Marshall VS65R signal path help?

[DrGonz78]

Right, ok so I have got measurements for all pins of all IC's with and without a signal input. AC and DC.

            No Signal        AC       Signal                                No Signal       DC         Signal

IC2
1            216mV                   207mV                                     [U]15V                              15V[/U]
2             0                            0                                  0                                  0
3             0                            0                                  0                                  0
4             0                            0                                  -15V                             -15V
5             0                            0                                   0                                 0
6             0                            0                                   0                                 0
7             0                            0                                   0                                 0
8             0                            0                                   15V                              15V

IC4
1             0                            0                                  [U]15V                              15V[/U]
2             0                            0                                  0                                  0
3             0                            0                                  0                                  0
4             0                            0                                  -15V                             -15V
5             0                            0                                  0                                  0
6             0                            7mV                                0                                  0
7             0                            0                                  0                                  0
8             0                            0                                  15V                              15V

IC5
1             0                            0                                  [U]15V                             15V[/U]
2             14mV                        16mV                                8mV                            8mV
3             0                            0                                  8mV                            8mV
4             0                            0                                  -15V                           -15V
5             0                            0                                  17mV                          17mV
6             0                            0                                  9mV                            9mV
7             0                            0                                  0                                 0
8             0                            0                                 15V                             15V

IC6
1             0                            0                                  [U]-15V                            -15V[/U]
2             0                            0                                   0                                0
3             21mV                        21mV                                 0                                0
4             0                            0                                  -15V                           -15V
5             0                            0                                 [U] -15V                            -15V[/U]
6             0                            0                                   0                                 0
7             0                            0                                  15V                             15V
8             0                            0                                   0                                 0

IC2, IC4 and IC5 has 15v DC on pin 1 which seems suspicious, I don't think that should be there. Also, IC6 has -15v on pin1 and pin 5. Now perhaps they use those null pins as a comparator circuit or something. Does anyone agree at least that we should not have 15v on pin 1 or IC2, IC4 or IC5?

[allante666]

IC2, IC4 and IC5 has 15v DC on pin 1 which seems suspicious, I don't think that should be there. Also, IC6 has -15v on pin1 and pin 5. Now perhaps they use those null pins as a comparator circuit or something. Does anyone agree at least that we should not have 15v on pin 1 or IC2, IC4 or IC5?

No idea whats happened here I cant even see the last few posts in the right order! Ok one thing I should mention is that on IC2 pin 1, the voltage swings between 15.3 and 14.9 depending on the channel switch position. If that helps? ( I figured it had something to do with the switching but cant see the diagram now for some reason)

[DrGonz78]

No idea whats happened here I cant even see the last few posts in the right order!

Look at the top right of this thread, right under the heart symbol "Likes" notifier, and look for the Display drop down box setting. Set it to linear mode so it will be less confusing. Many times as threads get longer the hybrid stuff messes it all up. Also are you on a computer or a phone looking at the site?

No Signal AC Signal No Signal DC Signal

IC2
1 216mV 207mV 15V 15V
2 0 0 0 0
3 0 0 0 0
4 0 0 -15V -15V
5 0 0 0 0
6 0 0 0 0
7 0 0 0 0
8 0 0 15V 15V

IC4
1 0 0 15V 15V
2 0 0 0 0
3 0 0 0 0
4 0 0 -15V -15V
5 0 0 0 0
6 0 7mV 0 0
7 0 0 0 0
8 0 0 15V 15V

IC5
1 0 0 15V 15V
2 14mV 16mV 8mV 8mV
3 0 0 8mV 8mV
4 0 0 -15V -15V
5 0 0 17mV 17mV
6 0 0 9mV 9mV
7 0 0 0 0
8 0 0 15V 15V

IC6
1 0 0 -15V -15V
2 0 0 0 0
3 21mV 21mV 0 0
4 0 0 -15V -15V
5 0 0 -15V -15V
6 0 0 0 0
7 0 0 15V 15V
8 0 0 0 0

[g1]

Does anyone agree at least that we should not have 15v on pin 1 or IC2, IC4 or IC5?

