Johaanus monarke analog amplifier

[nmarques]

Hello!

I purchased a Johannus analog Monarke in OrgelZentrum last March.
A month ago, when I played a long time (25minutes) with stop "Octaff 2' (and other great/pedal stops) I listened some strange noises from the loudspeaker of that stop. It happened always I played long time as I said. When I switch off that stop for a minute It goes normal.
But last Sunday without any noise the loudspeaker or the Pedal/Great amplifier (Octaff 2 is from great) started to blown off and no sound was played from that moment in pedal or great, only on swell (which have an inpendent amp). I have saw (and smell :) ) the pedal/great amplifier and all components are ok. When I switch on the organ, the lights bright on that amp but no sound, no nothing from that amp. If I replace the loudspeaker that make those old noises, can I fix the problem? Give me a suggestion...

Thanks and sorry for my bad English!

[indianajo]

A tutorial on analog amp repair is http://www.diyaudio.com/forums/solid...de-manual.html. Note stage equipment has way more power transistor failures than organs, because of the constant (and often incorrect) plugging and unplugging of the cabling to the speakers. You'll need a DVM with an ohms scale, at least.
The most common cause of failure on 40 year old organ amps is the electrolytic capacitor. When it shorts, it often takes out a power rectifier (diode or bridge) or a resistor in the circuit between the transformer and the capacitor.
Another problem can be corroded connections on the input side of the amp. Brass and tin oxidize in a decade or so and input level signals do not have the energy to burn the oxide off. Disconnect and reconnect all input connections. You can test the power amp alone by putting your finger on the hot pin of the input connector. This should make the speaker hum.
Speakers rarely fail unless the rubber suspension fails and causes the wires to scrape. You can test the speaker by pushing on the center gently, to see if you can feel the wires scraping, or if it moves smoothly. Speakers with noticable tears in the rubber suspension should be reconed or replaced. If one is careful, one can recone a speaker with rubber sheet and tire bead seal compound (liquid rubber)
Having repaired several hifi amps over the years, once a consumer electronic device has problems, I start by replacing all electrolytic capacitors over 20 years old, or 10 years in tube (valve) equipment. they sell 10000 hour service life capacitors now, that should last longer. I've had to re-cap my ST70 power amp 4 times in 40 years, and I put 71 of these capacitors in my 1968 H100 organ. The ones away from the power supply made a great improvment in tone.
You may learn to use a meter, and how transformer/diode/capacitor power supplies work, by reading Thomas Floyd, Electronic Devices, the Electron Flow version. I successfully repair amps and preamps with a DVM, and an analog meter with 2 VAC and 20 VAC scales. No oscilloscope is required.

[nmarques]

Ok. Many thanks for the answer!
I think that the speaker of that stop is dead, because I heard a noise from that speaker and I smell something burning for a while. But with that burned speaker all the amp don't work, maybe there is another "security" component that turned off to prevent more damage (tubes? Capacitor?... ). All the lights are on when I switch on the organ.
In orgelzentrum they said that the organ has about 13 years...

[indianajo]

Solid state amps that fail often have inadequate DC protection systems that allow DC voltage to come out on the speaker when anything fails and burn the speaker up. If the organ is 13 years old, it is possible a bad solder joint or other bad connection is whanging the output into DC over and over, causing the repairman to replace the speaker (symptom) over and over instead of repairing the cause. I recently repaired a 1994 amp I had bought for $55 with labels "DO NOT USE CHANNEL B" and the B power supplies disconnected. It had a bad solder joint on an op amp input that had been causing 170 VDC on channel B, over and over again since 1994. The B channel output transistors burned out the DC protection system instead of blowing the circuit breaker as it should have done. Alternately, DC idle current circuit failures for the output transistor can cause overheating of the output transistors, and repeated failure, also. Another possible cause of repeated failure of the output transistors is ultrasonic oscillations.

[nmarques]

I attatched some photos of the amp

Backwards

Amp backwards por nmarques2011, no Flickr

Front

Front por nmarques2011, no Flickr

When I switch on for a minute today I notice the following on the diodes of great/pedal amp

diodes and capacitors por nmarques2011, no Flickr

That amp continues no working.
In the swell amp (working normally) I noticed the reverse. The diode on the right was cold and the other two were hot.
Replacing the speaker solves my problem?...

I checked all connections and they haven't any corrosion.
I noticed that OrgelZentrum replaced all cables from the eletronics to the speakers and the keyboards they are like new
Tomorrow I will try using headphones.
What/Where is the protection for the amp? In the diodes? capacitors?

Many thanks!

[indianajo]

That amp continues no working.
Replacing the speaker solves my problem?...
Tomorrow I will try using headphones.
What/Where is the protection for the amp? In the diodes? capacitors?

