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Messages - Loudthud

#1
Amplifier Discussion / Re: Acoustic 134 reverb
March 22, 2024, 12:51:24 AM
The Acoustic tube amps probably use the same tank as the tube Fender amps with an 8 or 10 Ohm input impedance.

Check the 230 or 330 amp schematics to see if the drive circuit is similar to the 134. That would indicate that the 1BB1D1B is the correct tank.
#2
Amplifier Discussion / Re: Watts vs Volume (db)
March 20, 2024, 07:27:43 PM
What is VAR and what does it have to do with anything ?
#3
Problem 2 would seem to be Q9, part of IC2. I think you can still get these parts at Jameco in the USA. Check the -15V supply for ripple, it comes from A1A.
#4
Probably a bad connection on the preamp circuit board.

Do you have the schematic correct for you amp? Post it or a link.
#5
With a DVM, first check a cap in circuit for a short or very low resistance. Then, remove from circuit and check for a short again. Normally when you check a cap for resistance, at first it will seem like it's a low resistance but as the cap charges up, the resistance reading will increase until the DVM indicates over-range. If the DVM can measure capacitance, try that to see if you can get a reading close to the value and tolerance of the cap.

Verify that any rectifier(s) are good with the DVM's diode check feature.

Check for shorts around the power transistors.

A light bulb limiter will usually prevent fuses from blowing. A Variac is a better option, but should have a way to measure current to avoid blowing fuses. Sometimes you can use the Variac to apply a very low Voltage to the amp and find which circuit or part of a circuit is drawing excessive current. This is a procedure for a qualified and experienced technician. 

You should not connect a speaker to the amp until it can operate without blowing fuses and it is verified that there is no DC on the speaker output.
#6
Usually, the problem is electrolytic caps. They are generally available but usually smaller than they were forty some years ago. This means you may have to improvise how the caps are mounted.

Spare circuit boards will only be available from some amp that has been cannibalized, they will likely have the same problem your amp has.

See if you can find a tech who knows what a "Light Bulb Limiter" is and knows how to use it.
#7
The Newcomer's Forum / Re: LA4282 in BTL bridge mode?
February 26, 2024, 03:42:52 AM
Quote from: Maddus on February 26, 2024, 03:24:54 AMIs it bad for the chip when only one channel is mostly used? So my idea would be to hook up one channel to an internal speaker and the other to an extension speaker jack.
That's not a bad idea, it shouldn't harm the chip. The heatsink should be adequate for two channel operation.
#8
The input is simply a dual differential pair, one NPN the other PNP. Each pair has it's own active current source. The outputs of the diff-pairs feed current mirror driver transistors where most of the Voltage amplification takes place. Bias for the output stage comes from a Vgs multiplier which matches the temp-co of the output MOSFETs much better than a Vbe multiplier. The source follower type output clips softly because the output Voltage can only swing to about 5V less than the rail Voltage. In addition, the zener diodes across the feedback resistor can cause soft clipping depending on the diode's characteristics.

Hard to tell what the open-loop gain is without modeling or building one, I don't speak German or trust any translation on the net.

Designs like this can be found on the diyaudio.com forum.
#9
I would suspect Q1. Any big negative Voltage hitting the input will slowly degrade Q1 (making it hiss) as it has no protection.

Always plug the guitar cord into the guitar first, then plug the other end into the amp. That loud buzz you hear when you do the opposite is what damages the input transistor.
#10
The Newcomer's Forum / Re: LA4282 in BTL bridge mode?
February 20, 2024, 01:18:40 AM
The LA4282 has a max load of 8 Ohms per channel so you'd have to use a 16 Ohm load in bridge mode. If you want something louder than 10W, use both channels each driving it's own speaker.
#11
I like to use my meter's current mode to measure how much an effects pedal consumes. Simply use a 9V battery, unsnap one side of the battery connector and touch the probes between the battery terminal and the side of the connector you just unplugged.
#12
Most meters have a separate input to connect one of the probes to when you measure current. There is almost always an internal fuse that will blow if you forget to move the probe back to the Voltage jack when measuring Voltage.
#13
Hifi amps are OK for casual use, but will likely fail under heavy duty stage use. It would be best to operate the Hifi amps below clipping and below the maximum load.
#14
You might try to locate some "14 pin DIP component carriers". These can be used to adapt other IC's to the socket on the Heathkit PCB or solder the old 7472 with the mangled legs to one to see if the chip still works. The 7472 is not a popular chip and may be expensive to find.

It looks to me like the chip is just used in the divide by two mode where J and K inputs are both a logic High and the Set and Reset inputs are active. An old practice was to just leave inputs unconnected to anything to act as logic High inputs. (A bad practice). Half of a 7474 chip can be adapted to perform this function.
#15
443-4 looks like a Heathkit part number. Look for an assembly manual on the internet or possibly a Heathkit cross-reference. Be sure to observe the "1" printed on the PCB as the only indicator of which way to insert the IC. The socket doesn't appear to have any marking telling you which way to insert the chip.