HotRod Deville motorboats only on Drive channel

[Don Moose]

Gentlemen -

I have on my bench a HotRod Deville that motorboats, but only on the Drive channel. The Drive control, and the tone controls, control the frequency of the motorboating - it's a low frequency that approaches subsonic as certain controls are rotated.

As the Drive control is turned Up, the frequency goes Down - unless the More Drive is engaged - this reverses the direction of the frequency control.

The +/-16VDC rails have had their resistors changed to 330R - looks factory - and the switching TP test values all look reasonable. Not exact, but within a range that looks OK to me. The relay contacts operate correctly, as far as I can measure with an ohmmeter and the amp in standby.

Changing tubes in V1 and V2 changes the character of the motorboating slightly, but doesn't kill it.

Not seeing a feedback path that would let the Drive control and tone tone controls set the frequency of an oscillator.

Isolated the V1 board from the chassis - no change in operation.

Removing V1 still motorboats, but frequency doesn't change with Drive control - didn't try the tone controls in that mode.

Saw something in another thread about the V2 ribbon and some other wire. Gonna go look at that, but in the mean time assume that's not the problem.

Ideas?

[Don Moose]

OK - I was wrong about the direction reversal with More Drive engaged ... unless it's just messing with me.

With V1 removed, the tone controls still control the frequency of the motorboating, with the Bass control having the most profound effect.

[pdf64]

Motorboating usually means a bad power supply cap. Try bridging C36 with one of the same spec.
http://www.webphix.com/schematic%20h...od_deville.pdf
Pete.

[Don Moose]

I thought about that, but the entire preamp is fed from the same PS node - wouldn't the clean channel MB, too, if that were it?

[pdf64]

The clean channel may not have sufficient gain to satisfy the conditions necessary for oscillation.
It's a pretty quick thing to check.
I'd be very surprised if it was something other than C36! Pete.

[Enzo]

I thought about that, but ...

Never think up reasons not to check something.

[Don Moose]

After finding a 10u/450 cap in the pile that is my bench, I stand corrected ... about 10 minutes after my last post.

Thanks, fellas!

[UpScale Music]

C36 it is..

Thanks for the great posts!

[Ronsonic]

Never think up reasons not to check something.

This.

Print it out.

Post it over your bench.

It is The Law.

[jrfrond]

Never think up reasons not to check something.

Amen to that! Just did a 6G16 Vibroverb the other day whose 2nd channel broke into oscillation. As it turns out, one of the (seemingly) recently replaced Sprague filter caps (for that stage) was open, causing coupling through the power supply.

[Alex R]

Sprague filter cap? Open?

...does not compute. Don't they have self-healing mojo inside?

[Prairie Dawg]

What the man says.

[Fendermender]

Glad I happened onto this page! I picked up a HRDV that had taken a severe hit to the rear that crunched the panel, bent the chassis, and cracked the edge of the mainboard opposite the end where C36 resides. I suspected a broken trace that was somehow causing the drive channels to motorboat. I did a quick shortcut test of the 6 large electrolytics for continuity (a brief beep with a reading that momentarily dropped that would repeat if leads reversed ends). This told me that C36 was dead open. It's all fixed and sounds like new, thanks! Now I just need to complete the chassis repair and make a new panel.

[lowell]

Just fixed the same amp with the SAME exact symptoms and issue. Thanks!

[km6xz]

Learning to be an effective diagnostician involves thinking about a problem as it IS instead of what it often is or what sometimes is. Reserving judgement until basic circuit conditions are measured to determine the current state to give enough evidence to only then start to determine the problem. Fixing it is not just stumbling on a part change that seems to improve operation but determining by collecting the evidence of what the mechanism of fault is(was): how did it happen and what else would have been affected by that original condition. As experience with thinking like a diagnostician increases, the number of evidence measurements usually decreases to the bare minimum needed to determine the actual fault and why it happened.
People here get upset by my harping on not changing anything until it is proven to be defective but if they stick with that they will find they will become better a waiting for the evidence before jumping to a conclusion and will be better techs. It also saves a GREAT deal of time. Your job task one is not to increase the number of uncontrolled variables that cause your measurements to be ambiguous.
How much time? If a well proven mechanism of fault has not been found in 30 minutes 3 more hours is not going to help, in fact it will just add confusion. Spend the first 30 minutes taking logically appropriate measurements. If at the end of 30 minutes the real cause of the problem is not determined, notate the readings and put is aside and come back when you are fresh. Taking longer means you are not fully understanding how the circuit works so learn about it. That is not customer time, don't bill them for the time you need to study to be familair with what you are selling in the first place, diagnostic skills. After a diagnosis is determined, doing the repair is simply labor and craft skill. What takes the most skill and time usually is an accurate evidence supported diagnosis. Anyone, with a few hours of instruction can repair a unit based on an accurate diagnosis but only 1 in 20 techs progress from repair mechanic(parts replacer) to skilled diagnostician, mostly due to bad habits and not putting in the time to really learn the fundamentals.
The most skilled are fastest, and by far the least expensive for the customer because needlessly replaced parts are not installed or charged for. The customer loses value of the unit when allowing it to be worked on by beginners or techs who are not professionals in training, continuous education, properly equipped and with well developed diagnostic skills.
Having test equipment is essential for good tech work, measurements are less ambiguous and more repeatable. Being a tech is one of the lowest cost of entry professions, but even so, $10,000 for instruments and tools would be a minimum for quality work and diagnosis. Very few small businesses capable of net income of $60-75k a year can be engaged in professionally with less investment. Most seasoned techs who work on a variety of technologies have much more than that invested. Here is a home bench that would be typical for effective diagnostics of pro audio gear including wireless.