|mtlbasslad||4/15/2018 4:11 PM|
|This Week on the Bench - Peavey Vypyr 60 AND Fender Dual Showman 'Red Knob"|
Greetings, solder addicts,
This should probably be 2 threads, so I'll get to the Peavey later...
The Fender came in with a 'no reverb' complaint, here is what I've checked -
Accutronics 4AB3C1B tank - input 1 ohm, output 200 ohms, cables are good (& in the right place I think, white input red output)
Ran a 1000hz 3mv signal in, crazy loud 'sproing' if I tap the tank, so output seems good.
Checked the 12ax7 & 12at7 which are next to reverb trans, OK
Checked reverb trans - black/green secondary reads 1.2 ohms, blue/red primary is 1.065K (spec I read somewhere here said 1.9K... ?
Checked output AC voltage at input to tank - ziltch -
Interesting arrangement on the 12at7 - pins 1&6 are tied, as are 2&7, 3&8 and 4&5... so plates, grids, cathodes & heater (according to the 1953 GE data sheet)
Does that mean the 2 triodes are working together?
What do I look for now?
P.S. found the main board attached by only 2 of 6 original screws, one of which was broken off & stuck in the chassis (gorilla must have over-tourqed it). A little Dremel, drill & 6/32 tap and board is now as Leo would have liked it. I'm better at mechanics than electronics - but you guys already know that.
Waiting on your wisdom,
|mtlbasslad||4/15/2018 5:24 PM|
|52 Bill||4/15/2018 5:26 PM|
Test the reverb transformer with the tester at R.G. Keen's site. Resistance readings will not find shorted turns.
The two halves of the 12AT7 are wired in parallel, so they are both working together to be able to drive the reverb transformer. If you plug a small 8 ohm speaker into the reverb drive jack you should hear the guitar signal coming out of it if the drive circuitry is working correctly.
|g1||4/15/2018 6:17 PM|
|That schematic is for your other Peavey thread. |
Try the speaker trick like 52Bill mentioned for the reverb driver TX. Sounds like the recovery side is good as you heard the springs crash.
If the reverb TX is ok, one thing about these is that reverb assign switch on the front panel. Make sure signal is getting through it.
|mtlbasslad||4/15/2018 6:30 PM|
|Oops for the schematic post. |
I could try the speaker trick, but I'm not getting any AC output reading on the 'ol Fluke, so...
The R G Keen trick would be interesting too - if I had the neon bulb - am I not reading the meter correctly?
Forgot to mention I did check that reverb assign switch - ch1, ch2 & both.
Is not the low resistance reading on the trans telling?
|Enzo||4/15/2018 7:03 PM|
|My advice to my trainees: never think up reasons not to check something.|
WHen people imagine a transformer with shorted turns, they have some image of a dead short between the wires. But the reality is a bad transformer usually shorts two adjacent turns together. That means if a good transformer reads 1000 ohms, the shorted one reads 999.5 ohms. Your meter would never detect that. Also on the secondary side, it is a low turn count winding which will naturally have low impedance and low DC resistance as well. SO again we are trying to detect a difference of 0.003 ohms. it is like trying to measure the gauge of your guitar strings with a yard stick.
The classic Fender reverb drive is just a small power amp. It uses the two paralleled halves of the 12AT7 as the power tube, and the transformer matches the higher tube impedance to the low reverb impedance. The suggestion of connecting the drive cable to a small speaker is based on the circuit being a small power amp.
Apply a signal to the amp input, look for signal at the reverb drive. Is ther signal at the 12AT7 grid? at the plate? Is there B+ at the plate? Is there some reasonable DC on the cathodes?
Always remember your meter measures resistance, not impedance.
|mtlbasslad||4/15/2018 7:20 PM|
|Hi Enzo, |
I have run a signal into the amp & measured 0 VAC at the trans output - I would poke the meter around the 12AT7 if I knew what to look for...
DC B+ at the plate?
AC signal at grid & plate?
What is reasonable DC on the cathodes?
All measurements referenced to ground I guess?
