Mark III blue stripe redplating, quick diagnosis and repair

[woodbutcher65]

I will say that this little meter may not be outstanding below 100 pF but I haven't had cause to check any caps that low in value. They aren't typically found in tube amps.

 

[Jeffw5555]

Believe what you want. I'll believe the results I'm getting based on the evidence right in front of my face..

Show a photo of your cheap meter reading a 10pf cap. Evidence trumps BS.

 

[woodbutcher65]

Not sure if I actually even have any caps that small right now. I recently purged my parts inventory of everything I wasn't using in guitar amp work, but really, what does it even matter when talking about parts you won't even need in a guitar amp?
The smallest cap I've checked so far was 25 pf and it measured in spec. My HP bench LCR meter (Yes, I have one) agrees with it. So down to 25 pf I trust it to be reasonably accurate. Since I apparently don't have any caps handy that are smaller than that then I'll just limit my observations to this component value range for the time being.

 

[Henz]

I will say that this little meter may not be outstanding below 100 pF but I haven't had cause to check any caps that low in value. They aren't typically found in tube amps.

My c+ as a 47pf, two 120pf, 180pf, 1000pf, 250pf and 30pf which the code on it says 10. When I removed the 10pf out, my meter read .03nf. ( (30pf).

 

[woodbutcher65]

Could you do me a favor and post a photo of the bias circuit section of your Blue Stripe? I just want to confirm the stock resistor values in the bias circuit, as I aim to reverse the adjustable bias mod that was done to mine.

 

[Henz]

Rather than go to all that trouble, just remove and test the orange drops out of circuit. They should not read as low value resistors.

I got a couple of these and they tell me the ESR and resistance and capacitance of any common capacitor. Get one or two.

https://www.ebay.com/itm/2754341298...Fol5tKVkq9V//orXKQc1zNbF2juD|tkp:BFBMgtiA19Jh

Hey WB, which meter am I looking at to purchase? Ty

 

[Henz]

Could you do me a favor and post a photo of the bias circuit section of your Blue Stripe? I just want to confirm the stock resistor values in the bias circuit, as I aim to reverse the adjustable bias mod that was done to mine.

Is that question to me WB? I don't have a Blue stripe.

 

[woodbutcher65]

It applies to you if you have a Mark III. I don't think the bias circuits ever changed between the stripes.

 

[Henz]

It applies to you if you have a Mark III. I don't think the bias circuits ever changed between the stripes.

I have a MKII C+ DRG. Factory. I would like to know the time stamp when these became " The Holy Grails" of amps. When I purchased it back in 1984, it was just another amp to me. I was never floored with it. In fact, many ppl claimed that it was missing something, which I agree!

 

[woodbutcher65]

Generally, legends are built over time. The enormous popularity of Metallica made James Hetfield's guitar tone a de facto legend, and so people feel that they have to have that tone in order to be musically complete. Just as there are those who worship Eric Clapton and can't stand the idea of not having his tone, and so any original Marshall Bluesbreaker combo amp is now worth the price of a nice car.

As for me, when I got my first blue stripe Mark III, I knew that I should not play it much because it was SO good. It had the aggression and gain that I sought. Not because of anybody else's tone, but because it was to MY tastes. I knew I had to flip it and make some money on it, which I did, but I did so wishing that I could have kept it. Now, a few years later, I have another, and it's going to stay with me for, almost certainly, many years to come. It's as good as the first one.

The Mark IIC+ was identified as a special amp when it was still pretty fresh. I remember reading of comments about it in late 80s Guitar Player magazines. They were trading for higher prices then than other Mark IIs, and over time they have slowly and consistently increased in value more than any other Mark series amp. While your average Mark III probably sells for between 1800 and 2500 dollars today, a IIC+ will trade in the range of double that, or more depending on its combination of options.

I've never had the good fortune to encounter an available Mark IIC+ and having had the discretionary funds to spend on it at the same time. But I'd probably work quite a trade out to get one even though I can't guarantee that I'd like it any better than the Blue Stripe.

