Hot Rod Deluxe help

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Smokebreak
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Hot Rod Deluxe help

Post by Smokebreak »

I've got this HRD here. It works, but there is a crackle/fizzle/sizzle that's pretty prominent. Sounds like a bad preamp tube issue, but isn't.
This amp has all new tubes, and filter caps. When I take the preamp out to another amp, the slave amp is fine, and has no fizzle, denoting it's at the PI or the output section.
I come to the same quasi-conclusion when pulling preamp tubes. The fizzle remains until I pull the PI tube.
I had a thought that it could be coming from the reverb, so I lifted a leg of R38, but the fizzle remains. I'm about to hit screen resistors and things like that, but they all read in spec.
I hit all the joints around the PI, but haven't done a full re-solder yet. Chopsticking tells me nothing on this one, unfortunately .
Any ideas? I've got both boards pulled so it's party time.

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xtian
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Re: Hot Rod Deluxe help

Post by xtian »

Help me shoot down the mechanically-sympathetic vibration theory first. Do you have a sine-wave generator? Can you set it up so the "fizz" sound happens continuously while your hands are free to press on tubes, and chopstick everything?

Next, is it the speaker? Use another amp to power the HRD speaker.
I build and repair tube amps. http://amps.monkeymatic.com
Smokebreak
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Re: Hot Rod Deluxe help

Post by Smokebreak »

Wish I could help, no SW generator. I tried different a different speaker. After posting, I googled, and apparently this is a know issue with this amp, as it's usually the dropping resistors in the power supply, or the PI plate resistors, or socket solder issues. I was actually going to do the PI plates first, so I'll start there.
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sonicmojo
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Re: Hot Rod Deluxe help

Post by sonicmojo »

Yes, check for cracking on the traces near the sockets. I've seen this before. I had to run little jumpers.
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Bryan
pdf64
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Re: Hot Rod Deluxe help

Post by pdf64 »

A 'fizzle/sizzle' may indicate instability, so consider using a chopstick to tweak the lead dress whilst the issue is present.
Smokebreak
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Re: Hot Rod Deluxe help

Post by Smokebreak »

Turned out to be the PI plate load Rs
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xtian
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Re: Hot Rod Deluxe help

Post by xtian »

Glad you solved it!

To add to Pete's post about instability and oscillation, sometimes the HRD will oscillate with the rear panel removed, but works fine with the panel in place. That's a wicked pisser!
I build and repair tube amps. http://amps.monkeymatic.com
Smokebreak
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Re: Hot Rod Deluxe help

Post by Smokebreak »

I worked on a few of these but never had to take off the socket PCB. What a PITA. And those standoffs! I seriously though I was going to create an endless circle of repairs just moving those delicate boards around.
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Ron Worley
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Re: Hot Rod Deluxe help

Post by Ron Worley »

If you are removing either the main board or tube socket boards (or both), consider touching up the solder connnections on each board for the ribbon cables. The movement and flexing can cause dodgy solder connections to loosen or even disconnect.

Lyle Caldwell on Facebook posted a fairly comprehensive HRD repair guide, check it out. He's one of the best techs out there.
Ron
Smokebreak
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Re: Hot Rod Deluxe help

Post by Smokebreak »

Thanks a bunch Ron! That will come in handy for sure.
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