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Author Topic: 1980 Regal Bass Amp schematic  (Read 3912 times)

galaxiex

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1980 Regal Bass Amp schematic
« on: September 05, 2016, 09:48:02 PM »
Found this on ebay, price was right.  Came with the original owners manual including the schematic.  8)

Seller had already tested it, so when it got here I plugged it in and it fired right up.
Great! No noises, no hum sounds pretty good. Ok, so....

First thing I noticed was the "death cap" C7 so removed that, and added 3 wire safety grounded IEC socket.

I could not figure out where the circuit grounded to the chassis, so to be sure,
I added a ground wire from the circuit board direct to the chassis, right next to where I grounded the IEC socket.

 :grr :grr :grr Now the amp hums/buzzes like hive of angry wasps.

Fairly sure I introduced a ground loop somewhere/how.   ::)

So gonna pull the ground I added and see, but too tired tonight to work on this anymore.

« Last Edit: September 05, 2016, 09:49:57 PM by galaxiex »
If it ain’t broke I’ll fix it until it is.

g1

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Re: 1980 Regal Bass Amp schematic
« Reply #1 on: September 06, 2016, 11:46:23 AM »
  Yes you have introduced a ground loop.
The input jacks are connected by single conductor wire, and rely on the chassis for grounding.  If the circuit ground were not connected to chassis, you would have massive hum when turning up the volume.
Remove your ground strap you added, then measure ohms from chassis to circuit ground, I'm sure you will find it to be connected.

edit:  I see some kind of strap connected to the volume pot casing, does it go to circuit ground?
« Last Edit: September 06, 2016, 11:48:34 AM by g1 »

galaxiex

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Re: 1980 Regal Bass Amp schematic
« Reply #2 on: September 06, 2016, 10:28:21 PM »
Thanks g1

Yes, there is a bare buss wire from the circuit board ground to the volume pot metal case....

But.... the pots are all mounted on a separate metal plate, that is, separate from the metal chassis.
The pot mounting to that plate is with little tabs that poke thru holes in the plate, and those tabs are twisted to fasten the pots to said plate.

Hard to describe.... I'll get a pic up later... anyway from there the pots are mounted to the chassis with plastic bushings.

I didn't actually check if that metal "pot plate" is somehow electrically connected to the chassis.
On visual inspection it does not appear to be.

I have not had time to look further into this since last night.
I'll probably get back to it on Sunday as right now I/we are practicing for a gig on Saturday.  :)
If it ain’t broke I’ll fix it until it is.

J M Fahey

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Re: 1980 Regal Bass Amp schematic
« Reply #3 on: September 07, 2016, 04:16:47 AM »
1) ave you checked that dc voltage at speaker out rail is about 15V(before the capacitor)  ?

2) The chassis is basically a bent L shape, made out of phosphatized sheet iron, so front panel is same piece (so has continuity) with thechassisfloor where everything is mounted.

Isn´t the sub bracket holding the pots in contact with front chassis side?

I doubt it`s not, or it would never have worked.
Or PCB ground must be in contact with chassis ground one way or another.

galaxiex

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Re: 1980 Regal Bass Amp schematic
« Reply #4 on: September 07, 2016, 06:03:14 PM »
1) ave you checked that dc voltage at speaker out rail is about 15V(before the capacitor)  ?

2) The chassis is basically a bent L shape, made out of phosphatized sheet iron, so front panel is same piece (so has continuity) with thechassisfloor where everything is mounted.

Isn´t the sub bracket holding the pots in contact with front chassis side?

I doubt it`s not, or it would never have worked.
Or PCB ground must be in contact with chassis ground one way or another.

1. yes it is 15V.

2. No, the sub panel is not in contact with the front of the chassis.
It is a good 3/8 inch or more away from the front panel.
There are bushings that hold the sub panel away from the chassis front. (see below)

Well I had a few minutes today so looked at this again.

Slightly embarrassed here... the pot sub panel is indeed grounded to the chassis.
What I thought were plastic bushings on the pots, are in fact, black colored metal bushings.
Not obviously metal on first glance, but there you are.

So I removed my "extra" ground and.... surprise surprise, the amp works just like it did before.

I had created a ground loop.  duh!

All of the above seems obvious in hindsight.
At the time I was working on this to remove the death cap and install the 3 wire line socket,
I wasn't thinking too much about what was connected to where,
other than to focus on making sure I had the power line hot, neutral and ground right.

I then casually observed that there didn't seem to be a circuit ground to the chassis, so added one.

Again, it's all obvious now.
As you said JM, the amp would not have worked without a circuit ground.
It HAD to be grounded somewhere.  ::)

Cheers! :)
If it ain’t broke I’ll fix it until it is.

g1

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Re: 1980 Regal Bass Amp schematic
« Reply #5 on: September 08, 2016, 02:11:05 PM »
Black metal strikes again.  ;)
Glad to hear you got it sorted out.

phatt

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Re: 1980 Regal Bass Amp schematic
« Reply #6 on: September 09, 2016, 08:04:40 AM »
Yes the ground path is often the hardest part to keep track of when messing with circuits. :duh
One could say it's a "Common" problem. (bad pun) :crazy2:
Phil.

galaxiex

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Re: 1980 Regal Bass Amp schematic
« Reply #7 on: September 09, 2016, 08:14:35 AM »
 :lmao: :lmao: :lmao:
Good one!
I like puns  :lmao:  :duh
If it ain’t broke I’ll fix it until it is.

Loudthud

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Re: 1980 Regal Bass Amp schematic
« Reply #8 on: September 09, 2016, 04:46:09 PM »
I like the simplicity of this design. It might make a nice beginner project. If not the whole amp, at least the power amp. It could run from a 24VAC wall wart with a couple of simple changes.

What are the transistor numbers?

galaxiex

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Re: 1980 Regal Bass Amp schematic
« Reply #9 on: September 10, 2016, 12:36:54 AM »
I like the simplicity of this design. It might make a nice beginner project. If not the whole amp, at least the power amp. It could run from a 24VAC wall wart with a couple of simple changes.

What are the transistor numbers?

I agree, nice simple design.

The 2 output transistors are tip29 and tip30
Driver Q3 is marked GE1822 on the heatsink clamp that seems to be glued to the plastic body.

Q1 and 2 are marked 6080GE near as I can tell. Kinda faded and hard to read.

Here's the full owners manual with the "house" part numbers. Not that that helps....  ::)

Edit; I have the idea to maybe add a Accutronics reverb brick to one of the inputs since I'll never plug in 3 instruments all at once.
Seems a shame to not use all that space on the chassis.  ;)
It can be an "effect" channel, and done in such a way to not drill extra holes in the front so to keep the look original.
Maybe add a hidden "Reverb" control knob in the back somewhere. Or just have a pre-set reverb depth.
« Last Edit: September 10, 2016, 12:44:06 AM by galaxiex »
If it ain’t broke I’ll fix it until it is.