Marshall AS80R preamp trouble

[Randall]

Trying to trace the signal in this Acoustic Soloist amp. Problem is low output in Transducer channel. Piezo/preamp switch has no effect on output, so it seems it is stuck in preamp mode. I can see the switch raises and lowers the signal on the top preamp board thru IC 204A up to the top of R274, but at the bottom at the connection with R275 the signal is less and is not effected by the switch. Now I lose it at this point, because I don't see where it goes once it hits CON201.

Also, I have no signal on IC204B pin 6, but I do have signal that follows the Vol control on pin 7. C213 10uF is good. What to do next?

Marshall AS80R.pdf

[g1]

Are you assuming CN202 goes to CN201?
I don't think that is the case. (if so, the signal from R275 dead ends at -12V )
You have the amp, and it's a Marshall schematic , so you are going to have to trace which connector goes where and tell us.

[The Dude]

......You have the amp, and it's a Marshall schematic , so you are going to have to trace which connector goes where and tell us.

The thing I hate about Marshall schematics! They're a connector following nightmare.

[Randall]

Con202 goes to Con101 on the lower board. Then it goes to the Send and Return jacks, and then back to the upper board to R276 if I am not mistaken. So with a dummy plug in the Return jack I have no signal at R276 nor pin 7 of IC204B. Yet still I see a marked increase or decrease at the top of R274 and a smaller signal at the bottom of R274 that barely changes with the switch. I lifted the bottom of R274 and I get the response to the switch there. R275 and R212 measure good in circuit. What the hey is going on, why can't a get a change in level following the switch after R275?

Or, am I chasing the wrong thing? It has been known to happen!

[g1]

Sorry if I was not clear. JS201 is the main input jack?
R275 leads to CON202. Where does CON202 connect to?

[g1]

Ok, I think you meant CON202 goes to CON101.
Forget about the switch, it's not a level adjust, it's compensation for different impedances, pickup or preamp.
Are you saying you are injecting signal in the input, but have a dummy plug in the channel return jack?
Remove that dummy plug. You have signal at R274, how about at IC204 pin7, IC205 pin1,7,8,14? (adjust VR205)

[Randall]

Sorry, and edit above CON202 goes to CON101.

[Randall]

"Forget about the switch, it's not a level adjust, it's compensation for different impedances, pickup or preamp.
Are you saying you are injecting signal in the input, but have a dummy plug in the channel return jack?
Remove that dummy plug. You have signal at R274, how about at IC204 pin7, IC205 pin1,7,8,14? (adjust VR205)"

Really, the switch is just for impedance? A naked peizo vs a preamped signal is quite different level-wise.

Yes I am injecting signal into the input jack and see twice the signal level at R274 with the switch, which makes sense to me. If I remove the dummy plug I see signal at IC204 pin 7. Also at IC205 pins 1, 7, 8, 14, following VR205. But the volume doesn't seem loud enough, which is the main complaint, and the switch has no effect on the level, which seems to me it should.

[g1]

I'm just going by what it says in the manual. "with source select pushswitch for piezo or pre-amped input. Piezo - 10 meg input. Preamp - 100K input."
If the channel is weak in both switch settings, then no point in getting hung up on the switch.
If the channel sounds fine when using an acoustic that has onboard preamp (at normal guitar volume settings), then the channel itself is not weak and you are correct to focus on the switch.
I could be wrong about this, but I've chased my tail looking for lack of boost from onboard preamps, only to find that there wasn't much actual boost, the preamp was acting mostly as a buffer.

[g1]

And just to be sure, how about just doing a resistance check to make sure both sides (A&B) of SW203 are working, and solder is good.

[Randall]

This is getting on my nerves, this amp. The switch is confirmed functioning, resoldered it anyway. It makes a VERY slight change in treble, but that is all. I did realize I am testing it playing an acoustic guitar with a magnetic pickup, not a piezo if that makes a big difference. With a preamp engaged, it is loud enough, but nothing much happens on the volume control until about 12:00. So, perhaps I am trying to repair what is not actually broken? It SEEMS like the volume control should engage sooner, but I don't KNOW that it ever did. It SEEMS like the switch in question should do more than it does, but who really knows? Maybe this just wasn't Marshall's shining achievement, and it just is what it is?

So far after much time put into it, I have found nothing wrong in terms of bad parts or connections, etc. I did clean up the Return jack, and others, but I sure wish I had another one to A/B with. Someone talk me down of this ledge!

[g1]

All I can tell you is that was the same thing I found with the onboard preamp I repaired. I thought it should boost the signal but it did not. It just affected the frequency response a bit.
Is this the complaint it was brought to you for? I guess you have tried a jumper cable in that 'pedal loop' on that channel?

In case any others want to comment on the switch, it's SW203A and B in the picture attached. One thing I'm not sure of is how R210 factors in.

[J M Fahey]

All I can tell you is that was the same thing I found with the onboard preamp I repaired. I thought it should boost the signal but it did not. It just affected the frequency response a bit.
Is this the complaint it was brought to you for? I guess you have tried a jumper cable in that 'pedal loop' on that channel?

In case any others want to comment on the switch, it's SW203A and B in the picture attached. One thing I'm not sure of is how R210 factors in.

[ATTACH=CONFIG]45745[/ATTACH]

Donīt know what the front panel label says, but looks like a Piezo/Magnetic pickup selector switch.

With contacts to the left:
* input resistor net adds up to 101k (R209+R209A)
* IC204a gain is 1+(15k/14k7)=2X
* output is attenuated 3k3/(3k3+10k+470)=0.24X
* so total gain is 2*0.24X=0.5X

with contacts to the right:
* input resistor net adds up to 10M+
* IC204a gain is 1+(15k/4k7)=4X
* output is attenuated (6k8+3k3)/(6k8+3k3+10k+470)=0.5X
* so total gain is 4*0.5X=2X

so yes, there should be a not dramatic but noticeable gain difference.

If not, one of two switch halves is not working.

To test each on its own:
make a short pigtail wire, ground one end, test with the other:
* with switch on left, touch R210 top, gain should increase 3X
* with switch on right, touch top of R212, output should drop to about half.

Test with a steady tone; strumming a guitar might not be too consistent ... plus you need your hands free

As a side note:
* Op Amp input impedance is **very** high, maybe hundreds of Megohms, tens at least, because R251 is bootstrapped (there is basically same signal on both ends, so no current flows through it) so effective impedance is set by input net resistors only.
That said, any magnetic pickup is happy driving 10M so itīs not strictly necessary, but, hey, they had a spare switch section, so .....

[Randall]

OY! This is getting even more confusing. It doesn't seem the drawing follows the board regarding the switch (labeled Input Source piezo/preamp). With "switch on left" (actually pushed in) you would expect both R211 (10M) and R212 (6.8K) to be shorted to ground, but actually only R211 is shorted, R212 reads close to 6.8K. However in the opposite position (pushed out) the opposite is true, R211 reads 10M and R212 reads shorted. So what does this mean?

edit: I can now confirm the drawing is wrong. The B side of the switch is reversed, the top of R212 is connected to the right side of SW203B.

For some reason the circuit worked for a short while, using the switch made a noticeable change in gain, as does Juan's tests, yet now it has stopped working again. Yet the switch does seem to test good?

[J M Fahey]

It may be a flaky switch, flaky soldering, cracked track, many things.

If in doubt Iīd plain replace that switch and redo all involved soldering.