1976 Boogie SP3 restore help

[mark1406]

The presence pot is 10K, you won’t get much out of a 100K pot, it would be more like a switch.

The brand of the pots isn’t that important, what is important is the taper of the pots. I really like a 30% taper on the 250K treble pot. If the treble pot has a 10% taper I find the response is bunched up at the end of the pot.
I find a 10% taper works well on the 250K bass pot. The I like a mid pot with a 10% to 30% taper.

The taper of the volume controls depends on what response you want from the amp.

Regards

Mark

 

[user 41014]

Thank you. I should probably test a few pots as I would really like to crack the older cts pot code. I've got a bunch of cts pots- it seems like people prefer it to alpha.

 

[mark1406]

To measure the taper of the pot you put the wiper of the pot in middle and measure the resistance. If you have a 250K with the wiper in the middle and measure 25K then the pot has a 10% taper.

Regards

Mark

 

[user 41014]

To measure the taper of the pot you put the wiper of the pot in middle and measure the resistance. If you have a 250K with the wiper in the middle and measure 25K then the pot has a 10% taper.

Regards

Mark

You must have been reading my mind.

I pulled out my new pots with known tapers and was going to take some measurements. The audio/linear is easy enough to tell but I wasn't sure about percent of taper. I thought I had more large shaft pots than I do.

I need some pot! Some pots that is.

 

[user 41014]

The power cord was a little sloppy in the chassis so I ordered a handful of strain relief sizes. They were 10-12 pennies at mouser so don't buy on sleazebay, etc.

This is the 1st boogie I've had a loose mains on. Here goes in case you're in the same boat or need to change an older IIB cords falling apart. I've reused the strain reliefs so you may not have to get new ones.

The hole with straight sides is called double D.

The relief in the chassis was 6P3-4. Even though it was the well crimped cord causing the slop I used the 6N3-4 strain relief that my IIB's are using. Mains cord size matters. For Heyco 6N3-4 translates to their model 1200 and 1207 = 6P3-4.
For the most part these double D's holders hold up well.

 

[user 41014]

I got some .047uF Vishay polyester caps in. Given how tight testing the Mullard C280 was tho I decided to make an aquarium out of this chassis.

The 600V orange drop looked newer and tested 47.69nF so no reason to replace it but I did anyway with a 47.25nF testing tropical fish. The 59.6nF testing Black Beauty got yanked for another tropical fish coming in at 48.3nF. I'm digging the look.

Someone put 2W film R's for the power socket heaters. Sure looks red brown black to me but I pulled one and it tested 102ohm. My other Marks have 1/2W 100R CC's for heaters so I'm guessing someone stuck circuit in-spec testing heater R's there that were wildly out of spec to themselves. They looked hacked in so they'll get swapped out.

I pulled the Bass pot GH1048. It tested 196K ohm. With the shaft broke I got it close to center and measured 16K. I haven't cracked the old CTS codes but Mesa has a 591048 which is 250K ohm and unspecified audio taper. They also have a 598148 which is a 250K 10% taper short shaft which might be the one I need here.

That leads me to think GH1048 was a 250k 10% taper. Resistors usually drift up, but is it the case with pots they drift down? I'm wondering if the carbon is laid over a conductor and as the carbon wears the conductance increases with use/wear?

 

[mark1406]

The power cord was a little sloppy in the chassis so I ordered a handful of strain relief sizes. They were 10-12 pennies at mouser so don't buy on sleazebay, etc.

This is the 1st boogie I've had a loose mains on. Here goes in case you're in the same boat or need to change an older IIB cords falling apart. I've reused the strain reliefs so you may not have to get new ones.

The hole with straight sides is called double D.

The relief in the chassis was 6P3-4. Even though it was the well crimped cord causing the slop I used the 6N3-4 strain relief that my IIB's are using. Mains cord size matters. For Heyco 6N3-4 translates to their model 1200 and 1207 = 6P3-4.
For the most part these double D's holders hold up well.

The mains cable on my SOB amp went all sticky and weird. I decided to replace it with a thicker cable. It was all fine.

