Fender Hot Rod Deville 410 Clean Up
My son bought a used Fender Hot Rod Deville 410 original and we are going to go over it since he says it doesn't sound right. Googled it and many say that it it is similar to a Blues Deville but that one unused half of a 12AX7 was added for even more distortion in the drive channel and they went too far on gain. Some suggest replacing the tubes with 12AT7s but he tried that and one in the second stage seemed alright but that it still was not right.
Another person suggests changing the second pre amp tube to a 12dw7/7247 which puts a 12AX7 half in the clean channel and the 12AU7 half in the distortion channel where the lower gain fits well. We will try this.
I opened it and noticed a date of 1996 on the circuit board, not sure if that is the date that it was designed or the actual age of the unit which would place it just shy of 20 years old.
First impressions are that the jacks feel very cheap, and I don't like the circuit board from a reliability perspective but I suppose it works.
Speakers are four 10" Eminence for Fender. He wants two 12" but I don't think I'm up for making a new baffle board we'll see.
The red printing on the output tubes is burnt to brown in some areas and I have a feeling that it was red plating.
All the main signal paths are tubes 12AX7 preamp and 6L6GC outputs, but the reverb, pre out and power amp in all employ TL072 OP amps. Fender seems to like these as they are in many of their newer amps. I believe that these snap rail to rail when over driven, IIRC, anyone know for sure? Let's hope that they are never over driven.
Pulled the tubes and they are all Groove Tubes, 12AX7s say Sovtek on them not sure about the 6L6 GC but my friend noticed that they had welded plates - nice! One pre amp tube tested weak and the outputs tested fine - still I believe that they need to be tested at full power since the tester doesn't stress them as in circuit.
======== Schematic Walk Through
Schematic from Fender is here in .pdf:
http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_DeVille_schematic.pdf
Output plate supply is 485V! I'd prefer something closer to 440V.
Even the preamp supply seems high at 388V.
The input stage looks like typical Fender with the 1 and 2 jacks but they've combined the typical
two channels into one with relay switches to raise the gain for the dirty channel.
V1A has the typical plate and cathode resistors with a large cathode bypass cap of 47 uF providing
increased gain over the full frequency response. This is larger than needed but they probably are
going to roll- off the low end with the interstage coupling caps so that is fine. Looking closer all
four preamp stages have the typical 1.5K cathode and 100K plate resistors, smaller cathode bypass
caps are used to provide response shaping. The response effect can be calculated here:
Cathode Bypass Capacitor Calculator
There are two front end pots, gain for the clean and drive for the distortion channel allowing the player
to preselect two different volumes depending on which channel is selected from the foot switch.
It is interesting that the low frequency roll-off is quite different between the two channels and also
the much lower value than typical, grid leak resistor makes the roll-off frequency dependent on the
pot settings. Here are the values for low volume settings:
Clean: .022uF into 470K for 15.4 Hz
Dist: .0015 uF into 430K for 246.8 Hz
Minus 3 dB can be calculated here:
High Pass Filter Calculator
This blocks LF from the distortion channel since bass does not sound good when distorted but I have
to wonder if they went too far.
R44 could be increased to make the LF cut less dependent on volume position, perhaps it was deliberate
to keep it clean at higher volumes? To keep the cheap speakers clean, just speculation?
The Bright switch puts in the second stage cathode bypass cap but it is disabled when the distortion
channel is selected. The cap is a small value so that it only boosts the high end.
Output of the second stage feeds the tone stack which looks like the usual Fender arrangement.
The relay switch that disables the Bright control also further cuts the high end by enabling C3 as a
shunt at the input of the 3rd stage. Presumably to cut the highs that might get too harsh with more
distortion.
This amp has the amusing, "More Drive", option where a 1 uF cathode bypass is enabled on stage 3 by
Q1 and a 22 uF cap on stage four through Q2 - JFET switches. There are actually two foot switches, one
to select clean or distorted channels and the other to enable "More Drive".
