| Author | Message | ||
     Le Basseur |
As says the title,I'm on the edge of a nervous breakdown... Ten days ago I got a MusicMand "One Fifty" on repair.The owner complained that the amp was already on repair to several techs,but came back in the same state.From what I heard from the owner,the amp had an OPT problem (it burned out so it had to be rewinded).The first rewinding process was unsucessful (the isolations took fire again) so the amp went to a second tech who rewinded the OPT again.The problem is that never again,this head played properly.It strangely distorts on moderate gain/master settings,like it has some sort of an "edge" besides the guitar's signal. I had a hard time in order to identify this head...according to the site,this amp isn't what it supposed to be. (I must confess,I repaired only "mainstream" or "boutique" amps,alot of them...but this is the first MM I got on the shelf and it literally puzzled me for two reasons:1.I never expected a pair or quartet of output tubes to be CATHODE driven,and 2.this is the first amp ever I don't know where to begin with). Surely,someone "cosmetised" this head because the back plate is missing...so no clear chassis ID. Any other ID sign looks irrelevant in terms of what's described in some other threads in this forum,but here there are: Chassis serial (stamped on the back): A 004893 On the right edge of the chassis,handwritten with red marker: 1/150 4-8 AK Preamp PCB (underside): BB2 Driver PCB (underside): DB2 In the vicinity of the mains tranny,there's a chassis hole (as I understand,there were some PI/drivers' versions using a 12AX7 so this must be the intended tube socket's place). The output tubes' screen grid's resistors aren't "one piece of 470 Ohms/tube",but each pair has a common 750 Ohms hefty resistor. (For every other detail,I can post some pictures...just let me know how to do it on this forum because I don't see an "upload" option). One of the main issues is that when I replaced the driver transistors (they were of different types,something like 2N6488 and 2N6487 and I thought,that must be it,piece of kake),I saw that the supposed stage between the master volume and the PI chip WAS MISSING.I mean,the PI/driver's PCB didn't contain such a stage AT ALL. Please try to understand my frustration...I'm not a pro tech,I'm a pro musician who builds/repairs everything that moves and has tubes in it,for a long time,but this time I'm mad as hell because,I'd say it again,I don't have idea what's going on. Please gurus,tell me what to do...where to begin,what should I look for,what to measure? Thank you in advance for your advices and pacience! Best regards, Le Basseur | ||
| Steve Kennedy (admin) Username: admin Registered: 03-2002 |
The very first thing I would do is to download all of the 130 and 150 schematics from this web site and start comparing the basic circuitry blocks. Since the amp was hard to indentify in the first place, it is possible you might be looking at an HD130 chassis and not an HD150. They are quite similar but not exactly the same and there are several versions of each style! From your description, I think you are looking at the wrong schematic! The 2100-150 schematic has that circuit between the Master volume control and the PI... the 2275/2475-150 schematic does NOT have this circuit and may closer resemble your amp. Also, compare the feature set... the 2100, 2275 and 2475 chassis all have different complements of Bright/Deep switches and some of the controls (Middle tone control, for example) and some other circuitry variations as well. Start with the power supplies...low voltage (for the preamp) and the high voltage for the output stage. These must be the proper voltage and clean before you can expect the amp to work well. If in doubt, replace the electrolytic caps in the power supplies and in the preamp stages! I would pull the output tubes and turn the Master volume control all the way down to isolate the output stage from the preamp. Then using a signal generator and a scope, verify that you can develop a clean wide-swing signal without distortion at the Master volume control input. Note what kind of Gain (Volume) setting and input signal strength it takes to induce preamp overload distortion to ensure that a typical humbucking pickup won't overload the preamp. If there appears to be a preamp problem, walk through the circuit and find the offending stage. Once you think the preamp is working as it should, concentrate on the output stage. If the PI and drivers seem to be able to develop a good clean drive to the output tube sockets, consider the possibility that the amp isn't biased properly or that the tubes may not be in great shape. A 150 uses 6L6 output tubes and not the EL34s found in the earlier 130 series. Of course, this all assumes you are starting with the proper schematic for this chassis AND the amp is complete and correct. If the amp has been modified then you should restore the amp to the factory conditions. Make sure you have properly identified the correct chassis first! Also, get all the Music Man Factory Service Bulletins (at the top of this Message Board and in the Manuals area) for factory modifications and biasing information. Steve |