Yamaha B100 115SE repair

[BrianS]

This is another amp I picked up for $20 off Craig's List in the hopes of learning something, but also with the idea of re-purposing the power amp section.

First off, the previous owner had tried to replace a few of the control pots (vol., bass, etc...), and kind of messed it up.  It is missing the bass pot, too.  So, he told me the preamp was messed up, but the power amp was fine... :lmao:

So, I hooked it up with a jumper wire where the bass control was, and put my meter on the output jack to see if any DC voltage was present there (learned that from the TA-16!  Thanks Roly!).  Sure enough, there was +43 dcv there, but it dropped steadily to near zero (.135 or something).  However, it took probably a whole minute or more for it to drop all the way down.

After it dropped down, I hooked my speaker up and gave it a little test run with a guitar.  The amp was quiet and single notes sounded nice and clean.  Then, when I turned it up and hit a full, loud chord, the sound would drop out rather abruptly and the leak light would burn fairly bright, and the "on" indicator light would go dim...something is sucking current when a large signal is applied.

I unhooked the preamp, still hoping that is where the problem lay.    Nope.  I hooked up my OCD boost pedal directly to the power amp and plugged my guitar in.  Again, playing softly with the pedal's volume down a bit, I'd get a clean signal, but as soon as I turned things up and hit it hard, the leak light burns bright and the indicator lamp flickers.

So, that's where I'm at.  I did find a service manual to download for free: http://www.owner-manuals.com/B100115SE-service-manual-YAMAHA.html.  I've briefly looked over the schematic, which does not include any voltage measurements.  I do not have an o-scope or a signal generator, so I can't do their recommended tests.

Where to start?

[Roly]

there was +43 dcv there, but it dropped steadily to near zero (.135 or something).  However, it took probably a whole minute or more for it to drop all the way down.

If you look at the circuit on page 13 bottom-right, between the "DC Board" and speaker sockets "SP OUT1/2" you will see a 2200uF/160V and a 10k resistor.  This cap is the output coupling cap and with the speaker disconnected it can only charge via the 10k resistor.

That time constant is;

2200 * 10^-6 * 10 * 10^3 = 22 seconds, but this is for 66%, so fully charged would take about a minute.  In other words a) that looks okay to me, and b) with a coupling cap the speaker is safe from damage by a duff output stage.  So far, so good.

What you now need to measure is the voltage on the transistor side of this cap, the "MA Board".  Pin 6 should have the +ve supply rail, and the voltage on the output or "half-rail", pin 5, should be very close to half whatever the pin 6 voltage is, and it should jump there almost instantly at power up.  (Anything else and you have some dead silicone to find).

If this is the case then I would normally assume that the power amp is healthy, and the "collapse" on loud notes is an artifact of having the limiting lamp in series.  If, and only if, the voltage on pin 5 jumps to half the supply at power up I'd be inclined to try it without the lamp (or use a higher wattage lamp first and see if that gives you more headroom to collapse).

[J M Fahey]

What you see is the lamp limiter lamp limiting

[BrianS]

 xP

Yeah, that limiting bulb sure does a good job!!  I even had a 60watt bulb in there, and this is a 100watt amp!

I did your test, Roly, and it worked out right.  I've got about +104vdc on pin 6 and just over +51vdc on pin 5.  Swapped my limiting bulb out to the 150watt one that I use for tube amps and the amp worked fine.

FWIW, I've gotten one of my old textbooks out, "Introductory Electronic Devices and Circuits" and will be doing my best to "relearn" a lot of the basic stuff.  I won't ever be a math whiz or have a complete understanding of how all this fits together, but I do want to be able to get a lot better at troubleshooting problems...and not overlooking the basics!!

Now that I know the amp is working properly, it's on to my next step: "re-purposing" the power amp.

I want to have a fairly lightweight rig for rehearsals and small gigs.  My idea is to purchase a Tech 21 Flyrig5 (http://www.tech21nyc.com/products/sansamp/flyrig.html), which I can run right into a PA system.  However, I also want the option of driving my own speaker cabinet, so I'll need a power amp...which is where this Yamaha comes in.   I have a couple questions:

1)  I'd like to have a volume control on the amp.  How would I best implement this?  What about input impedance?  Obviously the power amp works just plugging a pedal into the front of it, but maybe there's something I'm overlooking?

2) I'm going to need a new chassis.  I'm really hoping I can build my own out of some scrap metal and I have some ideas, but I'm open to suggestions.  Right now, the plan would have the amp "top-mounted", so the heatsink would be sitting upright, rather than hanging down.  Will this be bad for air flow?

Any other advice regarding how to best go about converting this to a guitar power amp would be great.  Remember, I only paid 20 bucks for this thing, and my hope is that I won't have to spend too much more re-purposing it.

Thanks!

[Roly]

I've got about +104vdc on pin 6 and just over +51vdc on pin 5.  Swapped my limiting bulb out to the 150watt one that I use for tube amps and the amp worked fine.

Good, good.  Remember, these amps are only about 50% efficient so to get 100W out it needs at least 200W in, and to get that at a reasonable voltage would require a limiting lamp of perhaps 500 watts so ... nah, forget it.   :loco   :cheesy:

I won't ever be a math whiz

Multiply and divide for Ohms Law, and squares and roots for power.  To repair amps you don't really need much more than this.  Have a look at my Maths Phrasebook for Tourists.  With power engineering and exotic audio filters it can get a bit long-haired, but you won't encounter either in amp repair.


Your measurement tells us the power amp supply is 104V, and this is the peak-to-peak output voltage available, or 51Vpk.  Looking at the circuit the feedback network is 1.8Meg and 10k (in series with the 1/160 cap), so the AC gain is roughly 1800/10 or 180 times (which is a fair bit).  So the peak voltage at the base of TR302 should be about 51/180 = 0.28333333 or about 300mVpk, which means we need the same voltage on the base of TR301 for full output, so that is the sensitivity of the "MA Board" at pin 3.

This input is shunted by a 10k so the input impedance can't be any higher than this, so what you've effectively got is a fairly sensitive, fairly low impedance Line input.


With an input impedance of only 10k just sticking a low value pot in front will work but it may have an odd characteristic, lumped at one end, and it will also reduce the overall input impedance even more.  A simple buffer consisting of an emitter-follower (transistor) or source-follower (FET) would lift the input impedance considerably allowing the use of a much higher value pot.  Assuming that you are going to use the "DC board" the 30V supply previously for the preamp will be available to power such a follower stage.


The only real consideration with heatsinks is free air flow, and that they work better with the fins vertical than horizontal.  If you are really stuck physically then the normal answer is a fan, but to make it less noisy it can be speed controlled.

[BrianS]

Hmmm...you've given me a lot to think about!  I'll get back to this after my vacation...