My 112-75 preamp board is labeled FP with a date of 8-4-81. Trying also to determine which driver board. Because its not quite the DB-4 schematic that I have. Anyone have any ideas.
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Wed, 05/13/2015 - 17:51
#1
Which board is this one?
Sorry no one has answered you
Sorry no one has answered you. I don't have one and have no knowledge so I'm no help! Mike.
Compare schematics
From looking at the schematics, if the driver board has 4 diodes and 2 680 ohm resistors, then it's a DB4. If not, then it's the driver board shown in the big schematic. Like Mike, I've never seen one of these amps so I'm telling you more than I know. Never let ignorance get in the way of a good time. Good luck. -mgriffin
DB-4
Its a DB-4 I verified that by just lifting it up and looking at its designation. However, it's got a 150MF 3V cap on it, which doesn't match the drawing for my amp. Only driver board I can find with that cap at the wiper of the pot, is a 2100-b schematic. sheesh. I think I will put the 20MF at 25V that the schematic for my amp calls for. BTW it is a 2275-75 chassis. All voltages present and accounted for, and all pots and switches function quiet and nice. However the amp just seems way too weak, new tubes made no difference and it was biased a little hot like someone prior was trying to get a little more out of it. I know it needs all electrolytic caps etc and they are on the way from Mouser. Actually this amp looks like it was bought off the showroom floor, put a cover on it, stuck it in a musty mildewy closet and let it sit there for 34 years....lol I know its been messed with some, some of the chips have been changed and it looks like someone took out and replaced the very first input jack on channel one, other than that this thing is stock. I hope I can bring it back to life. If anyone has any ideas on what could be holding this thing down, please chime in.
About that Cap
That 150uF 3V cap will never see a voltage greater than 1.4V so it's OK to use that 3V value. The extra uFs vs the 20uF cap allow it to hold the bias voltage a little more stable when big bass signals hit those transistors. In actual use, it's probably not a big deal. I suspect they used 20uf 25V caps at the factory because they're used in several other places in this and many of the other amplifiers. In other words, manufacturing cost control. Good luck with the re-cap. If the caps are as old as the amp, it should sound a lot better when you're done. -mgriffin
Frustrating amp
All electrolytic s replaced. All IC sockets and IC's replaced. New tubes and tubes sockets. Driver board completely rebuilt, mainly because I happened to have all components and I hate carbon resistors. Output jacks replaced, old ones looked like someone had bent on them. Cleaned all Pots, put in new tubes. Amp sounds exactly as it did before all work....lol When sitting next to my deville, both amps set on 3 master and channel. The deville will run you out of the room. This musicman sounds nice, but has no balls at all. Sounds like my deville set on about 1. Only test equipment I have these days is a voltmeter and from reading and pretty much knowing whats going on, voltages all look good. The only improvement I've made is with all this work is the amp is nice and quiet, what little scratchyness and noise it did have is gone, but this amp is weak. Any ideas?
Speaker?
Hi Brian,
How does the MM sound when you crank it up? Setting Vol. controls to the same number may not have equal sound level, so maybe things are OK. One thing I'd try is to connect the Deville speaker to the MM and visa versa for a sound comparison. It could be the MM speaker is weak. Your ears are the best test equipment for this. -mgriffin
Speaker
The speaker is fine, sounds good hooked to the deville. I didn't go the other way because my deville is a 410 at 2 ohm, and didn't feel like messing around with the wires to only use 2 speakers. Cranked up a bit it seems okay I guess, just not what I remember from my old 212 65 from years ago. Just got done playing this thing for about an hour.
Master volume seems fairly linear throughout range, even though the schematic calls for a reverse audio. The channel volume controls seem like audio taper even though schematic calls for linear. Amp is kind of lifeless until you hit 8 on either channel then it comes roaring to life, regardless of master volume setting. You have to the channel volume on 9 or 10, and the master way down to get any overdriven tones at all. This is not how I remember my old 212-65 working. Again all voltages are good. Biased at 24.9 mv and both tubes are within .4 mv of each other.
Pots
Hi Brian,
I think you're on to something. Good job eliminating the speaker BTW. If the Channel Volume pots and/or the Master Volume pot are not the correct taper (Linear, Audio, Reverse Audio), you may get symptoms like you describe. Have you looked at the pots to see if they've been worked on? Sometimes hard to tell. Also, have you measured the pots while turning the knobs to see how they behave? Finally, have you googled the part numbers on the pots? I'm just guessing but perhaps you have Audio Taper pots for Channel Volume. Here's a web page that describes the taper transfer functions and may help you sort through the problem. http://www.geofex.com/Article_Folders/potsecrets/potscret.htm -mgriffin
Pots
They all measure okay and look original, that doesn't mean they are correct...lol. I really do think they have audio tapers in the channels, which is one explanation for the symptom. Mouser and a few places have replacements and they're cheap, but they are also alpha brand Taiwan junk, so the bushing will be small for the panel, I hate that. In all my years of messing around with this stuff, I have never changed so many things with very little change in the outcome. Either this is how this amp worked out of the factory, or its something I've just never experienced. Don't get me wrong. If I only needed a good sounding amp for the living room I would probably just stop and use it and not worry about where the pots are set...lol. This is not the case though. I need another gig amp that will get heavy use at club volumes and this one just doesn't feel right. I've got one guy telling me that an output transformer can be weak and behave like this. I've heard this before. However, I have never seen it. Has anyone ever experienced a weak output transformer???
Weak Xfmr?
