Studio 22 voltage query

Hi all,

Recently purchased a 2nd hand Mesa Boogie Studio 22 (with graphic EQ, but not Plus). I have done the lead master and gain mods, and am overall extremely happy with the sounds coming from this amp. Much prefer it to my old Fender Hot Rod.

However I have question about the high voltage secondary output from the main transformer. According to the schematic it should 360VAC but I am only measuring 275VAC. And this in turn reduces the expected DC voltages at points A, B and C (c53, c52, c51).

The amp still seems to work fine, but surely this cant be right. Running off 240 mains (UK). Could this be a faulty transformer (561022), or has someone deliberately reduced the secondary voltage?

Any insights would be appreciated.

Thanks
Dave

the studio 22 should have a rectified B+ in and around 375-400 volts...

so if you have 275 volts AC on the secondary that sounds right...

take a DC voltage reading after the rectifier diodes....

i do not see any AC secondary voltage notes on my Mesa shop schematics....

360VAC!!! ha! after that is rectified you would be in Mark I/Twin Reverb territory

Thanks for your reply. According to the schematic there should be 360VAC (R1 and R2) before the diodes and 390V DC after the diodes (at the standby switch). I am measuring less at both points. Just a bit odd.

Schematic here... http://www.classictubeamps.com/SchemMesa.html

i see the 360VAC on the .22+ schematic and I believe that is a typo...that would be the voltage for the .50 caliber amps...

some rectifier math would indicate that secondary should be around 300-315 VAC for the pair of EL84's

that full wave rectifier would bump it up to 390-400 VDC for the B+

there's hardly any filtering on these amps so there will be some +/- on those figures....

what is your B+ at the A,B & C feed points for the stages....

Just measured and am getting...
A = 313V DC
B = 229V DC
C = 180V DC

octavedave said:

Just measured and am getting...
A = 313V DC
B = 229V DC
C = 180V DC

Click to expand...

okay, yes, that's low...

some basic troubleshooting can eliminate the PT...

is this a Boogie export with a factory 230v PT, or is this an aftermarket from TAD(there are 3 taps on the primary which need to be correct), etc...?

with that said, you can lift your secondaries from the circuit and take all your AC readings...

with what you have described so far it sounds like the PT is struggling so you are near the problem/solution...

i would isolate the PT to rule out anything downstream...

Thanks again for your assistance here, it's reassuring to getting another person's opinion.

I don't have the amp opened up at the moment, but i did try isolating the secondary HV output from the transformer previously and i seem to remember it made no difference, in other words still 275VAC. So this suggests the problem is not upstream. I will double check this.

Not sure the history of the amp ie Boogie export or aftermarket (what is TAD?). But i can see that the model number of the PT is 561022, which seems to match the number on the schematic. Although i guess there may be different variations of this model.

Very weird.

No problem...wish i could be more helpful. TAD is Tube Amp Doctor which has a lot of Mesa Boogie parts for the non-US market including all transformers for all models... They are also very helpful. I believe their transformers are aftermarket but they have 220v, 230v & 240v direct replacements. I would check with Mesa direct first if it comes down to a bad PT...

Stupid question but is your wall voltage 240V? There is no step-up/step-down transformers mounted in the cabinet by someone who might have brought it over from the US?

Ok have got a little further with the investigation.

Sent support request to Mesa, still awaiting a reply.

Been in touch ith TAD, who have been very helpful. We have established PT is not one of theirs. It appears to be made by a US company called Schumaher, who have supplied parts to Mesa in the past. I have sent them an email too (no reply).

But what i have established is that the current transformer is definetly only outputting 275-0-275 as opposed to 330-0-330. The other thing I have noticed is that the transformer doesnt have the 50VAC tap (for powering the footswitch LDR circuit) that the TAD parts have and that is shown on the schematic. So clearly this is the wrong transfromer. But what is bizarre is that the model number engraved on the tranformer seems to be correct 561022. It appears that someone has added an additional 50VAC transformer to the underside of the chassis, to get round this issue.

Clearly the amp has been modified at somepoint (there is a servicing sticker inside the chassis from a service compnay relating to work done in 1990 - I might try and contact them today, assuming they are still trading).

Perhaps the orginal transformer died, and they replaced it with a lower rated PT and a separate 50VAC PT. perhaps to save money on buying an original Mesa part. There is evidence of an old incident of overheating/fire at some point on the PCB.

I am thinking I should buy the proper PT from TAD, and try and get back to how the amp was orginally designed. My only conern, other than the cost (I have already spent way too much money on this amp already!) is whether re-introducing 330vAC will cause other issues. But I am not sure I can sleep well at night knowing the amp is running on 80% voltages.

