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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Wed Nov 07, 2012 9:52 am
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Hi BMW,

Thanks for your reply.

Last year I did a near full recap of the amp so there could be an issue and I will confirm values etc. I also am now planning to investigate/replace all resistors.


I put all tube pins voltages in the files in the link above for each "tube in" scenario as you described. I will extract the values and repost. I don;t remeber, aside from Vibrato tube, any issues with readings, at least as compared to stated voltages on the schematic and layout diagrams.

With respect to coupling and bypass caps, which ones should I be targeting? I know about the two .1mfds at the left side of the layout board view. Are there some reasonable test points that I should be using for voltage reference points?

Also, in your original discussion around points A B C and the doghouse, what physical points am I supposed to be measuring to attain the test you describe?

Re: speakers. I have done no replacements since this is an investment heavy step given that I have no such spares lying around and the 2-ohm configuration makes plugging in an external cabinet something of a non-starter for me right now. If I can get the electrnoics sorted, a speaker replacement is probable, but until I get there I am not so interested in the investment until I am sure it will work out.

Thanks again for all of the help and guidance.

On quite a few occasions I have considered giving up on the project. But then I play my strat through the reverb channel and realize how special that sound, even with its currrent issue, really is. I have a Roland JC 77 as my "other" amp and, while I like it, it just has not got the magic and the reverb kinda sucks :-(

So I am looking forward to making progress on the project, but even with some of my electronics knowledge, soem f teh steps are not quite as clear to me as they used to be in my youth.

After I get the electronics sorted out, I plan to re-tolex the cabinet and re-grill the speaker baffle. I also have a replacement blackface panel for the final touches - yes it is a Silverface by current clothing, but the innards are all BF.

Chris
BMW2002Ti wrote:
taskerc, I agree with Shimmy. You should be able to pull V5 (vibrato tube), run a guitar into the NORMAL input and get tone (all other tubes in). If it's still funky... it's not the vibrato section or tube.

I'm starting to think you have a leaky coupling cap. None of the input/tone stage/phase inverter control grids (pins 2 and 7) had high readings?

Just looking at the amp, does it look like any coupling cap were replaced? Are they all in OEM mfd and voltage range?

And all of the bypass caps on the 12AX7 and 12AT7 cathode-to-ground were within normal OEM values?

And you had OK range of idle bias on those 6L6GC?

One last thing, can you sub another known, good speaker(s)? Just to rule out something funky with them. You may have already done this. :D

If all this fails, you may have to go section-by-section from the input to the output to find this problem. This can be done with a DVM. But, is easier with a signal generator and scope (read: good tech).

Sorry. I hope its a coupling cap or out-of-spec resistor or something else which is simple.


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Wed Nov 07, 2012 12:18 pm
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Chris,

Use ANY known, good full-range speaker. It doesn't need to be 2-ohms, for this test. You are looking for the same distorted tone, to rule out speaker issues. You want to do this first. To save yourself a TON of un-needed circuitry work.

Same distorted tone = circuit issues. No distortion = speaker issues.


Let's concnetrate on the NORMAL and VIBRATO inputs and the phase inverter section. See the drawing. Those caps circled in red are coupling caps. Those circled in green are bypass caps.

Image

In the photo the coupling caps are the big brown ones, some blue/white ones, and a couple orange ones . The bypass caps are the smaller green lettered/black ones. Now, your cap colors and shape could be different. I used this only as an example. Look at your LAYOUT (bottom of link). This shows where each component SHOULD be lying.

Image

http://www.webphix.com/schematic%20heav ... _schem.pdf

Points B, C,& D are the appropriate (+) points on each of the caps under the hood. Check the schematic and compare to cap layout. NOTE: polarity.

The reason amp power supplies are built this way is each cap/resistor section is a pi-filter network. Dropping noise a certain amount of dB. Now, each pi-filter network placed in parallel one after the other = additive filtering effect. And better decoupling. The most filtered section feeds the input tube. Which has the most gain. And is therefore most succeptible to noise.

Image

Good luck! Keep us posted. :mrgreen:


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Wed Nov 07, 2012 7:42 pm
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Thanks again BMW.

Now, I confess that I read your comment about tesing with any speaker and was considering that to be a wasted option. But for kicks and giggles, i wired up an old radio shack minimus 7 and NO DISTORTION. Beautiful, clean, jangly sound from the normal channel. Then over to Tremolo and same beautiful result. Plugged in original speakers and sure enough, top right speaker farting like cowboys around a fire inblazing saddles.

http://m.youtube.com/#/watch?v=R6dm9rN6oTs&desktop_uri=%2Fwatch%3Fv%3DR6dm9rN6oTs

Thank you for reminding me to go back to basics.

So now my only issue appears to be some motor-boating of reverb once the reverb setting passes 6. Any ideas on this one?

What a great day, and I am only kinda sad that i could have resolved this months ago.

Chris


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Post subject: Re: SRSF AB763 - revisited - crummy sound on low strings
Posted: Wed Nov 07, 2012 8:01 pm
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taskerc,

Good news!

Check the wiring layout and soldering points on the 12AT7 drive and 12AX7 recovery tubes in the reverb section. Also, the electrolytic cap powering those 12AT7 and 12AX7.

Smoetimes, even new caps can be under-spec'ed.


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