Traynor YGM-3 Dazed and Confused Input

[misterc57]

This is a two input version. See page 3 of attached schematic.

Plug inserted only to input one, tip is 1M to ground

Plug inserted only to input two, tip is 137K to ground (unless there is a plug also inserted into input one, then it is 1M to ground.

Issue I have is a very distorted input one, and a better only slightly distorted input two.

Both inputs feed pin 7 of V1 through a 68K resistor and a 1M ground resistor.

Scope pics below show input one and input two, 1K injected signal, volume set at one.

Traynor-Guitarmate-YGM3-Schematic (1).pdf

[Helmholtz]

Plug inserted only to input one, tip is 1M to ground

Plug inserted only to input two, tip is 137K to ground (unless there is a plug also inserted into input one, then it is 1M to ground.

That's normal.
Inputs are wired as with a typical Fender amp.
To make the inputs different sensitivity, the 68k resistors are wired as a voltage divider via the switching contacts for the lower sensitivity input.
So input impedance is essentially 2x68k = 136k for the low input and 1M for the high input.

Where did you scope signals? Speaker output?
Is your probe well compensated?
What does the sig gen signal look like and what is its level?

[mozz]

Where are you measuring the signal at? What kind of input signal? What you are describing is normal because second input is wired that way. Check on V1 output after the cap to the next stage.

[misterc57]

Measuring at the output/dummy load, direct cable into the scope (no probe).

Input is a 1K signal at 148 mV.

So then there is no issue? Use input 2 for regular guitar signal and input 1 for a high gain.

Thanks!

[misterc57]

Help me understand my readings.

When I said input signal measures 148 mV I was looking at my DMM AC reading.

I then connected the scope to the same measuring point (at input one), and if I am reading the scope correctly I am seeing 50 mV signal.

Which is it, 50mV or 145mV?

Arghh. Sorry my DMM was not grounded. Actually reading 223 mV on the DMM, not 145 mV

Thank you.

[g1]

50mV RMS is right about 148mV peak-to-peak. Is it possible that those are what is showing for the scope and meter?
Have you tried the amp with a guitar? Can you get a clean sound?

[Helmholtz]

A DMM reads RMS value.
With a scope we typically read peak-to-peak.
Peak-to-peak voltage is RMS voltage times 2.82, but only with a clean, undistorted sine.

[g1]

A DMM reads RMS value.
With a scope we typically read peak-to-peak.

That's what I thought, but I'm not familiar with all types test equipment. I figured at least the scope may be able to be set to display an RMS reading.

In any case, I don't see any obvious distortion from the test tone, so maybe the guitar volume just has to be backed off a bit?

[Helmholtz]

I figured at least the scope may be able to be set to display an RMS reading.

But that might require the operator to read the scope's manual .
All I can say is, that RMS value always is lower than peak-to-peak value.

I suggest to compare scope channels.
Also scope input impedance should not be set to 50 Ohm.

[misterc57]

A lot of the scope functions and when reading the manual are over my head. I have been weak on using the scope but I want to learn and make it a regular part of my diagnostics tools, so starting to use it every time and learn how to use it correctly.

Makes sense that peak to peak should always be higher than RMS. I must be reading the scope wrong when reporting 50 mV at the input jack.

When you say input imp at 50 ohms, where is that set? and compare scope channels?

Just tried a guitar at input two and sounds pretty good, no issues. There is a very slight background hum at idle (i think 60 hz). I have all new filter caps installed.

Thank you

[Helmholtz]

When you say input imp at 50 ohms, where is that set?

There should be a channel input settings menu - consult manual.

and compare scope channels?

Using CH2 might show if CH1 is shot/out of calibration.

BTW, one of the most revealing amp tests is checking output power.
Takes 5 minutes if well prepared.

[mozz]

Automatic Measurements: Period, frequency, cycle RMS, mean, peak-to-peak

You should be able to get on screen measurements of all the above, then save it as a preset. There will be a button or menu to let you choose between 50 ohm or 1 meg, but probably need the probe hooked in for that.

[loudthud]

If you don't know how to adjust "Probe Compensation", see my post #61 in this thread: https://music-electronics-forum.com/...4060-200s-sunn.

The TDS210 is a 60MHz scope, I doubt it has the option of 50 Ohm inputs.

[mozz]

I don't see 50 ohms in the owners manual. Most of the scopes I see always have 50 ohm options. Anyway I think he should be using the probe, put on 10x and 10x in the menus.

[g1]

How did the guitar sound with input #1, still distorted? At what guitar volume level does it start to distort? What kind of pickups in guitar?
It's possible the amp is working fine, but this is all good experience for setting up the scope and power output measurement procedures anyway.
Agree with the others that direct into the scope with no probe is not a good idea for now.