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Post subject: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 5:48 pm
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My hotrod deville took a dump on me the other day. Need some advice on what to try next.

Symptoms: Low distorted signal in all inputs (including power amp in)

I've been focusing on the power board since i'm getting the same results starting at power amp in.

What i've done to the amp so far:
Replaced the half watt R57 and R58 with 1watt salt licks. (the 82k was bad when i started this, i thought that was the problem, but no go)

I've also replaced the screen grid resistors with 5w ceramics.

I've reflowed every joint on the power board

Unrelated, but I've changed all the high voltage caps since i was already in there with some F&T's

My voltages read pretty good from the service diagram. The higher ones read about 10v low (483v and 485v), which didn't seem all that bad.

I've found no more dead resistors in the power section. I read them in the circuit with the power on by reading a before and after voltage to make sure I had continuity.

My next step is going to be pulling out the caps and checking them.

Any other thoughts? My tubes are brand new JJ. I have not tested them in another amp, but i've never gotten anything bad from eurotubes.com, I will be double checking the tubes in the next few days when i can over and pick up my back up set.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 6:08 pm
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Also wanted to make note, The main caps seem to discharge pretty fast after turning the amp off. The drop down to about a half volt in less than a minute. I expected these to hold their charge much longer, especially given all the warnings about making sure you discharge them. This is making me think coupling cap, but i really don't know anything about amps, I'm just a bicycle mechanic.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 8:13 pm
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You really should check the tubes in another amp, or try a known good set in this amp. Possibly your PI tube or output tubes are flaky.

No offense, but it seems that you are just using a shotgun approach and replacing components that might not even be bad, and really don't know what you are doing. You will get a voltage drop from one lead of a working resistor to it's other lead, that's normal, and you don't check continuity with a voltmeter.

Sounds like you are in way over your head, and you're probably doing more harm than good in this case. Quite a big leap from inner tubes to amplifier tubes. :wink:

"Salt licks"? :lol:

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 8:44 pm
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shimmilou wrote:
Sounds like you are in way over your head, and you're probably doing more harm than good in this case. Quite a big leap from inner tubes to amplifier tubes.


+1

Schwinn's are Schwinn's and Twin's are Twin's, and never shall the twain meet.

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(who rode a Huffy)

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 9:35 pm
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I'm not just up and changing components. the voltage divider was shot, so i replaced it and its still working counterpart (r57 & r58). the other components were replaced only because i was in there, and its not a bad idea since they have been known to fail. the caps were not bad, and the screen resistors were not bad. I'm well versed in electronics and a A+ certified computer technician. I don't often work on tube amps, but i have built over 200 audio effects pedals, this is not new stuff for me. Help is appreciated, your bashing is not.

I have switched the tubes to a known working set to no avail.

\I don't think you understood me when i described how i was measuring continuity across the resistor. I know there will be a voltage drop, thats what it does. measuring a resistor while it's in a circuit does nothing. I'm just checking that the current is passing through the resistor. If i have voltage on one side, but not the other, then i know it's toast. the salt lick comment was made regarding the 5w ceramic. no one calls them salt lick around here? i've been calling them that since the mid 90's, they look like a salt lick..


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sat Jun 15, 2013 11:58 pm
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Ok. So you've measured some voltages. What were the plate voltages on the PI and power tubes? What about on the grids and cathodes of the tubes? What do you have the power tube bias set to? Have you measured the bias supply voltage? Have you traced the signal through the amp using an oscilloscope? How do you know that R57 was bad? Other than telling us what parts you have replaced, you haven't really given us anything specific to say that those parts needed replacing. Don't just tell us that you measured a voltage. Tell us what the voltage measurements were. Trying to diagnose amp problems over the internet is not easy, especially when you don't provide any useful information regarding what you have found.

BTW, shimmilou is extremely knowledgeable when it come to tube amps. In your first post you said "i really don't know anything about amps, I'm just a bicycle mechanic". Then you said "I'm well versed in electronics and a A+ certified computer technician." Which is it?

And no, I've never heard of "salt licks" either.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 12:20 am
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Thanks bluesky636. I saw your post when I previewed mine, and since I already typed it, I'll go ahead and post.

I'm not bashing you spint3, just pointing out that you don't seem to be troubleshooting, but guessing. As bluesky636 pointed out, you first gave the impression of a novice. Not everyone is well versed in schematic reading or troubleshooting, it's no sin.

For example, with the power off and caps discharged, you can certainly check the resistance value of Plate resistors (such as R57 & R58), screen grid resistors and control grid resistors. In fact, in this amp, a great many components can be checked in the circuit before applying power, because the cold tubes are "open" and won't conduct, so it's just as if you've lifted one leg of these resistors.

After checking all component values possible, then apply power, standby switch in play position, and check the schematic voltages at the tubes themselves and this will verify whether or not your components are good. Negative meter lead connected to the chassis and probe the voltages with the positive lead. You must be very careful when checking voltage at the tubes pins, because a slip of the meter can short pins together easily, and some of the pins should have high voltage.

Is the bias set properly, what is the negative bias voltage at the output tubes' pin 5?

