Acoustic 150 Amp - Power Module 17-12 Oscillation

[TomCarlos]

MEF Members -

I picked up a rescue amp, an Acoustic Control 150 Head. This is the later version, the one with the 17-11F Preamp (Tremolo/Reverb preamp board).

The amp needs a heavy dosage of TLC - and I have begun to work on it. Thus far, I have done the following:

New filter cap - 3900uf, 100v (Cornel Dubilier)
New output transistors (MJ15003G) and attached with a new thermal isolator and grease.
Replaced all other Electrolytic Caps (on the Power Module and Preamp boards and Output coupling cap)
Changed the power switch configuration (put a new switch on the back panel) and removed the death caps (I will post a photo of this later on)
The pots on both preamp boards are in bad shape. I have tried to clean them but some will need replacing.

For now, I have Output!!!! Both preamp modules pass a signal but each board has issues and will need work, including figuring out why someone would solder ceramic caps to the bottom of the boards!! That is a good start for me considering the work I have done.

Before tackling the preamp boards, I want to make sure the Power Module (17-12) is working properly.

For now, I am testing my disconnecting both preamp boards and connecting a signal directly to the 22K input resistor R301 on the Power Board.

When I connect a dummy load, I am getting a clean signal. When I connect a speaker (8 ohms), I am noticing some oscillation on the upper edge of the signal. I have tried a couple different speakers, same result.

The output coupling cap is new, 1000uf, 50v. I checked the 22 ohm and 1.uf cap that hangs off the output jack.

The DC voltages around the board seem to be within an acceptable range and I am seeing a skosh under 70 watts before clipping. I think that is average for this amp.

Any ideas on why a speaker would cause the oscillation but the dummy load would be clean? I am reviewing the service manual but do not see anything in there related to this issue.

Thanks, Tom

[Helmholtz]

Any ideas on why a speaker would cause the oscillation but the dummy load would be clean? I am reviewing the service manual but do not see anything in there related to this issue.

A speaker is a reactive load. Its impedance changes/increases strongly at higher frequencies because of its inductive component. This affects the NFB stability of the amp.

I checked the 22 ohm and 1.uf cap that hangs off the output jack.

That seems to be the Zobel network. Its purpose is to compensate the inductive speaker load at high frequencies and thus stabilize the amp.

[TomCarlos]

Thank you Helmholtz....

I did some additional testing today with both 8 ohm and then 4 ohm dummy loads. At 4 ohms, the amp generated 100 watts before clipping- and that is the spec. ACC has this amp rated at 110 watts on a 3.2 ohm load.

There are a few oddities with the layout and wiring on the Power Module board that I would like to share.

The output coupling cap C311 is strapped to the side chassis, next to the output transistors, next to the phase inversion transformer, and under the Power Module. The negative side of the cap has a single white wire that goes to the Power Module board and makes a single connection - to the yellow wire, and that goes directly to the output jack. (There was a broken trace that so I soldered the wires together. I will clean that up). It seems there could have been a better way to make that connection (off the board). It is hard to flip the Power Board over to work on it with wires coming from other destinations.

The other odd thing I noticed is that the Power Ground (as identified on the schematic) consists of five components that run directly to the Bridge Rectifier. The emitter resistors R320 and R321 (mounted to the Output Transistors) terminate on a solder lug near the output transistos. A single black wire goes from that point to the Power Module board and lands on the edge that is furthest from the end connectors. If you look at the photo, there is a spot next to the PC Board number and just above the incoming red wire. That is where the emitter resistors land. From there, a long trace goes all around the PC board. It picks up the thermistor (R317), the 6.8 ohm resistor (R315), and the 500uf cap C310. This entire trace connects to the single black lead which is the Power Ground (the one that goes to the Bridge Rectifier). Intuitively (and to me), anything that is labeled as a Power Ground should have a thick trace and short run. With two empty and available traces on the edge of the board, it would make more sense to land the ground wire coming from the emitter resistor there. I think I'll play around with that change and see if it affects the signal. If nothing else, it is one less wire soldered to the bottom of the board and would make it easier to flip over for access and working on the board.

Anyway, we are in project mode and this is a good amp to experiment with.

[TomCarlos]

One other thing.... for the ground loop fanatics....

If you look at the mounting bracket at the bottom side of the photo, there is a trace from the Power Ground that connects to a screw on the bracket. That bracket is also connected to ground through the screws on the side chassis. So in essence, you have a ground loop. I cut that trace. There is no reason (that I can think of) that the mounting bracket should be grounded. All components on the Power Board are either connected to ground through the Power Ground or Signal Ground traces.

[TomCarlos]

Ack !!! I never posted the resolution to the Oscillation !!!

I pulled the Power Transistors and carefully inspected the TO3 mounting socket. A couple of them had residual Thermal Goop in them- so there was a poor connection to the Emitters and Base connections of the output transistors. I cleaned the sockets with 90% alcohol and remounted the transistors. Oscillation gone.

Next, I will take a look and see if I can minimize the audible hum. I'll disconnect the preamps once again to see if the issue is Power Module related, coming from one of the Preamps, or something in the Power Supply. I picked up a hint.... 120Hz is from the Power Supply, 60Hz is from grounding.

More to come...

[vintagekiki]

Check SIGGND and PWRGND that there is no poor connection.
Check the C307, C309, C310 that they have not changed the value.

