mark III bass head blows a fuse

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kwlw
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mark III bass head blows a fuse

Post by kwlw » Sat May 28, 2016 10:28 am

So, my mildly modded (preamp section only) mark III 400BH played quite a few gigs (because too many people started to like the sound), however it decided to go silent in the middle of the venue. I've got it back with the description "we did nothing, just playing and now it blows a fuse". As the cab, cable also belongs to me and was used as a set - they were proper impedance and gauge.

On the surgery table - the only fault I can find, is dead shorted Q6 transistor (power). It's emitter resistor tests OK, protection diode also. Same for the driver transistor Q3 - it seems to be intact also. Can't find any more faulty parts. WTF? Always thought, that transistors die in packs. Can it be, that some sort of protection circuit has done it's job?

As I am not very into semiconductors (tube guy) - need some help there, what to do next. I've browsed through few sites, with scary stories about SOA and semiconductor wearout.

Should I replace all 8 power transistors to be sure, that poor thing won't need any rest in the middle of the concert? Also - I see that Peavey put silicone-fiberglass insulators on power transistors. Would mica ones be some improvement there?

P.S. oscilloscope, variac, current limiting bulbs, etc. are available in my hobby workshop

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Re: mark III bass head blows a fuse

Post by Enzo » Sat May 28, 2016 7:56 pm

Always thought, that transistors die in packs.
Unlearn that, there is no "always".

In my experience, when an output transistor shorts E-C, it often but not certainly always will have one fail on the opposite polarity side. Plenty times only one is bad. The only times I see entire rows of them fail is when one shorts B-C. That usually takes out the group.

You have 8 power transistors, two groups of four. Note that the end one of each group is wired as a driver, Q3, Q13. The remaining three are parallel outputs. The amp will function with as little as one output per side, at least for test. Obviously it won;t produce full power that way, but good enough for tests. So pull the dead transistor and see if the amp runs without it. If so, it was probably all that was wrong. The remaining transistors, powered up and at idle, can be crudely checked. Measure voltage drop across each one's 0.33 ohm 10w ballast resistor. If they all measure about the same - and I have no idea what you will find, then they are likely OK. If one seems way off from the others, it is suspect. The transistor I mean, probably not the resistor itself.

This amp has worked for what. 35 years or so? Why would we feel that changing the heat sink insulators would be helpful now? Peavey never engineered their amps right on the edge, they always built in a large margin for reliability. Those silicone heat pads work just fine. messy heat grease and mica work fine too. But I see no reason to change.

Scary stories? Feh. These are basic solid state amplifiers. Worry about SOA when you are designing amps. When it comes to this one, the engineers at Peavey ALREADY designed it. Any necessary SOA considerations are already in there.

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Re: mark III bass head blows a fuse

Post by kwlw » Sun May 29, 2016 1:13 am

Enzo wrote:
Always thought, that transistors die in packs.
Unlearn that, there is no "always".
roger that! :)
Enzo wrote: In my experience, when an output transistor shorts E-C, it often but not certainly always will have one fail on the opposite polarity side. Plenty times only one is bad. The only times I see entire rows of them fail is when one shorts B-C. That usually takes out the group.
It's logical, but was not so obvious. Now I've got it, because could not understand why in some cases I have whole pack "genocide" inside the amps (including driver transistors) and sometimes just pair toast devices. That was missing part of the puzzle.
Enzo wrote:Measure voltage drop across each one's 0.33 ohm 10w ballast resistor. If they all measure about the same - and I have no idea what you will find, then they are likely OK. If one seems way off from the others, it is suspect. The transistor I mean, probably not the resistor itself.
I understand you are suggesting me to measure current in such way. Will try to do this, first with current limiting bulb and variac, which is not OK for this amp, because it uses stabilized +-15v in the poweramp circuit. Then will try to fire it up.
Enzo wrote: This amp has worked for what. 35 years or so? Why would we feel that changing the heat sink insulators would be helpful now?
Better heat transfer? I suspect that it could be that amp was too hot as it was pretty long venue. It's audible on cabinet, this amp is playing, when speakers get hot. Also, because it's wimpy powered by todays standards - it's always at full volume. And could probably got some clipping, because of non properly working compressor/DDT circuit (yea, needed to fix that, but forgot, because ppl wanted to play and I had too little time to look into that). Because of exotic chip - I still won't be fixing it now and continue to walk on the razor's edge with DDT disabled. I have only LM3080N available which is some sort of different animal from CA3094, regarding your post here.
Enzo wrote: Scary stories? Feh. These are basic solid state amplifiers. Worry about SOA when you are designing amps. When it comes to this one, the engineers at Peavey ALREADY designed it. Any necessary SOA considerations are already in there.
SOA seems to be dependent on the temperature. As far I understood - slight trip there (at the time of failing of other transistor in the pack) with hot heatsink and device is not OK anymore. I mean it still works and measures more less ok, however it has damaged sections which a likely to fail under extreme conditions. Some call it semiconductor wear, thermal fatigue. So I have doubts about reliability of survived ones. Eight transistors are cheaper than spoiled evening when a band looses bass during show. So now I am trying to deal with bad fame of "unreliable old stuff" here :).