If you don't measure the DC out on the speakers, you may replace the speaker several times before you are done repairing the amp. Also, the amp may blow up your headphones tomorrow, if the DC out the speaker terminal exceeds 0.1 V. I see you didn't read the tutorial in post 2. A speaker that scraped the insulation off the coil due to bad suspension could have blown the amp. An amp that puts out DC voltage due to an internal fault could have melted the insulation off the coil in the speaker, which could blow up your amp again after it is repaired. You have to check both, for the speaker coil scraping mechanically (a 2 second check) and dc voltage out of the amp on the speaker ( a 20 second check once you find the speaker terminals).
I see no signs of a DC protection circuit on this board. They usually include 5 to 20 components, including speaker shorting SCR's or triacs, or speaker disconnect relays, and a DC detection network of op amps or, often after 1990, a microprocessor and speaker current measuring transformers (coils),
When testing the amp I load amplifier speaker terminals with an 8 or 4 ohm resistor (depending on the designed speaker load, ie what resistance does the original speaker measure?) at a wattage higher than the amplifier can deliver, . I buy a cheap used car radio speaker, and protect it from DC with a series >1000 uf bipolar capacitor, to hear what is going on. Then I exercise the amp with the tone coming in on an organ, or a transistor radio on the workbench. This amp, with one pair of output transistors visible, probably doesn't put out more than 40 watts.
Oxidation on brass or tin connectors is not a visible phenomenon. Removing them and replacing them is the first level removal process.
After I checked the amp for DC out on the speaker, and found that was okay, I would check the + and - power supply rails that they were ~80% of the rating shown on the main capacitors. With a switcher power supply instead of a Mains transformer, you will have to use some other method to determine what the power supply rails should measure. Purchasing a schematic diagram is one sure way to find out. (or looking for a free one on the internet).
If those were okay, I would then disconect the input from the tone generator some way, and inject some hum with my finger to make the amp hum into the speaker. Or connect the input to the transistor radio. I would then take my analog voltmeter with the 2 VAC scale, and see if 1 VAC was coming into the amp from the tone generator with a key held down at full volume . This would prove whether the problem is the amp, or something before it.
This amp has those 10 boards with little components. Those might be some sort of a mixer, to add together several channels into one output. Each might put out a 1 VAC signal, going into a resistor adder that adds all the signals together. I'd have to trace the wires to figure out what they were. maybe speaker on/off boards or something.

[nmarques]

Where do I put the voltimeter to meausure the DC voltage?

I don't understand this line:
"If those were okay, I would then disconect the input from the tone generator some way, and inject some hum with my finger to make the amp hum into the speaker. Or connect the input to the transistor radio. I would then take my analog voltmeter with the 2 VAC scale, and see if 1 VAC was coming into the amp from the tone generator with a key held down at full volume . This would prove whether the problem is the amp, or something before it."
Where do I need to meausure?

The 10 boards are eletronics for each stop. They have compononents and volume regulator for each stop. They have this number TDA1514A

But if I burnt the amp, I could see it. I looked carefully into the amp and there's nothing missing or burnt...

[indianajo]

DC voltage on speaker will be measured on the two wires going to the speaker. One of these speaker wires will be connected to "analog ground" usually the same as the chassis of the amp but not always.
DC voltage power supplies for the output section can be meaured from the analog ground to the cases of the two TO3 (diamond shaped) output transistors. You should also be able to see it coming in from the power supply on a 3 terminal large gauge wire connection. The power supply will have the power transformer, or alternately be a switcher in a steel box with AC mains going in and 3 to 5 terminals coming out. the higher voltages of the 3 are the rail voltages for the output transistors. The other 2 terminals on the power supply, if present, are the +- 15 v or +- 12 v or something similar for the signal parts of the preamp and amp.
With the 10 board being for the the stops you should be able to see 1 VAC going in to them if the stop is on. Most DVM's will not do this, but VTVM's and 200kohm/volt analog VOM's will.
The last paragraph, starting with "if those were okay" is a conditional clause to be followed, only after all the previous statements have been proved to be true by measurement.
You have remarkable faith in your eyes. The bond wires in a TO3 transistor are about 36 gauge and can burn through with no visible damage to the case. Alternately, if you install a shorted wire instead of a speaker, some amps will blow the silicon die right through the top of steel case (but not always).

[nmarques]

Ok but when I heard the noise of the "explosion" it was on a loudspeaker (they are mounted in the top of the organ). The amps are near the floor.
Can the speaker burn some protection in the amp or something like that?...

[indianajo]

You haven't discussed the answer to the question "does the speaker coil scrape when you push the center straight in and out?". If it did, shorted turns could have blown the amp. If it didn't scrape and short some turns,and the speaker wire wasn't otherwise shorted, then the amp likely popped into the speaker as it failed. Bar band amps tend to get shorted speaker wires due to hookups in the dark, but organs don't unless they were recently moved and the speaker wire was hooked up incorrectly (creating a short circuit).