Peavey Transtube 112 EFX: No Output and DC on the Speaker Leads

[Johnrcurry]

I am working on a Peavey Transtube 112 EFX. It came to me with "a loud hum" but no output.

(I am trying to add as attachments a schematic of the power amp for this amp and a drawing of the pinout for the LM3886, but the files simply will not upload. I'll continue working on it and add them when I can.)

I disconnected the speaker, checked for DC on the speaker leads and measured -39vdc with my red voltmeter probe on the wire that connects to the positive speaker terminal and my black probe on the wire that connects to the negative speaker terminal.

I isolated the problem to the power amp by running the a cable out of the effects send to the power amp in of a known good amp and confirming that everything worked as it should.

This amp uses an LM3886 Audio Power Amplifier for its output.

I traced a signal through the power amp to pin 10 of the LM3886. From there, there is no signal out of the LM3886. I measured voltage on the various pins and got the following readings"

Pin 01: 39.3vdc
Pin 02: 0.00
Pin 03: 39.3vdc
Pin 04: -39.5vdc
Pin 05: 39.2vdc
Pin 06: 0.00
Pin 07: 0.00
Pin 08: -1.2vdc
Pin 09: -2.0vdc
Pin 10: 0.00

Looking at the power supply, I measured + and - 15vdc where the schematic shows it, and + and - 39vdc where the schematic shows + and - 35 volts.

I did not find any obvious damage or failed components, although C119 had a cracked solder joint, which I repaired.

I am looking for suggestions as to where to look next. The voltage measurements on Pins 3, 8, and 9 of the LM3886 look fishy. Does it look like the LM3886 may be bad? How would I test that device?

Does anyone have other suggestions as to what might be wrong here?

Thanks.

[The Dude]

If you have a rail voltage at the output of the LM3886 (pin 3) and +&- supply are correct, you almost certainly have a shorted IC. Try a new one.

[Enzo]

Beat me to it. A chip amp like LM3886 with DC on the output is almost always just a bad IC.

[Johnrcurry]

Thanks, The Dude and Enzo. I will replace it and report back.

[Johnrcurry]

It occurred to me to ask. . . I have 39vdc where the schematic shows 35vdc. Is that "close enough" or should I be concerned?

[Enzo]

Close enough.

[Johnrcurry]

Thanks, Enzo.

[Johnrcurry]

To close the loop. . . . I replaced the LM3886 and that fixed the problem. The amp now works as it should. Thanks for your help.

John

[alexisholson]

I'm having the same problem with my Peavey 112 efx transtube. I'm looking for either a service manual or schematic, parts list and circuit board layout!
Does anyone know where I can obtain this?

[Chuck H]

I'm having the same problem with my Peavey 112 efx transtube. I'm looking for either a service manual or schematic, parts list and circuit board layout!
Does anyone know where I can obtain this?

Have you contacted Peavey? It's been over a decade since I contacted them for information but at that time they were very receptive and helpful. More recent reports from other members here indicate this hasn't changed.

https://peavey.com/c/Contact-Us

[DrGonz78]

Well perhaps the 212 EFX transtube is close enough?

PV TRANSTUBE_212_EFX.pdf

TRANSTUBE_212_EFX_pa.pdf

[g1]

Well perhaps the 212 EFX transtube is close enough?

Thanks Doc, that enabled me to find my 112 schem.
attached