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These days, the best I get for clues on gear that gets routed into my shop from across the aisle of our inventory warehouse is a gaffer's tape label with a simple comment, like "Loud Hum when switched on from Standby", or "Fuzzy Pots", etc. This one in question simply stated "Sounds like Blown Speaker?"
I normally start by moving the amp up to the Check-out bench, plug it into the Power Analyzer, fed by a variac, and power it up. Switching out of S/B mode, with no signal connected, volume controls at full CCW, tone controls mid-point, Reverb and Vibrato or other effects off. A quick look at the power analyzer will tell you if you're in the 'nominal' range of power draw/current draw from the mains. Turning up the volume controls one at a time will tell you much.....if you have dirty pots, that normally is revealed, and how severe.....exercising the pots, racking them from CW to CCW quickly will reveal much. With the Volume pots at Full CW, then racking the tone controls will also reveal noisy pots. Same with the Reverb pot. There's usually enough 'white noise' with the Vibrato channel turned up to listen to the Tremolo function...Intensity and Speed, and if those work between the range of the controls ok.
I'll usually hear noisy tubes during this initial check-out stage. One amp before this one was up for service, the amp had Loud Hum as soon as I switched out of Standby. Unplugged the Driver Tube and tried again, Loud Hum gone. Plugged that back in, and removed V4 channel mixer tube, and Loud Hum still there. So, restored V4, and went to fetch a fresh 12AT7 driver tube, and the Loud Hum was gone. It had relatively fresh matched 6V6 power tubes installed. I tapped on both tubes, and wiggled them a bit in their sockets (Kevlar glove on my hand), and all was well there. No tell-tale signs of hum in the residual, so I shut down, fetched my bias probes just to have a look at the plate current level/balance. That looked nominal.
So, that 'repair' only took 15 minutes.
This one, with the 'blown speaker?' had three noisy pots, so I pulled the chassis out. Getting the chassis out of Re-issue Deluxe Reverb cabinets isn't always a simple exercise, depending on the roof-top foil inside the cabinet, as well as the mechanical fit of the baffle and width of the chassis inside dimensions, as well as the condition of the Tolex flaps folded and glued down on the inside cabinet walls. This chassis took some effort to coax out of the cabinet, and brought a sample of the roof foil with it as a prize!
I moved the chassis to the bench, pulled the front panel PCB assy and injected spray into the three noisy pots, this time NOT removing the pots to be more aggressive with that cleaning process. The fuse holder was loose (a traditional problem with late generation re-issue Deluxe and Twin Reverb amps and their cousins), so unplugged the mains wires to the fuse holder, removed it, and installed a pair of thin ITL washers..one outside, one inside, and spun the plastic mounting nut back into place and secured it with my 9/16" open end wrench. Plugged the Mains wires back onto it.
While I had the chassis on the test stand, I dressed the Power Xfmr secondary leads from how Fender installs them....twisting the pairs separately, then the pairs together, re-connecting as they came out, with adding the primary leads twisted together and around that bundle, usually eliminating the need for tie wraps as were in place before.
This amp didn't have a foot pedal, but, I was hearing the Vibrato function on. I fetched my TRS shorting plug used to activate Tremolo and Reverb without a pedal, and that made no difference, so it wasn't a jack issue. I found a small jumper between the Vibrato terminal and jack ground installed. That's why the Tremolo was on. Clipped that off, so a pedal IS again required to get the Tremolo to work. Removed that 'noise source' from the amp's residual
Then,drove the amp with Burst Pink Noise, after I had moved the chassis back into the cabinet. During that check-out, the output level abruptly increased a good 10dB or more, and sounded distorted. Shut it down, and removed the chassis to the test bench again. THAT must have been what the note was about.
With the amp on the test stand again, I drove it with both the pink noise source, as well as sine wave source, using 8 ohm dummy load and watched for a while under both conditions to see if I got any abrupt changes. No change yet.
Then, this morning, I fired up my B & K 1022 BFO Generator, as it can directly drive speakers...and set it for Warbled Sine....1Hz modulation, 10Hz Deviation, and tuned the dial for 30Hz, so it modulates from 25 Hz to 35Hz, and cranked it up to 6V RMS (max on the generator on that 6 ohm impedance-match setting). No change yet.
I removed the speaker from the cabinet to have a look at the voice coil wires. I found the fed thru an insulated rivet to the terminals, and terminated in a crimped U-shaped terminal, but no solder applied. I turned on my soldering iron, and applied solder to those joints at the crimped terminals. With my Fluke 8060A connected across the speaker terminals, reading 6.04 ohms DCR, I pushed on the cone watching the reading change from 6 ohms to over 100 ohms momentarily, in both polarities, before returning to nominal DCR when the cone is at rest or stopped at a new position. This takes you back to the first year of Electronics, where the instructor has a large DC Current meter, needle resting in the middle of the range, with a coil of wire wrapped around an insulated cylinder. Inserting a bar magnet into the cylinder, the current meter needle swings in the direction of the inserted magnet while moving it. If left in the new position, it returns to mid-point, as there's no change in magnetic flux until the magnet is moved again.
After listening to the speaker driven from the generator, I changed the setup, routing the generator to the Yamaha power amp sitting below the generator, and connected the output of the amp to the speaker, giving me a bit more drive current, and changed the Frequency to 40Hz, and deviation frequency to 16Hz (32Hz to 52Hz), and increased the drive voltage to 9V RMS. That's close to the max out of a Deluxe (around 11V RMS).
So far, no abrupt change in output.
So, I haven't yet found WHAT caused that abrupt change I heard yesterday. It will show up again if I haven't cured it by the soldering of the leads. The speaker's dust cover in the center covers the voice coil leads passing thru from the back of the cone. Both lead wires were snug where they exit the cone, and plenty of lead so nothing is under tension.
