Marshall TSL 602, loud hum...any ideas?

[Dave Johnson]

I have a Marshall TSL 602 combo that hums loudly. It never used to do this. If you turn on the power & stand by switches togeather, the hum comes on as the tubes warm. I normally turn on the power, then turn on the standby a minute or so later. As soon as I powered it up today and took it off standby, it started humming. The hum is there with or without a guitar being plugged in. I tried a 2 prong adaptor for the power cord, still hums. Removed the preamp tubes one at a time, no change. Removed the power tubes one at a time, the hum got quieter on one power tube, but was still there. I bought this amp new about six years ago, never messed with it, still original tubes. Marshall branded preamp tubes and Russian
EL 34's. This amp has no more than 50 or so playing hours on it. I don't have another set of EL 34's to try. Any thoughts would be appreciated. Thanks, Dave.

[stratman_el84]

Sorry to hear of your problem.

At this point, I'd recommend taking the amp to a tech. I'd advise against replacing the power tubes yourself unless you're able to set the bias correctly. It sounds like the power tubes are indeed the problem from your description, though it's impossible to be sure without having it on the bench. It's possible that a filter cap may be going bad, though not as likely seeing as the amp isn't all that old.

I'd recommend against replacing them with Sovtek EL34's, as they generally have poor lifetimes and performance at the higher voltages typically seen in Marshalls. I'd recommend a matched set of SED Winged-"C" or JJ as far as current-production/reasonably-priced tubes go. Good luck!

Cheers!

Strat

[rockon1]

Too bad you dont have a spare set of power tubes to try. Its fairly easy to bias to factory specs. Dug this up.

MARSHALL SERVICE BULLETIN

SUBJECT: Bias Info/Procedure for JCM2000 series Marshall amplifiers

THEORY OF OPERATION: We are setting the bias using the current method instead of cross over distortion method. The Bias PCB assembly is as follows:
PR1: Bias mini-pot adjustment for pin1 which is one side of the push pull amp (two output tubes in a 100watt amp and one output tube in a
50watt amp). Pin1: Connected to the cathode of the output tube(s) which then goes through a 1 ohm resistor then to ground. Pin2: ground reference.
Pin3: Same as pin1 but for the other side of the push/pull amp. PR2: Bias mini-pot adjustment for pin3, the other side of the push/pull amp.


PROCEDURE:
1. Make sure amplifier is connected to a load with the proper impedance
selected. 2. Power up amplifier on STANDBY and let the circuit stabilize for a couple of minutes.
3. Locate the male three pin molex connector (CON2) with the two mini-pots (PR1 and PR2) on both ends found on the bottom of the tube bay. 4. Connect DMM (set to read mV) with alligator leads, reference common lead to center pin (pin2) on molex connector CON2 and positive lead to pin1 on CON2.
5. Take amplifier off of STANDBY with no signal, adjust mini-pot (PR1) closest to pin that the positive lead from your DMM is connected to and set it to the mV voltage that is listed in the chart below. 6. Repeat steps 4 & 5 for pin3 and until both pin1 and pin3 mV are the same.

BIAS CHART mV SETTINGS
(pin1 and pin3)

DSL50 45mV TSL60/1/2 80mV
DSL100 90mV DSL401 1.375V
TSL100/122 90mV DSL201 .675V

[Dave Johnson]

Thanks for the replys. I think I need to get a new pair of power tubes. It's the quickest way to find out if I have other problems. The amp sounds O.K. full power and everything, I just cant stand that hum, it's not normal. It just started. I did switch the two power tubes around, still no change.

[Dave Johnson]

O.K. I put in a new set of power tubes today, no change. I pulled the chasis out and took a look around and found a couple of burnt spots on the main board where what appears to be a couple of small ceramic capacators, also burnt, called C26 on V5 & C27 on V6 . I found a schematic for this amp but I can't find a parts list that tells me what C26 & C27 is. Also can these parts be replaced, or do I have to replace the whole PC board. Any thoughts?:eek: I did a little more research and found that these are 500v 22pf capacitors. Where can I order these? Radio Shack was no help. Thanks.

