Hi all,
I have an Ampeg SS-140C (schematic here) with a few issues. Here are the main problems:
Here's what I've tried and the results so far:
This suggests to me that the problem is in the power amp / power supply. My next steps are to check for +40 and -40 VDC rails from the supply and +44, +15, and -15 VDC on the harness from the preamp board. My guess is the output transistors/filter caps need to be replaced, as this amp is ~30 years old.
Am I going about this the correct way? Is there any way to check which transistors, if any, need to be replaced without removing them from the circuit? Does the low output/no volume issue affect the reverb issue, or are they separate?
All help is greatly appreciated. Thanks!
I have an Ampeg SS-140C (schematic here) with a few issues. Here are the main problems:
- Low output from Channel A (still sounds good, but gain and level knobs have to be turned to ~4 to get it above whisper volume)
- No sound from channel B
- No reverb on either channel
Here's what I've tried and the results so far:
- Change guitar, cable, input jack (high vs low), speaker cabinet -> No effect
- Spray pots with Deoxit -> Less scratching, no change in volume
- Jump effects loop -> No effect
- Plug guitar directly into effects return -> Ever lower output on Channel A, no sound from Channel B
- Change speaker outputs -> Same symptoms using wet or dry speaker outputs
- Plug effects send of Ampeg into effects return of different amp -> Volume jump on Channel A, sound restored on Channel B
This suggests to me that the problem is in the power amp / power supply. My next steps are to check for +40 and -40 VDC rails from the supply and +44, +15, and -15 VDC on the harness from the preamp board. My guess is the output transistors/filter caps need to be replaced, as this amp is ~30 years old.
Am I going about this the correct way? Is there any way to check which transistors, if any, need to be replaced without removing them from the circuit? Does the low output/no volume issue affect the reverb issue, or are they separate?
All help is greatly appreciated. Thanks!
The reverb will be unrelated.
The correct way? No, Guessing is not the way to do it. Output transistors would not be the cause of low output, not by themselves anyway. The preamp seems to work, so look at the power amp, yes indeed check the power rails. Weak filter caps will cause hum or maybe distortion, but not loss of output.
Check those fractional ohm power resistors in the emitter lead of the outputs for opens.
The correct way? No, Guessing is not the way to do it. Output transistors would not be the cause of low output, not by themselves anyway. The preamp seems to work, so look at the power amp, yes indeed check the power rails. Weak filter caps will cause hum or maybe distortion, but not loss of output.
Check those fractional ohm power resistors in the emitter lead of the outputs for opens.
I can't read the schematic linked. They will be something like 0.47 ohm or 0.22 ohms or similar. JUst measure them in circuit, they will either look like a short to your meter or they will be open. very rare for the value to shift. Touch your probes together first to see what your meter reads for the probe resistance. Subtract that from the readings. In most cases, it will either be open or correct. it is hard for many meters to accurately measure half an ohm, so close probably means OK.
In the circuits you linked, I only see one pair of output transistors in the power amp. I assume the other power amp is identical. You are listing 10 resistors, while I would expect four total. I am referring to the resistors that connect directly to the emitter of each output transistor.
In the circuits you linked, I only see one pair of output transistors in the power amp. I assume the other power amp is identical. You are listing 10 resistors, while I would expect four total. I am referring to the resistors that connect directly to the emitter of each output transistor.
Way back in the old days, my teacher at school told me to spend some time looking at the inners of anything that needs service.
Well, electronic parts were big and there was a lot of cabling.
Finding damage caused by rodents was not unusual, hence the advice to take a very close look before even to start taking any measurement.
Of course, no power applied.
If all is clear, take notes of cabling, parts, mechanical things etc... if needed and start to replace the obvious damaged parts.
Only then apply power through a bulb current limiter and measure the supply voltages.
Make sure these are present and within the expected values.
So the repair is almost done. (so my teacher told me and as always he was right)
Now it's time to find the rest of the problem.
You tried to isolate the problem?
