| Author | Message | ||
     Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
I have a 112RD 100 with a chassis date of 81-6. It is the Canadian model, but made in USA. The Reverb has a loud annoying hiss when it is engaged (foot switch). Is there a fix? Any help appreciated. | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Could be a noisy IC or noise from the tank itself. Does it make noise when unplugged from the tank? | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
The hiss/buzz is evident both when the RCA plugs are disengaged from the chassis and when they are plugged in. No difference either way. The hiss/buzz disappears when the footswitch is used to turn the reverb "off". The hiss/buzz is most evident when the reverb is up above 2 or 3 and loud at 10. Also the reverb is really only evident above 6-7. The hiss on the reverb is evident when the Gain is low or zero. So bit does not appear to be the tank itself. Also, the Gain causes a hiss/buzz increase when it is Cranked up with or without the reverb engaged. What do you think? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Yup-I think noisy IC or componenets in that area. You could just yank out IC2 and see if the hiss goes away. That will let you know you're in the right area. Then start testing components in that area of the reverb send and receive. | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
Ok. I'll try that. The amp is the version with the 6L6's. I'll check the schematic to confirm the IC to pull. The chassis is hard to open because the bolts are tucked in between the transformer and the cabinet. The one time I did open it up, it took a good 30 minutes to get the thing bolted back together Thanks for the response. | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
Note to all Forum members...Take a picture before pulling the ICs!  It is hard to recall the correct orientation once they have been pulled and left on the bench for a while. I'll Post my own photos later. Pulling the ICs and swapping them didn't seem to solve the hiss issue on the reverb. But it did, however, give me a few heart paplitations when the amp didn't work when the IC is oriented the wrong way. | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
After pulling the ICs and swapping them around, I found no clear resolution ot the Hiss when the Reverb is engaged. What I did find was that there is a small Poteniometer mounted on the PCB that can be adjusted. Turning it all the way Counter-clockwise eliminated the hiss, but also badly affected the volume on the amp (decreased volume) and created distortion. Round vertical device in the centre of the image.  As a result I put the POT back into the position I found it, and put the chassis back together. Does anyone know what the purpose of the small POT is? Is a bias adjustment on the output tubes? Why does it affect the Reverb? | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
After pulling the ICs and swapping them around, I found no clear resolution ot the Hiss when the Reverb is engaged. What I did find was that there is a small Poteniometer mounted on the PCB that can be adjusted. Turning it all the way Counter-clockwise eliminated the hiss, but also badly affected the volume on the amp (decreased volume) and created distortion. Round vertical device in the centre of the image.   As a result I put the POT back into the position I found it, and put the chassis back together. Does anyone know what the purpose of the small POT is? Is a bias adjustment on the output tubes? Why does it affect the Reverb? | ||
| Mike Kaus (mm210) Username: mm210 Registered: 05-2006 |
Tha should be the driver pot on that amp. I am not too familiar with the rd's but they only show ONE trimpot on the schem so that has to be it. Lower and it goes into crossover distortion and higher brings up the drive into the power tubes. You have to balance the pot to get the right setting and see if the driver transistors are balanced into the power tubes. THAT'S where a scope wouold be nice to see the waveform being injected into the finals. Mike. | ||
| Rob Flindall (flinly) Username: flinly Registered: 12-2012 |
Thanks Mike. That makes sense (even to me). When I have the inclination I'll have it worked on by someone more knowledgable than I am. |