Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: AxeAmpTN on August 03, 2020, 05:07:53 pm
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I'm sure this has been addressed before, but in all my searching, I can't find it:
'65 Twin Reverb Reissue came into my shop with the reverb not working. The footswitch is good and the connections to the circuit all work. When I turn the reverb up, I can hear hiss on the channel. I can physically manipulate the tank and hear the springs. All this leads me to believe this is an issue with the driver/input to the reverb transformer. I've opened the chassis and examined the components, leads, solder connections, etc. and everything looks good. I've swapped out the tube to eliminate that variable. Still no progress.
So what am I missing?
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what's the volts at TP7 - 10
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Apply dummy-load to main output (or pull the PI tube). Adapt a speaker to the reverb send. Play. It should play like a very-very small guitar amp. If it is silent, why?
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Apply dummy-load to main output (or pull the PI tube). Adapt a speaker to the reverb send. Play. It should play like a very-very small guitar amp. If it is silent, why?
Sure enough, the speaker is silent.
what's the volts at TP7 - 10
DC volts = 0
Okay, so we're hitting on the issue. Looking at the circuit and schematic, it's looking like C13 may be kaput (or have bad connections. I'll try resoldering. PRR & shooter, does this sound like I'm on the right track or am I getting sidetracked?
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DC volts = 0
just running out the door so;
the TP's are AC - signal. I was interested where the signal quit.
when I get back i'll look at schematic again.
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DC volts = 0
just running out the door so;
the TP's are AC - signal. I was interested where the signal quit.
when I get back i'll look at schematic again.
Doh...you're right. I did check that and got about 35 mV if I remember correctly. That was with controls at 5 and no input signal (if I'm understanding the schematic directions correctly).
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Doh...you're right. I did check that and got about 35 mV if I remember correctly. That was with controls at 5 and no input signal (if I'm understanding the schematic directions correctly).
You will not get any ac signal voltage at those test points unless you inject a 37mV 1Khz sine wave at TP1.
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You will not get any ac signal voltage at those test points unless you inject a 37mV 1Khz sine wave at TP1.
Okay. Corrected that. Voltage at TP7 is .334VAC.
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Gotta do more than that. Read the notes at the bottom left of the schematic. Set the knobs accordingly. Now compare your AC voltage readings for TP7, 9, and 10 with the schematic. But first, measure and post the DC voltages for V3 pins 1, 2, and 3.
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Gotta do more than that. Read the notes at the bottom left of the schematic. Set the knobs accordingly. Now compare your AC voltage readings for TP7, 9, and 10 with the schematic. But first, measure and post the DC voltages for V3 pins 1, 2, and 3.
Doublechecked my setup with lower-left instructions.
DC voltages on v3 (no input signal per instructions): 428VDC on pin 1, .003VDC on pin 2, 4VDC on pin 3.
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Gotta do more than that. Read the notes at the bottom left of the schematic. Set the knobs accordingly. Now compare your AC voltage readings for TP7, 9, and 10 with the schematic. But first, measure and post the DC voltages for V3 pins 1, 2, and 3.
Doublechecked my setup with lower-left instructions.
DC voltages on v3 (no input signal per instructions): 428VDC on pin 1, .003VDC on pin 2, 4VDC on pin 3.
AC voltages with 37mV 1kHz input signal: .701VAC at TP7, 28VAC at TP9, .429VAC at TP10.
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4VDC on pin 3.
that's about 1/2 what schematic shows, 7.8v
pull the ECap from cathode, see if it goes UP, NO signal
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V3 and the RT are working OK. You should have had guitar sound when you rigged up a speaker to the reverb send (white plug on schematic). Try that test again. You must be plugged into the VIB channel and set VOL, TRE, MID, and BASS to 10. Be sure to kill the main amp by pulling the PI tube or replacing the main speakers with a dummy load.
If this test passes then you have bad tank or cables.
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4VDC on pin 3.
that's about 1/2 what schematic shows, 7.8v
pull the ECap from cathode, see if it goes UP, NO signal
No change.
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V3 and the RT are working OK. You should have had guitar sound when you rigged up a speaker to the reverb send (white plug on schematic). Try that test again. You must be plugged into the VIB channel and set VOL, TRE, MID, and BASS to 10. Be sure to kill the main amp by pulling the PI tube or replacing the main speakers with a dummy load.
If this test passes then you have bad tank or cables.
Okay, that worked. (Stupid me, I used tried hooking up an RCA plug with jumpers to the speaker instead of just clipping into CP31 and CP30.)
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I would swap out the cables next. Steal a stereo patch cable if need to. If cables don't fix it, then you need a tank.
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You checked the tank IN and OUT? If reversed the tank works about 1/10,000 as good as it should. The side that measures an Ohm or two goes to the RT. The side with hundreds+ ohms to the recovery grid and kill switch.
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Okay, so I've pulled out the tank and all again and started doublechecking everything. I found the in/out reversed, but the tank still won't work. Output side measures open. Pulled off the connection to the output transducer - still open.
So it's looking like the tank itself is the problem. Looks like this whole thing really boils down to making sure I'm not missing details when looking over everything.
Thank you all!