Remember, I have some nice tools but not a lot of experience... at least I deduced the paralleled 12AT7 thing (patting myself on the back LOL)
|Enzo||4/16/2018 12:34 AM|
|The plates are pins 1 and 6 of the 12AT7, I want to see something like 300volts DC there. Not zero. What does the schematic say? Looks like 440v. So that. Cathodes want to have not zero, a couple volts or more. Schematic say 6v.|
|Jazz P Bass||4/16/2018 12:39 AM|
|Here is a crop of the Twin Reverb schematic that shows test point voltages.|
Voltages in an oval are Vac.
In a rectangle, they are Vdc.
If you have a decent signal voltage at the 12AT7 plates and nothing at the reverb secondary, then the transformer is bad.
|mtlbasslad||4/16/2018 3:56 PM|
|@ Enzo - I've got 350Vdc at the plates, 16Vdc at the cathodes...|
@ Jazz P Bass - thanks for that schematic, tried to get the full one from one of your old posts but it came up blank?
The partial you posted says 67Vac at the cathodes (pin 1 & 6)... colour me confused - I should be looking for 2 different voltages there, ac & dc
@Everbody - does this amp need the reverb footswitch need to plugged in or is default (no switch) reverb on? Because the owner sent me a mono cord, not TRS as required. Lets see if I can scrounge a headphone cord & a couple of 1/8 to 1/4 adapters around the house...
Really showing off my ignorance here, I know, but every night I go to sleep less stupid...
|g1||4/16/2018 4:17 PM|
|Default condition should be reverb on with no footswitch.|
The AC voltages shown in the ovals are conditional on the AC signal applied to the input. See notes in lower left of schematic. DC voltages are shown in square boxes rather than ovals.
I think the Red Knob "The Twin" schematic should be very close, the red knob dual showman schem I have does not have reverb.
|mtlbasslad||4/16/2018 4:41 PM|
|Thanks g1, this looks like the schematic the owner brought - but this unit has 2-button footswitch. I find both almost impossible to read (old eyes don't help)...|
The one Jazz P Bass posted a partial of is much clearer.
If default is reverb on then I still have a problem.
Although there have been opinions about the resistance of the trans, no one has commented on the low primary resistance I measured - 1.065K not spec 1.9K ??
Another thing, where can I get a Hammond retail in Canada - they make them here after all...
|g1||4/16/2018 4:45 PM|
|I don't think you need to worry about the reverb tx until you verify there is signal getting to the reverb driver tube grids.|
I re-loaded that schematic with a reverb version.
The one JazzP posted is for the 65 re-issue I believe.
|g1||4/16/2018 6:24 PM|
As far as Hammond distributors in Quebec, see here: Hammond Mfg. - Electronic Distributors - Canada
One thing that does bother me a bit, you said you have 16VDC at the reverb driver tube cathode. That is a lot. Does the resistor there still measure 1K2 ? Have you tried a different 12AT7 there?
But back to previous post, you need to verify AC signal is getting to grid of reverb drive tube.
|mtlbasslad||4/16/2018 6:40 PM|
|Thanks again g1,|
Now that I know where to look for signal (pin 2&7) I will... & check R?
Each new schematic has something different, I have 3 now, some with more & some with less info...
I just found a new (3 yrs) online site in Canada called Next Gen in Nepean Ont - & they have the Hammond for $20CDN, also all the bits I need for the Peavey repair YAY
Going to brave wicked ice storm & get back to the shop...
|mtlbasslad||4/17/2018 10:59 AM|
|OK it's either bad technique or bad eyes (or both)...|
Now I have a signal at the grid that varies as I adjust gain, and through the cables to the tank input - so trans is good I guess..
And the tank tests good - input 1 ohm output 200 ohm - and lots of sproing when I tap the tank, so input transducer is toast even if it tests good?
Still have 16Vdc at cathode of 12AT7, resistance from pin 3-8 to gnd is 1K2...
|g1||4/17/2018 1:38 PM|
|Can you measure the AC voltage at the plates of reverb driver?|
Compare to example from 65 Twin in post #9, should be similar. Try to set up so you have test signal of 2V AC at the grids, should give ballpark 67V AC at the plates.
And have you tried another AT7 for the driver tube? Still 16VDC at cathode? (you can swap in the PI tube to test)