Op amp stage clipping badly - Page 1
Forum section: electronics / repair

author: Mojoampguy
author_rank: Contributor
timestamp: August 21, 2025, 06:53:31 pm
content: I’ve been working on trying to repair a Fender FM100H 100w guitar amp for weeks and I’ve hit a brick wall. The op amp at the front end of the power amp section of the circuit is clipping really badly on the negative half cycle. Even though the input signal is a clean sign wave. The output signal clips badly on the negative half cycle. The overall dc offset at the output is also approx. 100mv. I measure -1.7mv at the input pin 5 with no AC signal presented to the amp.In attachments.1. Scope view - yellow trace pin 5 + I/p signal. Blue trace pin 7 o/p signal. Clipped negative cycle fault.2. Screenshot off op amp circuit3. Overall amp schematicI done all the obvious. Checked power supplies, checked all connections and components in this stage of the circuit. Swapped op amp, twice. No in a socket.Any advice? Thanks

author: Benta
author_rank: Super Contributor
timestamp: August 21, 2025, 08:17:33 pm
content: It would be easier if you:- please rotate the photo so we don't have to break our necks.- focus the camera to avoid the French Impressionist styling.

author: moffy
author_rank: Super Contributor
timestamp: August 21, 2025, 10:59:11 pm
content: Because of the asymetry you could try replacing D16 and D17.P.S. Check that the supplies are what they should be.

author: thm_w
author_rank: Super Contributor
timestamp: August 21, 2025, 11:46:21 pm
content: We have a repair section, you can move it there with button buttom left:https://www.eevblog.com/forum/repair/What are the rail voltages on the TL072? I assume you measured them as +/- 16V?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 22, 2025, 08:38:37 am
content: Check every semiconductor (and diodes near opamp first).Every p-n junction mast have 0.65+-0.05 VDC drop in right direction.And, of cause power supply voltages (that is a very first move).Then you may replace opamp itself. It is cheap, it only takes some time to unsolder lt.Upd.: opamp may clip if one of output transistors is failed too.

author: mikerj
author_rank: Super Contributor
timestamp: August 22, 2025, 08:50:34 am
content: You say you've checked the power supplies, but did you measure the actual voltages on the op-amp supply pins? The schematic shows a test point on the op-amp output with a 33mV signal level, presumably this will be under specific test conditions so are you able to re-create those test conditions and measure the signal level?

author: Mojoampguy
author_rank: Contributor
timestamp: August 22, 2025, 12:03:04 pm
content: I will replace the diodes, but I doubt it will help. I’ve already clipped the legs to disconnect them out of the circuit. No change to output signal.

author: Mojoampguy
author_rank: Contributor
timestamp: August 22, 2025, 12:04:57 pm
content: Can’t see an option to move the post. Yes the rails are 16v at the chip.

author: Mojoampguy
author_rank: Contributor
timestamp: August 22, 2025, 12:06:05 pm
content: Already done all these steps, to no benefit

author: moffy
author_rank: Super Contributor
timestamp: August 22, 2025, 12:26:40 pm
content: Quote from: Mojoampguy on August 22, 2025, 12:03:04 pmIf you haven't already tried it replace C40 (0.47uF electrolytic) with a 0.47uF metal film capacitor, an electrolytic can act as a rectifier and low value electros are prone to drying out.

author: Mojoampguy
author_rank: Contributor
timestamp: August 22, 2025, 02:20:07 pm
content: I’ve already replaced C40. No change.

author: nfb
author_rank: Contributor
timestamp: August 22, 2025, 04:00:52 pm
content: This op amp operates in a negative feedback loop with the power amplifier. I would check for a fault there.

author: Vicus
author_rank: Regular Contributor
timestamp: August 23, 2025, 06:41:52 am
content: Exactly, there is no feedback resistor so the behaviour of the opamp could be ok. Maybe the output of the opamp has to be like that to compensate a non linearity of the output stage. Also try the measure with a lower voltage, the result may change. Have you scope the output of the power amp?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 23, 2025, 07:31:24 pm
content: The opamp is in the feedback loop of the power amplifier.It may be a problem downstream in the circuits it drives or a voltage fed back from the PA it's self.Tricky to isolate the failure point.What is the actual problem with this amp?

author: Mojoampguy
author_rank: Contributor
timestamp: August 24, 2025, 08:35:35 am
content: Clipped output on positive cycle of a sine wave.

author: magic
author_rank: Super Contributor
timestamp: August 24, 2025, 08:51:23 am
content: If positive is clipped then why are you worrying about the negative part of this waveform?There is some problem with power transistors (Q9 and/or further to the right) driven by this chip, particularly the high side ones.

author: Mojoampguy
author_rank: Contributor
timestamp: August 24, 2025, 11:13:51 am
content: Thanks for the advice. I had got laser focused on the op amp stage. Fault was elsewhere as suggested. Finally found a duff resistor in the power amp circuit. R88. What a slog that was. Relearned a lot about transistor Biasing, forgotten since university for me was in the late 70’s, so valuable for the future repairs. Thanks again for pulling me out of a trance. 😄