The fourth preamp stage is only used for the distortion channel and it is removed and muted by the
second relay. A Master Volume pot is provided at the output of this fourth stage and therefore does
nothing when the clean channel is selected. This allows the distortion channel to be driven very hard
without being forced to drive the power amp to full output. Turn up the drive to get as much distortion
as is needed then turn down the Master for a comfortable playing level. Interesting to note that the AC
signal voltage shown at the plate of the fourth stage is 17.1 V. This is probably enough to blow out the
inputs of the TL072 OP amps since they only run on 16V supplies, so back to back Zeners clamp the
signal at +/- 15V, I might have chosen 10V to keep the levels more reasonable.
The output of the final output relay is attenuated to feed a TL072 buffer U1A for the Preamp output
jack, this is not in the path to the power amp.
Interesting that the relay output is not attenuated to feed the TL072 driving the reverb spring, is it
normal to distort the reverb driver in a Fender amp? If not I'd probably attenuate that drive signal
by 6 to 12 dB. I'd also use a more robust driver, something like a headphone driver for the reverb
drive OP amp. I'll have to check the gain of that stage perhaps it is unity gain, not sure since I don't
know the impedance of the reverb tank.
OP amp U2B amplifies the output from the reverb tank to the Reverb pot the output of which is
mixed into the power amp input. Some response shaping is used.
OP amp U1B is an input buffer with gain for external sources feeding the power amp.
The power amp is very close to a standard Black Face, no ultra linear connection, but with a few
very small 47 pF caps added probably to tame the massive harmonics from the distortion, and
a few diodes added probably to protect the tubes and output transformer from arcing if the amp is
ever operated without a speaker load.
My son bought a used Fender Hot Rod Deville 410 original and we are going to go over it since he says it doesn't sound right. Googled it and many say that it it is similar to a Blues Deville but that one unused half of a 12AX7 was added for even more distortion in the drive channel and they went too far on gain. Some suggest replacing the tubes with 12AT7s but he tried that and one in the second stage seemed alright but that it still was not right.
Another person suggests changing the second pre amp tube to a 12dw7/7247 which puts a 12AX7 half in the clean channel and the 12AU7 half in the distortion channel where the lower gain fits well. We will try this.
I opened it and noticed a date of 1996 on the circuit board, not sure if that is the date that it was designed or the actual age of the unit which would place it just shy of 20 years old.
First impressions are that the jacks feel very cheap, and I don't like the circuit board from a reliability perspective but I suppose it works.
Speakers are four 10" Eminence for Fender. He wants two 12" but I don't think I'm up for making a new baffle board we'll see.
The red printing on the output tubes is burnt to brown in some areas and I have a feeling that it was red plating.
All the main signal paths are tubes 12AX7 preamp and 6L6GC outputs, but the reverb, pre out and power amp in all employ TL072 OP amps. Fender seems to like these as they are in many of their newer amps. I believe that these snap rail to rail when over driven, IIRC, anyone know for sure? Let's hope that they are never over driven.
Pulled the tubes and they are all Groove Tubes, 12AX7s say Sovtek on them not sure about the 6L6 GC but my friend noticed that they had welded plates - nice! One pre amp tube tested weak and the outputs tested fine - still I believe that they need to be tested at full power since the tester doesn't stress them as in circuit.
======== Schematic Walk Through
Schematic from Fender is here in .pdf:
http://support.fender.com/schematics/guitar_amplifiers/Hot_Rod_DeVille_schematic.pdf
Output plate supply is 485V! I'd prefer something closer to 440V.
Even the preamp supply seems high at 388V.
The input stage looks like typical Fender with the 1 and 2 jacks but they've combined the typical
two channels into one with relay switches to raise the gain for the dirty channel.
V1A has the typical plate and cathode resistors with a large cathode bypass cap of 47 uF providing
increased gain over the full frequency response. This is larger than needed but they probably are
going to roll- off the low end with the interstage coupling caps so that is fine. Looking closer all
four preamp stages have the typical 1.5K cathode and 100K plate resistors, smaller cathode bypass
caps are used to provide response shaping. The response effect can be calculated here:
Cathode Bypass Capacitor Calculator
There are two front end pots, gain for the clean and drive for the distortion channel allowing the player
to preselect two different volumes depending on which channel is selected from the foot switch.
