67 Super Reverb ugly distortion

[vintagekiki]

Praise The Scope!

Dear colleague
We have a similar work biography (72 years old engineer) From an early age I started with electronics as a radioamateur.
For a while, I was a traveling TV repairman, where the only instrument was an analog AVO meter.
When servicing through practice, I learned that the cause of failure must first be found, and then the defective part replaced. Every other procedure (applying any components not provided by schematics) is a eliminate consequences, and the actual failure still exists.

You will agree that when in a device all the passive and active components are correct, the voltage points are correct (as it is written in schematics), the device is correct. I have an oscilloscope since the '80s and as I said I used it in 5 - 10% of the time, mostly for servicing the SS amplifier.

Despite the language barrier, I regularly follow this portal, but it is just not clear to me except that with instruments with whom colleagues take photos, for repair they still use the advice from portals. Where is knowledge, where is experience gained?
In addition to being by profession qualified, I play bass from hobby, and just when I started playing, I came to the conclusion that a music equipment repairer, in addition to measuring equipment must have a guitar as a measuring tool.
Repaired amplifier using measuring instruments,doesn't have to mean it's 100% usable for a musician. The final test is to "stab" the guitar and hear if it sounds right.

And as you said back to the Super Reverb.

[Tone Meister]

I am just grateful that it doesn't cost me anything to learn from the many hundreds (thousands?) of years of experience that is available to the every day hobbyists/hacks like myself.

The give and take among the uber-experienced designers, engineers, and techs here is a post-graduate-level education for those who will pay attention.

[SoulFetish]

The fastest, accurate method of testing a transformer for a short is to disconnect any load from all the windings. Then connect a 9V battery through a momentary switch across the lower impedance winding. Then connect a neon lamp across the higher impedance winding. The tap the momentary switch a couple of times to pulse dc across the coil. This will cause a flyback voltage proportionally on every winding, and If the transformer is functioning properly, then the neon bulb will flash over and indicate there is no shorted turn.

[SoulFetish]

I built this little rig so all i have to do is run 2 pairs of test leads to the transformer

[vintagekiki]

[ATTACH=CONFIG]55387[/ATTACH]

[earache]

Hello again - back from vacation and gonna start the testing again.

First thing I will do is review all previous comments and make sure I have answered any questions, like:

“Quote Originally Posted by vintagekiki
Questions
- When FSR has ugly distortion, do you play it without pedals only guitar (clean) or through some pedals (effects).
- After the "general" service in 2011, is it the FSR was put in proper condition and how it worked then.
- When it appeared and what preceded the ugly distortion.”

The distortion is when played without any pedals or effects, just a guitar and a cable.
The Amp worked great after the service in 2011 and was played and gigged regularly.
The Amp began showing these symptoms at a band practice. It manifested only as the distortion, no smells or flashes or smoke.

[earache]

If you are one of the interested and helpful parties - I respectfully request that you might look over the previous posts again as well , this has been a long thread so far with lots of info.

Thank you for your interest, your knowledge and your time!

[earache]

I’ve got a dummy load put together and mounted to a heat sink.
The three resistors equal 2.8 ohms in parallel.




Also made a speaker cable with clips to connect it all



Tomorrow I will continue with some testing and will post results.

[vintagekiki]

The distortion is when played without any pedals or effects, just a guitar and a cable.
The Amp worked great after the service in 2011 and was played and gigged regularly.
The Amp began showing these symptoms at a band practice. It manifested only as the distortion, no smells or flashes or smoke.

Put new tubes in power amp GZ34, 12AT7 and 6L6GC.
Disconnect .001uF from mix network 2 x 220k and temporarily connect to guitar jack (red on schematics) as it is on schematics.
Plug the guitar into the jack. You should hear 6-7W clear sound from the guitar.