Preamp, poweramp? Sure? OK then.
Let's see how the voltages of your supplies are.
Well, electronic parts were big and there was a lot of cabling.
Finding damage caused by rodents was not unusual, hence the advice to take a very close look before even to start taking any measurement.
Of course, no power applied.
If all is clear, take notes of cabling, parts, mechanical things etc... if needed and start to replace the obvious damaged parts.
Only then apply power through a bulb current limiter and measure the supply voltages.
Make sure these are present and within the expected values.
So the repair is almost done. (so my teacher told me and as always he was right)
Now it's time to find the rest of the problem.
You tried to isolate the problem?
Preamp, poweramp? Sure? OK then.
Let's see how the voltages of your supplies are.
Ok I'll try that next. My plan is to run a tone generator into the effects return (or the rear power amp in jack) and use a signal tracer (plugged into input of battery powered Pignose amp) to trace the circuit downstream from the signal input. How far down the power amp signal chain can I go before I have to worry about damaging the tester amp?
Put a cap in series with the tracer amp input. That will block any DC. You can then listen to signal perched atop a DC voltage. If you are in a very high signal level area and don't want to overdrive the test amp, put a "volume control" ahead of the input. A high value pot, like 250k or 100k 0r something. The test signal comes in at the top of the pot, the wiper to the test amp input, and the low end of the pot to common. Cap value not critical, I use 0.047uf because I have bags of them. Make sure the cap voltage is high enough for any circuit you touch.
Your problem is missing output, not distorted, right? So keep the signal turned down during test. Missing signal is just as easy to notice at 2 watts as it is at 200 watts.
Your problem is missing output, not distorted, right? So keep the signal turned down during test. Missing signal is just as easy to notice at 2 watts as it is at 200 watts.
Tried sending 500 Hz signal into effects return jack and got sound through speaker cab on both channels. The only difference between this test and the first was the removal of the reverb tank. Volume output is still low, but at least sound is present. Next test is to check volume with guitar input. Is there any danger running the amp for an extended period of time without the reverb tank connected?
And any ribbon cables? Pull them off and push back on. In fact, just pull them half way off and push back down. That refreshes the contact surfaces on the pins.
Get some Chinese takeout, and save the wooden chopsticks. Excellent for insulated probing. Now while it runs, push against the components on the board, as well as just push down on the board itself here and there. This flexing can reveal poor connections.
Get some Chinese takeout, and save the wooden chopsticks. Excellent for insulated probing. Now while it runs, push against the components on the board, as well as just push down on the board itself here and there. This flexing can reveal poor connections.
Chopstick test and harness reseating did not reveal any bad joints. However, I did notice that the output volume is higher when running 500Hz tone into power amp in jacks on rear of amp instead of effects return jack on front of amp. Unfortunately, the effects return jack is hidden under the preamp board, so next step is to pull out preamp board and check for corrosion on effects return jack.
Can someone help me locate the effects send and return jacks on the schematic? Are they J3 and J4 on page 1?
Can someone help me locate the effects send and return jacks on the schematic? Are they J3 and J4 on page 1?
Another interesting wrinkle: I plugged my tone generator into the chorus power amp in jack on the rear of the amp. I can get a tone on my test amp with the Ampeg off, but as soon as I power the Ampeg on I just get buzz and hum into the test amp at the same location (tip and sleeve of power amp in jack). Is this expected?
I like to jump in...
I have the impression that the OP is trying whatever and hope that the signal comes back in one way or the other.
Also a random cleaning and replacing parts will not solve the problem.
Let's take it back from the beginning and place all we know on a row.
1. Voltages are present but confirm that you find these on each connector.
That's on both the pre-amp boards and the power amp board.
2. Insert a signal form your generator into a input connector on the left of the front panel. Witch one is not important right now.
Connect the line out to another amp in.
Confirm if all the controls of channel 1 and channel 2 are working?
Do not bother the reverb or chorus right now.
