control which determines the amount of pre-amp gain. The new Master Volume controls the signal into the output tube. This provides crunch/break up at low volumes.Gain/AA764/5E1: The Fender Champ 600 has tone stack rather like the AA764 circuit but with fixed values. The Fender Champ 600 Gain/5E1/AA764 kit is extremely simple to install and allows the player to switch between the standard ‘AA764’ circuit and a close match to a ‘5E1’ circuit which gives the amp a different tone and more gain. The two capacitors supplied with the kit improve both the ‘5E1’ and ‘AA764’ tone.Negative Feedback: The Fender Champ 600 has a Negative Feedback (NFB) circuit that works to eliminate distortion. The NFB mod that lets you choose whether to run the amp with or without negative feedback. Switching out the NFB circuit opens up the amp’s tone and gives it more crunch.

ICBM

Well, bypassing the tone stack certainly adds gain and it's quite likely that's what the switch does, but it does not turn it into a close match to the 5E1 circuit - or even a 5F1, which is closer but still significantly different. Both those circuits have a valve rectifier, the 5E1 has a choke, and the NFB arrangement is different (if not also turned off).

TforTele

@ICBM  Intriguing. Quite beyond me but fascinating.  What I thought was simple is clearly not.  I’m happy to share the full kit instructions and schematics by email if you’re interested.  The mods were carried out by a pro for the last owner, incidentally.

If I open up the amp, what would be the plan with the chopstick?

ICBM

TforTele said:

If I open up the amp, what would be the plan with the chopstick?

To carefully poke components, move wiring etc until you can find what reliably makes it cut out and come back, then you'll have found it. You'll obviously need it open in front of you with the speaker connected - the combo box on its front with the amp chassis resting on the back of it is usually the easiest - and a guitar or other signal source going into it.

TforTele

@ICBM  Got it, many thanks, will give that a go.  Thanks again for your patience and help.