Hot Rod DeVille 410 issue

[stevek]

Hi all. Noob alert, I just signed up. I took the time to search through the forum before asking for help from anyone, but I can't find anything related.

Here's the problem:

Bought a used HRD 410 about a month ago. Amp is about 4 years old, and was virtually immaculate when I bought it.

Switching from my drive channel to my clean channel. The clean channel volume will blast well above the level where I have it set, then drop to the level I have it set, where it stays. Adjusting the volume, either up or down, during the "loud" period does nothing, even if I cut it to 0. The volume stays at the "loud" level until it drops into "quiet" level on its own, when the volume control works again.

The problem is intermittent. Sometimes it switches correctly, sometimes it just has a quick volume pop, sometimes it stays in "loud" mode for long enough for me to walk over to my amp, tweak the volume a few times, scratch my head, try it again, then it goes to "quiet" mode.

Mind you, this is during band practice, so it's not like I'm trying to make a 60-watt tube amp behave itself at low volume. I typically run at about 3-4 on the drive channel and 2-3 on the clean.

I am sure the issue is isolated to the amp.

Never had anythign remotely similar with any other amps I've owned, so I'm not sure what's going on. Any help would would be appreciated.

[bnwitt]

Well first of all you amp is really only one channel. The "drive channel" so to speak is really just a signal boost circuit carried out by a couple of relays. They are K1 and K2. Each relay has two sets of normally open and normally closed contacts. That means there are 4 actual switches taking place.

On the K1 relay section A, pins 4,6,8 will bypass the clean channel's volume control and 220K resistor and re-route the signal through a 100K resistor and the drive control. This boosts the signal on it's way to V1B by reducing the resistance in the path by 120K ohms.

The fact that your clean channel volume control does nothing during the problem period indicates that the relay contacts are not switching back after the relay is de-energized. Sounds like you have a sticking contact in the K1A relay.

[stevek]

Now that makes much more sense than anything I was coming up with (bad tubes/input jacks, etc.), because it explains the volume-independent part. I will definitely check it out. Thanks a ton for the tip.

[bnwitt]

Do you have a multimeter? There are some voltage tests you can do to make sure it is not the footswitch jack or faulty low voltage power supply components. The Fender schematic shows these tests and test points. It is available at

http://www.mrgearhead.com