Hi all,
I had a Super Reverb (early 70's) circuit AA270 on the bench with a few problems, I've fixed a bunch but all that remains is weak reverb.
- Tubes are OK
- New filter caps, plate load resistors, cathode resistors, bypass caps
- The voltages on both the reverb drive/recovery circuits are within spec.
- Cabinet vibrations/tapping the reverb tank send spring noises through the speakers so I think the recovery circuit is fine.
- Hooking up a speaker to the driver circuit results in some signal heard, but not a lot.
- 500pf cap, 1M grid leak, cathode resitor/cap are all new.
Which leads me to think the driver circuit is fine, but I'd like to confirm this by knowing what signal strength I should be seeing - what is a good test signal volume (1K sine) at the channel input? What voltage should I be seeing on the Reverb transformer secondary (reverb send)?
Thanks!
edit: reverb transformer measures Pri: 200ohms Sec: 1.5ohm, no leakage between windings or chassis.
I had a Super Reverb (early 70's) circuit AA270 on the bench with a few problems, I've fixed a bunch but all that remains is weak reverb.
- Tubes are OK
- New filter caps, plate load resistors, cathode resistors, bypass caps
- The voltages on both the reverb drive/recovery circuits are within spec.
- Cabinet vibrations/tapping the reverb tank send spring noises through the speakers so I think the recovery circuit is fine.
- Hooking up a speaker to the driver circuit results in some signal heard, but not a lot.
- 500pf cap, 1M grid leak, cathode resitor/cap are all new.
Which leads me to think the driver circuit is fine, but I'd like to confirm this by knowing what signal strength I should be seeing - what is a good test signal volume (1K sine) at the channel input? What voltage should I be seeing on the Reverb transformer secondary (reverb send)?
Thanks!
edit: reverb transformer measures Pri: 200ohms Sec: 1.5ohm, no leakage between windings or chassis.
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Is the reverb pan the proper type? Should be 4AB2C1B (Or 9AB... or 8AB...) Not 4EB...
Are the cables plugged into the correct ends of the pan. The return cable is the one that hums when you touch the tip, it goes into the OUTPUT jack om the pan side.
The pan is mechanically intact inside? All springs present? No transport foam block or anything blocking springs?
Did you test the drive transformer as a transformer? I don't offhand know what resistance they need to be, but one shorted turn can kill the levels and not show up on a meter. Go over to RG Keen's Geofex web site and find the transformer tester. A very simple thing made with a battery and a small neon lamp, it will detect shorted turns.
In my shop if I suspected the little tranny, I would sub it. If I had a reverb transformer, I;d tack it in in place of the original and see. But as a functional test, most any transformer would work. Like a single ended one for a Champ or something. And even if I lacked that, using a small push pull tranny, just use half the primary, and connect that to the pan. Any difference?
Are the cables plugged into the correct ends of the pan. The return cable is the one that hums when you touch the tip, it goes into the OUTPUT jack om the pan side.
The pan is mechanically intact inside? All springs present? No transport foam block or anything blocking springs?
Did you test the drive transformer as a transformer? I don't offhand know what resistance they need to be, but one shorted turn can kill the levels and not show up on a meter. Go over to RG Keen's Geofex web site and find the transformer tester. A very simple thing made with a battery and a small neon lamp, it will detect shorted turns.
In my shop if I suspected the little tranny, I would sub it. If I had a reverb transformer, I;d tack it in in place of the original and see. But as a functional test, most any transformer would work. Like a single ended one for a Champ or something. And even if I lacked that, using a small push pull tranny, just use half the primary, and connect that to the pan. Any difference?
Hi Enzo,
This is a new symptom - the reverb always played fine.
The tank is good, springs intact, cables swapped, tested with pan removed. The recovery circuit definitely works.
All that's left is the transformer or the input electronics of the pan.
