Jordan Entertainer J110

[joshdfrazier]

Hello all, (obviously) my first time posting here. I've been building and repairing tube amps for some time, but solid state technology is uncharted territory for me.

I recently acquired an old Jordan Entertainer amp. There is very little information available online regarding this amp. In fact, the schematic inside of the amp is actually wrong. The amp was working for awhile, until it was jostled while playing, and made a loud hum. I found that the rectifier diodes were physically damaged. i replaced them, and the amp worked for awhile. I was playing, and the same thing happened, this time taking a resistor with it. this resistor is wired in series with a diode, connecting the base of two output transistors. the schematic inside of the amp does not show this resistor, only the diode. the resistor was so completely burned, that it couldn't be identified. i replaced the resistor with a 22k and replaced the diode as well.

so now, the amp powers on again, and stays on with no issues, but no sound. only the slightest bit of 60hz hum, which is not noticeable until your ear is placed right on the driver. with the volume maxed out, the hum gets only SLIGHTLY louder. the amp has spring reverb, and you can actually HEAR sound coming from the springs/tank itself. So, SOMETHING is working, but it isn't the output section.

Seems it would be obvious to replace the output transistors, but before I do that I want to make sure there isn't something else I might be missing?

EDIT: I suppose I should clarify, when I say "the same thing happened" I'm referring to the symptom (hum), and not the physically damaged rectifier diodes.

[DrGonz78]

Welcome to the site! First thing first is to make sure the output section of the amp is not putting DC voltage onto the speaker. If you have DC at the output then it would be wise to disconnect the speaker. Solid state amps do not require a load or speaker when powered, much different than the tube amps you might have worked on. In fact, connecting the speaker to an amp that is producing DC voltage on the output can damage your speaker.

So with the speaker disconnected you will then want to check the speaker leads with a voltmeter. Do you have a multimeter or something to check the voltage? Connect the black probe to the negative part of the speaker output jack/terminal and red probe to the positive output jack/terminal. We need to know if there is DC voltage being fed to the speaker terminals. If there is then DO NOT connect the speaker to the amp until it is resolved.

Research light bulb limiters as this device comes in handy if the amp is ever blowing fuses and will come in handy in the future working on solid state amplifiers.

[teemuk]

How "wrong is the schematic"...? Few minor circuitry revision changes excluding a rather helpful reference? Or entirely unusable?

It would help tremendeously if we were even slightly familiar with what kind of circuit we're dealing here. Is it possible for you to scan or photograph the schematic and post the image here?

[J M Fahey]

Agree and add: the resistor in series with diodes is the bias network.
22K is impossibly high, usual value is 10 to 100 ohms and it can even be shorted without danger.
But 22K overbiased and certainly burnt output transistors.
Bu to stop this guessing game, do as Teemu suggested and post a clear, readable,sharp, well illuminated schematic picture.

[joshdfrazier]

Thanks for the replies. I drew in red the location of the "mystery" resistor that does not appear on the schematic.

teemuk; it is more likely revisions were made and the schematic was not updated. It is definitely the schematic for this amp, I was unclear on that.

There is no DC voltage being sent to the driver. I use (impedance matched) "junk" speakers when testing, anyway. I had a bulb limiter that I built, but it was lost in a move and have yet to build another.

[J M Fahey]

I'd start by checking that you still have the +/-19V power rails, replace Q14/15/16 which are most probably shot, plus Cr1, leave the mystery resistor shorted for testing, and reapply power with a 25W`series bulb lamp limiter with no other load connected.
I also suspect the speaker might have died.
Anyway fire the amp up, rechechb power rails and absence of DC at the output.
It *might*  be all that amp needs, and if not, measurements now will be more meaningful.
Remember heatsinked transistors use insulating micas, grease, and probably nipples.

[joshdfrazier]

my power rails are fine, and cr1 has been replaced, +19vdc/-19vdc on either side of the diode. speaker is most definitely good. Is there any one "good" source to purchase these transistors? Mouser has these:

2N5190G: http://www.mouser.com/ProductDetail/ON-Semiconductor/2N5190G/?qs=sGAEpiMZZMshyDBzk1%2fWi7XVVUd4Z%2fsloAY%2fGWX12hs%3d (unsure if the g suffix renders this unusable)

2N5193: http://www.mouser.com/ProductDetail/STMicroelectronics/2N5193/?qs=%2fha2pyFaduhbgd5uuHMvGRKH%252bqgO6VbWRtgqLDohffA%3d (appears out of stock)

2N4921G:http://www.mouser.com/ProductDetail/ON-Semiconductor/2N4921G/?qs=sGAEpiMZZMshyDBzk1%2fWi%2fPUgtclNldlaxfxMPQa9Mo%3d

Thanks for the help!

[Roly]

+19vdc/-19vdc on either side of the diode.

Eh?  CR1?  That doesn't look good; sure you have it in the right way around, cathode to Q16?  It's supposed to be in forward conduction.  If you have full supply between the output transistor Bases then one or both are stuffed (and the diode is either reversed or open).

The voltage on the Base of Q15 should be no more than a diode drop, ~0.6V, above ground, and on the Base of Q16 no lower than ~-0.6V below ground.

The driver Q14 should also be tested out of circuit.

Replace the resistor with a wire link (this will only give you a bit more crossover distortion).

The value of this resistor sets the idle current through the output pair, and as JMF says, it will be somewhere between 10 and 100 ohms.  Once you get the amp going again you can try fitting a 10 ohm and see what the idle current and crossover distortion are like.  The idea will be to try larger values, 12, 15, 18, etc., until you get acceptable crossover distortion at the least possible idle current (say 10 - 40mA).  The diode is there as thermal compensation.