IC2,4,& 5 are M5201 switching op amps, pin 1 is a control pin, so DC voltage there is fine.
OC Disorder posted a real nice signal schematic in post #58. It appears that signal is getting through IC3, but not through the IC2 switcher. (assuming green channel selected and VR1 turned up)

[allante666]

Hadnt actually noticed that that was a signal path diagram, I just saw the thumbnail and thought it was just a repost of the diagram. VR1 wasnt turned all the way up when I took measrements but was probably on about 2-3 so that I could hear the tone.

[allante666]

So, I tried again but with this time I had VR1 fully up and also VR4 and had the green channel on. (signal of 120mV into guitar input) I measured AC on the pins of IC2 and also IC 5
IC2 came out differet this time, I had (all in mV ac) 207 on pin 1 same as before but with the volume full up I had 880 on pins 2, 3 and 5 (and that changed with the volume so wasnt just spurious noise) Nothing on 4, 7 or 8 and 466 on pin 6. IC5 however showed nothing AC on any pin except for tneh 16mV on pin2 as before. Does this point to somewhere around the tube? (I did swap that for a new one to check it)

[g1]

Caution as tube is running on high voltage.
Might as well check that F3 fuse is good before going any further.
Then check DC with no signal at tube pins 1,2,3,6,7, & 8.
Then check AC at same pins (with signal applied same as conditions for post #66).

[allante666]

OK so, F3 is ok.

No signal (but still plugged in to jack) I have DC readings of:-

1=355
2=196
3=199
6=196
7=0
8=1.6


With signal input and VR1 up (same as #66) I have AC values of:-

1=0
2=35
3=20mV
6=35
7=840mV
8=355mV

[g1]

So 35VAC at pins 2 & 6? Have you verified they respond to VR1 changes and no ghost readings?
It appears the signal is lost between pins 2 and 3. (shown as right side of tube on drawing)

[allante666]

Just checked and yes the 35vac does respond to VR1 nicely. Are you saying that pins 2+6 are correct and that pin 3 should have a signal output on it of more than 35vac?

I havent got time to dig further tonight now but if thats the case woudl that make R19 suspect? It doesnt measure low but then again it doesnt measure anything, Its still in circuit though. If it was open would that cause it?

[DrGonz78]

Just checked and yes the 35vac does respond to VR1 nicely. Are you saying that pins 2+6 are correct and that pin 3 should have a signal output on it of more than 35vac?

I havent got time to dig further tonight now but if thats the case woudl that make R19 suspect? It doesnt measure low but then again it doesnt measure anything, Its still in circuit though. If it was open would that cause it?

Yes pin 3 should have the signal output coming out through the cathode. R19 is a cathode load resistor and the overall arrangement for that tube is what we call a cathode follower. Typically the AC signal will ride on the DC voltage of the plate and the DC voltage will be stripped off by a coupling cap that feed the next stage. In the cathode follower the AC signal rides the DC voltage at the cathode similarly to how the plate does in the example before. Not sure exactly what would happen is R19 was extremely high resistance or open, but you need to verify it since replacing the tube did not fix the problem.

[allante666]

SUCCESS! It was R19 o/c. All I can say is many many thanks to all who have helped with this. Valves arent my strong point hence why I only do solid state stuff, I wouldnt have found it wthout you guys. I really appreciate it, its been an enlightening and at times frustrating journey. Thanks also to oc disorder for the signal map, not sure if you just happened to have it ready done or you did it specially!

[allante666]

Just a quuick addendum to this (Already typed this once and it disappeared so apologies if it pops up twice). After replacing R19 and getting full sound back I found it to have a little bit too much hiss at idle. It only hissed with a lead plugged in and and didnt respond to any controls. Cure was to replace IC7 wich must have been noisy. Ironically I had replaced this with a brand new one to begin with!