Peavey used top of the crop aluminum cased Motorolla MJ15003, which are not available today. I am not so optimistic regarding ON Semi ones, which use steel cases or something.

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Re: mark III bass head blows a fuse

Post by kwlw » Sun May 29, 2016 8:39 am

Fired it up without failed transistor. Starts and plays ok. Plugged my japanese LP copy into it - beautiful tone, even with some chineese no-name speaker I use for amp testing. No strange that people using these heads for guitar. Really need to fix this baby ASAP :).

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Re: mark III bass head blows a fuse

Post by Enzo » Mon May 30, 2016 7:05 pm

OK, so aside from the associated resistor potentially being damaged, we have demonstrated that the rest of the amp works. So install a new transistor, and out it goes back into the world.

Yes SOA involves temperature, and that is what the ample heat sinking is about. Sure they warm up when cranked, but not so far as to overheat. In fact there is a thermal cutout on there to prevent it from killing itself. By design, it won;t heat itself to the point of heat failures. Not unless some part is otherwise bad. "Better heat transfer" sounds great, but we are talking cutting an inch off a journey of miles. if you want to go with mica and grease, then go ahead. I bet the amp will run another 35 years that way too.

PV has used the steel cap TO3s from Moto/OnSemi for years now, and they are just as reliable. I have no doubts about them myself.

I don;t assume the bad part failed from heat. Not in the sense it overheated. Every time you plug a speaker cable into the amp, you hit those outputs with a static discharge. You might not always feel a spark or see one, but your body and surroundings almost always have a charge on them. That could have damaged a transistor. A shorted speaker cord a year ago might have stressed the part, but not to the point of failure, and now whatever little last straw finally broke the camel's back. Point being, we usually never know exzactly why some part fails. We can rationalize something, but we never really know.

I have found all those 1980s PV solid state amps to be real workhorses, and once I fix the obvious tuff, they almost never have any subsequent failures from the initial trouble.

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Re: mark III bass head blows a fuse

Post by ScottMarlowe » Wed Jun 01, 2016 1:53 pm

Wow you guys are bringing back memories. This was my first amp ever, and it too suffered a shorted power transistor and blown fuse. At the time I was no electronic tech, but my girlfriend's dad was and he showed me what to look for.

Was a simple fix. I went to the local electric supply house (back when we had those) and got the right transistor and it was back up and running that day.

Same amp fell out the back of an airplane once. Head / amp section separated and yanked out a few pins when the cables disconnected. Soldered new pins on, straightened out the rest and it kept working for another decade before I sold it.

Glad you see it looks like you have gotten yours up and running!

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Re: mark III bass head blows a fuse

Post by kwlw » Sun Jun 12, 2016 4:49 am

Interesting thing about that particular amp - after replacing all MJ15003's with new ones, same batch, (low power) hFE matched - I cannot replicate the issue with DDT. Which was significant sound level difference with limiter on and off. That's why guys on the stage played with compressor off (and clipping the amp). It's cold day today, however I clearly remember oscillograms with strange spike on sinusoid when DDT was on. Now sinus looks like sinus when DDT is limiting signal. And is limiting on right threshold. So weird chip seems to be not the one to blame. Enzo was right once again :). That's why I've left silicon insulators and haven't replaced them with mica ones (because Enzo is right too often :lol: )

Still like this amp with guitar + deluxe cab with alnico speaker in it. It has not the best drive-crunch tone there is, however still suitable and some parts of the tone are exceptionally good. Love it.

Decided to add some pics. First - the reason why the amp was taken apart (small burned patch on crystal near the shadow of the wire, can believe that it was because of static damage to the part):
Image
Half way done (something around 15 minutes of labor with proper tools ;)):
Image
Finished board, just before first power on:
Image

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Re: mark III bass head blows a fuse

Post by kwlw » Sun May 07, 2017 1:24 pm

As I love Peavey Mark III BH for their sound with clean guitar, but one in posts above is always "not home", so I picked just another patient in pretty bad condition to fill the void.

After some fixing it came alive, however initial problem still exists - it plays just fine at room volume, but at medium gig volume (30-50W output) - blows fuse after few minutes of playing. First clipping appears (DDT light comes on at volume level it should not and you need to reduce volume for it to go off), then after minute or two - fuse goes bang. When fuse replaced - everything goes exactly the same. If I play at room volume - it works for at least hour or more and sounds great. I can play guitar and replace fuses time to time, but I would like to play some bass on it.

I suspect cracked solder or resistor somewhere, but maybe symptoms ring a bell for someone (Enzo?). It's pretty tedious to go through all the PCB and reflow every pin :(. Even bigger nightmare is to test every component. Maybe there is shortlist of "usual suspects" somewhere?

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Re: mark III bass head blows a fuse

Post by Enzo » Sun May 07, 2017 2:00 pm

SO this is a new to you amp? And without reading through the old thread, it blows fuses, and if I recall, so did the other amp.

