A request from a non member

[AlanBeckett]

That's all very interesting but I don't see what it adds to getting at the answer to the problem.
How big Physically was the Resistor?
Have you still got it?
Is it a Wirewound?
Clearly it's been overstressed and it isn't protected, so even taking a good guess at it's value is likely to result in the failure of the replacement.

Alan

[Refugee]

Well it looks like a fully electronic instrument and that is what I wanted to know.
The pictures are telling me that is is not just an ordinary instrument with a pickup and amplifier.
It is an analogue unit with all those little PCBs connected to the keys.
The power supply was most likely not faulty when the resistor started to burn.
There are several complex PCBs in it and those power lines with the power resistors will go to the various plugs in the top of the cabinet but not the audio power amplifier as that will not have a dropper resistor in the power supply as it will have separate ones on the transistors.
I would first use a meter to check out all the wires and plugs and make sure that all the power is getting there and that none of them are frayed and short circuiting.
Then I would follow the wire from the burned resistor to the PCB it feeds and check for short circuits between the plug pins and the negative point that joins all the negatives on the capacitors.
I would expect it to power up without the burned resistor and produce sound if the head phone wires of an MP3 player are touched on the volume control. Try this test as it will rule out the audio amplifier for certain.

It does look a bit complicated for anyone whom is new to any electronics either vintage or modern.

[pwdrive]

My Guess is that the missing resistor is quite low in value as it's in a high current low voltage situation, as has been said some details about the resistor might help, as was explained earlier it might be possible to work out with a reasonable amount accuracy what its value was.

From the PSU schematic you posted it looks like the AC supply to the rectifier that supplies the PA supply is 40 volts? and that the AC input to the other rectifier is 7 volts?
If that is the case then I would say that the 40 Volts is for the PA and the 7 volts is for the preamp and processing.

In the absence of a current limiting variable bench PSU I would start with the amplifiers PSU and get that up and running under a load in isolation from the rest of the amplifier, if you know what the continuously rated RMS output of the PA is you can get a rough idea of the PA's current draw as you can roughly work out approximately what the B+ supply is by calculating it from the AC RMS input to the PSU.

From all that you can work out the value of a suitable dummy load to connect to the PSU, I would work out a value that is suitable to simulate the PA's current draw at half it's rated continuous RMS output and would run it like that for an hour to make sure the transformer etc is ok.

Lawrence. EDIT: Whoops...missed refugees post.

[richard296]

the physical dimensions. its the same pin centres as the existing resistor that is a neohm 15 watt ceramic the body is oblong 48mm long x 12mm x 12mm no resistace markings just the 10% tolerance i believe this resistor to be on the 7 volt side. The missing resistor is on the 40v side. hope this helps.

[pwdrive]

Still need to know if the resistor is missing (from the board) or weather it's gone AWOL, ie: is it still extant?

Lawrence.

[richard296]

the unit is a combi for a galanti elec accordion. i purchased the accordion as it has reeds also. i play accordion and know that the reeds seldom get used as they tend to use the electronics so new it would be in good playable condition. i got the amp/etc free because it wasn't working. the man said his son had tried to use it for his elec guiter as it has 2 mic inputs. i think this is how it's damaged. i have checked the transformer for circuit continuity and plugged in (with a dimmer first for safety) then on the mains by itself and it gave the correct outputs. i checked the speakers on another amp they work fine. i checked the bridge rectifiers in the power supply the one on the 70v side was shorted so i replaced it i removed and checked the caps on a cap meter and they are within tolerances. i read the article by another member regarding esr so i have ordered a meter to recheck them. i realise that this is a big project for me but i am certain that i can get it going again even if i have to make a kit amp and utilise the power supply. in any event the knowledge i will gain will be ten fold. i have learned a lot from this site already many thanks

[richard296]

no resistor when i got the unit.

[Refugee]

Lets identify identify the resistor in the low voltage supply first.
I have used the following link to look up the part number on my example.
http://www.datasheetarchive.com/
The example I looked up is a jolly big one and it is 15 ohms and 17 watts.
Yours looks like about 7 or 5 watts going by the size.
You will notice that the values are printed 15R on the one i looked up.
The 10 ohm and 33 ohm ones are 7 watts and 5 watts going by size.

[pwdrive]

You should be able to trace the connection from the output side of the missing resistor and find out exactly where it goes and what it's connected to if it goes to the PA board then that board can be reverse engineered if needs be to obtain a schematic. You mentioned 70 v in post # 26 is that an error?

Lawrence

[Refugee]

I would guess the amplifier will be something like 70 to 100 watts and also even though it is not a very good photo there does not look like there is any burning around the output transistor resistors. The two transistors with round heat sinks will be the drivers and there will be two big NPNs on the big heat sink under the PCB. They will most likely be 2N3773 but could be 2N3055. The emitter resistors don't look at all burned.
I would first do a careful inspection of that connector on the power supply and make sure the wires are not frayed. They don't look good in the photo.
If I was fixing it I would replace the AC connections with a ghetto blaster mains inlet and keep them well away from the DC power as the connector does not look like it is rated at more than 50 volts. The tracks look like they also go far too close to the rectifier and could well have caused the fault in the first place.