You should have approx:
Output tubes:
485 VDC at both pin 3
480 VDC at both pin 4
-50 VDC at both pin 5
60 mVDC at the bias test point

PI tube:
280 to 380 VDC at pin1
280 to 380 VDC at pin 6
42 VDC at pins 3 and 8

These checks can give you some direction. However, the first thing that I would check is that your speakers are plugged into the correct jack (the one on the left). If they are plugged into the ext speaker jack, the result will be, "Low distorted signal". :idea:

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 12:52 am
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I'll post my voltages tomorrow. all supply voltages i measured were in spec with what was as listed on the data sheet. I am running my bias voltage at 70mv, I will dial it back and retest everything and post the voltages tomorrow.
when i started this task, i started at the speaker and worked back. I started by testing everything in the circuit how it is, and as you guys have said, with the amp off most of the power components are open. when i hit the 82k 1/2w i got infinite ohms. i pulled one leg off and confirmed it was dead. I was advised to replace the pair of them with some 1w parts because this is a common issue, and most techs like to up it (i did the screen resistors at the same time because i was in there, and i've heard of them being an issue. i didn't want them to become a problem later. they measured fine). which catches us up to now, i just replaced the burnt resistor and the trouble persists.

I've got it in my head that the problem lies somewhere past TP23. I had planned to check c24 out, from looking at the schem it appeared to me to be a coupling cap holding the dc in the power stage. but word is that it is most likely not one of the caps, and upon looking at their composition, i guess i agree. i rarely see tant, ceramic, and poly caps fail in my pedal building. its always been the electrolytic that develop issues. which i guess is what inspired me to swap out the gray caps from 1996 to new f&t ones.

but yeah, i'll post those voltages up tomorrow and if you guys could tell me what you think, that'd be awesome.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 1:02 am
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Now you sound more like a competent tech. I agree, the ceramic caps just don't fail, but a good idea checking for DC on the other side of any coupling caps. BTW, 70 mV is fine, I wouldn't worry about dialing it back. That's close to 16 watts idle per tube and is just about right, maybe even a little cool which is fine. That is also a clue that your power tubes and related circuitry are likely working well.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 12:43 pm
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Here's what i've got on the voltages you've requested:

V5
3. 467VDC
4. 467VDC
5. -49.6

V4
3. 468VDC
4. 468VDC
5. -49.6

V3 (PI)
1. 282.7VDC
6. 271VDC
3, 8. 39.1VDC

Bias was set to 60mV, tubes were pulled from a working Hotrod Deluxe.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 12:57 pm
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i did check the speaker jack orientation and impedance (though i just measured its resistance), cones feel fine with no evidence of the voicecoil hitting the magnet. for purposes of testing, since it is on my bench, it is plugged into a known working external cab.

When i started reading the test point voltages the other day, my readings were way different than what the schematic said. I don't know if i misread the setup or not, it didn't specify whether or not the amp was supposed to be in standby or not. That was the point where i knew i needed some help from people that specifically knew amps.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 1:32 pm
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spint3 wrote:
i did check the speaker jack orientation and impedance (though i just measured its resistance), cones feel fine with no evidence of the voicecoil hitting the magnet. for purposes of testing, since it is on my bench, it is plugged into a known working external cab.

When i started reading the test point voltages the other day, my readings were way different than what the schematic said. I don't know if i misread the setup or not, it didn't specify whether or not the amp was supposed to be in standby or not. That was the point where i knew i needed some help from people that specifically knew amps.


If the amp were in standby, all high voltage would be shut off. Voltages seem reasonable. How is the amp performing now?

How do you have the external cab connected? Plugged into the main jack or the external speaker jack? If it is plugged into the external speaker jack, you must have either the main speaker plugged in or an open plug inserted, otherwise the external jack is shorted to ground.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 1:52 pm
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I'm still at the same point as far as sound goes. the speaker is indeed connected to the main output using a proper speaker cable.

The confusion i had was referring to the 30 odd TP readings on the schematic portion of the documentation. 'all controls set to 50%, reverb full ccw, yellow channel LED on' is what the schem says (from memory) but no mention of whether the high voltage was on or off. For instance, going from memory here, on TP2 says i should have something in the mV range, but i was reading high voltage there. That's what made me decide to go on the cap spree to see if any were passing dc. I guess we'll get to that when we get to that.

I'll check for dc leak on the coupling caps later when i get home, and i want to make sure those ribbon cables are good. other than that, i'm not sure what check next. I guess i can take the preamp out into another power amp and verify that that section is functioning properly.


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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 2:49 pm
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spint3 wrote:
I'm still at the same point as far as sound goes. the speaker is indeed connected to the main output using a proper speaker cable.

The confusion i had was referring to the 30 odd TP readings on the schematic portion of the documentation. 'all controls set to 50%, reverb full ccw, yellow channel LED on' is what the schem says (from memory) but no mention of whether the high voltage was on or off. For instance, going from memory here, on TP2 says i should have something in the mV range, but i was reading high voltage there. That's what made me decide to go on the cap spree to see if any were passing dc. I guess we'll get to that when we get to that.

I'll check for dc leak on the coupling caps later when i get home, and i want to make sure those ribbon cables are good. other than that, i'm not sure what check next. I guess i can take the preamp out into another power amp and verify that that section is functioning properly.


It is not clear from the schematic, in fact it is not even stated, but all voltages inside ovals are AC voltages and all voltages inside rectangles are DC voltages. At this point, I would say you need to input a 4mV 1kHz sine wave at the input (I am assuming it is peak-to-peak) as shown on the schematic and trace it through the amp, preferably with an oscilloscope.

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Post subject: Re: Service Advice. Hot Rod Deville 2x12. Need Help.
Posted: Sun Jun 16, 2013 3:12 pm
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Since you say that you have the same distorted sound when plugging into the Power Amp In, that would seem to indicate a problem in the power section, or maybe even in the reverb section, as the preamp is bypassed when plugging into the Power Amp In. But, it couldn't hurt to verify the preamp section as you said, by plugging Preamp Out into another amp. I am still thinking about the PI tube, or the circuit around the PI, especially since you had a bad Plate resistor (82 K).

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