Regards

Mark

 

[user 41014]

I've had to turn the oven to 420 and bake my head into these pots. I see no documentation on things like GH1048 but I'm starting to crack some of the Mesa code- I believe all the 598xxx mesa pots (CTS) are round short shaft as will be needed in these amps as it turns out.

I installed the 598147 to replace the Bass pot.

Pots in general kinda suck for quality at these price points I'm finding. I tested about 30 various new pots and not all are landing in the 10% tolerance of the 450S series. The majority are lower than rated and not slightly.

I checked all the pots in circuit not knowing how that would go. Right or wrong I used the outside lugs reading matched to the combined wiper to each outer lug as a check for test trustworthiness. The volume pots and maybe the treble pot can't be trusted in circuit. The new Bass pot measured the same 239K in and out.

The master was at -18% of assumed 1M, while the tone and back panel pots are -21.8 to -30% off spec. This begs the question of when people consider them out of spec. I'm sure some pots are spec'd +/- 20% so does a reverb pot at -30% of spec (69.9K) matter?

And if presence is supposed to be 10K, my in circuit test of assumed 250K makes me wonder what someone was thinking and if it has anything to do with the unusual PI/FB config.

 

[user 41014]

The old ugly hacked in heater 2W film R's are out and the 1/2W CC heaters in looking as inconspicuous as normal. I was thrown by the film R's but it was a good lesson or reminder that gold and silver aren't the only tolerances and red is 1%. Seeing the brown black brown in my hand was the realization that it was not gold striped.

With the old Bass pot measuring 196K there was no sense in trying to rebuild it. And as much as I like the look of the way the guts are shaping up I was thinking I might dress the outside up with new knobs.

Am I the only one that thinks a vistalite top for a headshell is the way to go? But then you would need LED's and metal grill, maybe some chains......

 

[user 41014]

There is a detached light blue wire that has me baffled. It originates at the V6 plate with a 220K ohm voltage dropper to a blob just beyond pin 6 on the board where the blue wire attaches.

The length is right for a TS jack where on later Marks there would be a Class A or 60/100W switch. The wire is solid which makes me think AC and there is a solid wire remnant broke off at that jack.

The purple wire to the same jack is coming from the RCA jack at the chassis center in between the reverb connects. There it meets with a dark blue wire that after touching the mid pot heads to the Gain Boost pot switch. After tracing the dark blue wire I was thinking the RCA must be for a gain boost footswitch and the TS double jacking is also for the GB since no one uses RCA for footswitching.

When I originally saw the jack on the chassis I was thinking pre-out but I don't think so.

And what would someone be doing with plate volts through a 220K? Does anyone know the current from the combined pair of plates to calculate the volts downstream of the 220K?

 

[James]

There is a detached light blue wire that has me baffled. It originates at the V6 plate with a 220K ohm voltage dropper to a blob just beyond pin 6 on the board where the blue wire attaches.

The length is right for a TS jack where on later Marks there would be a Class A or 60/100W switch. The wire is solid which makes me think AC and there is a solid wire remnant broke off at that jack.

The purple wire to the same jack is coming from the RCA jack at the chassis center in between the reverb connects. There it meets with a dark blue wire that after touching the mid pot heads to the Gain Boost pot switch. After tracing the dark blue wire I was thinking the RCA must be for a gain boost footswitch and the TS double jacking is also for the GB since no one uses RCA for footswitching.

When I originally saw the jack on the chassis I was thinking pre-out but I don't think so.

And what would someone be doing with plate volts through a 220K? Does anyone know the current from the combined pair of plates to calculate the volts downstream of the 220K?

Both amps I posted earlier have the same wire. The other end isn't connected to anything, but wrapped around a couple of the shielded cables.

I vaguely remember someone on thegearpage asking someone at Mesa what the purpose was. I don't think there was really an explanation, other than it being done for a reason.

As for the gain boost, I'm not sure. I know when I got my 76 it had the three RCA jacks (two reverb, one disconnected) and the gain boost wired to a 1/4" on the back... it didn't look stock though:

 

[user 41014]

Both amps I posted earlier have the same wire. The other end isn't connected to anything, but wrapped around a couple of the shielded cables.

I vaguely remember someone on thegearpage asking someone at Mesa what the purpose was. I don't think there was really an explanation, other than it being done for a reason.

