Markbass Little Mark II

[binito]

I'll change the IRFP240 & irfp9240 damaged a Little Mark III. IRFP27n60K damaged too in power supply.
Where I can measure the BIas to set the new transistors?
The schematic of power amp very similar at little II.
http://elektrotanya.com/parsek_mark_.../download.html
Thanks

[Leooooh]

Hey there!!

Need some help here..... A few weeks ago I had a Mark Bass Combo Head II in my bench with the fuse blowing. I opened it and started to measure the power MOSFETs. It uses 3 IRFP240 and 3 IRFP9240. One IRFP240 and one IRFP9240 were shortened. I also measured the other main transistors to ensure everything was ok, before turning the amp on, and found that the two MOSFETs (IRFP27N60K)in the SMPS were shortened too. I also measured the switiching diodes and all other caps and transistors. Nothing wrong. Replaced both of them, cleaned and changed the termic paste to ensure it would dissipate the heat correctly . Reasembled the whole amp and power it. Amp was sounding as supposed to. Called the customer and delivered the amp.

To my surprise, the amp came back a few days later, with he same problem. Blowing fuse. Opened it and found the same issue. The two MOSFETs in SMPS were shortened as well as one IRFP240 and one IRFP9240 in the power section. This time, I replaced all the 6 MOSFETs in the power amp section as well as the MOSFETs in the SMPS. Turned the amp on and the issue was solved. Delivered it to the customer.

Now, to my surprise, after a few more days, the customer told me the amp was blowing fuses AGAIN. Told him to bring it to me, and the same issue happened. Two MOSFETs in the SMPS and another two in the power amp section.

I measured again all the power transistors, as well as diodes and caps. Nothing more is shortened or open. Everything seems to be fine, but my experience says that if replace those shortened MOSFETs, the amp will return again, cause I didn't find the actual problem. I also considered not using enough termic paste in the power MOSFETs, but after checking them again, everything is fine. Thanks to this thread, I found the schematics of this amp and tried to found any resistor or other component that are connected to those failing MOSFETs to find the cause of the problem, but nothing is wrong.

Does anybody saw anything like that?

I read on this thread, that an user had the power board replaced with a similar issue, and then the issue appeared again!!! Can't understand this.

I live in Brazil, and we don't have any Mark Bass authorized tech. If I can't solve this issue, I don't see any other option then sending the amp to Italy, to Mark Bass (I contacted Mark Bass, and this was the suggested action by the manufacturer).

Any help will be greatly apreciated.

Thanks a lot.

[nickb]

My guess would that there is a problem with the power amp. Just to throw some suggestions out - Did you check the idle current? Does the fan operate correctly? Could there be a problem with the user's speaker cable ( intermittent short) or the speaker impedance too low?

[binito]

Hey there!!

Need some help here..... A few weeks ago I had a Mark Bass Combo Head II in my bench with the fuse blowing. I opened it and started to measure the power MOSFETs. It uses 3 IRFP240 and 3 IRFP9240. One IRFP240 and one IRFP9240 were shortened. I also measured the other main transistors to ensure everything was ok, before turning the amp on, and found that the two MOSFETs (IRFP27N60K)in the SMPS were shortened too. I also measured the switiching diodes and all other caps and transistors. Nothing wrong. Replaced both of them, cleaned and changed the termic paste to ensure it would dissipate the heat correctly . Reasembled the whole amp and power it. Amp was sounding as supposed to. Called the customer and delivered the amp.

To my surprise, the amp came back a few days later, with he same problem. Blowing fuse. Opened it and found the same issue. The two MOSFETs in SMPS were shortened as well as one IRFP240 and one IRFP9240 in the power section. This time, I replaced all the 6 MOSFETs in the power amp section as well as the MOSFETs in the SMPS. Turned the amp on and the issue was solved. Delivered it to the customer.

Now, to my surprise, after a few more days, the customer told me the amp was blowing fuses AGAIN. Told him to bring it to me, and the same issue happened. Two MOSFETs in the SMPS and another two in the power amp section.

