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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 11:39 am
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You might want to try hooking a voltmeter to the DC power supply for that side of the circuit. You should get a steady DC voltage before you play anything into the amp. The Amp will be using power more or less off of that DC supply depending on the frequency and amplitude demand of the signal coming from the guitar. IMHO the amp will need more power for the BASS than it would for the higher notes because it has to drive the speaker cone harder and farther to keep up with the demand for low notes.

So if your voltage drops much at all when you throw some base into the vibratto, then that's an issue.

If the power supply is holding a steady DC with loud Bass, then look further downstream in the circuitry.

The DC supplied to the vibratto itself may be an issue.

As far as where the power loss is coming from, the electrolytic caps on old electronic equipment are 90% of the problem in most cases. Solder joints too, but if the solder (rosin core only) is good, you should see fairly shiny solder joints and no frayed wire strands anywhere.

In order to test the caps, (UNPLUG THE AMP FIRST)!!! Make a 10K ohm bleeder resistor with an alligator clip lead in series with the resistor and insulate the whole thing so that you won't get zapped.

ground one end of the bleeder line to the chassis and with the amp OFF , touch all the leads of the caps and bleed off any residual energy that might be lurking in the circuit. Hook your voltmeter to these bleed points and check that they are in fact Zero volts before you start testing anything.

Now that includes the power supply too!!! Safety first !

Now you can unsolder the caps on one end only and put the Ohm meter in the 2K Ohms range. This will put about 1.5 volts of DC on the Ohm meter leads. This will charge any "good" caps and the resistance reading will increase as the cap takes a charge from the ohm meter. If the meter does not show an increasing resistance as the charge goes into the drained capacitor, the cap is leaky and must be replaced.

The polarity of the cap should be observed and usually the Black lead of the ohm meter is negative voltage out, so put that on the negative end of the electrolytic. Usually this is the end with the crimp ring around the casing at one end. some of the caps may also be labeled on one end.

Some of the larger caps may take a minute or two to fully charge. But if any of them fail to charge, the ohms reading on the meter will not move..or be very low such as 200 ohms or less.. as the meter leads are touched to the isolated cap leads (both ends of the cap with one end lifted out of the circuit) the reading should go from showing 1 (for open) very quickly towards 0 but then it will reflect the changing resistance as the healthy cap charges up. Eventually it will stabilize if it is good at something above the 2K reading and you can change ranges above 2K if you want to watch it go all the way up to a full charge but this will take longer.

A schematic should help with all this if you know how to read it and a parts location diagram.

Wish I had your amp to play with it should be a piece of cake.

Good luck!


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 1:57 pm
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I really don't think this is a power supply issue. The original post stated that it only happens in the Vibrato channel. No problems with the Normal channel. A PS unit issue would effect both channels.


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 2:24 pm
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You guys are awesome! Really appreciate you taking time to walk the elecro-rook through the steps!
Bass drops out with any guitar (none have active circuitry), and no the problem does not occur in Normal channel (thankfully cuz at least I've got a lower powered plan B in a pinch.)

Of course, the problem seems to have disappeared as quickly as it came. I will pull the chasis to take look for any obvious issues (not entirely certain how to correctly discharge capacitors so will have to be super careful). Will use insulated needle nose to gently check any suspect connections.

I will definitely check the jack connections to make sure it isn't something as simple as that...tho I am NEVER that lucky! But theres always a first time!

While I have volt/ohm meter, I have used it primarily for home circuit checks, auto work, etc. Never on electronics like this so ought to be an adventure!

Thanks again for your guidance!
I'll keep you posted on progress. If you think of anything else I can check please let me know!


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Sun Feb 22, 2015 2:56 pm
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BMW2002Ti wrote:
I really don't think this is a power supply issue. The original post stated that it only happens in the Vibrato channel. No problems with the Normal channel. A PS unit issue would effect both channels.


+1

I'm dubious that it's related to the power supply.

Arjay

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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Mon Feb 23, 2015 7:21 am
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kevink wrote:
Bass drops out with any guitar (none have active circuitry), and no the problem does not occur in Normal channel (thankfully cuz at least I've got a lower powered plan B in a pinch.)

Of course, the problem seems to have disappeared as quickly as it came. I will pull the chasis to take look for any obvious issues (not entirely certain how to correctly discharge capacitors so will have to be super careful). Will use insulated needle nose to gently check any suspect connections.

I will definitely check the jack connections to make sure it isn't something as simple as that...tho I am NEVER that lucky! But theres always a first time!

While I have volt/ohm meter, I have used it primarily for home circuit checks, auto work, etc. Never on electronics like this so ought to be an adventure!

Thanks again for your guidance!
I'll keep you posted on progress. If you think of anything else I can check please let me know!