As for the gain boost, I'm not sure. I know when I got my 76 it had the three RCA jacks (two reverb, one disconnected) and the gain boost wired to a 1/4" on the back... it didn't look stock though:

View attachment 896

Lmao-
Ah, there's a loose open voltage wire "being done for a reason". Great.

Apparently it's used to stabilize the coaxial inputs in case one should run out of plastic zip-ties. 460V must be too much for zip-tying hence the step down 220K ohm R.

.....maybe it wraps the coax for inductance to interfere with the input signal but someone couldn't figure out where to terminate a closed loop or forgot their right hand rule for flux and just abandoned it for a live tie off.......... um........
For now I'll just add a load- to my bong!

 

[user 41014]

So this is what I'm seeing regarding the fetron/12ax7. Am I missing something?

1) Parallel V1 22uF/25V caps and IN5303 diodes for fetron; for 12ax7 use normal parallel 22/25 bypass with 1.5K resistor off the cathodes like the other 12ax7 stages. This amp has already been changed to 1.5K R's.

2) V1 plate resistor's of 100K (I think bluebug had 82K's) and as low as 56K for fetron use as per schematic. Mine has the dual || Dale 100K's which is 50K, so all I have to do is yank one Dale from each plate and I'm at 100K like the Mark I RI schematic if it can be relied on.

3) That schematic calls for a resistor after the brown wire (260V) hits the preamp board. It shows "33K - 15K wl fetron" and 225V post resistor before the V1 100K plate R's. I don't see this resistor at all. Is there a V-dropper I'm looking in the wrong place for?

The repair manual states the fetron circuitry will work for 12ax7 despite the above and adds:
"If the amp experiences a cutting out when in the heavy distortion, loud driving mode after a 12AX7 has been substituted for the Fetron, then install a 250 pf 1000v disc capacitor across the plates of the driver tube (12AX7) next to the 6L6's. The plates are pins 1 and 6. If an oscilloscope is available and the Fetron shows a lopsided wave form or uneven clipping, it can sometimes be corrected by changing the value of the plate load resistor. Values should fall in the range of 33K to 100K."

That statement, if the last sentences reverted to discussing V1 plate loads and not the PI, a big IF, suggests my currently effective 50K plate loads are within a unknown sweet spot range that only a scope (or maybe human transducers) can dial in. Putting a pot in series could do the dialing but it sounds like I need a scope. In the meantime (which is a great song) I could leave as is vs lift a Dale leg from each plate to compare audio response of 50K vs 100K.

"...changing the plate load resistor" is said singularly tho. The only other option I see is the pair of R's coming off the PI plates to the yellow wire (345V feed) where the coupling bias caps are.

 

[user 41014]

As for the gain boost, I'm not sure. I know when I got my 76 it had the three RCA jacks (two reverb, one disconnected) and the gain boost wired to a 1/4" on the back... it didn't look stock though:

James, do you know the month of that '76 and serial? I noticed it has a RP-4D. I'm trying to get a loose timeline of pre-boards up through the IIB's. With my late '76 SP-3 it would seem the RP board went through more updates. I can't see the date of the board.

I do like the "amps for dummies" approach labelling the ground trace, B+ and whatever else might be labelled.

 

[user 41014]

The taper of the volume controls depends on what response you want from the amp.

I should check the volume taper of my DC5 for what taper not to use. My take is the DC5 goes from not enough signal to juice the circuit at volume of 1 and by 2 jumps to gig level rather abruptly. Does the higher taper % correct that or do I have it backwards?

 

[James]

Lmao-
Ah, there's a loose open voltage wire "being done for a reason". Great.

Apparently it's used to stabilize the coaxial inputs in case one should run out of plastic zip-ties. 460V must be too much for zip-tying hence the step down 220K ohm R.

.....maybe it wraps the coax for inductance to interfere with the input signal but someone couldn't figure out where to terminate a closed loop or forgot their right hand rule for flux and just abandoned it for a live tie off.......... um........
For now I'll just add a load- to my bong!

The weird thing is that multiple amps have it. I've seen at least one amp that didn't, but it was one of the later ones built by someone else.

