Hot Rod Deville BAD Oscillation On More Drive

[PepeSilvia]

Well, I screwed something up this time. I've got a Hot Rod Deville, was worknig fine except for the occasional pop which I attributed to a power tube being on the way out. I got a new set of 5881s for it, popped them in, biased it, and took it to the practice pad to try 'er out. I didn't have a Fender footswitch, so I tried using a Marshall two button I had. I'm not sure if this is where I broke something or not....

Now, whenever I've got the More Drive channel engaged there's a really bad oscillation. I'm fairly certain it's somewhere AFTER the Volume, but before the Tone Circuit. The first volume doesn't do anything. The tone controls can shape the sound of the oscillation, like you would expect a tone circuit to do. Here's the funny one, the drive knob controls the frequency of the oscillation. It can go from fairly high pitched and sweep down to a really deep bass, all the way down to below the frequency I can hear it when the drive knob is all the way up. I tried swapping the old tubes back in, and the problem is still there.

The problem is NOT there on the Clean or (Less) Drive channels. Also, when I turn the master volume all the way down, I can hear a somewhat high pitched sound coming from the circuit board. Not sure if that's related but I figured it might be the source of the noise, I just can't track it down to which component it is. What say ye, oh smart ones?

[PepeSilvia]

After doing some research, it sounds like a filter cap is to blame. I'd still appreciate any input before I jump into a cap job...

[Alex R]

If you take the back off the filter caps are there in front of you. If you have a steady hand and nerves of steel (which of course you do), you can jumper another cap - a 450, or preferably 500v cap of 20uF or more, across each one in turn, and see if one stops the oscillation. This will find most bad caps, though if it's really bad, maybe not. Check polarity, watch the series caps, remember the dangerous voltages yada yada.

[pdf64]

As with the thread a few weeks ago, C36 may be the baddy. Pete

[Prairie Dawg]

That's right Alex. I've got one with alligator clips soldered onto it for just that purpose.

[jrfrond]

Just one precaution: I've seen bad solder joints on the filter caps as well, which will cause the same symptoms as an open cap, but a new one clipped across the leads will not help because there is no physical connection.

[stokes]

I've seen way too many of these newer Fender amps with this symptom.In each case it was one of the filter caps,usually one of the pre-amp caps,but I always change all of them when I see it.They use some real crap caps in these amps.I've changed all the caps in two amps that had no symptoms and the customer was pleased with the improvement in the amps response.I cant say for sure if Fender got a bad run for a period or if those caps are just that bad,I wish I had noted the exact build dates on the amps so I would see a pattern,but I've never been a fan of those asian electro's anyway.I would recomend re-apping the entire power supply with Sprague or F&T's,the oscillation will go and the amp will sound much tighter.

[badbuzz]

As with the thread a few weeks ago, C36 may be the baddy. Pete

I had the same issue as the OP. This Deville is also the first one I ever serviced that had a bad K2 relay. Thanks for the tip on C36. It made my day! I love the Internet.

[nevetslab]

Well, I screwed something up this time. I've got a Hot Rod Deville, was worknig fine except for the occasional pop which I attributed to a power tube being on the way out. I got a new set of 5881s for it, popped them in, biased it, and took it to the practice pad to try 'er out. I didn't have a Fender footswitch, so I tried using a Marshall two button I had. I'm not sure if this is where I broke something or not....

Now, whenever I've got the More Drive channel engaged there's a really bad oscillation. I'm fairly certain it's somewhere AFTER the Volume, but before the Tone Circuit. The first volume doesn't do anything. The tone controls can shape the sound of the oscillation, like you would expect a tone circuit to do. Here's the funny one, the drive knob controls the frequency of the oscillation. It can go from fairly high pitched and sweep down to a really deep bass, all the way down to below the frequency I can hear it when the drive knob is all the way up. I tried swapping the old tubes back in, and the problem is still there.

The problem is NOT there on the Clean or (Less) Drive channels. Also, when I turn the master volume all the way down, I can hear a somewhat high pitched sound coming from the circuit board. Not sure if that's related but I figured it might be the source of the noise, I just can't track it down to which component it is. What say ye, oh smart ones?

Have you taken your Marshall foot pedal out of the pedal jack, reverting back to the panel switch buttons? The Fender Pedal isn't just a shorting pedal, it is a diode network that selects voltage levels of the switching network that controls the relays and FET's to do the channel and Drive/More Drive selections. Shorting that jack, which is what your pedal does, prevents anything from changing, leaving the default state of Normal Ch, if I'm not mistaken. I don't see that shorting the 27.3VAC source via 820 ohms to ground would do any harm to any of the switching networks.

Of course, you still now have this new condition, which I assume is again using the panel buttons and not the pedal. Usually when there is a circuit problem in the channel or Drive/More Drive circuits, it's U3 4560D Op Amp, and sometimes either xstr Q3, Q4, or perhaps one of the zener diodes.

As to the component-side filter cap additions, all too often in servicing the Hot Rod Series, I find solder joint fractures on the power supply buss caps. Many generations of their PCB's have changed the solder mask to a very tiny circle of solder plate, while the copper layout shows much more copper available underneath the solder mask. I scrape that crap away, and tin the now-exposed copper, pull a bit more of the cap leads thru and fold them over (or with replacing the inferior Illinois caps, put better quality buss caps in, folding their leads over and soldering in place. The PCB quality on the Hot Rod series is horrible; far too easy to have traces and pads lift in spite how careful you are.

Bad solder joints/solder joint fractures all across that board is really common. PITA to extract the board, but worth the effort from what you find on the back side. Don't forget to tighten the PCB standoffs, which invariably aren't tight!

[Enzo]

Just to let you know, pepe has not logged on in about six years, so he may not get this message.

[nevetslab]

Just to let you know, pepe has not logged on in about six years, so he may not get this message.

Opps! I just now saw the date on the original post, being from 2011. So much for being attentive to details.