Magnatone M15 reverb circuit problem.

[dragonworks]

I am not very good at this and need help. First off, on the schematic, which is the in and out to the tank, the 12AU7 or the 7025. When I hook a scope up to the 2.5k 10W resistor in line with the plate of the 12AU7 I can see a good strong signal but the depth control has no effect on it. Same when I hook up to the output across the 1UF 450 coupling cap. When I hook up to pin one and two on the 12AU7 I see a clean strong signal which the depth control doesn't affect. When I hook up to the 100pf cap across the 2.2M resistor  on the other side I see a smaller signal that is being affected by the  depth control. I have tried three different tanks, two of them brand new. When I smack them I can hear the reverb very slightly. Before this there was lots of verb but it would feedback if turned up. All new tubes. I have also substituted tubes. One of the reverb tanks is an new accutronics that is supposed to be the direct replacement.Any help is greatly appreciated. The schematic should be attached. Not the clearest, if anyone has any questions about components I have clearer drawings so I could probably answer. The "A" in circle designates channel one, the "A" in a square designates channel 2. I have yet to figure out what the "F" in a square is?Thanx in advanceJ.D.

[sluckey]

Last things first... The circle letters (A-L) are all for CH 1 vibrato board. They represent wires that connect the vib board to the rest of the amp. The square letters (A-L) are all for CH 2 vibrato board. They represent wires that connect the vib board to the rest of the amp. There is a table that shows wire color for each letter.

All of those strong signals you mentioned are in the reverb driver circuit and will not be affected by the depth control. This is normal.

This cap coupled reverb driver requires a tank with a high input impedance A typical fender transformer driven tank will not work. What is the model number of the accutronics tank you have?

On the schematic the left side of the tank is the input and the right side is the output.

Here's a clearer schematic...

     https://el34world.com/charts/Schematics/files/Magnatone/Magnatone_m15a_early.jpg

[dragonworks]

I checked the signal on both plates and both grids of each tube and the depth control did not seem to affect any of them. The depth control affects the signal on one side of the 100pf cap across the 2.2 M resistor that runs to the "A" in the square on the schematic (channel 2), but seems to do nothing on the other side, the side connecting to the "A".  If you look at the schematic just above the color chart is states "A" in a circle is channel one, "A" in a square is channel two. Does it also relate to the color code in the wire chart? I will double check when I get back downstairs and look at it again. The reverb tank was working. I ordered the accutronics described as a replacement for the original tank in there. The tank in there was definitely working not long ago, but as stated, it would feed back if the depth control was turned up. How can I measure the impedance of a tank? I can hear a slight bit of reverb when I smack the tanks.Thanks for the help.J.D.

[sluckey]

Study the schematic. I know it's complicated. The reverb only goes to the CH2 amp/speaker. Reverb is not sent to the CH1 amp/speaker.

Again, what model is your accutronics tank?

[dragonworks]

I will have to check the tank. I bought it quite awhile ago. I still have the original that was working. I was getting reverb on both channels. I will look at the tank tomorrow, it will be a busy day.

[sluckey]

Both preamps are fed into the reverb so in that context you have reverb on both channels. But the output of the reverb circuit only feeds CH 2 power amp. If you disconnect CH2 speaker you will lose all reverb.

[dragonworks]

The replacement tank I was sent has an elliptical black logo with white uppercase letters MOD in it.the model number is P-RMOD-4FB3D1B
Here is one on reverb. Says it is for fender, ampeg, marshall.
https://reverb.com/item/1411439-mod-p-rmod-4ab3c1b-spring-reverb-tank-black

                                 
                     

[sluckey]

4FB3D1B is the number I was looking for. That tank should work with that Maggie circuit.

[dragonworks]

Reading your responses I get the feeling you think it is the tank? I was reading at another site about a gent with the exact same problem. Turned out to be a ground on the tank?  I was going to post a link to it here but now I cannot find it. I was reading the original tanks were about 175 ohms in and out? I think the one I have is like 1470 in? If I find the other thread again I will post a link to it here. Like I said, I can hear a faint bit of verb through the original and the accutronics tank. I rebuilt the power supply, replaced all electrolitic caps and some others. This is the last problem, it is frustrating when you don't have much experience.

[Lectroid]

dragonworks,

Here's a link to a useful document if you don't already have it:

https://www.amplifiedparts.com/tech-articles/spring-reverb-tanks-explained-and-compared

Every number and letter in the 4FB3D1B number has a meaning and this doc will give you all of the technical specs, including what the "D" means. (hint)

Your tank is probably okay.  It has a nominal input impedance of 1475 ohms, (refer to that doc) but if you measure the input resistance with a DVM you should get a reading in the 175 - 200 ohm range for a good tank.