If it's still ugly distortion:
- Check 1k / 1W and 4k7 / 1W and all solder joints (metal threads) in the filtration chain from stby sw to point + 410V.
- Output transformer
- May be oxidize the ground from speaker jacks. Clean the chassis below speaker jacks (replace jack) ¹⁾

Perform all measurements on the power amp via temporarily guitar jack.



¹⁾ https://music-electronics-forum.com/showthread.php?t=49447&p=535958&viewfull=1#post535958

[vintagekiki]

[ATTACH=CONFIG]55475[/ATTACH]

Mount female guitar jack on Heat Sink.
The clips connection is unreliable.

[Helmholtz]

The clips connection is unreliable.

I was going to say the same thing. These clips are not suitable for high currents and may add considerable contact resistance (easily 0.5 Ohm or more). The load current will be up to 5A! So the lousy clip contacts would steal e.g. P=5*5*0.5=12.5W from your output and get very hot.

Best is soldering the speaker cable to the dummy resistors with such high currents and low load resistance.

[earache]

Vintagekiki - gonna take a stab at this, but a couple comments/questions first:

I have already installed new power tubes and a new 5AR4.

I donÂ’t have a new 12AT7, but I do have other ones that I know work (from other amps)

I may have to rig up something so I can use my IPod Touch as the signal source, unless I can figure out some other way/source...I have a small multimeter than puts out a square wave, could that work? Here is its signal:

[earache]

Put new tubes in power amp GZ34, 12AT7 and 6L6GC.
Disconnect .001uF from mix network 2 x 220k and temporarily connect to guitar jack (red on schematics) as it is on schematics.
Plug the guitar into the jack. You should hear 6-7W clear sound from the guitar.

If it's still ugly distortion:
- Check 1k / 1W and 4k7 / 1W and all solder joints (metal threads) in the filtration chain from stby sw to point + 410V.
- Output transformer
- May be oxidize the ground from speaker jacks. Clean the chassis below speaker jacks (replace jack) ¹⁾

Perform all measurements on the power amp via temporarily guitar jack.

[ATTACH=CONFIG]55477[/ATTACH]

¹⁾ https://music-electronics-forum.com/showthread.php?t=49447&p=535958&viewfull=1#post535958

Does this look ok?



I have the shield connected to ground...

[earache]

So with the guitar plugged in as illustrated above, there’s no “ugly distortion” - just the sound of an amplified guitar.

The volumes on either channel do not work as you would guess, so I can’t turn it up at all.
I hope that this indicates that the output section is ok...

I have a video of the test if you need to see it

You said I should “Perform all measurements on the power amp via temporarily guitar jack”

What measurements and how? Should I try to inject a signal there and measure at the output?

[vintagekiki]

So with the guitar plugged in as illustrated above, there’s no “ugly distortion” - just the sound of an amplified guitar.

The volumes on either channel do not work as you would guess, so I can’t turn it up at all.
I hope that this indicates that the output section is ok...

I have a video of the test if you need to see it

You said I should “Perform all measurements on the power amp via temporarily guitar jack”

What measurements and how? Should I try to inject a signal there and measure at the output?

Excellent.

No ugly distortion indicates that the output section is ok.
Inject a sine signal to the power amp via temporarily guitar jack, and see what signal and how much amplitude is on the dummy load.
If you don't have a clear sine signal, get it from here:

https://music-electronics-forum.com/showthread.php?t=49793&page=2&p=540317&viewfull=1#post54 0317

Increase the amplitude from the source signal slowly until on the oscilloscope which is connected in parallel to the dummy load appears the clipping. The first sign when a sine signal goes to clipping is the pure power of the amplifier.
If the output section is correct, on the dummy load (2 Ohm) should appear voltage of approximately 10VAC (voltage is read on an AC voltmeter)

When found that the output section is correct, return .001uF in its place and put new 12AX7 in socket for normal channel (first tube right on layout).
Plug the speaker into the spkr jack, and plug the guitar into the jack of the normal channel.
What's the sound on normal channel clean or ugly?

Tip
All tests on the amplifier are done with a clean sine signal.