I measured again all the power transistors, as well as diodes and caps. Nothing more is shortened or open. Everything seems to be fine, but my experience says that if replace those shortened MOSFETs, the amp will return again, cause I didn't find the actual problem. I also considered not using enough termic paste in the power MOSFETs, but after checking them again, everything is fine. Thanks to this thread, I found the schematics of this amp and tried to found any resistor or other component that are connected to those failing MOSFETs to find the cause of the problem, but nothing is wrong.

Does anybody saw anything like that?

I read on this thread, that an user had the power board replaced with a similar issue, and then the issue appeared again!!! Can't understand this.

I live in Brazil, and we don't have any Mark Bass authorized tech. If I can't solve this issue, I don't see any other option then sending the amp to Italy, to Mark Bass (I contacted Mark Bass, and this was the suggested action by the manufacturer).

Any help will be greatly apreciated.

Thanks a lot.

the new MOSFETs that installed had the same gain 3? were they matched?
the bias equal in all those transistors?

[diydidi]

I had exactly the same experience.
I found cold solder joints on some of the large film resistor packages. Reflow all the larger smd resistors and make sure the bias changes when you turn the bias pot.

[Leooooh]

My guess would that there is a problem with the power amp. Just to throw some suggestions out - Did you check the idle current? Does the fan operate correctly? Could there be a problem with the user's speaker cable ( intermittent short) or the speaker impedance too low?

Hey...thanks for the tips man.

Checked the speaker impedance and it is ok ( 8 ohms). Tha cable that connects the module to the speaker is ok too (no shorts). The fan operates correctly too.

One thing I could not measure was the bias. Even using a similar schematic that i found here:

https://elektrotanya.com/parsek_mark.../download.html

There is no indication about, what Bias I should set the amp. I tried to measure the dc voltages in D3, D4, D5 and D6, but found a different value of the schematics. D5 is measuring 0,4V and D6 is -3,0. D3 is 0,3 and D4 is -2,9. Those 4 diodes are ok. No shorts or opened.

How do I check the iddle current?

Thanks again

[nickb]

I would measure the voltage across the 0.33 ohm resistors - 3.3mV would be 10mA and I suggest that is a reasonable number. These FETs are designed for switching applications and, to be frank, I really have no idea what a corrent bias current would be. I can say that if they run hot at idle, that that is too high. If you see a crossover notch when scoping the output then it's too low.

The best way would be to use a distortion analyzer and gradually increase the bias, stop when you no longer get any rapid improvement in THD but taking care not to let it get too high.

You're real problem might be a matching issue. These FETs come with a very wide +/-50% in gate threshold voltage and that could lead to significant variations in the current between them when operating. At full 500W into 4 ohms the peak source current in each FET is over 5A i.e the drop across each 0.33 ohm resistor is getting on for 2V and that will help to match the currents. At a full output into 4 ohms each FET will be dissipating something like 50W. That is a lot of heat to dispose of and any mismatch could make one much hotter than the next leading to failure.

[MarkusBass]

It uses 3 IRFP240 and 3 IRFP9240. One IRFP240 and one IRFP9240 were shortened.

You actually haven't specified how many MOSFETs you replaced. 2 or 6? Were they matched? As Nick says, matching MOSFETs in such amps is extremely important. Even if MOSFETs are from one production batch. Did you match them?

Mark

[Sowhat]

I must love Australia although I have never been there. I use an LOPT ring tester to do these checks. It's one I built from plans made available by Dick Smith electronics. I don't think Dick Smith (the Australian Radio Shack) is in business anymore but the plans are out there on the Internet somewhere. I used to do a lot of video game monitor repair and their pesky troublesome flyback transformers. This tester works amazingly well... I guess I built it correctly. Oh yeah, you don't have to remove the transformer, this tester does it's thing in circuit!

[buschman]

I have a tube amp repair friend working on my Mark Bass 2. There is a relay on the output that seems to have failed. It cuts the output. When the relay is bypassed the amp works fine. He tried to replace the relay but could not find the correct one. He has bypassed it. Is it going to work for long? It seemed to be a protection circuit maybe in case the amp was turned without a speaker?

