Noisey pre-amp in an old SS bass head. I need ideas.

[BasicBassVI]

I picked up a mid 70's Cerwin Vega BG 250 bass head the other day. I ordered it through my local GC and it arrived today. Initially, I pulled up an old thread to find info and schematics (there's not much out there) but I think it's time to start a new thread on this one.

After a quick shake down, everything appears to be pretty much in order and original. I already plan on replacing all of the electrolytics (everything's nearly 40 years old) and also plan on replacing all of the caps in the preamp. There seems to be a lot of white noise in the pre. When I turn the master down (or the noise gate up) the head goes dead silent. However, at a good playing level (this thing gets LOUD) there's what seems like an inordinate amount of noise. It's not unusable noisy, just more than I care to hear. Do you think the re-cap will be enough, or should I look at putting in a fresh set of metal-oxide resistors as well?

http://music-electronics-forum.com/t...1395/#poststop

That's all the info I've found, so I'm having to build my own parts list.

[J M Fahey]

Neither.

I'd say troubleshoot and repair what's actually wrong, not replace things at random just because you read it on the Net.

If the amp works, plug and play with it.

If at some time you have a real problem, then address it.

[BasicBassVI]

Fahey, you are an abrasive individual, but you have knowledge and experience on me so I'll take your advice and troubleshoot.

I know for certain that the noise originates in the preamp. I don't have a scope, but I do have a meter and a soldering iron. Given what I know (it's a solid state amp full of 40 year old components), I'm trying to determine what component or components are likely to generate excess hiss. The noise is much louder when I turn up the eq sliders, but adjusting the tone knobs has no effect. Aside from refreshing old electrolytic caps and carbon comp resistors, I'm at a bit of a loss. Can you point me in a direction?

[The Dude]

Agree with Juan. In fact I rarely even post in threads that start with anything referring to random across the board cap replacement. I'm not going to write a dissertation on why I think it's silly, but look around at some other threads. Quite often an amp that starts off working just fine ends up with problems after a "cap job" because somebody botched something up. If it ain't broke, don't fix it.

Anyway,......does the noise go away when you unplug the bass? We know at least the problem is before the noise gate circuit, so I see no reason to mess with anything past that. Power supply filters aren't going to fix a white noise problem. The fact that they thought it necessary to include a noise gate in the amp makes me wonder if the noise you're hearing isn't normal. Have you played through any other amps of the same model or talked to anybody else who has one?

Edit: Just looked at the schematic. It's unfortunate they didn't see fit to label any of the components other than circuit board designations. It's not going to make it easy for us to help. There may be a less noisy op amp you could try at the input, but without knowing what's there to start with, it's hard to say.

[BasicBassVI]

Unfortunately, this is a pretty rare piece. From what I'm to understand there were less than 300 made from start to finish. The noise is there with nothing plugged in. I understand that power supply filters aren't going to fix a white noise issue, but those bad boys are 40 years old. This is my gig amp and it needs to be reliable.

I'm working up a parts sheet as we speak. I'll try to have some of those components identified soon. I'll also try to find someone who's played one of these before. I appreciate the help.

[Leo_Gnardo]

Fahey, you are an abrasive individual, but you have knowledge and experience on me so I'll take your advice and troubleshoot.

Juan and I are going to co star in an upcoming movie "Grumpy Old Technicians." Bound to be a hit, at least we can sell 100 copies or so right here on MEF.

I'm thinking, if they needed to put in a noise gate, maybe, just maybe there was a reason for it. It seems silly now that a professional MI company would release a product where they had to apply a band-aid to "fix" a poorly designed preamp, but silly things happened then (and sometimes still do.) Possible nobody was much impressed with that amp and that's why they only sold 300. On your behalf I do hope one of our other correspondents can supply you with a solution that will drop the woosh noise and you won't need to rely on the gate. Worse comes to worst you could rip out that pre and build a new one that's much quieter. Bass doesn't need all that much gain and lots of amps have been designed and built without objectionable levels of noise.

[The Dude]

Which of you is going to put the dead fish in the other guys car?

[Leo_Gnardo]

Which of you is going to put the dead fish in the other guys car?

I was wondering what that smell was. It's gone away now we're in the deep freeze.

[J M Fahey]

I know for certain that the noise originates in the preamp. I don't have a scope, but I do have a meter and a soldering iron. Given what I know (it's a solid state amp full of 40 year old components), I'm trying to determine what component or components are likely to generate excess hiss. The noise is much louder when I turn up the eq sliders, but adjusting the tone knobs has no effect. Aside from refreshing old electrolytic caps and carbon comp resistors, I'm at a bit of a loss. Can you point me in a direction?

Ok .
I suspect much more an active element, either a transistor (which were very good by '75 ) or an Op Amp, which still were quite poor.

Standard workhorse, used *everywhere* , including expensive recording mixers, was uA/RC/LM 741 .

There was not much else and yes, it was noisy.

So you might have some '75 technology Op Amp which would definitely benefit from an upgrade.

That said:

1) thanks to JPB posting "BG250.zip" , unfortunately it only shows the power amp output board, its driver board and a display sub unit.
No preamp there

2) I vaguely remember that amp "when it was new" (do the Math ) but never met one in the flesh so please take a front and back picture, and a couple sharp readable closeups of the preamp board (s) , might help identify parts and suggest some testpoint.

Suspect might be an IC located close to the inputs, although I'm somewhat baffled that tone controls do not affect it, yet graphic EQ does.