So, it's still fun in the physics lab for now.
I normally start by moving the amp up to the Check-out bench, plug it into the Power Analyzer, fed by a variac, and power it up. Switching out of S/B mode, with no signal connected, volume controls at full CCW, tone controls mid-point, Reverb and Vibrato or other effects off. A quick look at the power analyzer will tell you if you're in the 'nominal' range of power draw/current draw from the mains. Turning up the volume controls one at a time will tell you much.....if you have dirty pots, that normally is revealed, and how severe.....exercising the pots, racking them from CW to CCW quickly will reveal much. With the Volume pots at Full CW, then racking the tone controls will also reveal noisy pots. Same with the Reverb pot. There's usually enough 'white noise' with the Vibrato channel turned up to listen to the Tremolo function...Intensity and Speed, and if those work between the range of the controls ok.
I'll usually hear noisy tubes during this initial check-out stage. One amp before this one was up for service, the amp had Loud Hum as soon as I switched out of Standby. Unplugged the Driver Tube and tried again, Loud Hum gone. Plugged that back in, and removed V4 channel mixer tube, and Loud Hum still there. So, restored V4, and went to fetch a fresh 12AT7 driver tube, and the Loud Hum was gone. It had relatively fresh matched 6V6 power tubes installed. I tapped on both tubes, and wiggled them a bit in their sockets (Kevlar glove on my hand), and all was well there. No tell-tale signs of hum in the residual, so I shut down, fetched my bias probes just to have a look at the plate current level/balance. That looked nominal.
So, that 'repair' only took 15 minutes.
This one, with the 'blown speaker?' had three noisy pots, so I pulled the chassis out. Getting the chassis out of Re-issue Deluxe Reverb cabinets isn't always a simple exercise, depending on the roof-top foil inside the cabinet, as well as the mechanical fit of the baffle and width of the chassis inside dimensions, as well as the condition of the Tolex flaps folded and glued down on the inside cabinet walls. This chassis took some effort to coax out of the cabinet, and brought a sample of the roof foil with it as a prize!
I moved the chassis to the bench, pulled the front panel PCB assy and injected spray into the three noisy pots, this time NOT removing the pots to be more aggressive with that cleaning process. The fuse holder was loose (a traditional problem with late generation re-issue Deluxe and Twin Reverb amps and their cousins), so unplugged the mains wires to the fuse holder, removed it, and installed a pair of thin ITL washers..one outside, one inside, and spun the plastic mounting nut back into place and secured it with my 9/16" open end wrench. Plugged the Mains wires back onto it.
While I had the chassis on the test stand, I dressed the Power Xfmr secondary leads from how Fender installs them....twisting the pairs separately, then the pairs together, re-connecting as they came out, with adding the primary leads twisted together and around that bundle, usually eliminating the need for tie wraps as were in place before.
This amp didn't have a foot pedal, but, I was hearing the Vibrato function on. I fetched my TRS shorting plug used to activate Tremolo and Reverb without a pedal, and that made no difference, so it wasn't a jack issue. I found a small jumper between the Vibrato terminal and jack ground installed. That's why the Tremolo was on. Clipped that off, so a pedal IS again required to get the Tremolo to work. Removed that 'noise source' from the amp's residual
Then,drove the amp with Burst Pink Noise, after I had moved the chassis back into the cabinet. During that check-out, the output level abruptly increased a good 10dB or more, and sounded distorted. Shut it down, and removed the chassis to the test bench again. THAT must have been what the note was about.
With the amp on the test stand again, I drove it with both the pink noise source, as well as sine wave source, using 8 ohm dummy load and watched for a while under both conditions to see if I got any abrupt changes. No change yet.
Then, this morning, I fired up my B & K 1022 BFO Generator, as it can directly drive speakers...and set it for Warbled Sine....1Hz modulation, 10Hz Deviation, and tuned the dial for 30Hz, so it modulates from 25 Hz to 35Hz, and cranked it up to 6V RMS (max on the generator on that 6 ohm impedance-match setting). No change yet.
I removed the speaker from the cabinet to have a look at the voice coil wires. I found the fed thru an insulated rivet to the terminals, and terminated in a crimped U-shaped terminal, but no solder applied. I turned on my soldering iron, and applied solder to those joints at the crimped terminals. With my Fluke 8060A connected across the speaker terminals, reading 6.04 ohms DCR, I pushed on the cone watching the reading change from 6 ohms to over 100 ohms momentarily, in both polarities, before returning to nominal DCR when the cone is at rest or stopped at a new position. This takes you back to the first year of Electronics, where the instructor has a large DC Current meter, needle resting in the middle of the range, with a coil of wire wrapped around an insulated cylinder. Inserting a bar magnet into the cylinder, the current meter needle swings in the direction of the inserted magnet while moving it. If left in the new position, it returns to mid-point, as there's no change in magnetic flux until the magnet is moved again.
After listening to the speaker driven from the generator, I changed the setup, routing the generator to the Yamaha power amp sitting below the generator, and connected the output of the amp to the speaker, giving me a bit more drive current, and changed the Frequency to 40Hz, and deviation frequency to 16Hz (32Hz to 52Hz), and increased the drive voltage to 9V RMS. That's close to the max out of a Deluxe (around 11V RMS).
So far, no abrupt change in output.
So, I haven't yet found WHAT caused that abrupt change I heard yesterday. It will show up again if I haven't cured it by the soldering of the leads. The speaker's dust cover in the center covers the voice coil leads passing thru from the back of the cone. Both lead wires were snug where they exit the cone, and plenty of lead so nothing is under tension.
So, it's still fun in the physics lab for now.
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