Have we tried gigging with this head and a different speaker cab and a different speaker cord?

Since it is level sensitive apparently, are we using a real speaker cord and not just a guitar cord for the speaker?

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Re: mark III bass head blows a fuse

Post by kwlw » Sun May 07, 2017 11:26 pm

Enzo wrote:SO this is a new to you amp? And without reading through the old thread, it blows fuses, and if I recall, so did the other amp.
Yes, that's "new" one. Other amp works fine for a year now, at all volumes needed.
Enzo wrote:Have we tried gigging with this head and a different speaker cab and a different speaker cord?
Of course. After first time, amp went to inspection (and turning on on current limiting bulb) and will not leave my "workshop" until it's stable. All further fuse blows were in well controlled environment. Both with resistive or known good speaker loads.

I would say it's either going into some wild oscillation or something breaks contact inside. If it would be tube amp - it would be fixed by now ;). But I still don't get well into transistors.
Enzo wrote:Since it is level sensitive apparently, are we using a real speaker cord and not just a guitar cord for the speaker?
This sounds insulting :P. And at these power levels (50w@8ohm=2.5A of current) average guitar cord would be hot but should hold few minutes quite easy without melting through insulation and solder. But answer is yes. Real speaker cord 2.5mm2 was used. And other amps play through setup well.

I have pretty good skill in guitar tube amps and have OK equipment in my "workshop". But semiconductor amps - are terra-incognita for me :(. Can distinguish basic blocks on the schematics, but have only slight understanding about their relationships. So need some help there ;).

Sau

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Re: mark III bass head blows a fuse

Post by Enzo » Mon May 08, 2017 2:07 am

Insulting? I don't know you from Adam, I cannot assume anything at this point. I have to start at the start. Even people who have been around a long time don't always cover all the bases, I have to ask.

Scope the output to see if ther are any parasitics going on. The bulb is great for amps blowing fuses, but we cannot operate an amp at any real power levels while on the bulb.

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Re: mark III bass head blows a fuse

Post by kwlw » Tue May 09, 2017 6:05 am

Enzo wrote: Scope the output to see if ther are any parasitics going on.
Burned handful of fuses until got shorted output transistor, that stopped experiments. In the progress - checked output signal with the scope, no noticeable parasitics or my scope is too slow (20Mhz). After going through all suspicious PCB traces with soldering iron (inspecting visually, have still pretty good eyes), idle power usage of amplifier dropped a little (my lab socket shows average consumption per second). Playing radio around 50W for a hour went ok, so I started to add load. Still fuse blowing at high output (100+W).

Interesting thing, that with DDT off - clipping comes not slicing peaks horizontally but slopping upper top of sinus with sloped line. Should it be like this or it is a symptom? I have not taken the shots, but slope is similar like plate in this picture, but occurs ONLY on tops of sinus:Image

Thought maybe voltage stabilizers powering opamps lacking source voltage to provide stable +-15v, that proved to be wrong. Opamp PSU is rock solid always having enough source voltage.

One interesting thing - one of predriver transistors is like twice as hot as another one in opposing shoulder (like +60C vs +30C). That one with inductance in emitter. Does that say anything to you?

Maybe I should check something after I'll replace failed components (will do that next week).

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Re: mark III bass head blows a fuse

Post by Enzo » Tue May 09, 2017 10:20 am

Generally, when I replace an output transistor, I also replace its driver transistor, because the driver faced some stress trying to drive the shorted part. Even if it tests OK< it still was abused.

20MHz is more than fast enough to see parasitics. You won't need to resolve anything into individual cycles anyway. Also, the parasitics in these amps won't be higher than 20MHz anyway.

Why are we clipping the amp? No reason to do that. DDT is ther to prevent that. I turn it off during troubleshooting, but I don't clip the amp. As to why it might be assymetrical, might be your aching driver.

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Re: mark III bass head blows a fuse

Post by kwlw » Sun May 14, 2017 1:46 pm

Issue was mechanical. There was dry solder joint on double diode. After just another fuse lost, I have made some jig with automatic circuit breaker. This speedup search process. After preamp board was carefully inspected once again, I have noticed, that sometimes amp even is not starting, unless I touch wire which goes to the double diode on poweramp board. What's interesting, while amp was working, I could move the wire, tap everything with chopstick - nothing.
But after it "blew" circuit breaker (followed by start with bulb in series), sometimes it stuck in "shorted output" mode, when bulb lights full power. Then some vibration in area of double diode was needed to restore connectivity.

Now amp passed all my tests and happily playing rock radio station for more than hour at resistive load and monitoring speaker. Pretty hot for such low power output, but if I remember - his brother did the same :). It is interesting that turning off DDT and letting amp to clip a bit - is quite perceivable volume increase. Should it be like this?

Actually it's just curiosity, this amp is for solely use by me - would rather see something more than 50W. Let's hope it will keep working stable and I don't plan to switch off DDT :).

Thanks for ideas and help!

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