It is interesting that the low frequency roll-off is quite different between the two channels and also
the much lower value than typical, grid leak resistor makes the roll-off frequency dependent on the
pot settings. Here are the values for low volume settings:
Clean: .022uF into 470K for 15.4 Hz
Dist: .0015 uF into 430K for 246.8 Hz
Minus 3 dB can be calculated here:
High Pass Filter Calculator
This blocks LF from the distortion channel since bass does not sound good when distorted but I have
to wonder if they went too far.
R44 could be increased to make the LF cut less dependent on volume position, perhaps it was deliberate
to keep it clean at higher volumes? To keep the cheap speakers clean, just speculation?
The Bright switch puts in the second stage cathode bypass cap but it is disabled when the distortion
channel is selected. The cap is a small value so that it only boosts the high end.
Output of the second stage feeds the tone stack which looks like the usual Fender arrangement.
The relay switch that disables the Bright control also further cuts the high end by enabling C3 as a
shunt at the input of the 3rd stage. Presumably to cut the highs that might get too harsh with more
distortion.
This amp has the amusing, "More Drive", option where a 1 uF cathode bypass is enabled on stage 3 by
Q1 and a 22 uF cap on stage four through Q2 - JFET switches. There are actually two foot switches, one
to select clean or distorted channels and the other to enable "More Drive".
The fourth preamp stage is only used for the distortion channel and it is removed and muted by the
second relay. A Master Volume pot is provided at the output of this fourth stage and therefore does
nothing when the clean channel is selected. This allows the distortion channel to be driven very hard
without being forced to drive the power amp to full output. Turn up the drive to get as much distortion
as is needed then turn down the Master for a comfortable playing level. Interesting to note that the AC
signal voltage shown at the plate of the fourth stage is 17.1 V. This is probably enough to blow out the
inputs of the TL072 OP amps since they only run on 16V supplies, so back to back Zeners clamp the
signal at +/- 15V, I might have chosen 10V to keep the levels more reasonable.
The output of the final output relay is attenuated to feed a TL072 buffer U1A for the Preamp output
jack, this is not in the path to the power amp.
Interesting that the relay output is not attenuated to feed the TL072 driving the reverb spring, is it
normal to distort the reverb driver in a Fender amp? If not I'd probably attenuate that drive signal
by 6 to 12 dB. I'd also use a more robust driver, something like a headphone driver for the reverb
drive OP amp. I'll have to check the gain of that stage perhaps it is unity gain, not sure since I don't
know the impedance of the reverb tank.
OP amp U2B amplifies the output from the reverb tank to the Reverb pot the output of which is
mixed into the power amp input. Some response shaping is used.
OP amp U1B is an input buffer with gain for external sources feeding the power amp.
The power amp is very close to a standard Black Face, no ultra linear connection, but with a few
very small 47 pF caps added probably to tame the massive harmonics from the distortion, and
a few diodes added probably to protect the tubes and output transformer from arcing if the amp is
ever operated without a speaker load.
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I have this amp and I had to fix R58. It was a partial open. Fender said they had a vendor problem with the plate load resistors,R4,11,16,22 and many of them either develop a partial or full open. Check voltage drops on especially R57 and R58, the PI plate load resistors.
I also developed a loose connection on one of the ribbon cables that connect the main board to the smaller board with the tubes mounted on it. Use a chopstick and prod these and other points when powered on to see if you can find any loose solder joints or bad resistors. And yes, 485 volts could kill you. Don't do this unless you are confidant about it. Be careful!
Fender also used a linear taper pot for the master vol. instead of an audio taper, so the volume comes on strong fast. A 12AY7 in the V1 slot can help this. I've even used a 12AU7 also. Yes, try the 12DW7 you stated as well. Try the 12AU7 or 12AT7 for the PI also.
From what I hear, those TL072 chips are doing OK, just reverb and pre out and power amp in duty. They used these in the red knob amps too.
