Mark III - reverb troubleshooting help

[Tuna141]

Hey guys,

I'm not having any luck troubleshooting the non-working reverb on my newly acquired Mark III blue stripe and hope you can give me a few more ideas. Here's what I know so far:

- reverb itself works (worked fine when connected to my Mark IV)

- the reverb cables and connections are OK (making good contact with phono jacks, measure 200ohms between them, grounds are good)

- I hear the springs through the speaker when I bang the reverb tank, so the ouput of the reverb and circuitry seems fine. So, my problem must be with the circuitry feeding the reverb.

- tried swapping V4 (reverb tube) and it didn't make a difference.

Any additional trouble shooting ideas/tips? Does anyone have a schematic?

Thanks in advance.

 

[Boogiebabies]

Yes, there's a small input transformer inside the tank. When you have spring noise with no input the output reverb tank transformer uis still sending the spring noise to the to the Reverb trannie. Open the tank and see if the RCA jack connector wires are still soldered to the mini transformer.

 

[Tuna141]

Thanks for the reply BB.

I guess I should have added that I opend up the reverb and everyhting looks good - all internal wires connected (including wires to transformer), contacts clean & making good contact with RCA cable, etc.

I also plugged the Mark III's reverb into my Mark IV and it worked fine. The cables also appear to be OK. Therefore, unless there is some kind of strange intermittent connection, the problem is most likely related to the circuitry driving the reverb.

Could a messed-up reverb footswitch jack or related circuitry cause no reverb? Any other ideas?

 

[Boogiebabies]

A schematic would be useless. It does not list any of the functions, but may help you in mapping out the V4. I was thinking the 1/4 inch reverb footswitch jack may be bent, I would check ot out, but it may be an unlikely since a 1/4 cable should short even a large gap. It may have corrosion or is severely bent. Follow the two gray wires off the reverb pot and see how the cable goes in and check for continuity. I would also check the reverb pots sweep just to see if you have a severe drop in resistance, especially early on the sweep.

It would also be useful if you had a Mesa EQ/REV footswitch to see if the EQ kicks in.

 

[Tuna141]

The reverb switch jack looked OK to me, but I wasn't sure if it included some kind of "switch connection" (i.e. when a cable is plugged in it breaks a portion of the reverb circuit or something like that). I cleaned the contacts and bent them a little more to give good contact with the cable when plugged in.

I'll rig up a little test cable to try to footswitch the reverb using the footswitch jack, to see if anything happens. I'll also check the pot resistance to see if that acts strangly.

I'm going ot be adding the R2 volume mod soon, and will mess around with the reverb again then. I'll let you know how I make out.

Thanks.

 

[Boogiebabies]

This is strange.

Just for giggles, try checking for cold solder joints on the reverb input where it jumps to the main circuit board.

 

[Tuna141]

I already checked continuity from the tip of the cable through the jack and through the traces to the transformer - basically as far as I could follow the signal. It was fine.

I actually just got off the phone with Mesa because I wanted to order a 250K pot for the reverb mod and some rubber feet which my amp was missing. The salesman wasn't sure of the pot's part number, so he transfered me to Mike Bendinell. He knew the part number of the pot off the top of his head (no surprise there :lol: ). I figured I'd take advantage of the situation and ask him if he had any ideas regarding my reverb problem. As usual he was very personable and helpful - Mesa's customer service is the best and he's a great guy (as you already know). He was talking so fast, I had trouble writing evrerything down quick enough. Hopfully I got most of it right

Mike suggested: 1. with a clean low gain sine wave running, probe the grid of V4 (pin#2) with and without the reverb connected. I should see about a 5% change. Must be due to the load fo the reverb.
2. check the contiuity from the tip of the white cable to the sleeve/shiled of the gray cable. It should measure 1-2 ohms. 3. Check the grounding of the reverb internally (someone may have changed the reverb and put in the wrong type, which has a different grounding scheme)

I'll try this stuff after my parts arrive and I'm installing the R2 volume mod.

 

[Boogiebabies]

The tank should be a 9AB2A1B.

You may want to try the old 1 wire mod to share the ground of the chassis with both cable sleeves. If you have a scope you would be set, like we all have a scope handy. He's probably looking for a load from the reverb transformer.

In the mean time I would be testing resistors on the V4 to be sure none are blown. In the original Mk's Mesa used a 2 watt 100K. In the 3 they use a 1/2 watt that sees 308V. I would start testing coupling caps and cathode caps for open shorts as well. The worse case is a new reverb transformer for $ 12.00.

 

[Tuna141]

My parts from Boogie finally arrived last week and I was able to install the R2 volume control mod. What a great mods. It works perfectluy. I also kept the direct out, but added an internal trim-pot to replace the external one I removed to do the R2 mod. If I ever need to change the direct out level, I just need to slide the chassis out and turn the pot to get the gain I need.

After installing the R2 mod, I went to work torublehshooting the reverb problem. The cause of the dead reverb was a blown (open) R116. This is a 33K, 1/2W resistor that's on the input side of the reverb circuitry. While probing around, I found that this resistor had infinite resistance, so I knew something was up. After a trip to Radio Shack and a little soldering, I powered the amp up and the reverb worked perfectly.

P.S. Thanks for your troubleshooting tips Boogiebabies.

 

[Boogiebabies]

I need a new hobby. This online repair stuff is getting too easy !!!

 

[Shep]

my blue stripe's reverb died the other day.. it was the input transformer in the tank... i had a old fender tank laying around so i fitted that..it was the same size and the ohmage was 198 ohms which wasn't to far off my working mk IV's tank.. but it didn't work i found out that the tank input had to be grounded.. i so i had to make a new line to go to my amp the only thing is the reverb send isn't internally ground so i had to do that as well..It works now and sounds super Good..but in doing all of this will i burn anything out ? is this un safe? or is it all good.

 

[Tuna141]

I don't think you'll hurt anything by grounding the reverb input.

When I was on the phone wiith Mike B, a couple of weeks ago picking his brain for reverb trouble-shooting tips, he suggested checking to make sure that the reverb was the correct type; he told me that some are grounded and some are not, and that the Mark's needs to be gounded. He said if someone had put in the incorrect type of reverb, that all I had to do was ground it and it should work fine.

 

[Shep]

Wonderful.. Thankyou