Ampeg BR4 low output

[J M Fahey]

i will get those measurements shortly for you. One question is there ever a reason to use 440Hz Vs 1kHz?

It depends on your meter.
98% of meters are rated flat/precise up to 400Hz because thatīs still "mains frequency" (I guess in planes, so they keep transformer weight low) so 440Hz "should" be read accurately; at 1000Hz it may or may not be accurate.

I used a drum machine on the 0 db input to get the 34 volts probably not ideal but I guess did show the power section could do it.

Drum sound being basically audio bursts I donīt know how can you use that, suggest you download some 440 or 1000 Hz MP3 ; I publised one such source many times.

[nosaj]

It depends on your meter.
98% of meters are rated flat/precise up to 400Hz because thatīs still "mains frequency" (I guess in planes, so they keep transformer weight low) so 440Hz "should" be read accurately; at 1000Hz it may or may not be accurate.

Drum sound being basically audio bursts I donīt know how can you use that, suggest you download some 440 or 1000 Hz MP3 ; I publised one such source many times.

Ok input 680mv 1k into FX return is 18.89vac into 8ohms

-0db input 380mv 1k into input is like 11.65vac into 8 ohms.

Plan of Attack
Input at 0db ch
check ic1 pin7
Ic2 pin2, pin14
J12 pin7
j10 pin1,4
It occurs to me a dirty S1 switch could dull the signal quite a bit above measurements should clue me in.
380mv was as low as my function generator would go. i need to make a voltage divider for it
Thanks,
nosaj

[g1]

You can carry on with the 680mV at PA IN jack. Look at the right side of schematic pg.1 at the chart with the TP's. It has a column '40VRMS at 8 ohms'. The TP voltages in that column are (approx.) what you should get with 680mV into the PA IN jack.

[J M Fahey]

I see "reasonable" sensitivity at the loop input, it might show a problem or not, we are talking line level signals here, but I *definitely* see a problem at the Instrument input.
380mV at the 0dB input not even making half (or worse, 1/3rd) of expected output votage equals "deaf as a brick" in my book so start there.

Too busy just waking up to open schematic and analyze it in detail, but that measurement alone stands like a sore thumb.

As G1 said, inject whatever they ask for at the input, setup controls as needed (usually all tone on 5, no bright/boost/contour/whatever) and all gain/volume/master controls set to 10, and check signal level at all TP starting from input jack.

Overpraised Ampeg unjustly despised Crate (funny because both areb the same except labels) issue the best service schematics in the Industry,chock full of Test Points and .... look Mom!!! .... scope waveforms!!!!!

And yes, answer very well may be something "silly" like a dirty pot or switch.

They call it "silly", after the fact, those who didnīt have to waste long bench hours until finding it, that is.

Or even worse, those who waste time and money, even weeks waiting for parts to arrive, to , say, replace *all* caps, pull and measure *allĻ* resistors and semiconductors ... only to find that none of those was the problem.

Just last week I got a Guitar combo to repair, "guitar type" (mixed feedback for low damping) amplifier had been replaced by a generic "EBay" , HiFi type which "works but has no guts".
I rebuilt the original one, better but still no bite or presence.
Mind you, all controls worked.

Finally found a hairline almost invisible crack in one track leding to the treble pot.
It still worked end to end, which had me thrown off the track, just that, say, instead of +/-15dB range was now 0/-30dB

Same full range as before but instead of a Treble control it had now become a "Dullness" one.
Treble was fully 15dB down always, relative to what it should have been.

My point being that all of your controls may "work" , which makes you think "no problem here" and look elsewhere, but you may have a gross gain/signal loss somewhere in between, only path is to go checking from TP to TP until you have the WTF? moment.

[g1]

Here are the preamp schematics. Test conditions and voltages for testpoints on pg.3

[nosaj]

Here are the preamp schematics. Test conditions and voltages for testpoints on pg.3

Test points 1st value is what should be, 2nd value what it be.
Input 31mv close as I could get to 26 on the schematic, Gain and Master and Line Out on 10. Tones at 5. All buttons out.
tp1 .110 .09
tp2 .25 .208
tp3 12 4.53
tp4 7 .03
tp5 5.5 .03
tp6 1.8 .04


The next batch are on the eq board which I need to remove to check. The parts locations on Peavey schematics would be great here. Some of the other test point are harder to find with the copius amounts of glue spread over the boards.

So in between TP2 and TP3 we've got a big drop. Initial inclination is the IC . Rails to IC are good.

Thanks,
nosaj

[g1]

That's strange. At least TP3 is giving about a third of expected. But the next 3 TPs are almost flat line. If it's an IC problem it may be more than 1.
I have a diode check scheme that I run on those TL and other series op amps. If it doesn't check out I know it's bad. If it checks out it's maybe good. Basically I check ins and outs to supply pins, reverse probes and do again, compare to new good IC.

[nosaj]

That's strange. At least TP3 is giving about a third of expected. But the next 3 TPs are almost flat line. If it's an IC problem it may be more than 1.
I have a diode check scheme that I run on those TL and other series op amps. If it doesn't check out I know it's bad. If it checks out it's maybe good. Basically I check ins and outs to supply pins, reverse probes and do again, compare to new good IC.

Ok replaced IC1 Signal dropping at s2 found resistance between r73 and pin1 roughly 1meg. Scratched back some and resoldered much better now. From the input with .336v 1k input at 0db jack get about 34v into 4ohm from CHA and CHB So I think both power amps are fine sounds a lot better with a bass now. Now when I put it in Mono mode i use the binding posts the 2 reds ones It is here I am unsure of measuring the voltages. Do I put a meter lead on each of the reds? When I do this the voltage still shows right about 34 volts. So I'm kinda not sure what to do at this point.

Thanks as always,
nosaj

[nosaj]

Can anyone tell me the proper way to get the voltage output measurements when it is in mono?

Thanks,
nosaj

[g1]

Your load has to be floating. You can scope one of the reds to see that you aren't clipping. (scope ground clip to amp chassis)
Measure AC volts between the 2 reds with a floating voltmeter.

[nosaj]

Your load has to be floating. You can scope one of the reds to see that you aren't clipping. (scope ground clip to amp chassis)
Measure AC volts between the 2 reds with a floating voltmeter.

A floating voltmeter could you explain? When in Mono I have the cable hooked up to the 2 red jacks and the mono option selected. Volt meter red clipped on one red and the other black lead clipped on the other red jack but only get the 34v. So I am confused on how when not bridged I get about 34v from ch A and 34v from ChB and Bridged I get the same voltage.
Any ideas?

Thanks,
nosaj

[g1]

By floating I mean no ground connection. So your load can't be grounded, nor your voltmeter.

[nosaj]

By floating I mean no ground connection. So your load can't be grounded, nor your voltmeter.

Ok so in bridge mode you measure from red to red an in the non bridge mode one on red and one on black. So when since I'm getting 34v red to red in bridge and 34v on chA and 34v on chB red to black in non bridged mode. What am I doing wrong or not seeing?

Thanks,
jason

[g1]

There is no ground of any type on your load?

[g1]

A dirty mono switch could be the issue.
If you scope the 2 channels as if testing stereo, but engage the switch, (using only ch.A input), the 2 channels should be out of phase. Are they?