3. Insert a signal into the Line in and check if it's available on the Line output jacks at the rear of the amp.
These jacks are named:
3.a. Left (Chorus) output.
3.b. Right (Dry) output.
3.c. Mono Line Output.
Confirm that all these jacks carry the correct signal.
3.d. Confirm that the chorus controls are working.
4. The signal is still connected to the Line Input jack.
Confirm that the signal is available at the speaker jacks of
4.a. Channel A.
4.b. Channel B.
Do not care about the correct levels; that's for later.
First you have to be sure the signal line is working from front to back.
Leave the reverb circuit for what it is right now. Address that later.
If you follow these guide lines, you will be able to isolate the problem and then you can carry on.
Now let's see what you find on ALL the points as mentioned (1 .. 4b).
Good luck
I have the impression that the OP is trying whatever and hope that the signal comes back in one way or the other.
Also a random cleaning and replacing parts will not solve the problem.
Let's take it back from the beginning and place all we know on a row.
1. Voltages are present but confirm that you find these on each connector.
That's on both the pre-amp boards and the power amp board.
2. Insert a signal form your generator into a input connector on the left of the front panel. Witch one is not important right now.
Connect the line out to another amp in.
Confirm if all the controls of channel 1 and channel 2 are working?
Do not bother the reverb or chorus right now.
3. Insert a signal into the Line in and check if it's available on the Line output jacks at the rear of the amp.
These jacks are named:
3.a. Left (Chorus) output.
3.b. Right (Dry) output.
3.c. Mono Line Output.
Confirm that all these jacks carry the correct signal.
3.d. Confirm that the chorus controls are working.
4. The signal is still connected to the Line Input jack.
Confirm that the signal is available at the speaker jacks of
4.a. Channel A.
4.b. Channel B.
Do not care about the correct levels; that's for later.
First you have to be sure the signal line is working from front to back.
Leave the reverb circuit for what it is right now. Address that later.
If you follow these guide lines, you will be able to isolate the problem and then you can carry on.
Now let's see what you find on ALL the points as mentioned (1 .. 4b).
Good luck
Thank you for the detailed write-up Tarzan. I apologize if I gave off the impression of shooting in the dark. Here are the results of what you suggested:
1. Voltages are present and correct on both boards, what do you mean by each connector? White wiring harness connectors?
2. Ran 10 mV 500 Hz signal from tone generator into “Low” input jack on front of Ampeg. Using “Line out” jack on front of Ampeg, ran signal to effects return jack on amp 2. All gain, eq, and level pots on channel 1 and 2 were responsive, and both channels had output. No reverb tank was connected, and chorus was not touched.
3. Ran 10mV 500 Hz signal from tone generator into “Line in” jack on front of Ampeg.
a. Ran “Right (Chorus) Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
b. Ran “Left (Dry) Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
c. Ran “Mono Line Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
d. Chorus engages and chorus knobs affect tone during tests.
4. Ran 10mV 500 Hz signal from tone generator into “Line in” jack on front of Ampeg.
a. Signal present on chorus output.
b. Signal present on dry output.
Thoughts?
1. Voltages are present and correct on both boards, what do you mean by each connector? White wiring harness connectors?
2. Ran 10 mV 500 Hz signal from tone generator into “Low” input jack on front of Ampeg. Using “Line out” jack on front of Ampeg, ran signal to effects return jack on amp 2. All gain, eq, and level pots on channel 1 and 2 were responsive, and both channels had output. No reverb tank was connected, and chorus was not touched.
3. Ran 10mV 500 Hz signal from tone generator into “Line in” jack on front of Ampeg.
a. Ran “Right (Chorus) Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
b. Ran “Left (Dry) Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
c. Ran “Mono Line Output” from rear of Ampeg into effects return jack on amp 2. Signal present through amp 2 speakers.
d. Chorus engages and chorus knobs affect tone during tests.
4. Ran 10mV 500 Hz signal from tone generator into “Line in” jack on front of Ampeg.
a. Signal present on chorus output.
b. Signal present on dry output.
Thoughts?
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