And no, those jacks aren't anything special, that's for sure.
Also, check the bias. Fender biased these cold, so you might want to liven it up with hotter bias. They call for 60mV at TP30. Try 70 or a little more. It says "Bias test point" right on the small board by the power tubes.
I also developed a loose connection on one of the ribbon cables that connect the main board to the smaller board with the tubes mounted on it. Use a chopstick and prod these and other points when powered on to see if you can find any loose solder joints or bad resistors. And yes, 485 volts could kill you. Don't do this unless you are confidant about it. Be careful!
Fender also used a linear taper pot for the master vol. instead of an audio taper, so the volume comes on strong fast. A 12AY7 in the V1 slot can help this. I've even used a 12AU7 also. Yes, try the 12DW7 you stated as well. Try the 12AU7 or 12AT7 for the PI also.
From what I hear, those TL072 chips are doing OK, just reverb and pre out and power amp in duty. They used these in the red knob amps too.
And no, those jacks aren't anything special, that's for sure.
Also, check the bias. Fender biased these cold, so you might want to liven it up with hotter bias. They call for 60mV at TP30. Try 70 or a little more. It says "Bias test point" right on the small board by the power tubes.
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Thanks for the warning but I've worked on tubes for many years, learned on them.
A lot of good info there thanks!
Might it not be better to drop the gain somewhere rather than putting in so many low gain tubes?
12AT7 was common as the output driver in the Black Face amps, did they go to a 12AX7 just for parts commonality and a bit more gain?
A lot of good info there thanks!
Might it not be better to drop the gain somewhere rather than putting in so many low gain tubes?
12AT7 was common as the output driver in the Black Face amps, did they go to a 12AX7 just for parts commonality and a bit more gain?
Various thoughts...
HR DeVille REISSUE? Your schematic is the original. The 1996 date is the design series, not the date it was made. Though it could also have been made that year too. There is an HR DeVille III, which is a reissue, but it has much later dates on the boards. So I am confused about a reissue. But if the schematic you posted matches your amp, and the layout drawing looks like your boards, that's good enough.
Your son said it doesn't sound right. That implies to me that it sounds like something is wrong. Not the same thing as not liking the tone. The approach is totally different when searching for a problem rather than searching for tone.
In my own opinion, I think the Hot Rod series amps sound great on the clean channel, and well, I don't think Fender can design an overdrive channel to save their souls. So clean channel and an overdrive pedal for me. But that is personal taste.
People love to toy with their amps, so there will be no end of ideas you will find on the internet for ways to "improve" the amp. Certainly sticking other preamp tubes in it won't hurt, and if you like the results that is all that counts. Speakers are the one thing that will change the sound of an amp more than anything else. Since the chassis has a jack on it, you can easily unplug the internal speakers and connect a 2x12 cab or other alternative.
If you look at the op amp circuits, you will see that the preamp out IC is not in the signal path. Likewise the power amp in IC is only in circuit when using the power amp in jack. If the jack is empty, the IC is not in the loop, so to speak. The reverb IC circuits saved them the cost of a transformer and another tube. There are test points on the drawing that give the signal levels in the area. For example the preamp out stage. The input has a voltage divider R28/29, a roughly 1/16 divider. That IC sees a small signal. The power amp in IC would have to see something like 12v RMS of signal at the power amp jack to overdrive the IC input. So I wouldn't worry about anything snapping over anywhere. And the reverb drive would have to receive a similar level from the preamp, which doesn't occur. But even if it wanted to, at the output of the preamp is a zener clamp at 15v, CR1, CR2. (Just upper left of the power amp jack on the print)
If your tube paint is browned, then the tubes could have been red plating (turn it on and look) or just run hard over time. 485 volts sound like a perfectly happy B+ and VERY common on 6L6 amps. Have you checked the bias on the amp? There is a convenient test point on the corner of the tube socket board. Factory setting is 60ma (30ma per tube) for a fairly cool 14 watts each. Well within the tubes happy place. Fresh power tubes ought to make the amp sound more lively, make sure to set the bias per spec.