[sluckey]

Reading your responses I get the feeling you think it is the tank?

No, I just wanted to verify that you were using the correct tank.

[dragonworks]

Found the thread I was looking at here on the site. https://www.thegearpage.net/board/index.php?threads/measuring-reverb-tank-impedance.568658/ I will have to look into this I guess.Thanx for everyones interest and help.J.D. This link doesn't seem to work? The title of the thread was Measuring reverb tank impedance started by Leonc.
Here was the fix:Problem solved, thanks to help from Juke Amps' Gary C!
 
 This was indeed the correct reverb tank....but it is a bit different from the old tank in my M10 (and apparently, the original tank in the M13). Turns out that the sleeve of the input jack on the old tank was grounded but the input jack on the new tank was isolated and no grounded. Consequently, I had to ground the shielding from the input cable.
One of my connections to the tank is not grounded, the one from the 2.2M 10 watt resistor. But the tank worked before. I have been in and out of this amp quite a bit and could have broken the ground without noticing, but I have two of these and the other one is wired the same way.

EDIT by sluckey... Fixed boogered link

[dragonworks]

It is time to bring it to a real tech who can figure it out..J.D.

[dragonworks]

Thanx for fixing the boogered link. Never heard it referenced that way, made me laugh.

[sluckey]

Thanx for fixing the boogered link. Never heard it referenced that way, made me laugh.

That's a deep south country boy term. But you got it. May not be globally recognized. 

[dragonworks]

I just moved from upstate NY around Woodstock to the Tenn. Va. border near Kingsport. I am hearing lots of colloquialisms I have never heard before, and sometimes I get to hit back with some of my own like noisier than a skeleton jerking off on a tin roof.

[dragonworks]

I am getting a lot of help on another board but still have not found the problem. I am beginning to think it is just me!

[dragonworks]

I have reverb now and too much of it. As I turn it up she starts to feed back.

[dragonworks]

In response to the gent who was helping me at the other site when asked what I did. He did say the feedback was probably from improper mounting and probably acoustic feedback from vibrations:The tank was not mounted, just sitting on top of the amp. Now for the juicy details. I probably went all through this for nothing. I replaced both connectors, the .01uf @400v cap and the 270K resistor on pin 1 of the 7025. Before I had done that, I needed the room in the one side of the basement, so I moved the amp to the other side. Before I had the cabinet facing down. Now I have the cabinet facing up. I noticed one of the speakers was not working. I started checking. There is a knob for a switch for internal or external speakers. That knob was missing. The knob for the on and off/polarity switch keeps falling off when the amp is upside down.  I didn't realize it was set to external speakers. As soon as I set it for internal speakers both speakers started working and the reverb kicked in. Now, I have to look and see why once speaker cuts out when it is switched to external speakers. I am guessing the reverb cuts out if that speaker is not working since the return only goes through one channel. One other thing, when switched to internal speakers, there is considerably more hum in the amp. At one time while changing electrolytics ect., while I had the amp upside down the knob probably fell off, I must have put it back on the internal/external switch and switched  the external speakers on and much of the hum went away and I probably thought I had turned the polarity switch  which is right next to it. I don't really know. This is a weird amp if you read the literature. I don't know if that is normal or not. I will also have to look at the schematic and see if I can figure out if that is the normal operation of the amp or if there is something wrong with the internal/external switch. Why would the amp only work with one speaker and the reverb cut out when switched to external? If I were within driving distance of you, I would come over and let you kick me in the ass. The reverb was feeding back before all of this with the original tank anchored in the cab. I was using the accutronics reverb which I had sitting on top of the amp. I hooked up my "Ruby" tank which is about half the physical length of the accutronics an it doesn't feed back but doesn't sound as good. The hum that is introduced when turning on the internal speakers is unacceptable. It may be just that the hum that is driving the reverb into feeding back? We live and we learn or we get electrocuted. I am pretty sure that is how I am going to go at this rate, but I could think of worse ways.
While working with the cabinet upside down, although it didn't have far to go, the thought of both speakers not working did cross my mind, but I couldn't really tell because the cabinet was upside down. In the heat of the moment I forgot about that and didn't catch it until I moved everything and had the cabinet facing up. Then I thought, let me check and see if both speakers are working, doh. After looking at the schematic there must be something wrong with that switch (rotary knob), it appears that both speakers should still be working with the external speakers turned on.