VHS reverb low volume (solved)

[twebb6778]

Hi all, currently trying to troubleshoot a VHS that was previously working perfectly for a good week or so. The 3PDT seemed to fail so I've replaced that, now everything is fine except turning on the reverb significantly reduces volume.

The effect is still there, but much lower output. Anything obvious I might be missing or ideas on where to check?

 

[Robert]

Turn off the reverb.
Switch the chorus on and off, does the volume drop?

Turn on the chorus.
Switch the reverb on and off, does the volume drop?

 

[twebb6778]

Thanks for the quick response! I'll test it out when I get back from work and let you know.

 

[twebb6778]

Turn off the reverb.
Switch the chorus on and off, does the volume drop?

Turn on the chorus.
Switch the reverb on and off, does the volume drop?

No. With the reverb off, switching chorus on and off does not drop volume.

Yes. With the chorus on, volume drops whenever reverb is engaged.

I also noticed that if the reverb is turned on first, the volume is always low, regardless of whether chorus is on or off.

It's a shame, because the reverb sounds so damn good.

 

[twebb6778]

Bumping this.

This is my first FV-1 build so I'm a little lost as to how to troubleshoot. I'm really confused why it would work flawlessly then the reverb suddenly die.

 

[twebb6778]

I've reflowed all joints, just in case, and started digging around with an audio probe. With just the reverb on I'm getting clean signal on pins 1 and 2 of the FV1, input, and wet signal with volume drop on pin 28, output, so the problem seems to be related to the IC.

Interestingly, I'm not getting any signal at all from any other pins on the FV1.

 

[twebb6778]

After looking at the schematic and not getting any signal from the FV1 pins, I had a penny drop moment and realised that the pots aren't actually attenuating signal, but rather the regulated 3.3 volts, then feeding that voltage into the FV1 to tell it how much or little of the effect to apply.

I'm going to check voltages with my DMM tonight, hopefully I can pin down where the problem is. As frustrating as this is, I feel like it's been a great learning experience.

 

[Robert]

Yep this is a tricky one since the signal always follows the same path regardless of the mode it's in....

Even more confusing is the fact that Reverb Only, and Reverb + Chorus are two completely different algorithms. I can't think if any reason changing a footswitch would have any effect on the reverb volume.

Is it possible the volume was low before that repair?

 

[twebb6778]

I haven't had a chance to check voltages yet, but it was definitely working as intended. I was using it in a full band and using all effects in different combinations to put it through it's paces.

I can't imagine how a faulty footswitch would affect the circuit, but I wanted to include the fact that I'd swapped it for context.

 

[twebb6778]

Got my IC voltages. These were taken with with only the reverb switch on, reverb pot wide open. Power supply measuring 9.4v.

All seems to be within spec, but I'm no expert.

FV1
1 - 1.6
2 - 1.6
3 - 1.6
4 - 4
4 - 3.3
6 - 3.3
7 - 0
8 - 3.3
9 - 1.2
10 - 0.3
11 - 0
12 - 0
13 - 3.3
14 - 3.3
15 - 3.3
16 - 3.3
17 - 0
18 - 0
19 - 0
20. 3.3
21 - 0
22 - 0
23 - 3.3
24 - 0
25 - 0
26 - 3.3
27 - 1.3
28 - 1.7

TL072
1 - 4.6
2 - 4.6
3 - 4.1
4 - 0
5 - 4.1
6 - 4.6
7 - 4.6
8 - 9.1

24LC32A
1 - 0
2 - 0
3 - 0
4 - 0
5 - 3.3
6 - 3.3
7 - 0
8 - 3.3

L78L33
Input - 9.1
Ground - 0
Output - 3.3

 

[twebb6778]

So given that the audio probe isn't particularly useful for this circuit, and IC voltages seem to be ok, is there a possibility that this might be a fault with the FV1 or the EEPROM?

 

[chongmagic]

Have you tried re-seating the EEPROM by chance?

 

[zgrav]

Have you built any other FV-1 pedals that use an eeprom? You could swap out the eeproms and see if you are still getting a volume drop in the two switch settings that use the reverb and the reverb+chorus.

 

[twebb6778]

This is my first FV1 build so I don't have another EEPROM I could swap it with. I've already reflowed all my suspect joints but I'll take another look and focus on that IC.
Thanks guys!

 

[zgrav]

Focus on the connections for the reverb control and the parts that connect to it.

 

[twebb6778]

I managed to get this one working over the weekend!

I think the problem was a short on one of the reverb pot lugs. Because I placed my jacks on either side of the enclosure, I had to install the pcb on a slant. When I replaced the footswitch, I believe that the lugs from the reverb pot made contact with the spdt below it and shorted.

I've now insulated all the spdt's with electrical tape and the pedal is working great. Thanks for the support all! I feel a bit silly that it was such a simple mistake, but it's a good lesson in offboard component placement.

 

[PKRPedals]

I know this is an old thread but I also notice a volume drop when switching on the reverb. It's only with the reverb. I have gone back through and don't see anything obvious. Anybody else experience this? I built this pedal awhile back and it's just been sitting. I decided to put it on my board and noticed the drop.

 

[twebb6778]

Were there any volume issues when your first finished it?

 

[PKRPedals]

It's been quite awhile since I did it. If I remember correctly it did but I kind of put it on the shelf to do other stuff. It's a good sounding pedal and I can deal with the volume drop but if it can be fixed, that would be good.

 

[twebb6778]

Your best bet would probably be going through the circuit with an audio probe to find where the volume drop occurs to isolate your issue. Could be as simple as an incorrect resistor.