I don't know in your case, but in most cases, the Fender/Groove 6L6s are Sovteks.
HR DeVille REISSUE? Your schematic is the original. The 1996 date is the design series, not the date it was made. Though it could also have been made that year too. There is an HR DeVille III, which is a reissue, but it has much later dates on the boards. So I am confused about a reissue. But if the schematic you posted matches your amp, and the layout drawing looks like your boards, that's good enough.
Your son said it doesn't sound right. That implies to me that it sounds like something is wrong. Not the same thing as not liking the tone. The approach is totally different when searching for a problem rather than searching for tone.
In my own opinion, I think the Hot Rod series amps sound great on the clean channel, and well, I don't think Fender can design an overdrive channel to save their souls. So clean channel and an overdrive pedal for me. But that is personal taste.
People love to toy with their amps, so there will be no end of ideas you will find on the internet for ways to "improve" the amp. Certainly sticking other preamp tubes in it won't hurt, and if you like the results that is all that counts. Speakers are the one thing that will change the sound of an amp more than anything else. Since the chassis has a jack on it, you can easily unplug the internal speakers and connect a 2x12 cab or other alternative.
If you look at the op amp circuits, you will see that the preamp out IC is not in the signal path. Likewise the power amp in IC is only in circuit when using the power amp in jack. If the jack is empty, the IC is not in the loop, so to speak. The reverb IC circuits saved them the cost of a transformer and another tube. There are test points on the drawing that give the signal levels in the area. For example the preamp out stage. The input has a voltage divider R28/29, a roughly 1/16 divider. That IC sees a small signal. The power amp in IC would have to see something like 12v RMS of signal at the power amp jack to overdrive the IC input. So I wouldn't worry about anything snapping over anywhere. And the reverb drive would have to receive a similar level from the preamp, which doesn't occur. But even if it wanted to, at the output of the preamp is a zener clamp at 15v, CR1, CR2. (Just upper left of the power amp jack on the print)
If your tube paint is browned, then the tubes could have been red plating (turn it on and look) or just run hard over time. 485 volts sound like a perfectly happy B+ and VERY common on 6L6 amps. Have you checked the bias on the amp? There is a convenient test point on the corner of the tube socket board. Factory setting is 60ma (30ma per tube) for a fairly cool 14 watts each. Well within the tubes happy place. Fresh power tubes ought to make the amp sound more lively, make sure to set the bias per spec.
I don't know in your case, but in most cases, the Fender/Groove 6L6s are Sovteks.
I guess I should have made myself more clear. I would only do one of these at a time to see what it does. Then if it isn't what you want, try another place.
The 12AX7 as you know will give you more gain in the PI so I suppose Fender did what they did for design goal reasons. I agree with Enzo about Fender and their overdrive channels. Bur for clean, they can't be beat. And if you crank the clean channel, it will break up around noon or a little sooner if you can put up with the volume. Enzo is also right, this is the original Hot Rod DeVille. This was the next amp to succeed the Blues DeVille and you're right, it had an unused 12AX7 side and they used it in the new DeVille series.
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I added a walk through to my first post, and I only just saw these last two posts.
I tested tubes first because I was visiting a friend with a very good tube tester.
We will get back to just trying it measuring the bias and going from there. I'm fairly
certain that it needs to be recapped.
I've studied Fender schematics a lot, but I don't know these amps by model number all that well. It has a circuit board and relays so I figured it was a reissue, since I think of all real Fenders coming from the 50s to 70s.
Just checked yes the schematic does match so it seems you are correct that it is not a reissue. Thanks for the correction.
I tested tubes first because I was visiting a friend with a very good tube tester.
We will get back to just trying it measuring the bias and going from there. I'm fairly
certain that it needs to be recapped.
I've studied Fender schematics a lot, but I don't know these amps by model number all that well. It has a circuit board and relays so I figured it was a reissue, since I think of all real Fenders coming from the 50s to 70s.
Just checked yes the schematic does match so it seems you are correct that it is not a reissue. Thanks for the correction.