 

[woodbutcher65]

I have observed some slightly weird behaviors in my Blue Stripe that initially looked like just bad potentiometers, but the behavior changes in different channels so now I'm looking for a second opinion.

Please refer to your Mark III and set it to the clean channel. Tell me, when you turn up the bass, does this make the midrange and treble get weaker? Or does it just add bass?

On mine, when I turn the bass way up, the midrange/treble pretty much disappears. It's not adding bass, it's subtracting treble. By the time the bass is turned up all the way, there IS no treble.

Bad pot? Not sure. I'd rather diagnose before ripping parts out and replacing them.

Now, while still on the clean channel. turn up volume 1 across its full range from minimum to maximum. Does it seem to work as it should across its full range?

Now switch to the lead channel and set the lead drive to 7 and run volume 1 thru its full range again. Does it work as expected, smoothly increasing the level of distortion from very little to "all of it"? Or is there a segment where it seems to not be adding more gain?
On mine, gain DROPS when V1 is above 7. In fact, V1 seems to only operate properly from 3 to 7 on the knob, when in the lead channel.
BUT...V1 seems to operate normally on the clean channel.

So I'm not sure exactly if I've got pot problems or something else is going on. If you can check these tests out on your own Mark III and report back on what you get, it might help me to diagnose and repair this properly.

 

[Henz]

I have observed some slightly weird behaviors in my Blue Stripe that initially looked like just bad potentiometers, but the behavior changes in different channels so now I'm looking for a second opinion.

Please refer to your Mark III and set it to the clean channel. Tell me, when you turn up the bass, does this make the midrange and treble get weaker? Or does it just add bass?

On mine, when I turn the bass way up, the midrange/treble pretty much disappears. It's not adding bass, it's subtracting treble. By the time the bass is turned up all the way, there IS no treble.

Bad pot? Not sure. I'd rather diagnose before ripping parts out and replacing them.

Now, while still on the clean channel. turn up volume 1 across its full range from minimum to maximum. Does it seem to work as it should across its full range?

Now switch to the lead channel and set the lead drive to 7 and run volume 1 thru its full range again. Does it work as expected, smoothly increasing the level of distortion from very little to "all of it"? Or is there a segment where it seems to not be adding more gain?
On mine, gain DROPS when V1 is above 7. In fact, V1 seems to only operate properly from 3 to 7 on the knob, when in the lead channel.
BUT...V1 seems to operate normally on the clean channel.

So I'm not sure exactly if I've got pot problems or something else is going on. If you can check these tests out on your own Mark III and report back on what you get, it might help me to diagnose and repair this properly.

I'll do that test for you on my MKII C+ if it will help you.

 

[woodbutcher65]

It might. The two amps are not identical but they should be similar enough that the test should be relevant. Thank you.

 

[Henz]

It might. The two amps are not identical but they should be similar enough that the test should be relevant. Thank you.

I'll get to it asap. I didnt do test yet but one of your issues you have with Vol 1 , I have . If I turn down my treble pot and I believe Mast. 1 pot, while turning up Vol 1 , all is fine till nearing 7 . from 7 to 9 hum and buzz is introduced. Then on 10 it disappears. Side note. That meter that you posted for me to get, does it test ESR in orange drops? Ty, Jim

 

[woodbutcher65]

No, that meter is not specifically an ESR meter but if a capacitor is shorted it will read as a resistance.

 

[Henz]

No, that meter is not specifically an ESR meter but if a capacitor is shorted it will read as a resistance.

Ty. As resistance in circuit or out? A regular ohm meter will determine that?

 

[woodbutcher65]

Yes, that's what they do, but your typical meter does not put high voltages through the part to check for faults that don't appear at low voltages, which is what a bad coupling cap sometimes does. It may measure fine to a meter that tests it at 1.5 volts, but at operating voltage it could be a dead short.

 

[Jeffw5555]

Ty. As resistance in circuit or out? A regular ohm meter will determine that?

I posted an explanation earlier, but don't think you are yet grasping some of the basic principles of electronics.