James, do you know the month of that '76 and serial? I noticed it has a RP-4D. I'm trying to get a loose timeline of pre-boards up through the IIB's. With my late '76 SP-3 it would seem the RP board went through more updates. I can't see the date of the board.

I do like the "amps for dummies" approach labelling the ground trace, B+ and whatever else might be labelled.

A53x from 5-76. As far as I can tell, the SP-3 wasn't revised further during MK 1 production

 

[user 41014]

The weird thing is that multiple amps have it. I've seen at least one amp that didn't, but it was one of the later ones built by someone else.



A53x from 5-76. As far as I can tell, the SP-3 wasn't revised further during MK 1 production

Thank you. I'd really like to get the first and last Boogie RP/SP, 1st and last IIA RP/SP and 1st IIB RP/SP's for bookmarks. I have yet to see a RP/SP-8A and have never owned a IIA.

Seeing a '78 with SP-3 I'm inclined to agree with you. When the person gets their A-47 back from Mike it should shed light on the original commercial boards.

Maybe the loose open wire is an antenna to monitor possible china attacks on Ft Bragg in an effort to steal our Colossal Redwoods and petaluma national secrets.

And how does one live there making amps and not offer the 'Colossal' model amp? "It towers above the competition". I would have a coastal redwood between the e and s of the mesa logo if it was me.

 

[mark1406]

The wire around the co-ax cables is strange. The wire goes around the input cable, the output from the first stage cable and the input to the second input. The signal on the second two cables are out of phase with the input cable. The wire from the output stage should be only seeing the preamp earth potential. So I don’t see what is to be achieved with this wire.

I rewired my SOB and rerouted the wiring so I only have the input wire in that position.

Regards

Mark

 

[James]

Thank you. I'd really like to get the first and last Boogie RP/SP, 1st and last IIA RP/SP and 1st IIB RP/SP's for bookmarks. I have yet to see a RP/SP-8A and have never owned a IIA.

Seeing a '78 with SP-3 I'm inclined to agree with you. When the person gets their A-47 back from Mike it should shed light on the original commercial boards.

Maybe the loose open wire is an antenna to monitor possible china attacks on Ft Bragg in an effort to steal our Colossal Redwoods and petaluma national secrets.

And how does one live there making amps and not offer the 'Colossal' model amp? "It towers above the competition". I would have a coastal redwood between the e and s of the mesa logo if it was me.

My 1981 IIB (74xx serial) has the SP-8B board, which I believe is the first revision. When I sent the amp to Mike B, he told me how there were three versions of the IIB and the second two were higher gain.

He didn't say if this amp was the first revision, but I know it uses all 12AX7s. The 1982 I used to own used a 12AT7 in V2, but I can't remember which PCB. It was definitely SP-8X, but somehow I don't think it was the 8B. That amp was in the 8600 serial range.

 

[user 41014]

My 1981 IIB (74xx serial) has the SP-8B board, which I believe is the first revision. When I sent the amp to Mike B, he told me how there were three versions of the IIB and the second two were higher gain.

He didn't say if this amp was the first revision, but I know it uses all 12AX7s. The 1982 I used to own used a 12AT7 in V2, but I can't remember which PCB. It was definitely SP-8X, but somehow I don't think it was the 8B. That amp was in the 8600 serial range.

@sloanthebone just posted his IIB with a RP-9B. I'm jealous.

I have a few SP-9A's, SP-8B, RP-8C, maybe a RP-8B (I'll have to check) and the RP-9C is the one the thrash/metal guys seem to be discovering lately. I can't help but wonder if it's simply the IIB power section that not even the IIC+ can touch and then of course the 9C is the closest to the RP-10/11.

So that means there is a 8B, 8C, 9A, 9B and 9C at least. Most of mine are coli so it doesn't help with serial #'s but the early ones are 58x (long chassis IIB started at 550) and K12x and having only heard of one exception I don't think sub-K100 coli's exist.

74xx is almost 2000 amps into the IIB. It would be nice to see what's under the hood of 5575-6000 just to see if the 8A exists. It wouldn't make sense for the SP/RP-8's to span the IIA's and IIB's. So did the IIA's end on SP/RP-7x?