Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument amps.

Super Reverb AA270 reverb weak
Super Reverb AA270 reverb weak
Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 11th September 2014, 11:13 PM   #1
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
Default Super Reverb AA270 reverb weak

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.

Last edited by cancon; 11th September 2014 at 11:32 PM.
  Reply With Quote
Old 12th September 2014, 04:16 AM   #2
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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?
  Reply With Quote
Old 13th September 2014, 05:25 AM   #3
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
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.
  Reply With Quote
Old 13th September 2014, 05:37 AM   #4
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote
Old 15th September 2014, 11:41 PM   #5
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
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?
  Reply With Quote
Old 16th September 2014, 12:07 AM   #6
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote
Old 16th September 2014, 11:45 PM   #7
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
I appreciate the help, as always.
Should have a replacement by next week. Will update.

Cheers!
  Reply With Quote
Old 16th September 2014, 11:57 PM   #8
Magnetmaz is offline Magnetmaz  United States
diyAudio Member
 
Magnetmaz's Avatar
 
Join Date: Nov 2002
Location: Pittsburgh, PA
Send a message via AIM to Magnetmaz
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*
  Reply With Quote
Old 17th September 2014, 04:23 AM   #9
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote
Old 26th September 2014, 12:01 AM   #10
cancon is offline cancon  Canada
diyAudio Member
 
Join Date: Mar 2011
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!
  Reply With Quote

Reply


Super Reverb AA270 reverb weakHide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Burned up "choke" resistor in old Fender Super Reverb guitylerham Instruments and Amps 11 23rd January 2014 06:31 PM
1966 Super reverb problem child kingoftheblues8 Instruments and Amps 5 26th November 2012 07:05 PM
unusual noise on AB763 Super reverb clone krayon Instruments and Amps 8 13th February 2012 04:43 AM
Fender Twin Reverb VIBRATO PROBLEM - weak vibrato gibsonripper Instruments and Amps 2 19th January 2008 05:23 AM
Fender super twin reverb jazz Instruments and Amps 1 4th September 2007 09:30 PM


New To Site? Need Help?

All times are GMT. The time now is 03:30 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio
Wiki