Jansen 1225 Combo

[TIMBO]

Hi guys, Doing another fixup.
Replaced all Ecaps, pots and input jack.
Sounds pretty good.
But as usual the trem is not working.
Contacted Jansen and I was able to get a schematic, component list and test points.
The test point for the trem is the emitter of T11 this being 20v P/P on the scope, but I was unable to get a measurement.
I replaced the transistors with BC109 as they are a good replacement.
Still nothing.
Any ideas where to start.
Thanks

[shooter]

What are the DC volts on T12, Collector and base?  see if there is any AC at those points also.

[PRR]

> I was unable to get a measurement.

Means what? Steady DC? Off the chart? You were out of town?

"Something" is there. What do you see?

[TIMBO]

Hi PRR, Some funky stuff going on.
What I mean is that the test point for the trem is the emitter of T11 this should show a reading of 20v P-P on the scope.
The scope is showing a flat line.
BUT whilst doing some prodding the sine signal stopped at The junction of the 180k and .1u after the volume pot.
Thinking that I did damage I replaced T10(BC109 and T9(BC178) and signal was now passing but again at some point the signal halted again at the same junction.
Even while the signal was passing there was change to the scope reading at T11 emitter.
T8 maybe.

[shooter]

at some point the signal halted again at the same junction.

what happens at the wiper of the depth pot?  do you have a constant sin wave there?
The trem should be a free standing oscillator, if you disconnect the .47uf does the osc come alive?

[sluckey]

BUT whilst doing some prodding the sine signal stopped at The junction of the 180k and .1u after the volume pot.

This sounds like you may be injecting a test signal into the amp? Don't do that. That circuit has nothing to do with the trem oscillator.

What I mean is that the test point for the trem is the emitter of T11 this should show a reading of 20v P-P on the scope. The scope is showing a flat line.

Stick to troubleshooting this symptom. The 20V_pp signal at T11 emitter will be a sine wave about 3Hz to 10Hz. It is not dependent on you to inject a signal into the amp. The circuit shown below is the only part of that amp you need to look at to get that trem signal to work.

[TIMBO]

The funky thing is that not only is not passing the sine signal, it's not passing the guitar signal.
I'm am getting the guitar sounds but this is through the reverb circuit that is connected the volume wiper.
Is injecting a 1k sine at the input doing damage???

[sluckey]

There's no problem putting a test signal into the amp, that is, as long as it's an appropriate signal level.

But you had said the amp sounded good, just no trem. And also you had no trem signal at the emitter of T11. So, the trem issue is in the little pic I posted.

[TIMBO]

Thanks sluckey, I seem to be having intermittent problems with T10 and T9 and I am assuming that if these are not working the trem won't work.
I have now replaced T10 with a BC109 and T9 with a BC177 and the amp is sounding great but NOooooooooo trem.
I understand that the trem circuit is based around T11 and T12 and similar to a valve trem.
I have replaced both T11 and T12 with NOS BC109.

[sluckey]

I have now replaced T10 with a BC109 and T9 with a BC177 and the amp is sounding great but NOooooooooo trem.

OK, so we're back to the only symptom is no trem. There cannot be any trem effect until the oscillator circuit (T11 and T12) is working. If you have +26V on the collector of T11 but don't have a 3Hz to 10Hz sine wave at T11 emitter, then the oscillator is not working and the problem has to be in the little pic I posted. It may be more convenient to connect a scope probe to the top side (ungrounded end) of the depth pot) to monitor the trem sine wave. Set the scope vertical to 1v/div and set the time base to .1s/div (may be called 100mS/div). Set triggering to internal and adjust for stable display. Use internal/auto trigger mode if you have it.

Any general purpose Si NPN such as 2N2222 or the ones you used should work, provided you connected them properly. If you still don't have a trem sine wave, replace the three 0.47µF caps between T11 and T12. If the speed pot is bad, the trem oscillator will not work.

There are other things that would prevent you from having a trem effect even if the oscillator circuit is working. T8 FET must be OK. It's not likely to be an issue. Or that bias adjust could be set wrong and kill the effect. But none of this matters if the T11/T12 oscillator is not working.

This is the order that I would check things...

1. Check 26V
2. Replace T11 and T12
3. Check speed pot
4. Replace three .047µF caps

That 100K and 1M resistor in T12 circuit seem like odd values for a transistor circuit.

[sluckey]

I just saw your thread about this amp on another forum and you had a pic of the chassis. Anyhow, seeing that thread brought a couple things to mind. We don't have a complete schematic for this amp. Are you sure this is the correct schematic? Is there a footswitch jack for the tremolo? If so, be aware that some (not all) trem circuits require a footswitch to operate.

[TIMBO]

Hey sluckey, Checked those points.
Got a little lost with the scope setting, but had fun anyway.
Was able to find three .47u.
NOW works a treat.
Those peskie caps had drifted all over the place.
With the speed pot turned to 0, a perfect 20vpp at the emitter of T11

There is another five of those caps in the circuit and only a couple were close.
Checked the other value caps and they weren't too bad.
If I get a bit board at sometime I'll complete the schematic.

A HUGE thanks for another job well done.

[sluckey]

Good deal! I'm always amazed at the brand name amps you keep finding. Most are unknown to me. Always enjoy your projects. Keep on postin'!

[shooter]

I'm always amazed at the brand name amps you keep finding

Same, must be all that island isolation and inbreeding

[Willabe]

Those peskie caps had drifted all over the place.

Bad caps! 

[TIMBO]

Hi guys, Removed all of the coupling type caps just to avoid any future problems, so everything is working and sounding great.

It has a distortion switch that uses two AC126 transistors as diodes (base-emitter junction) and has a very valve like quality, but I think it needs a little smoothing out.
From what I can find a small cap in parallel could do the trick.

[TIMBO]

Hi guys, Got the distortion sorted, not great but much better.

The amp was put through some tunes and came up a winner.
This one is done.