Ampeg B4R weirdness

[Gtr_tech]

Complaint on this thing is "weak output". Indeed it seems to lack "balls" for lack of a better term. This thing should knock you over without advancing the master vol too much. Everything checks out on the preamp board, and a ton of cracked solder was reflowed on the main/output board. Output section rails and bias section checks out. Running an external signal into the pwr amp inserts results in plenty of output.

All normalizing jacks have been cleaned and verified. Anyone seen anything like this before?

Ampeg B 4R.pdf

[Jazz P Bass]

What is the amplitude of the Power Amp In signal, that drives the output to full power?
Compare this to what the preamp is putting out. ( sounds sexist)
That may be your first hint.

[Enzo]

In the interest of isolating the problem, looks like you could unplug the graphic and jump pin 1 and 4 together to bypass it.

[J M Fahey]

There seems to be signal loss or lower than normal signal along the path, but thereīs *a lot* of different points where this could be happening; youīll have to inject a good known signal in the instrument input jack, say, 100mV 1KHz, and trace it step by step.
Just guessing wonīt do much help, besides the 2 or 3 classic points such as loop in/outs or the connector Enzo suggested..

And ... good luck and patience.

[MarkusBass]

On the schematic (page #3) there are very detailed AC voltages specified. So finding the problem is just a matter of plugging a generator and measuring voltages in the test points (with an oscilloscope). The preamp should be tested with 26mV RMS. And the power amp with 0.68V RMS (this is written on page #4). This will allow you to isolate the problem.
I don't know what you mean by "a ton of cracked solder". I fixed many Ampeg amps and only in one of them the problem was caused by a cold solder. Was the amp treated so bad that there is a ton of cracked solder?

Mark

[J M Fahey]

Agree and add: on many "working/travelling" amps (as opposed to those that mostly sit at the garage or rehearsal room), whatīs usually called "cold/cracked solder", often means "cracked track just by a solder pad".
Itīs usually repaired (even unknowingly) by resoldering the adjacent pad (which by itself didnīt really need it) because the little solder added, plus the fresh flux, bridges the tiny gap.
Thatīs why signal path must be followed, and if it mysteriously disappears somewhere, pads along the suspect track must be resoldered even if on close inspection they look bright and shiny.
The cracks themselves are practically invisible.
They appear because when solder on a pad cools, it contracts and tends to "pull" its edge from tracks reaching it, applying a pre-tension (mechanical) there.
The fact that tracks usually arrive forming two perfect 90š angles increases that, because sharp angles concentrate tensions.
Old hand drawn Peavey , Acoustic, Kustom, etc. PCBs were absent from this (as do Military PCBs which specify 45š joints, avoiding 90š ones) but most regular PCB software is guilty of that.
A few can be setup to avoid that.
We are talking resistance to flexing and heavy vibration here; there are also RF considerations which do not usually apply in our area of work.

[g1]

Maybe a limiter malfunction? As Markus said, lots of voltages to check at the test points . Get's my vote for Yikes! in the simplicity sweepstakes, guess I've been out of the loop too long.