These were handy little mixers. Unpowered, with a very reasonable DSP reverb/delay in them. Straightforward to use, and still repairable. This one had an output chip with a leg broken, that made it intermittent. The effects send/returns can be a problem as they are in the main signal path. A good clean up doesn’t go amiss.
These amps are little known, go back to the mid sixties, and have a clean sound and power output to die for. They were an odd design of hybrid (transistor and valve). The preamp stages were transistor of a ‘bootstrapped’ design. This gives a transistor input stage a lot higher impedance which gives a better match to a guitar input.
The rest of the amp is pretty standard valve design with ecc83 phase splitter, and four EL 34’s output. The quality of the transformers is top notch, as is everything else in it.
This one had developed a strange, even harmonic distortion. The input stages have an unusual adjustable biasing arrangement, which turned out to have the fault.
These amps go for not a lot of money, and if you find one with the original cabinet, it’s worth an investment. Inside the cab are two beautiful JBL 15” speakers.
The whole setup was something like £600 in 1965. Phew!
1500 watts rms in a powered mixer format is hard to believe, but that’s what these things kick out: or in this case 750 watts, because one side of it didn’t work. There are still a lot of these about, and that’s getting on for twenty years since they first came out. They must be reliable, as there aren’t all that many find their way to the workshop.
A major power amp fault is usually a rebuild. These are bipolar amps using MJ15022 and MJ15023 transistors in a complementary design. If these go down, they’ll often go collector-emitter short, and blow back through the base which takes out the drivers and a fair bit of the bias chain and even before that. It’s safest to replace most of it and certainly the bias chain components, because if certain of those get fed up after the repair, it will knock the whole lot out again. You’ll also expect it to take out the switching rail power supplies with it.
Here it is, a sneak peak at the place where things are fixed!
This fault was an absolute bugger. It had a tiny output on one side, that wasn’t affected at all by the master output pot. It turned out to be a rotted track (under the laquer!) on the graphic pcb, that connects the output idc connector to that which takes from the aux pcb. It took me a lot of hours and no parts at all. Hard to explain that to a customer, sometimes.
This was a fairly modern version (Chinese?) and it didn’t work on one side. All the power amp stuff seemed alright. There are two load isolating relays inside as with many mosfet amps, and if you can hear those relays click in after a second or two, the power stages are likely to be ok.
At the back of the unit, all the input stuff is soldered directly to a pcb, which is populated with surface mount stuff. Surface mount components are small, light, and a pain in the arse to repair. If Peavey repaired this, they would replace the whole thing, and either chuck the pcb away, or send it to be reworked, where the components are stripped off, the pcb then flow soldered, and then the components stuck back on by a friendly robot.
Anyway, I replaced two surface mount TL072 chips, and it was fine. But my eyes weren’t. You can fit three or four of those chips on a thumb nail, and they have eight legs each. Some soldering job.
This one was a very early H//H, it wasn’t even called an IC100, but that’s what it was. Anyway, the reverb had packed up, and was desperate for a clean out/service. This is a spring line reverb in these, (in common with just about every other reverb amp of the time) and it used the accutronics version of it, a lot like Hammond used in many keyboards.
This has a device called a ‘transducer’ on either end of the spring, which is basically a tiny coil of wire on a laminated former, that the end of the spring moves inside. There is a transducer on either end and they’re not the same. They look the same but the impedances are quite different. The ‘drive’ end, (the input) is about 20 ohms or so, and the output transducer is about 200 ohms. Connect it the wrong way around, and it will do next to nothing.
Anyway, these transducers are connected to the outside world via two small wires, which might go to phono sockets, or just a little tag strip if it’s a very early one.
If you get a break in those, it’s repairable, and if you get a break into the coil, it’s usually not. In which case, you have to replace the unit.
The power on this amp had faded to next to nowt. It’s a while since I did this, but the fault was in the screen grid resistors on the output 6L6’s. I don’t know if this is a common fault in new ‘reissue’ type Fenders, but I’ve certainly seen it before. The original Fender output stages in things like the Tremolux, Twin reverb, Delux, and the like, used 5 watt ceramic wirewound resistors (470 ohm I think) which are pretty bullet proof, and able to withstand some high surge currents. Whereas the modern incarnation uses metal oxide resistors rated at 3watts or so. And a metal oxide resistor is not up to much in the way of surges.
The theory behind this design replacement I think, must be to do with the fact that a wire wound possesses elements of inductance and capacitance. But the values are minimal, and in this context, could even be an advantage.
Be that as it may, this one had a couple of 5watt wire wounds fitted, as Leo Fender intended. Leo was a genius (in my book) and people who do ‘mods’ to the product of genius, well, be it on their own heads.
I can’t go overboard with praise for these. The output stage is a power amp chip they use in hifi amps, and is mounted on a small pcb with a single inline multi connector. Not a very inspiring piece of gear to carry a hundred watts.
This lot is screwed to a mini heatsink with a fan on the back. If the fan gets tired the cooling fails and the chip blows. Compared with rebuilding a traditional power amp stage, it’s a pretty cheap job, but then, so is the whole amp. This one had blown its chip, as usual, and after moving the fan a bit in its fixings, to centre it again, it was fine.
I’ve not seen any Amcron/Crown gear for a long time, and this reminded me what great amps they were, and still are. This sort of quality just isn’t to be found in current gear. The customer suspected that it had developed a dc fault and blown his speakers on one side. The dc offset on these things is microscopic (that’s the dc value at the speaker outputs), and for a 2400 watt amp, unbelievable.
Anyway, wherever the fault was, it wasn’t in this amp. It had a cleanout, bias current setup (that’s the standing current through the output stage, and is the semiconductor equivalent of biasing in a valve amp output stage) and power transistors secured. It’s a good idea to slacken the output devices off, and then tighten them down again, when it’s getting on a bit. It makes sure that the heat transference is good to the heatsink, and also cleans up any contact resistance to the cases of the power transistors. That’s a good idea because they get their rail connection through the case. Ideally, removing the devices and replacing the heatsink compound is best, but this costs a lot.