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Post subject: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone!!!
Posted: Mon Sep 15, 2014 12:28 pm
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This ^#$*%$ amp is driving me nuts!

It was blowing fuses:

1) Installed new tubes....no more fuse blow.

2) But now its starts red plating (Lowered bias to no avail)

3) I check all the electrolytics (They are way out of spec) 47uf reading 20 and the 100 uf are reading 59) So I replace those.......all good.

4) Still red plating, find a bad 470 Ohm resistor so I change it....still red plating.

5) Reading 475 on Pin 3 of each tube socket......

6) Checked Bias resistoer R66? 1 ohm....reading fine.....

7) D3 Diode.....if its bad will it affect the bias circuit to cause red plating?

>Scratching my HEAD!!! :shock:

P L E A S E H E L P ! ! ! !


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 1:11 am
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Red plating need only a bias adjust or ......... good tubes.

If you are shure you 6L6's are good ,you need to replace a resistor (s) in bias circuit to lower the bias .

First replace R83 . It may do the job .
If not R76 , R77

Where do you read bias on the amp and how much you read ? Mv/Ma ?
Do you know what is red plating ?
Sorry for these questions , I don't know your experience in tubes amp fixing.

Shematic here;

http://support.fender.com/schematics/gu ... ematic.pdf


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 7:21 am
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Stratele, I think he can try first adjusting the bias (R82?). It's next to the C- point off the power tranny.


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 11:35 am
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BMW2002Ti wrote:
Stratele, I think he can try first adjusting the bias (R82?). It's next to the C- point off the power tranny.


I tough he try to adjust bias pot and no lower bias avalaible ...!!!!

"...2) But now its starts red plating (Lowered bias to no avail) "


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 11:49 am
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Stratele, yeah you are correct. Must read entire thread, first. :lol:

Maybe try cleaning the tube pins and sockets with contact cleaner. Could be a hinky contact with number 5 pin.


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 12:00 pm
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BMW2002Ti wrote:

Maybe try cleaning the tube pins and sockets with contact cleaner. Could be a hinky contact with number 5 pin.



+100

Bad contact may stop / drop negative bias voltage to the 6L6 grids


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 1:29 pm
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Guys = there is not resistor located in R83 (nice goose chase lol :lol: )

R77 Tests Fine and obviously there is no R76 either :P

With my DMM i measure bias as -003 ??? WTH????

With my bias probe (Bias Master) ZERO!!! after 15 seconds its starts red plating!


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 1:42 pm
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Ryan41018 wrote:
Guys = there is not resistor located in R83 (nice goose chase lol :lol: )

R77 Tests Fine and obviously there is no R76 either :P

With my DMM i measure bias as -003 ??? WTH????

With my bias probe (Bias Master) ZERO!!! after 15 seconds its starts red plating!



Look you don't have any negative bias voltage .
Remove all you 6L6s and trouble shoot your bias power supply . Something wrong there .

You can safely operate you amp with no 6L6 ( or any tube ) to work


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Tue Sep 16, 2014 9:52 pm
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stratele52 wrote:
Ryan41018 wrote:
...With my DMM i measure bias as -003 ??? WTH????...


Look you don't have any negative bias voltage .
Remove all you 6L6s and trouble shoot your bias power supply . Something wrong there .

You can safely operate you amp with no 6L6 ( or any tube ) to work


+ 1 stratele52

You should have close to -45 VDC on each pin 5 of the output tubes. Sounds like an open circuit somewhere, maybe a poor solder joint or broken trace?

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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Wed Sep 17, 2014 7:53 am
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It never hurts to double check your work many times over.


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Post subject: Re: Hot Rod Deville - Mystery.....Update
Posted: Sat Sep 20, 2014 1:47 pm
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Went back to the main board looked at all solder joints with magnigier and found bad solderpads on R74 and C74 (they were actually lifting with the solder still attached)

Fired her up and now I get a measurable bias reading :P

Obviously no more red plating...BUT................Now I got no output.

Tested V1 - V3 all failed, replaced them with New ones still no output.

Lined of the preamp out into another amp and I have signal.

Lined into power amp input..........NO OUTPUT :?: :?:

Any suggestions would be ABSOLUTELY APPRECIATED.


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Post subject: Re: Hot Rod Deville - Mystery.....Update
Posted: Sat Sep 20, 2014 1:54 pm
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[quote="Ryan41018"
Lined of the preamp out into another amp and I have signal.

Lined into power amp input..........NO OUTPUT :?: :?:

.[/quote]

Circuit open between Line Out and power amp .

Check V3 you put a new one ? , phase inverter circuit . V4-V5 output tubes could be bad

You may have broken something ......A amp hard to work on

Amp was playing before ? When red plating ?
Speakers are connected ?


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Sat Sep 20, 2014 2:07 pm
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R74 is on the main power supply , nothing to do with bias power supply . Or you mean R 77 ?

C74 ?? I don't see this capacitor on my schematic .

You do something that bring back the bias voltage ,what ? I don't know , but you may disconnect main power supply with your work with or around R 47 .

Some skill in soldering ?

Look carefully your work and all on the circuit .

Bad solder ; you don't need magnifier to see them, but experience . Look on Google , bad solder pictures.

Good luck


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Sat Sep 20, 2014 2:16 pm
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With my eyesight YES need magnifier LOL!!!


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Post subject: Re: Hot Rod Deville - Mystery.....Calling Shimmlou or anyone
Posted: Sat Sep 20, 2014 2:26 pm
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Ryan41018 wrote:
With my eyesight YES need magnifier LOL!!!


Need experience too , what look a defective solder .


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