Hi Brian,
Xfmrs usually either work or don't work. In the case of not working, there is usually smoke involved. I guess it's possible that it could be weak but that would be weird. One possibility is that the Out Xfmr has been replaced with the wrong one, but again, that would be weird and extra work. Did you check the pots rotational readings? Do they behave like an Audio Taper? Check them at 0, 10 and 5. A linear taper will measure 50% full value at 5. An Audio taper will measure about 10% of full value at 5. See the graph in http://www.geofex.com/Article_Folders/potsecrets/potscret.htm I have an MM 112-65 RD. It's not dual channel but output stage is nearly identical to your 112-75. It even has the same compression amp (the ones with two transistors in the feedback loop) between the pre-amp and power amp. It also has no master volume control so that's like your amp master vol. set at 10. My amp is living room loud at 2 and outdoor loud at 6. Does your amp have a line out jack? If so, take the line out with a master/slave cable and connect to your DeVille (assuming it has a line in jack) or another amp with a line in jack. That may help you isolate where the problem lies. Good Luck. -mgriffin
Pots
After I had found they way the channel pots were behaving I tested them, and they are in fact linear and work smooth. The master is in fact a reverse audio taper and works like it should as well. This is why I feel kind of stumped. Lets say I set the master at any level but for this say around 4, but will get same results setting it anywhere just at different levels of volume. I turn up the channel volume and it will increase as one would expect even though I feel its weak, then at 8 the amp roars to life and sounds great, however now too loud. Its like something has to get to some threshold then it works right. All electrolytics are new and good and correct value, behaved same before all were changed, however not as noisy now....lol So whatever was flaky is still flaky.
No line out of the pre-amp or I would have done that days ago.
I agree about the OT, I've never witnessed a weak one as well, but I sure hear about it a lot since I ran into this.
Not Pots
If nothing else, you're getting to know the innards of your amp probably more than you wished. You've checked all of the obvious things but to no avail. The way you describe the behavior, it's like something becomes active with volume at 8. If I remember right, the amp behaves as described on both channels. Looking at the schematic, IC3 is common to both channels. IC-3 also appears to be part Ch1&2 final preamp stage compression amps with 2 transistors and diodes in each feedback loop. Postulating IC-3 not working and with Volume at 8, it could be there's enough signal to turn on the IC-3 feedback transistors and conduct a larger signal to the power amp giving that sharp increase in volume. Next thing to possibly try is swapping IC-3 with IC-4 from the reverb circuit. I believe they're the same IC P/Ns. Question: Have you cleaned the IC sockets with something like DeOxit or pot cleaner (not an album cover)? Always a good maintenance tip when servicing amps. For now, I'm out of ideas but I encourage you to persevere. Keep sending clues. -mgriffin
Except
I've put in all brand new military grade IC sockets and replaced all IC's, transistors and the 1n914 diodes, its all cheap stuff and probably better quality control on them nowadays, so I just change them. That's stuff I normally just do when restoring something like this. Normally by now it's fixed working great and walking out the door cause some friend of mine wants it, and talks me out of it....lol Thus the confusion, with all that you would think something would change. But, nothing other than quieter operation. The best way I can describe this is "no sparkle" kind of just dry and sterile, no feeling to it, weak lifeless and uninteresting. I taught high reliable soldering in the service, so I've gone through and fixed a bunch of questionable solder as well, absolutely no change. lol
Due Diligence
Good grief. You have covered all the bases. I'm a retired EE and back in the 70's I was NASA certified for soldering. Some of my chit was on the first Space Shuttles. So now we know we're both over-qualified for this. Here's a dumb question:Where are your tone controls set? I thought I had a problem with my 112-65RD after cleaning the noisy pots. I had the volume set pretty high and not very loud. What happened to the power? Then I looked and all 3 tone controls were at 0. Doh! After adjusting them up, the solid sound came back. I felt like an idiot. Rumor has it that Johnny Winter ran his MM at full volume (10), Treble at 10, Mid and Bass at 0 and Bright On. I tried the tone settings and really had to crank the volume to get some noise. Works for Johnny but not me. -mgriffin
Ex Navy
I was an AE in the Navy, then after I got hurt I worked as a civilian for the AF on flight sims for 20 some years. Had much more fun co hosting a jam sessions weekly for the last 20 some years also, thus the reason all my rebuilds are in someone elses living room....lol
Tone pots all midrange for testing things. There's only one area of the thing I still don't like the looks of and thats all those ground lines jammed into two eyelets, but it won't fix this, they are all making good connection, just look crappy. Heck I even built a new turret board for the caps and diodes in the dog house because I hate eyelet boards. Mine looks lots better now, doesn't help a damn thing, but looks cool if you take the cover off....lol
Restored to nearly new now
Decided to just restore the whole darn thing. She's back to her showroom glory!
The only thing that I would
The only thing that I would comment about is leaving those old electrolytics on the board. Unless you found some new ones that are metal! Mike.
PS-It's beautiful!
All new
Nope they're all New from Mouser. I was surprised that the 20MF and 2MF were of the silver nature. Mouser part numbers.
75-TE1206-E3
75-TE1301-E3
That IS wild. Never knew
That IS wild. Never knew they made them that looked like that anymore. Good to know! Mike.
Very nice!
Nice to see a MM restored to as-new condition. Good job!
Cheers,
Lars Verholt
Heat sinks
Thanks for the comment Lars. Has anyone ever found a good heat sink for those 390 ohm resistors? I've looked and can't find anything.