It will not be a lartge signal on the output, but if you want to check it, connect a cord to the OUTPUT jack on the pan and simply feed it to some other amp.
In fact an old trick for amps like a Deluxe reverb was to unplug the cable from the pan output up to the amp chassis, and instead run a cord from the pan out to the (Usually unused) "normal" channel of the amp. Now that plain channel becomes the reverb controls, with tone and everything. Just shows the idea of plugging a pan into an amp to test is valid.
In fact an old trick for amps like a Deluxe reverb was to unplug the cable from the pan output up to the amp chassis, and instead run a cord from the pan out to the (Usually unused) "normal" channel of the amp. Now that plain channel becomes the reverb controls, with tone and everything. Just shows the idea of plugging a pan into an amp to test is valid.
With amp on, probed reverb circuit with a bare wire connected to finger, listening for buzz/hum through speaker cab. Results shown below:
Ran reverb tank output into input of second amp. No tone was present through second amp, but I could hear the spring sound when I jostled the reverb tank.
Does this point to a faulty fourth op amp on IC1? Also, shouldn't the CON1 connector on page 3 of the schematic be CON2, or am I reading the schematic incorrectly?
- No buzz when touching tip of PH2
- No buzz when touching pin 12 of IC1
- Slight popping (wouldn't call it a buzz) when touching pin 14 of IC1
Ran reverb tank output into input of second amp. No tone was present through second amp, but I could hear the spring sound when I jostled the reverb tank.
Does this point to a faulty fourth op amp on IC1? Also, shouldn't the CON1 connector on page 3 of the schematic be CON2, or am I reading the schematic incorrectly?
Post #59:
"Jumped finger to pin 12 of IC1 and heard buzz through speaker. "
Post #83:
"With amp on... No buzz when touching tip of PH2, No buzz when touching pin 12 of IC1"
What went wrong between these two posts?
OK, The reverb pan is working as you said in post #83.
The level of hum you introduce at pin 14 is a bit low to hear it through the speaker. Allthough you should here something.
The connector numberig is correct. Write on your copy that con1 on page 4 go's to con2 on page 2.
Better is to say that con1 on drawing V100-61-02 go's to con2 on drawing V100-60-02.
You will notice the numbering is not the sme; one is -61- and the other is -60-.
These are two different boards and hence the numbering of the connectors isn't the same.
But back to your problem.
IC1d can be bad. But the output of IC1d (pin 14) go's to pin 6/con2 - pin 6/con1 and then to IC2. That one is controlled by a switching voltage on pin 1. The output of that IC go's to pin10/con1 - pin 10/con2 and then called REVOUT and fed into the grid of V1 via C6(?) - should be a resistor of 10K.
But I assume that's ok unless there is a cap of 10n instead of a resistor of 10K.
But the unit would never pass the audio test at Marshall...
So, first thing first.
Is there any dc on pin 12, 13 and 14 of IC1d.
If so, you are sure it's dead and you have to replace it.
If not; check for a voltage on pin 1 of IC2. I can't tell from memory whatthe voltage should be at that point.
Is there a change in voltage if you push the switch clean / overdive channel?
And, pardon me asking, are the reverb potmeters wide open?
"Jumped finger to pin 12 of IC1 and heard buzz through speaker. "
Post #83:
"With amp on... No buzz when touching tip of PH2, No buzz when touching pin 12 of IC1"
What went wrong between these two posts?
OK, The reverb pan is working as you said in post #83.
The level of hum you introduce at pin 14 is a bit low to hear it through the speaker. Allthough you should here something.
The connector numberig is correct. Write on your copy that con1 on page 4 go's to con2 on page 2.
Better is to say that con1 on drawing V100-61-02 go's to con2 on drawing V100-60-02.
You will notice the numbering is not the sme; one is -61- and the other is -60-.
These are two different boards and hence the numbering of the connectors isn't the same.
But back to your problem.
IC1d can be bad. But the output of IC1d (pin 14) go's to pin 6/con2 - pin 6/con1 and then to IC2. That one is controlled by a switching voltage on pin 1. The output of that IC go's to pin10/con1 - pin 10/con2 and then called REVOUT and fed into the grid of V1 via C6(?) - should be a resistor of 10K.