Question
Does Acoustic oscillate, hum if the connection is PREAMP OUT - POWER INPUT interrupted.
Then POWER AMP only works.

Connect the guitar to POWER INPUT. If the POWER AMP is the correct, sound of the guitar must be
not distorted and clean.

[TomCarlos]

Vintagekiki... Thank you.

This note should probably be addressed separately or in a general "Preamp Hum" thread. But since I am addressing the ACC 150 Power Amp module, here it goes.

For this particular Acoustic amp, it seems there is some type of inherit hum issue with the preamp design. I now remember that years ago, I noticed the same problem with an Acoustic 135 - same boards, same components. For both channels, the Volume is at "0". Both channels, all knobs are at 12:00 O'clock with the exception of the Tremolo/Reverb which are at "0". At those settings, I would say the amp is acceptably quiet (for it's age, design, etc). BUT... the minute you alter the "Bass" control on the Tremolo channel, moving Left or Right of Center, you can see and hear a noticeable difference in the output. It sounds like 60Hz. I see the Frequency cycle altering and the counter cannot decide if this is 60Hz or 120Hz. But like I stated, at 12:00 O'clock, the hum is at a minimum.

Tom

[vintagekiki]

Acoustic 150 use the ferrite coil L101, L201 of 1.5H for Bass regulation. The hum you hear is 60Hz are inducted from the power transformer T401. Hum exists on both channels, and when Volume is at "0" because Volume control is before the tone control.
Simply this is something that can not be avoided. Maybe the hum would be reduced if instead of the ferrite coil used the gyrator.
The same problem exists with the Ampeg (V4 ...) that uses the ferrite coil for Bass regulation.

[TomCarlos]

Thanks again...

I see that the member "Tigeramps" addressed the Gyrator option in this thread - https://music-electronics-forum.com/...ad.php?t=41801

[vintagekiki]

Gyrator as replacements for inductors

https://www.tapatalk.com/groups/unofficialacousticcontrolcorpmessageboard/acoustic-126-bass-amp-inductors-t2296.html

Gyrator in Acoustic amplifiers

http://music-electronics-forum.com/attachments/29882d1407106112-acoustic-b450-b600h.zip

https://music-electronics-forum.com/attachment.php?attachmentid=36844

About Gyrator

http://www.geofex.com/Article_Folders/EQs/paramet.htm

http://www.muzique.com/lab/gyrator.htm

https://circuitswiring.com/bandpass-filters-gyrator-frequency-calculator/

http://sound.westhost.com/articles/gyrator-filters.htm

[nickb]

One other thing.... for the ground loop fanatics....

If you look at the mounting bracket at the bottom side of the photo, there is a trace from the Power Ground that connects to a screw on the bracket. That bracket is also connected to ground through the screws on the side chassis. So in essence, you have a ground loop. I cut that trace. There is no reason (that I can think of) that the mounting bracket should be grounded. All components on the Power Board are either connected to ground through the Power Ground or Signal Ground traces.

I might suggest the purpose of the track was to provide a low impedance path to ground for the power module. Now the only path is the high inductance wiring.

[vintagekiki]

To eliminate unwanted ground loop, check all ground wiring according to the drawing Cassis Wiring 150 (page 13)
When in question the ground loop there is no improvisation. Strictly takes account what is input ground is and what is output ground.

[TomCarlos]

Thank you Nick and VK -

I will admit that in the back of my mind, I keep thinking about "Star" grounding concepts.

VK - I did review page 13 of the Chassis Wiring Diagram. I looked for points on the chassis where the circuit ground physically connects to the chassis (with one of the black wires that comes from the negative (-) side of the power supply.

#1 The first point (and seemingly only point) is the output jack
#2 The 17-10 Preamp (Vol, Tre, Bass) board get's it's ground through the chassis, which comes from #1
#3 The 17-11 Preamp (Reverb/Tremolo) board get's it's ground through the chassis, which comes from #1
#4 The reverb tank get's it's ground from the 17-11 ground point, through a shielded cable
#5 The 17-12 Power Module has a Power Ground that goes directly to the (-) side of the Power Supply
#6 The 17-12 Power Module also has a "Signal Ground" that goes back to the 17-11 board and that connects to the chassis ground
#7 And that leaves a second ground path for the 17-12 components I mentioned previously, the second path to ground through the mounting bracket to the chassis. You have (IMO) high ground devices which have two paths to ground.

In the end, I doubt making significant changes in grounding will make much of a difference in terms of "noise" and "hum". But, as a prototype/project amp (which is now sounding like it did from factory), it would be interesting to experiment a bit.

[vintagekiki]

ACC 150 Ground wiring

All jacks are mount directly to the chassis.
17-12 Power Amp Board - Power Ground point goes directly to the (-) C401 and through gnd point Speaker Outputs jack to the chassis.
17-12 Power Amp Board - Signal Ground point goes directly to gnd point Signal Input jack to the chassis.
17-11 Preamp Board - Chassis Ground point goes directly to gnd point Signal Input jack to the chassis.
The reverb tank output ground directly to 17-11 Preamp Board - Chassis Ground point through a shielded cable
17-10 Preamp Board - Chassis Ground point goes directly to gnd point Signal Input jack to the chassis.

Edit
Mechanically clean from oxides (steel wool) places where is contact ground point with chassis (input / output jacks, solder lug washers ...)