HI all.
I've recently pulled my old Guitar amp out of the garage only to find it's dead.
First observations were:
When turned on:
Latest Observations:
I'd love to get this thing working again. Any ideas as to what the problem is or where to look next?
(i am armed with a soldering iron and a multi-meter and an engineering degree)
Cheers,
Chris
I've recently pulled my old Guitar amp out of the garage only to find it's dead.
First observations were:
When turned on:
- No Lights,
- Slight speaker hum,
- speaker pops when turning off
Latest Observations:
- Power plug fuse in working order
- Measured continuity in both primary and secondary coils of transformer
- Cant see any obvious faults in soldering etc, circuitry
- If left on for a minute or two i can smell a slight burning smell - a lot like a hot glue gun smell
- From inspection it looks as if the two large(physically) resistors in the centre of the chip (see pic) are whats burning the glue around and on them. (i was able to measure an appropriate resistance across these resistors)
I'd love to get this thing working again. Any ideas as to what the problem is or where to look next?
(i am armed with a soldering iron and a multi-meter and an engineering degree)
Cheers,
Chris
Disconnect load - speakers may burn out or be damaged. Disconnect the output transistors. Check transitions (diode mode). Check fuses. With a 60-100 watt lamp in series in the primary circuit, check the voltages. Inspect the details and P C B.
If the speaker makes sound, it is working. Maybe not well, but working. The pop at turn off tells me the power amp is likely working. All that tells me the power transformer is probably fine - they usually are. I'd suspect your transformer puts out a simple supply for the power amp and step down zeners for the preamp. If you have a separate winding for low voltage rails, check it.
No lights makes me think a low voltage supply us gone -- like 15v for op amps or some such. Are both rails present at any op amps?
No lights makes me think a low voltage supply us gone -- like 15v for op amps or some such. Are both rails present at any op amps?
Agree and add: FULLY lacking any data about your amp, please upload at least a couple gut pictures showing:
* the power supply , large filter cap labels should be readable, diode rectifiers too.
* show the power amp, clearly visible chipamp if used, or power transistors, whatever´s attached to heatsink.
* preamp so we see whether it´s discrete of Op Amp based.
Since some labels will not be visible/readable, make a small list stating: chipamp / power transistors / filter caps / Op Amps so we can identify them and suggest "measure voltage on such and such pins"
We can and will guess a lot, but we need at least a *minimum* of data; so far we are absolutely blind.
* the power supply , large filter cap labels should be readable, diode rectifiers too.
* show the power amp, clearly visible chipamp if used, or power transistors, whatever´s attached to heatsink.
* preamp so we see whether it´s discrete of Op Amp based.
Since some labels will not be visible/readable, make a small list stating: chipamp / power transistors / filter caps / Op Amps so we can identify them and suggest "measure voltage on such and such pins"
We can and will guess a lot, but we need at least a *minimum* of data; so far we are absolutely blind.
Last edited:
Additional Information + Photos
Thanks JM
Here we go:
Chip amps:
LM1875T x1 (pre amp?)
NJM5532LD x1
NJM4458LD x5
Transformer Voltage outputs:
33V - Across secondary coil
16V - To centre tap
Thanks JM
Here we go:
Chip amps:
LM1875T x1 (pre amp?)
NJM5532LD x1
NJM4458LD x5
Transformer Voltage outputs:
33V - Across secondary coil
16V - To centre tap
LM1875T main amplifier, other opamp for preamps. Tone control, effects. PCB LM1875T can be tested separately - connect the power supply and apply some signal to the input. Measure voltages. Solder 2200 mF in parallel (replace on the board).
Hi All
I've made some more measurements:
LM1875T
The low supply to the Pre-amp makes me think the fault lies before this stage.
The two physically larger resistors on the chip pictured get very hot very quickly when the power is on - could they or something adjacent to them be shorted/damaged?
I've made some more measurements:
LM1875T
Vcc -21V
Vee 21V
AC InVee 21V
0V
16.6V
33V
Supply to Pre-Amp16.6V
33V
-15V -> measured -6mV
gnd -> measured +9mV
+15V -> measured 24mV
gnd -> measured +9mV
+15V -> measured 24mV
The low supply to the Pre-amp makes me think the fault lies before this stage.
The two physically larger resistors on the chip pictured get very hot very quickly when the power is on - could they or something adjacent to them be shorted/damaged?
Attachments
Large resistors supply 22v to zener diodes and + -15v source capacitors.
The resistors should be warming up.
What is the voltage at the speaker output (without a speaker)?
The resistors should be warming up.
What is the voltage at the speaker output (without a speaker)?
Last edited:
Getting 2 to 37mV to the speaker outputs
The pre-amp voltage supply measure the same voltage whether connected or not.
The pre-amp voltage supply measure the same voltage whether connected or not.
If the preamp supply doesn't even work with the preamp disconnected, then it has a problem. Both sides gone? Unusual, but looking for something either bridging one to the other, or both regulating elements shorted. (Zeners?)
With boards reassembled on chassis and everything connected *except* the Red Blue Black cable (which I guess is the +/-15V supply from power board to preamp) turn amp on and measure as indicated, red probe to test points straight on resistor legs, black probe on supply ground or chassis ("should" be same thing) you "should" measure the voltages I suggest.
Post results.
Post results.
Attachments
i guess i'm likely being critical (ocd a little) but judging from the picture and the orientation of caps shown the polarities would be reversed,no?
Why? 🙂
Main +/-21V work fine and suspect +/-15V caps all point same way, negative strip to the right.
Main +/-21V work fine and suspect +/-15V caps all point same way, negative strip to the right.
i guess it would depend on the actual PCB layout so it 50/50 odds as to which is correct, sorry i just envisioned it differently....
hope the trouble gets sorted for the OP!
hope the trouble gets sorted for the OP!
I no longer think the problem lies on that board.
I realised the measurements i was making for the -+ voltage supplies would only give the desired -+15V when an appropriate load was applied at the other end of the supply (i.e. the pre-amp circuit board)
When i measure the outputs of the connected 15V supply i get approximately 100mv across the whole thing. (similar voltages regardless of which channel is on)
The resistance between the supply pins of the pre-amp board is very large when measured while disconnected which explains why the voltage supply was so low when connected.
So somewhere on the pre-amp board there must be something contributing to this? Not sure what the culprit could be.
I realised the measurements i was making for the -+ voltage supplies would only give the desired -+15V when an appropriate load was applied at the other end of the supply (i.e. the pre-amp circuit board)
When i measure the outputs of the connected 15V supply i get approximately 100mv across the whole thing. (similar voltages regardless of which channel is on)
The resistance between the supply pins of the pre-amp board is very large when measured while disconnected which explains why the voltage supply was so low when connected.
So somewhere on the pre-amp board there must be something contributing to this? Not sure what the culprit could be.
Why?
Definitely NO
Did you MEASURE what I WASTED time suggesting on post# 4?
Including downloading, editing and uploading the picture? 🙄
Who asked you to do that?
Measure VOLTAGE.
Please do the suggested tests.