Interesting transformer tester...I haven't built it yet but I'm wondering how this thing works? I thought you needed almost 100v to achieve striking voltage inside those bulbs. I tried just running that circuit with a 9V and couldn't get the lamp to work.
This is a new symptom - the reverb always played fine.
The tank is good, springs intact, cables swapped, tested with pan removed. The recovery circuit definitely works.
All that's left is the transformer or the input electronics of the pan.
Interesting transformer tester...I haven't built it yet but I'm wondering how this thing works? I thought you needed almost 100v to achieve striking voltage inside those bulbs. I tried just running that circuit with a 9V and couldn't get the lamp to work.
That is how it works, just like the 12v ignition on a car makes thousands of volts across a spark plug. When you energize a winding with the battery, then break the connection, the inductance of the winding tries to keep the current flowing, and that creates a voltage kick. That voltage spike does reach the turn on threshold for the neon lamp and so it flashes.
But when ther is a shorted turn on the thing, it dampens the action, and the spike that lights the bulb won't happen.
try it on a good transformer, you must have some sort of transformer around.
But when ther is a shorted turn on the thing, it dampens the action, and the spike that lights the bulb won't happen.
try it on a good transformer, you must have some sort of transformer around.
Nice!! This is a cool trick. I found a 120-30v PT in the heap-o-stuff which tested with a nice bright flash. I get no flashing at all with either side of the reverb transformer, all taps unsoldered. Does it seem strange for both coils to go at once? This amp was previously worked on plus the reverb transformer is not stock.
Something weird happened - I performed this test yesterday with a 9V battery. Today the batter was physically hot and dropped down to 6+ volts. Is this a side effect of this tester or a coincidence?
Something weird happened - I performed this test yesterday with a 9V battery. Today the batter was physically hot and dropped down to 6+ volts. Is this a side effect of this tester or a coincidence?
The battery should only be "ON" momentarily. The low DC resistance of the transformer will short the battery and it will indeed get hot. The battery only needs to be engaged long enough to see the blink... or not.
Think about this: If you take a regular power transformer and short out the primary, the fuses blow, right? Now if we instead short out the secondary, what do you know, the fuses also blow.
If one winding is shorted in a transformer, then the other windings will be affected too. That tester tells you something is shorted in the windings. It doesn;t tell you where in the windings. Once you have determined the tranny has a problem, it doesn;t much matter wher inside.
Think about this: If you take a regular power transformer and short out the primary, the fuses blow, right? Now if we instead short out the secondary, what do you know, the fuses also blow.
If one winding is shorted in a transformer, then the other windings will be affected too. That tester tells you something is shorted in the windings. It doesn;t tell you where in the windings. Once you have determined the tranny has a problem, it doesn;t much matter wher inside.
i hand built a Twin Reverb once... and everything was awesome on it, except a REALLY WEAK reverb. Like, you had to crank it to 10 and then it would start to feed back internally.... no good.
One day i had the reverb pan off and noticed the foam shipment blocks which immobilize the springs in transit were still inside the pan.
*smacks head*
One day i had the reverb pan off and noticed the foam shipment blocks which immobilize the springs in transit were still inside the pan.
*smacks head*
Guilty here too.
On the other hand, I do warranty work for Fender and others, and some of the easiest repairs I have done were store stock new from box, no reverb. Et voila, remove the foam block, charge half an hour, send it back to stock.
On the other hand, I do warranty work for Fender and others, and some of the easiest repairs I have done were store stock new from box, no reverb. Et voila, remove the foam block, charge half an hour, send it back to stock.
Conclusion: faulty reverb OT.
For those interested, the measured resistances on the new replacement are pri: 1K, sec: 1.3 ohms. I was getting something like ~200ohm on the pri, and the neon bulb test confirmed it!
For those interested, the measured resistances on the new replacement are pri: 1K, sec: 1.3 ohms. I was getting something like ~200ohm on the pri, and the neon bulb test confirmed it!
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