But I assume that's ok unless there is a cap of 10n instead of a resistor of 10K.
But the unit would never pass the audio test at Marshall...
So, first thing first.
Is there any dc on pin 12, 13 and 14 of IC1d.
If so, you are sure it's dead and you have to replace it.
If not; check for a voltage on pin 1 of IC2. I can't tell from memory whatthe voltage should be at that point.
Is there a change in voltage if you push the switch clean / overdive channel?
And, pardon me asking, are the reverb potmeters wide open?
Checked DC voltage on IC1 with amp powered up, clean channel selected, all reverb pots set at 10. All measurements with respect to chassis ground.
IC1 DC Voltage
Pin 12: 5.7 VDC
Pin 13: 10.7 VDC
Pin 14: 11.3 VDC
Voltages remained the same with or without signal through amp input.
Should I replace IC1 and recheck voltages?
IC1 DC Voltage
Pin 12: 5.7 VDC
Pin 13: 10.7 VDC
Pin 14: 11.3 VDC
Voltages remained the same with or without signal through amp input.
Should I replace IC1 and recheck voltages?
Marshall Valvestate VS100 head - 20VDC on speaker output - help to diagnose
This is my first post on this fantastic website. I have found several solutions to problems over the years and didn't have to post until now. I am a tube amp tech so Solid state issues other than obvious non-circuit type problems (bad pots, bad solder joints, bad jacks & speakers - fixed no issues) I have limited experience - I have repaired simpler foot pedal circuits, but this one is a real component level issue. I am working with a colleague who also repairs tube amps on this Marshall VS100 - he called me for guidance and is ready to ship me the amp chassis. The symptom is loud hum on speaker output, he is reading 20 DCV on speaker output and he has ordered new output transistors. We walked through the power PCB after verifying the preamp is functioning correctly. found a couple of bad diodes and 2 burnt resistors that read correctly but were obviously overheated.
I can provide details as to which parts were replaced, we have a schematic but no voltages are listed on this schematic so I am kind of stuck trying to find the cause of this problem. we have not replaced power transistors yet because I want to make sure everything else is OK before we risk burning new ones up. As soon as I am on your radar for troubleshooting, I will have him send me the chassis so I can work with you - I have many decades of electronics repair experience so I have the necessary test equipment - Transistor tester (Peak Atlas Pro 75), Heathkit Operating voltage Cap Tester and DER DE 5000 LCR as well as analog and digital VOMs and a scope & frequency generator.
Can you walk me through the diagnosis process? Should I start a new Post? Not sure of the correct process, my apologies
Thanks
Chris
This is my first post on this fantastic website. I have found several solutions to problems over the years and didn't have to post until now. I am a tube amp tech so Solid state issues other than obvious non-circuit type problems (bad pots, bad solder joints, bad jacks & speakers - fixed no issues) I have limited experience - I have repaired simpler foot pedal circuits, but this one is a real component level issue. I am working with a colleague who also repairs tube amps on this Marshall VS100 - he called me for guidance and is ready to ship me the amp chassis. The symptom is loud hum on speaker output, he is reading 20 DCV on speaker output and he has ordered new output transistors. We walked through the power PCB after verifying the preamp is functioning correctly. found a couple of bad diodes and 2 burnt resistors that read correctly but were obviously overheated.
I can provide details as to which parts were replaced, we have a schematic but no voltages are listed on this schematic so I am kind of stuck trying to find the cause of this problem. we have not replaced power transistors yet because I want to make sure everything else is OK before we risk burning new ones up. As soon as I am on your radar for troubleshooting, I will have him send me the chassis so I can work with you - I have many decades of electronics repair experience so I have the necessary test equipment - Transistor tester (Peak Atlas Pro 75), Heathkit Operating voltage Cap Tester and DER DE 5000 LCR as well as analog and digital VOMs and a scope & frequency generator.
Can you walk me through the diagnosis process? Should I start a new Post? Not sure of the correct process, my apologies
Thanks
Chris
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