Let's start with ESR. You don't seem to understand that it is a quality that is only really associated with large electrolytic capacitors, it never is associated with orange drop caps in these amps. (ESR is sometimes used as a quality factor for film caps used in power inverter and the like, but never with coupling capacitors in audio amps)

The reason ESR is tested for filter caps in amp power supplies is that electrolytic caps can measure just fine with a meter, but fail to act as a filter (the capacitance drops way off) with frequency. Lower ESR = Less ripple on DC B+ line = Less hum.

Coupling caps (like orange drops) can leak current across them, that is variable with voltage. In other words, they can pass DC voltage from one side to the other. As I posted earlier, many amp manufacturers provide schematics with typical voltages at various points to enable quickly finding leaky caps, Mesa does not. If you have a schematic and know a bit about electronics, you can usually easily guess what DC levels you should see along the circuit. Woodbutcher65's situation was extreme, the cap shorted hard and therefore easy to find.

Best way to check these orange drops is to isolate them or remove them, and first check resistance with a DMM. If it passes the basic resistance test, then on one leg put a 1M resistor to ground, then connect the other leg to DC voltage at the rated voltage of the cap, then measure the voltage at the junction of the leg and 1M ohm resistor. Initially the voltage will be at whatever voltage you applied on the other end, then drop to zero over time. If the voltage doesn't drop, the cap is leaky. As Woodbutcher65 said, these caps can test just fine at the low voltages that a meter applies, but leak at higher voltages.

 

[Henz]

Could you do me a favor and post a photo of the bias circuit section of your Blue Stripe? I just want to confirm the stock resistor values in the bias circuit, as I aim to reverse the adjustable bias mod that was done to mine.

Here's my DRG C+ bias on power supply. Over on power out pcb, bias in is governed by 2 220k's.

 

[Henz]

I posted an explanation earlier, but don't think you are yet grasping some of the basic principles of electronics.

Let's start with ESR. You don't seem to understand that it is a quality that is only really associated with large electrolytic capacitors, it never is associated with orange drop caps in these amps. (ESR is sometimes used as a quality factor for film caps used in power inverter and the like, but never with coupling capacitors in audio amps)

The reason ESR is tested for filter caps in amp power supplies is that electrolytic caps can measure just fine with a meter, but fail to act as a filter (the capacitance drops way off) with frequency. Lower ESR = Less ripple on DC B+ line = Less hum.

Coupling caps (like orange drops) can leak current across them, that is variable with voltage. In other words, they can pass DC voltage from one side to the other. As I posted earlier, many amp manufacturers provide schematics with typical voltages at various points to enable quickly finding leaky caps, Mesa does not. If you have a schematic and know a bit about electronics, you can usually easily guess what DC levels you should see along the circuit. Woodbutcher65's situation was extreme, the cap shorted hard and therefore easy to find.

Best way to check these orange drops is to isolate them or remove them, and first check resistance with a DMM. If it passes the basic resistance test, then on one leg put a 1M resistor to ground, then connect the other leg to DC voltage at the rated voltage of the cap, then measure the voltage at the junction of the leg and 1M ohm resistor. Initially the voltage will be at whatever voltage you applied on the other end, then drop to zero over time. If the voltage doesn't drop, the cap is leaky. As Woodbutcher65 said, these caps can test just fine at the low voltages that a meter applies, but leak at higher voltages.

Ty Jeff! I tested all of my OD's that I can in my amp for DC leakage when fully powered. They all tested fine. Now as you mentioned, I really shouldn't be concerned with ESR with the OD's? What if my amp had a voltage spike or some other accident? A while back after I recapped it, my stupid ars had the amp fully powered and I was using that canned air stuff to blow away dust and such from inside. Then I accidentally hit a high voltage trace and had a mini explosion. All of my OD's turned tan, they eventually turned back to orange. I'm guessing the epoxy interacted with the volatility of the canned air. Point being, could/ would that be any concern for certain components? I measured every component that I could. They all measured in specs. Thoroughly cleaned all pcb's. That happened 4 yrs ago. Thoughts? Thanks for your time! Jim