Dave

Well that kind of answers alot... the only way to right it is to put it back to stock with a correct PT. It is fairly straightforward...

Would you mind posting a pic of the supplemental transformer for the low voltage feed?

What country are you in?

Sounds like you are confident enough to get in there and do the PT replacement correctly and wire up the secondaries according to spec.

If I can help please let me know...

Thanks. I think I will, although just want to officially hear back from Mesa first.

Following this link for photos...

https://plus.google.com/115167333513380687370/posts/48nrPKQBWHF

Just a final word on this, if anyone is intererested, and thanks to Lovetoboogie for your posts. Mesa Support are advising that the transformer is definitely not an orginal part, hence why the HV outputs are lower than expected. However their advice is that if its working and sounds good, why go to the hassle and expesnse of fixing it. Running at lower HV just means output volume may be slightly limited.

So for now I have decided to save the £100 on a new replacement tranformer, and enjoy the amp as is.

octavedave said:

Hi all,

Recently purchased a 2nd hand Mesa Boogie Studio 22 (with graphic EQ, but not Plus). I have done the lead master and gain mods, and am overall extremely happy with the sounds coming from this amp. Much prefer it to my old Fender Hot Rod.

However I have question about the high voltage secondary output from the main transformer. According to the schematic it should 360VAC but I am only measuring 275VAC. And this in turn reduces the expected DC voltages at points A, B and C (c53, c52, c51).

The amp still seems to work fine, but surely this cant be right. Running off 240 mains (UK). Could this be a faulty transformer (561022), or has someone deliberately reduced the secondary voltage?

Any insights would be appreciated.

Thanks
Dave

Click to expand...

Dave, sorry I can't help with your current problem but I recently picked up a studio 22 which I hope to do the lead vol and gain mod to sometime soon. I have looked at various posts but some of the images have been deleted, can you let me see a schematic for the mods? Any other worthwhile mods while I'm at it? I intend to add another 4 ohm output in parallel with the current one to have the same setup as the 22+.

Hi,

well it seems that PT has been replaced, and schumacher PT cover end bells fitted to new one !

20% voltage less isn't a great issue on tone itself, only will reduce headroom volume a bit

mecury magnetics sells exact replacement for Studio22+ the PT MESAB-STUDIO22-P for 175 bucks, TAD sells to 150$,
i would personally go for mercury one.

i hope it will help,

Hi both,

Mods:
The Lead Gain mod is recommended for both Studio 22 and Studio22+. See picture here
https://www.dropbox.com/s/j8zg5vwyu56zobc/F6748E06-05CC-48E6-A28D-44138398E39F.png?dl=0

If your amp is the earlier Studio 22 then, I would consider the LEAD MASTER volume mod as well. see this article
http://www.premierguitar.com/articles/boogie-tricks-a-fuse-fix-1
Personally I adapted this mod to make it 'pre-insert' as opposed to 'post-insert', otherwise is doesn't work very well it you have a looper pedal in the insert chain (as I do). If you are interested in this variation, let me know (its not documented yet).

I have also built myself a 'dual circuit booster' external pedal that always me to boost the input signal pre-preamp whilst simultaneously attenuating the insert loop signal. This allows me to further overdrive V1/2 tubes, whilst controlling the overall change in level. Similar to how the Maxon DB10 Dual Booster works (mine was actually a modified EHX Soul Food pedal). This works wonders as I now have three stages on overdrive: RHYTHM (CLEAN), LEAD (OVERDRIVE), BOOSTED LEAD (HEAVY OVERDRIVE).

BlackBoxy: thanks for you feedback here. I finally got to the bottom of this problem. Someone in the past had replaced the transformer for a slightly lower rated transformer, but had kept the old chassis mounting bracket (where the model number was still stamped on). Hence the confusion. In the end I decided not to fork out $150 for a new transformer and risk messing things up. The amp sounds absolutely beautiful at the moment, so no need for me to 'fiddle'

Dave, thanks for the quick reply, will take notes of both mods to my repair guy, they seem fairly simple. I am also going to add another 4ohm output in parallel with the existing one so it has the same format as the 22+. R131 will be uprated to 2 watts at the same time. The only other mods I have considered is a chassis mounted reverb tank and adding a 12v cooling fan as these things run extremely hot, just to protect the circuit board. A bypass for the fx loop might be an idea too, mine has a lead permanently in place to stop the amp cutting out.

This is my 4th 22/22+ and I've also owned an F30 head, a Rocket 44 and a Mark IV so if I do the mods this one will be a keeper! Rocket 44 was nice as a single channel amp, F30 too clean/dirty, nothing in between, Mark IV I got cheap and was wonderful but way too loud and heavy for me so I flipped it.