Markley CD-120/JMF Spectra Channel Switching

[Rhodesplyr]

Last weekend, I bought a Dean Markley CD-120 amp, apparently the same as the JMF Spectra 120-7. The previous owner reported to me that the overdrive channel didn't work, and I read online that the problem is usually in the solid-state switching. Someone had replaced at least one of the JFETs in a previous attempt at repair, but I also found a 220k plate resistor to one of the 12AX7 sections completely open. After replacing that resistor, the channel switching works.

However, I know from experience that it's possible for things to "work" without working correctly, so does anyone know how to go about determining if the channel switching circuits are really working 100% as they were designed to? The hybrid tube/SS nature of the circuit combined with a sketchy schematic has me a bit confused. For example, the schematic I have doesn't show the footswitch wiring or connections, and it notably lacks most static DC voltages (for obvious reasons, given that the amp has a provision for disabling two of the output tubes for reduced power).

I understand that the circuit is designed to short the input of one channel or the other by using an N-Channel JFET (2N4393) as a grid-leak resistor. With the JFET 'On,' that channel's input is shorted to ground. But I find around -3V on the grid of the overdrive channel's input AX7 with that channel turned off, putting that tube section into complete shutoff, which is a bit different from just shorting the grid to ground.

And then there's the confusing part about the hybrid tube/solid state preamp section's ground being isolated from the main amp ground via a 47 Ohm resistor, so I'm not sure if some imbalance of the +/- 18V supply with respect to one ground or the other is expected/acceptable.

Any help from anyone who's had to think this through in the past is much appreciated!

Thanks!
David

P.S. I have seen an active grid-leak resistor used this way before; Sherwood's tube FM tuners used a triode as a grid resistor to implement interstation muting. And, frankly, based on what I've seen, Sherwood's design is more reliable :-)

[Enzo]

perhaps if you posted or linked to the schematic you are using, we could refer to it for commenting?

[Rhodesplyr]

Enzo,

Good point. I only have a paper copy, so I'll have to scan it.

Funny story--I read online that Terry Laul of Kansas was the designer of these amps and the ultimate authority on them, so I made up a list of specific questions, so as not to waste his time, and called him last evening to see what I could find out. I assumed that I was asking him about a 25 year old out-of-production design, but, completely unknown to me, Dean Markley is apparently planning to reissue some of these in the near future. I was told that they had called Mr. Laul last week, and, apparently, he is angry with how they are planning to proceed. They also reportedly asked him some specific questions about the design that he was unwilling to answer since he was unhappy with them.

The schematic is a bit unclear on some points, so it was only logical that I asked some of the same questions, but because I did, he became suspicious that I was someone sent to conduct industrial espionage for Dean Markley and hung up on me!!

Given the situation, I can understand his suspicions, but he hung up on me as I was trying to explain myself.

I guess I need to bring him the broomstick of the Wicked Witch of the West....

David

[Rhodesplyr]

I've uploaded the schematics. It's on four separate pages.

Terry Laul mentioned a "floating ground" for the preamp section; on the schematic, the preamp ground is separated from the output tube plate voltage ground by a 47 Ohm resistor. I think that's what had me confused since I'm used to there being a single ground potential in a given amp.

[Enzo]

Oh no, some peavey amps have as many as 6 grounds. other amps as well. They may all be at chassis level, but they are kept separate and only join by way of jack grounds and the like.

Your two grounds may have a 47 ohm resistor, but they may also be grounded at the jacks. Measure that 47 ohm resistor while it is in the circuit. if it measures abiout 47 ohms then the grounds are not continuous. If it measures shorted, then all the grounds go to chassis, the resistor is just insurance against a poor jack ground.

Impedance issues won;t silence a whole channel.

If your lead channel is dead, trace a signal through it. The channel is basically V2. Are the DC voltages reasonable around that tube? Apply signal to the amp, is it present at each plate? neither? Just one? Is it present at one side but not the other of either volume controls?

Are both +15 and -15 power supplies working?

Is the JFET shorted? Leave the amp powered, and set for lead channel, then measure resistance from that grid to ground. or basically from grid to ground. If the resistance is low, then either the FET is shorted, or the control circuit is stuck.

[Tubeampsonly1]

Sorry to cut in on this thread, I am new to this forum. I am looking for help. I recently acquired a Dean Markley CD-40[tan] I have been looking for a channel switcher and can't find one or any information on alternatives[jumping?] to access the drive channel. The clean channel sounds great and jumping the effects loop actually gives a little more grit but its just too loud for most of the venues I play. Can anyone point me in the right direction to either buy a foot switch[best option ] or build one? Again sorry for butting in.

[Jazz P Bass]

The schematic shows that the Distortion Channel footswitch pin must be pulled to ground to activate it.

The footswitch itself is a simple 2 button/ maintained switch.
The switches are on the tip & ring of a stereo plug.
(the schematic does not indicate the specific functionality. It is either Distortion or Reverb)
The sleeve contact is chassis ground.

[g1]

JPB, that's for a 30, the 40 seems to be like the 120, 3 button latching, 2 LED's and from what I gather, a multi-pin connector of some sort.

[Jazz P Bass]

So then the 3rd button would be for turn the FX on & off.

I cannot find a decent rear panel picture of the amp, so the OP will have to inform us of the plug configuration.

It still holds true that grounding the Ch, Rvb & FX inputs will change there state.

[Jazz P Bass]

Well I found a pic of the internals of the 3 button footswitch.
Ch & FX have an indicator LED.

4 wires.

My take on the LED's is that they would be in series.

The connector is on the underside of the chassis, beneath the FX loop connectors.

[g1]

My take on the LED's is that they would be in series.

Yep, in your pic the blue wire goes to one leg of the led which is covered by the yellow heatshrink. The cathode of the led then goes to the switch. Other side of switch is going to common ground.

Edit: That would be for the channel switch led (red). For the fx switch led, in the cd120 schem. it looks like the anode of the led would go to the switch?

[Gtr_tech]

I have a Signature 60 and love it to death. I made my usual changes to the front end circuitry and changed the bias circuit so it doesn't rely on the pot's wiper to pass voltage (a very common source of failure in these). Terry really hit the mark on this one with the active mid circuit. I just wish they'd left more clearance to the top cover...most current prod tubes won't clear it.

One thing I would have done different is to have a seperate winding for the low voltage stuff. I don't like hanging op amps and zeners off the bias tap. Carvin did this on the early X amps but wised up by the mid 80s and gave the op amps and switching stuff its own winding.

I currently gig a '82 X60 1-12 for smaller venues. Its getting time to replace main filters, so when I take it down for maintenance it will get a small transformer added for a low voltage supply.

[Charmless]

I have a Signature 60 and love it to death. I made my usual changes to the front end circuitry and changed the bias circuit so it doesn't rely on the pot's wiper to pass voltage (a very common source of failure in these). Terry really hit the mark on this one with the active mid circuit. I just wish they'd left more clearance to the top cover...most current prod tubes won't clear it.

One thing I would have done different is to have a seperate winding for the low voltage stuff. I don't like hanging op amps and zeners off the bias tap. Carvin did this on the early X amps but wised up by the mid 80s and gave the op amps and switching stuff its own winding.

I currently gig a '82 X60 1-12 for smaller venues. Its getting time to replace main filters, so when I take it down for maintenance it will get a small transformer added for a low voltage supply.

I'd love to hear about the changes you made to the front end!

[SoulFetish]

I'd love to hear about the changes you made to the front end!

Hopefully he still remembers