1.) Problems coming and going often means there is a bad solder point(s) in the circuit. Movement causes the hinky point to contact & connect sometimes, then disconnect at other times. Prolly the best way to test for this is the wooden chopsticks method. CAUTION: amp will have to be on the bench & hot to do this test. So, if you feel very uncomfortable dealing with 500VDC circuits --- you might want a tech to do this test.

You have amp on, speakers connected. No guitar input. Moderate volume. Start tapping components. WOODEN chopstick. Keep your free hand off the chassis. The bad solder point(s) or faulty component will sound like a sharp static noise.

Plug your guitar in and gently shake the jack. Hear static? Maybe the jack is bad. Solder to input resistors is bad. Main input line from jack to tube is bad. Ground is bad.


2.) To discharge stored voltage in a Fender amp, all you need to do is turn off amp. Pull power line from outlet. NOW, turn the amp's power and standby switch on. The amp will drain the stored voltage through the PS rail into ground. It will take a few minutes. Place a DVM from B+ side of the PSU rail (red lead of DVM) and black lead to chassis. Measure the VDC. Once the voltage ~ 0VDC, you can work on amp (really, less than ~20VDC). Be sure to turn both switches you off, before plugging amp back into wall outlet.

Positive (B+) side marked by (+) on main caps, under the doghouse. Side with the power rail resistors.

Image


HTH! Keep us informed. :)


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Mon Feb 23, 2015 4:19 pm
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Agree it sounds like poor solder connection. That proved to be the case shown from last two visits to tech. It also makes sense to me as have come across similar sporadic failure working autos which often was bad wiring harness or damaged but not broken wiring elements. Have found most/all auto elec components either work or they don't. Gotta love binary problems!

I love the chopstick idea ... wood isn't much of a conductor. I am in final throws of finishing my home studio and will then lug the brute to my shop in basement and begin. Hopefully I will discover some obvious issues and can address, but since the "problem" appears to be hibernating, I'm not sure I'll hit pay dirt.

Thanks again to all! I may not end up finding and fixing the problem but am certainly smarter from this discussion!

Electro-Rook


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Tue Feb 24, 2015 8:30 am
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Other places to look... the solder on V2's socket and solder on the bass and volume pots. Here's a layout diagram with some places to look. Tone stack caps, bypass caps, and cathode resistors off V1 and V2. V2's socket tangs. Controls on Vibrato channel.

I could only find the layout of the AA270 TR, but the input/gain stage in the Vibrato channel should be similar to yours (AC568?)

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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Tue Feb 24, 2015 10:43 pm
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kevink wrote:
... wood isn't much of a conductor...


Think again. It depends on the amount of moisture in the wood. I would never depend on wood as an insulator, and I would never poke around any live electrical circuit with a wooded chopstick. :roll: Plastic is a much better insulator, and glass is even better but can break easily. There are still a few old electricians that can tell you some horror stories involving wooden handled screwdrivers. :shock:

Notice that there are no wood insulators on power poles, they are made of glass/ceramic. :wink:

Also, if you want to discharge the caps as advised above, be sure to do it while the tubes are still hot, otherwise they will not conduct and will not discharge the caps. Always check with a meter to be sure.

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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Tue Feb 24, 2015 11:15 pm
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shimmilou wrote:
kevink wrote:
... wood isn't much of a conductor...


Think again. It depends on the amount of moisture in the wood. I would never depend on wood as an insulator, and I would never poke around any live electrical circuit with a wooded chopstick. :roll: Plastic is a much better insulator, and glass is even better but can break easily. There are still a few old electricians that can tell you some horror stories involving wooden handled screwdrivers.


+1

I have an assortment of dielectric tuning tools left over from my days in radar maintenance. They're non-conductive and non-inductive, perfect for probing and diagnosing a microphonic component.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 25, 2015 12:39 am
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Yeah. Maybe one of these nylon probes would be better.

http://www.amazon.com/gp/offer-listing/ ... dition=new


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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 25, 2015 1:09 am
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Retroverbial wrote:
...I have an assortment of dielectric tuning tools left over from my days in radar maintenance...


I have a set that is probably similar, from my old TV repair days. Various shapes for tuning coils, variable pots and caps, and embarrassingly, I still have the pocket protector that held them. Does that mean that I am still a nerd? :oops: :lol:

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Post subject: Re: 1968 Twin Reverb - bass cuts out
Posted: Wed Feb 25, 2015 1:40 am
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shimmilou wrote:
Retroverbial wrote:
...I have an assortment of dielectric tuning tools left over from my days in radar maintenance...


I have a set that is probably similar, from my old TV repair days. Various shapes for tuning coils, variable pots and caps, and embarrassingly, I still have the pocket protector that held them. Does that mean that I am still a nerd? :oops: :lol:


LMAO

Mine are stored in a large roll-up kit......no way would it fit in my shirt pocket.

:wink:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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