[stratman_el84]

O.K. I put in a new set of power tubes today, no change. I pulled the chasis out and took a look around and found a burnt spot on the main board where what appears to be a small ceramic capacator, also burnt, called C26. I found a schematic for this amp but I can't find a parts list that tells me what C26 is. Also can this part be replaced, or do I have to replace the whole PC board. Any thoughts?:eek:

No, replacing a PCB is extreme. Very very doubtful a PCB replacement is called for here. That's usually discussed shortly after the part in the story of what happened when the firemen left. :eek:

The first thing you'll need to do is get a positive identity on that part. Are you *certain* that it's C26, not another part near it that burned and that's the correct number? The Marshall schematic I'm looking at here:

http://www.drtube.com/schematics/marshall/tl60-61-02.pdf

says that C26 is on the Rear/Jack PCB Assembly rather than on the Main PCB Assembly. Try to confirm and also see if you can read anything printed/painted on the part. Look carefully around at nearby parts for signs of damage. Some snapshots might be a big help too.

To help you diagnose the problem and whether you should tackle it yourself, we need as accurate info as possible, especially seeing that this amp is chock-full of integrated circuits, voltage-regulated, multiple low-voltage DC supplies and rails mixed with high-voltage high-gain vacuum-tube circuitry, with foot-switching, triple-channel insane-gain goodness served up on several modern high-density PCBs with connectors up the wazoo. *Without*complete documentation online. Not exactly an easy long-distance troubleshoot assist!

These things *can* be a challenge even for an experienced guitar amp tech to troubleshoot when it's sitting on his bench due to the construction, so please be patient.

I'm sure anyone who can help including myself will. We just need more info at this point. Good luck.

Cheers!

Strat

[Dave Johnson]

I see what you mean, C26 is on the rear jack PCB, but I was looking at another schematic that clearly shows the mother PCB
http://www.schematicheaven.com/marshallamps/jcm2000_tsl60_60w.pdf (http://www.schematicheaven.com/marshallamps/jcm2000_tsl60_60w.pdf)
C26 is on V5 and C27 is on V6. The values are printed on the schematic. The capacitors are too burned and crusty to get any info off of them, but the PCB is clearly marked C26 & C27 at the crispy caps.:confused:

[stratman_el84]

I see what you mean, C26 is on the rear jack PCB, but I was looking at another schematic that clearly shows the mother PCB
http://www.schematicheaven.com/marshallamps/jcm2000_tsl60_60w.pdf (http://www.schematicheaven.com/marshallamps/jcm2000_tsl60_60w.pdf)
C26 is on V5 and C27 is on V6. The values are printed on the schematic. The capacitors are too burned and crusty to get any info off of them, but the PCB is clearly marked C26 & C27 at the crispy caps.:confused:

According to that schematic you posted, which as far as I can tell is the same one I looked at, clearly shows C26 on the third page drawing labeled: Rear/Jack PCB. If that's not what you're finding, then your amp and this drawing are two different things. It's possible that drawing was based on an earlier or later design revision than when your amp was built. If there's not even an accurate schematic, then I don't know how to help. :confused: This isn't like an old Bassman or Champ where the amount of stuff, hence the number of things that could go wrong, is relatively small.

At this point, I'd recommend a trip to the amp shop for a date with the tech. There just seems to be too little information, and what we have isn't jiving. I'd recommend against repeatedly firing it up in attempts to somehow find out what's wrong. Something may give way, short, and do some real serious damage, including destroying an entire PCB assembly for all practical purposes by toasting the PCB 'lands', and/or take out a power or output transformer.

Cheers!

Strat

[Dave Johnson]

This is strange, in the first page of the schematic (page 1 of 2) I'm seeing C26 & C27 (500v 22pf) at the power tubes V5 & V6. Then on the third page, titled rear/jack PCB I'm seeing another C26, which is, according to the schematic a 63v 100n capacitor. Could there be two different parts sheets, with the same reference numbers? There is no doubt in my mind that I am looking at the main (mother) board, and seeing the 500v 22pf capacitors. I have the board out of the amp now. Replacing the burnt caps doesn't seem like much of a problem, easy access now, but what could have fried them in the first place. The amp still worked with the burned caps. It just had that loud hum. :confused:

[Dave Johnson]

Well it's fixed... Huge thanks go out to all that have given me input on this, it's a complicated amp with multiple PC boards, and not a lot of info available, but I think it's O.K. now. I replaced the two burned capacitors on the main board with 1000v 22pf capacitors. Put every thing back togeather, held my breath, crossed my fingers and powered that sucker up. IT WORKS!
The hum is gone. It idles as quiet as a church mouse. My only concern now is, what caused this in the first place. I did find two seperate wire sets that had gotten pinched when the amp was assembeled. I unfolded the pinched areas and rerouted them away from harm. Time to make some noise and see if i can blow it up. Woo Hoo. :rotflmao