[The Dude]

Without looking at the schematic: If you're talking about a speaker relay and not a power relay, bypassing it is also bypassing your speaker protection. The relay could be bad, or it was doing it's job and your amp has a problem. There are relay sources everywhere. It should not be that difficult to find a proper relay and replace it (if bad). If it's not bad, you need to find out why it's not closing. I'd start by seeing if there is voltage at the relay coil.

[Sowhat]

I have a tube amp repair friend working on my Mark Bass 2. There is a relay on the output that seems to have failed. It cuts the output. When the relay is bypassed the amp works fine. He tried to replace the relay but could not find the correct one. He has bypassed it. Is it going to work for long? It seemed to be a protection circuit maybe in case the amp was turned without a speaker?

That relay which should be something like a 12 volt coil SPST relay (plus associated circuitry) monitors the speaker output and disables the speakers if a more than acceptable level of DC potential is on the output. The relay may work perfectly well but if the output section is "misbehaving" then you got any one of several other problems more serious than just a seemingly faulty relay. This is monitoring the BIG power in the amp, the power that the output MOSFETS are controlling... be very careful here. If your tech is unfamiliar with how to troubleshoot a MOSFET output section I might suggest finding someone who is. Personally, I would not apply mains power to this amp until the problem is solved.

Oh yeah, I almost forgot. You do not need a speaker load on a solid state amp... that's just for tubes that use an impedance matching output transformer. This relay is not there to protect your amp but instead is there to protect your speakers in case your amp decides to go ballistic and send massive amounts of DC power to that fragile little speaker voice coil, it can happen in a microsecond!

[buschman]

That relay which should be something like a 12 volt coil SPST relay (plus associated circuitry) monitors the speaker output and disables the speakers if a more than acceptable level of DC potential is on the output. The relay may work perfectly well but if the output section is "misbehaving" then you got any one of several other problems more serious than just a seemingly faulty relay. This is monitoring the BIG power in the amp, the power that the output MOSFETS are controlling... be very careful here. If your tech is unfamiliar with how to troubleshoot a MOSFET output section I might suggest finding someone who is. Personally, I would not apply mains power to this amp until the problem is solved.

Oh yeah, I almost forgot. You do not need a speaker load on a solid state amp... that's just for tubes that use an impedance matching output transformer. This relay is not there to protect your amp but instead is there to protect your speakers in case your amp decides to go ballistic and send massive amounts of DC power to that fragile little speaker voice coil, it can happen in a microsecond!

Thank you for the reply. I knew there had to be more to it. I have a tube and tech background as well but not like this. The amp is free to me so might be worth sending off. Are the mosfet the components mounted to the heat sink? One looks like it got hot. He played loud and asked a lot from the little amp. When it went out he gave it to me. I could just buy another head.

[The Dude]

Since you have the relay bypassed already, unhook the speaker and check the DC level on the output.

[Sowhat]

Thank you for the reply. I knew there had to be more to it. I have a tube and tech background as well but not like this. The amp is free to me so might be worth sending off. Are the mosfet the components mounted to the heat sink? One looks like it got hot. He played loud and asked a lot from the little amp. When it went out he gave it to me. I could just buy another head.

Yeah, but these heads are nice pieces of equipment, see if you can get it fixed first.

The power output MOSFET's are indeed the components attached to the heatsink and they can get a little hot, thus, the heatsink. The big thing here is they need to be balanced or conducting synchronously, kind of like a matched set of tubes. If one or more starts drifting off spec due to whatever, the DC level on the output node start's to rise because they are loosing their push-pull balance. Ideally you should have 0 DC volts on the output but there's a tolerance, nothing's perfect but it needs to be real real close to 0 volts. You can easily replace all the power MOSFET's and source resistors for a couple bucks but you still have to "match" the components which takes some time, gear and knowledge.

There's threads right here on MEF, other websites, and I think, even YouTube videos on how you do the deed, check around. Nothing's too difficult once you know how.

Oh, yeah, and as the Dude suggests: assuming that the relay is still bypassed, determine if you have zero volts DC on the output of the amp with power on, no signal applied to the input and no speaker attached. If you read zero volts DC then maybe the relay really is busted.