We started a Fender Twin Reverb copy many years ago and finished the box with 2 Weber 12" in it and then just transplanted his Line 6 amp into it.
So he can plug those into this amp.
We should just finish the amp, do you know of a source for a bare Twin Reverb chassis?
And fix the HR Deville and sell it.
So he can plug those into this amp.
We should just finish the amp, do you know of a source for a bare Twin Reverb chassis?
And fix the HR Deville and sell it.
Pete, reissue is a specific term. There was originally the Blues DeVille and Blues Deluxe, those from about 1993. Then there was the Blues DeVille Reissue in about 2004. Reissue means they took the basic performance and features of some older model and worked up a new version. In recent years, Fender has brought out modern reissues of various old favorites. Many have the original year as part of the name. For instance the "65 Twin Reverb Reissue" came out about 1991.
A number of places sell chassis for the Twin reverb, for example:
Mojo Chassis Blackface Twin Reverb Style Chassis
A number of places sell chassis for the Twin reverb, for example:
Mojo Chassis Blackface Twin Reverb Style Chassis
Here is our box, it is a bit deeper than a real twin:
http://baselaudiolab.com/PTB-FEND2C.jpg
Here with the pair of 8" replaced with Weber 12s:
http://baselaudiolab.com/G_AMP_FRT_BIG.JPG
http://baselaudiolab.com/PTB-FEND2C.jpg
Here with the pair of 8" replaced with Weber 12s:
http://baselaudiolab.com/G_AMP_FRT_BIG.JPG
Soldanos sound great, but a little too much for me. I like a Marshall JCM 800 type backed off some or a little more towards bluesy overdrive rather than full on distortion. Fender's overdrive sounds flabby and not crisp to my ears. I owned a red knob Super 112 and it had the same issues, but it's clean can't be beat just like this amp. Since I fixed the issues with the loose connection in the ribbon cable and R58, it sounds much better. It worked,but didn't sound great. As it heated up, the sound improved. I think early on these developed a high resistance connection and compromised the sound somewhat. Does your amp sound different when run for quite a while? If so, it could be a sign of a partial open. When the components get hot, they expand and the connection can get better as it gets hot. After all, it is an almost 20 year old PCB amp.
Also, in your walkthrough, you commented about the reverb distorting from too hot a signal. I found the same to be true as I never run my reverb past 3. Dime the reverb and it is unusable.
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Thanks for the info. I don't know much about opamps. Generally, what is the way to reduce gain in the TL072 or other generic opamps? Actually, if you don't mind, you could refer specifically to this circuit, pin 5&6 in and 7 out. Reduce R36? Is that the feedback resistor?
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I don't like 15 Vpp swings going into the first reverb OP amp so I would add 1M resistor in series with C13 since it will cut the input level in half (-6dB) due to the load resistor, R32 also being 1M ohm. Then if there is still too much gain, and there probably will be, reduce R36 to 100K even 51K or just lower it until the reverb control is reasonably centered for a lot of reverb.
Non-inverting OP amp voltage gain (AV) = 1 + R36/R37
Take 20 * log of AV to express it in dB
So stock AV = 1 + 220/2.7 = 82.5 or 38.3 dB
With 100K AV = 1 + 100/2.7 = 38.0 or 31.6 dB
and 51K AV = 1 + 51/2.7 = 18.9 or 25.5 dB
Theory about OP amp gain:
http://www.electronics-tutorials.ws/opamp/opamp_3.html
Non-inverting OP amp voltage gain (AV) = 1 + R36/R37
Take 20 * log of AV to express it in dB
So stock AV = 1 + 220/2.7 = 82.5 or 38.3 dB
With 100K AV = 1 + 100/2.7 = 38.0 or 31.6 dB
and 51K AV = 1 + 51/2.7 = 18.9 or 25.5 dB
Theory about OP amp gain:
http://www.electronics-tutorials.ws/opamp/opamp_3.html
WHAT 15v p-p swing? 15v p-p only gets half way to either rail, so no overshoot. Plus 15v p-p means over 5v of signal. You getting that much there?
