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Post subject: Blues Junior (made in mexico, cream board, 2003)
Posted: Tue Jun 25, 2019 7:17 am
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Hi All,

I am working on a Fender Blues Junior amp. Symptoms are low output volume and no reverb. Looks like the low volume is due to a bad input jack and the reverb was a bad connection inside the reverb tank. The reverb tank connector is a press fit thing. Anyone know where to get a better connector that will fit?

However, while looking around I see that the output tubes look like they have been really hot, markings almost burnt off and carbon inside the tube. R47 has gotten hot enough to scorch the PCB although it still reads OK and is running cool now. DC voltages look OK, but the AC voltage on D11/TP17 varies and goes up to 40VAC, seems like moving the ribbon cable affects the voltage. On D12/TP18 the voltage reads a consistent 3.5VAC. Any suspects for these readings? Obviously I need to remove the tube PCB and find the ribbon cable issues but is there anything else that is a likely culprit?

Thanks!


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Wed Jun 26, 2019 5:08 am
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Bad input jack = intermittent sound cutting , not low volume.


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Wed Jun 26, 2019 6:48 am
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stratele52 wrote:
Bad input jack = intermittent sound cutting , not low volume.


It is the input jack. When I jump a signal directly to the board the amp works fine.

What I am trying to figure out is the voltage readings around the output tubes.


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Wed Jun 26, 2019 10:06 am
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Those AC voltages are signal reference levels. They are based on a 10mv ac 1khz input signal.
The controls must be set according to the notes on the schematic in order to get meaningful measurements of AC signal levels.
They are useful to see if the signal chain is up to snuff.
The DC voltages are more useful for troubleshooting.
In your case, you are describing fried power tubes. Pull one at a time and run it.
It won't sound right, but each power tube should sound the same if both are good.
Get new ones with low gain rating to prevent frying the next pair.


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Wed Jun 26, 2019 11:38 am
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madlabs wrote:
stratele52 wrote:
Bad input jack = intermittent sound cutting , not low volume.


It is the input jack. When I jump a signal directly to the board the amp works fine.

What I am trying to figure out is the voltage readings around the output tubes.


Or solder.
What happened when you plug guitar in input jack ; steady low volume, no intermittent sound ?


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Wed Jun 26, 2019 12:53 pm
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stratele52 wrote:
Or solder.
What happened when you plug guitar in input jack ; steady low volume, no intermittent sound ?


Steady low volume. Jumping a signal to the junction of R1&R2 (with a plug in the input jack) gives me normal volume.

Quote:
Those AC voltages are signal reference levels. They are based on a 10mv ac 1khz input signal.
The controls must be set according to the notes on the schematic in order to get meaningful measurements of AC signal levels.
They are useful to see if the signal chain is up to snuff.
The DC voltages are more useful for troubleshooting.


Well, that is embarrassing. I somehow missed the test conditions. Thank you.

Quote:
In your case, you are describing fried power tubes. Pull one at a time and run it.
It won't sound right, but each power tube should sound the same if both are good.


Will do. Would this explain R47 being scorched? And the metal hold down piece seems to be really stuck on the output tubes. Is this normal and is there an easy way to get it off without breaking the tubes?

Quote:
Get new ones with low gain rating to prevent frying the next pair.


Any recommendations? The amp has the original groove tubes.

Thanks a lot for the help!


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Thu Jun 27, 2019 8:04 am
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Look at your schematic. You must be using a Blues Junior III schematic.
R47 connects the plate voltage to the screen voltage in the pwr supply. If an output tube fails - screen grid shorts to ground, that puts approx. 330 volts across R47, what happens - smoke. Replace both output tubes, R47, the two screen grid resistors and bias correctly. Note the FR in the schematic for the resistors, that means something, look it up and use the correct parts.


Last edited by jackhammer on Thu Jun 27, 2019 8:17 am, edited 1 time in total.

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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Thu Jun 27, 2019 8:11 am
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jackhammer wrote:
Note the FR in the schematic for the resistors, that means something, look it up and use the correct parts.


"FR"......flame resistant

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Blues Junior (made in mexico, cream board, 2003)
Posted: Fri Jun 28, 2019 9:16 am
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As Retro described, FR means flameproof. These are essentially a fusible resistor that is designed to fail when overloaded. Flameproof means they won't start a fire in the process.
You will find them in the Mouser catalog where they list 1/4 watt resistors rated for 1/2 watt. There are 1/2 watt rated for 1watt etc.
They are commonly used in power supplys and power amp chains to prevent cascading failures.
While they usually fail with obvious blast marks, they also can fail with only a tiny crater hidden underneath the body.
While I always try to replace with FR, a lower wattage carbon film will accomplish the same protection.


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