A suffix on a type number generally doesn't mean a lot, somewhat improved specs normally, but it may have a different pinout.

[joshdfrazier]

That was a mistake, the voltage is positive on both sides! While poking around, I noticed that Q13 is getting very hot, and is likely shot. I'll update after I replace the suspected components.

[Roly]

the voltage is positive on both sides!

How much "positive"?

I noticed that Q13 is getting very hot, and is likely shot.

Well it's a sign that something 'ain't right, but paradoxically a transistor that gets hot often isn't the problem - transistors that are short or open don't dissipate power and it's often the cold one (or IC) that's the problem.  Test Q13 out of circuit, but don't be surprised if it tests okay and the real problem is nearby (Q14?).

How does Q14 test out of circuit?

Generally speaking when you find a dead semicon in a S.S. output stage it pays to have a good look at the stage before because a failed OP device can often take its driver with it.

[joshdfrazier]

sorry to revive this. just getting around to replacing the dead transistors. I haven't had *too* much trouble locating modern equivalents, but I'm having a difficult time with the 2N5227. (http://alltransistors.com/transistor.php?transistor=4648)

I'm wondering if this is an appropriate replacement: http://www.mouser.com/ProductDetail/Central-Semiconductor/MPSA64/?qs=sGAEpiMZZMutXGli8Ay4kHKRdIvtrnJvR850nOB1k%252bo%3d

Really, all of the to-92's in this amp are hard to cross reference. So, to avoid asking this question EVERY time I'm stuck, what are the most critical parameters when it comes to replacing old transistors?

[Roly]

what are the most critical parameters when it comes to replacing old transistors?

Max voltage
Max current
Max power
Gain (hFE)

Possibly the least important thing is the case type (as long as it fits in the available space), then lead order (as long as you can rotate/wangle the right lead into the right hole).

There are a huge range of transistors and diodes out there, but in reality for the vast bulk of applications a few common types will cover just about any application you are going to encounter in a guitar amp.  It's really only when you get to the Outer Limits of electronics, gigahertz, microamps, ultra low noise, ultra high gain, that you have to search for special types.

Output transistors can be pretty demanding but are still fairly substituable, however for things like medium power drivers I have a bunch of BD139/BD140's because they are common and cheap here, and have voltage, current power and gain specs that exceed almost all needs.

In low level service the choice is even wider.  Elektor magazine for some years tried to rationalise this situation by simply specifying TUN, TUP, DUS, DUG for Transistor Universal NPN", "Diode Universal Silicon" etc, and frequently publishing huge tables of devices that met their "Universal" specs.

The 2N5227 is characterised as a "low noise PNP", 30 volt, 100mA, hFE (gain) 50-700.  While the gain spread goes quite high it also goes fairly low, so you will find that just about any small signal transistor will substitute.  This looks quite similar to a BC109 but in PNP which would be a BC179, however the difference between "low noise" types and normal types often makes no real difference in guitar amp service.  Note also that there are case variants of the BC179 as BCn79 where n may be 3, 5, etc denoting different cases and leadouts, but are electrically identical.  The lower gain BCn78-series should also suffice.

Have a look through the tables of transistors carried by your favorite company, filtering for at or over 30 volts and 100mA, and then for the highest gain and specific low noise types, then pick the cheapest and away you go.  Be careful of the lead arrangements, they are all over the place, bang it in and see of it does the job.  In the unlikely event it turns out to be too noisy just repeat for another common type, but I'll bet that just about any transistor worthy of the name will do the job.

Caution: the MPSA64 is a Darlington type, and while it exceeds the required specs it will also require circuit modification to bias correctly, and its ultra gain may give rise to other problems.  It is not a good idea to try and substitute a Darlington for a normal transistor or v.v.


{In reality this particular amp is running only a -9V rail in the preamp and the current through the 2N5227's are less than one milliamp (look at the load resistances, R5, R16, R41 and apply Ohm's Law using the supply voltage to find the maximum possible current) so the power will be microscopic.

They also seem to have used the B-E breakdown voltage of Q4 as a "zener" regulator so the supply could be somewhat less than 9V.

Um ... as something of an afterthought, just why are you replacing these transistors?}

[J M Fahey]

Same here
The 3 power transistor are probabloy shot, either shorted or open, and even Q13, but why mess with the *preamp*  transistors.
I guess you are shotgunning, that is replacing everything in sight without a real motive.
Don't, it *adds*  new errors to old problems.
Rather than that, post what's actually wrong with it.
Describe the problems you see/hear.

[joshdfrazier]

Wow Roly, thanks for that super informative post. It answered alot of questions for me. I didn't know that Darlingtons doubled the gain, so that is very good to know.

I guess I am "shotgunning" this amp. Some of the rectifier diodes crumbled and one of the transistors fell into pieces when i removed the heatsink. some resistors also seem a bit "crumbly". I'm not sure if its due to age, cheap components, or a combination.. I figured I'd replace all of the critical components. If it fails again it will probably just go in the trash, it's simply not worth the labor, hah.

[Roly]

A Darlington is actually a compound or double transistor and the gain of the two elements multiplies, hence the huge gain or hFE, and so is kind of a poor man's op-amp.



A particular problem in putting a Darlington in a circuit designed for a conventional transistor is that the two B-E voltages add, so they need around 2 * 0.65 = 1.3V of bias on the base to get them to conduct.  Another problem is that the huge gain will also tend to make a circuit designed for a conventional transistor misbehave (unless there is already a high degree of local negative feedback around the stage).

Generally speaking Darlingtons and conventional transistors are not interchangeable.

it's simply not worth the labor

Perhaps not, but it may be worth it for the experience/self education.   8|