If you want to tame the reverb other than simply not turning it up as far, why hobble the drive side. All that does is shake the springs. Unless it is driving some unwanted distortion into the springs, I want as hot a drive as possible. The place to reduce it is on the recovery side. Playing with R36 and its friends ought to be all you need to do.
If you want to tame the reverb other than simply not turning it up as far, why hobble the drive side. All that does is shake the springs. Unless it is driving some unwanted distortion into the springs, I want as hot a drive as possible. The place to reduce it is on the recovery side. Playing with R36 and its friends ought to be all you need to do.
We put the tubes in and set it up on the bench, we didn't have a lot of time.
The #1 input jack has no nut on, it came that way and it feels very loose on the circuit board.
Turned it on and we saw 25 mV at the bias test point, very low, we had his guitar plugged into #1 and he started playing and at some point the outputs red plated in a huge way. We both also heard a faint, maybe 10 KHz oscillation that was happening right when we came out of standby.
I thought that perhaps the oscillation might have had something to do with the input jack so we went to #2 and then he mentioned that the clean channel never sounded right. I mentioned how the bright switch is defeated when on drive and that the Master volume is defeated on the clean setting.
I pointed out that when you turn the mid control all the way up the treble control does nothing. The tone controls are very strange but I think that this is normal for the Fender tone stack.
The outputs did not red plate again and we concentrated on the clean channel, he showed me how as you turn up the volume the clean channel crackles, very nasty sounding distortion. We were running out of time and I'm fairly certain that I'll see that distortion with a signal generator and scope. The input jack and the cracking are the main problems so far.
I don't know if we'll see the oscillation again when we go back to the drive channel.
I noticed that R10 was gone but that a small electrolytic had one lead in the spot for R10 and there was a 1.5K resistor across it, so it looks like someone put in a bigger cap on V1B cathode and as expected the bright switch now does not work because of it. It seems to be the same as the mod discussed here:
Loose Red Wire From R17 in Hotrod Deville - Any ideas???? - Telecaster Guitar Forum
The wire off off R17 goes to the top hole where jumper W4 (I think it was W4) went on our amp.
Anyone know about this mod?
The #1 input jack has no nut on, it came that way and it feels very loose on the circuit board.
Turned it on and we saw 25 mV at the bias test point, very low, we had his guitar plugged into #1 and he started playing and at some point the outputs red plated in a huge way. We both also heard a faint, maybe 10 KHz oscillation that was happening right when we came out of standby.
I thought that perhaps the oscillation might have had something to do with the input jack so we went to #2 and then he mentioned that the clean channel never sounded right. I mentioned how the bright switch is defeated when on drive and that the Master volume is defeated on the clean setting.
I pointed out that when you turn the mid control all the way up the treble control does nothing. The tone controls are very strange but I think that this is normal for the Fender tone stack.
The outputs did not red plate again and we concentrated on the clean channel, he showed me how as you turn up the volume the clean channel crackles, very nasty sounding distortion. We were running out of time and I'm fairly certain that I'll see that distortion with a signal generator and scope. The input jack and the cracking are the main problems so far.
I don't know if we'll see the oscillation again when we go back to the drive channel.
I noticed that R10 was gone but that a small electrolytic had one lead in the spot for R10 and there was a 1.5K resistor across it, so it looks like someone put in a bigger cap on V1B cathode and as expected the bright switch now does not work because of it. It seems to be the same as the mod discussed here:
Loose Red Wire From R17 in Hotrod Deville - Any ideas???? - Telecaster Guitar Forum
The wire off off R17 goes to the top hole where jumper W4 (I think it was W4) went on our amp.
Anyone know about this mod?
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This could very well be the source of the crackling. Whoever did the mod probably didn't know how to solder on a PCB which is completely different than turrets. I would take the board off and check the solder joints and maybe even put it back to stock. Fender partially bypassed that stage for tone shaping and this could also be the source of the bad sound, too much low end from increased value of the cap. Then your bright switch will work also. When you saw only 25mV, which test point were you talking about, TP3 or 6?