The power amp shows an unlabelled single Op Amp in a DIP8 case, which is not a 741 because it has an external compensation cap, please read what it says , as well as any you find in the preamp.

The first designed-for-audio Op Amp , which was used in mixers and Hi Fi equipment, was uA739 , fast and low noise, external compensation caps (similar to what's shown in BG250 power amp) but it was a 14 pin double one, while the schematic shows an 8 pin single one.

So let's hope labels show standard searchable products and not obscure house numbers.

So please post those pictures, they will help a lot

EDIT: the idea was to send you straight to the main problem, I shuddered when you mentioned "replacing all caps in the preamp" ... ouch !!!

Caps do not cause hiss, 40 years in a hot tube amp is a torture chamber, but in a cool SS amp is not big deal, as long as they are reasonable commercial quality.

Carbon composition resistors are noisy and unstable, doubly so if dropping some 150V as plate loads, but by '75 they had dropped out of the picture, standard in an SS amp would have been carbon film, which are already very good and they would hardly have dropped more than 15 or 20V so they are very low in the list of suspects.

Now active parts and corroded contacts are something else.

[mikeydee77]

I think if it were me I would build an audio probe (google to find out more - lots on DIY stomp boxes) and use it to see if you can work out where the noise starts. You say that it is in the first half of the amp (I guess from testing via the send/return jacks), well then apply the same fault finding logic again (it's called a binary search - same principle as folding a piece of paper in half) and find out is the noise it in the first half of the pre-amp as well as the second? Keep going until you think you have identified where the noise starts. If the noise has a clear start point then then you can think about substitutions reasonably economically.

[Leo_Gnardo]

Caps do not cause hiss, 40 years in a hot tube amp is a torture chamber, but in a cool SS amp is not big deal, as long as they are reasonable commercial quality.

Juan, Basic, & others - I have found failing small electrolytic and tantalum caps in 40 year old gear, notably Acoustic, Marantz (hi fi) and Kelsey mix consoles. When goop is dripping out, and leads are corroded, some to the point of being totally missing, not such a bad idea to replace the obviously failed ones and while I'm at it, all the rest because after 40 years they may be failing without showing visible signs. It's time consuming but at least not awfully expensive in parts. Functions are the usual DC blocking, filters-EQ, local DC supply decoupling, op amp gain programming. If a cap was used to roll off noise, and it isnt' working anymore, or an interstage DC blocker & it's randomly leaking charge, there's a couple of noise sources. I have unfortunately had to "shotgun" all the small caps in some mid 70's Acoustic bass amps to get them to behave. I figure, how much different could a CV amp be. This one's certainly the right age to show such "geriatric" symptoms. (put grey haired grampa smiley thing here.)

[J M Fahey]

Yes, of course, I have replaced failed electrolytics as any other part, point is that replacing *all* before finding the noise source will only delay the main repair.

If testing shows electrolytics leaking/dry/open/whatever then by all means replace it, but I suspect active parts or maybe a poorly designed EQ which AFAIK might have been "grafted" on the preamp as an afterthought.

As mykeydee suggested, dividing the preamp in functional blocks will greatly reduce the search area.

But can't suggest anything specific until we have a little more information

[BasicBassVI]

Pictures with controls labeled



Here are some interior and exterior shots of the pre section, top and bottom of the board.

[BasicBassVI]

last pic



One more picture due to the 9 attachment limit.

After some further evaluation, I've noticed 3 things:

1. The inputs seem to be very high gain. I use a Fender "Pawn Shop" Bass VI and the neck pickup (an S/D Hot Jag) overdrives the low gain input at even the lowest settings on the first volume (not the master) control. Setting the "Normal/Attenuated" switch to "Attenuated" seems to fix this, although I'm not sure why it wouldn't be placed closer to the inputs if it was meant to affect input gain. Really wish I could find an owner's or service manual for that one.

2. As far as I can tell, the "Bass" control isn't doing anything. No difference in tone whether it's on 0 or 10. Here's what I can read from the legs in-situ, facing the back of the pot with the lugs pointed down: Set to 0: Center lug to Right lug: 5.3 ohms
Center lug to Left lug: short (0 ohms)
Set to 10:Center lug to Right lug: 31.6k ohms
Center lug to Left lug: short (0 ohms)
The pot is labeled BW7798 1377807 which tells me it's a CTS from '78 and that's about it.

3. The noise gate works, but it's awfully slow to close when I stop playing, regardless of where the control (which is a threshold) is set.

It still seems noisier (hiss) than it should be. The noise is made considerably worse by moving any of the graphic sliders away (up OR down) from zero. Say I pull the first three sliders down, the noise increases just the same as if I had moved the last three sliders up by the same amount. Selecting "B" or "C" on the "Tone Shape" switch increases the treble and also cranks up the hiss, about as much as the graphic sliders do. I hope this helps in some way. I realize that replacing all the electrolytic caps in this amp almost certainly won't fix what's going on, but there's a grand total of 16 in the whole amp and 12 of them comprise about $2.50 worth of parts on my next order, so why not renew those?

[BasicBassVI]

Ok, so there are 4 IC's in the EQ section of this pre. 3 of them are TI branded and one is not. There are three located directly behind the graphic EQ that are labeled "RC4136N", "RC4136DB" and "RC4136N". The "DB" variant is the no-name chip. Is it possible that this one was originally supposed to be an "RC4136N" and somebody put in an inferior replacement? Could I damage anything by swapping them and seeing if the noise moves or changes?