Ampeg BA115 Trouble Shooting

[ajg7]

I am new to trouble shooting IC's, but not other problem solving / logical situations (automotive work, computer programming, etc) and could use a little guidance. My amp is blowing the fuse instantly on start up and I have determined that the drivers and outputs are blown, but my problem is trying to figure out which resistors to replace. I have measured the resistance of every resistor on the board, but did not disconnect them from the board so parallel paths exist for some of them. But there are many that do not have any parallel paths but still show low resistance. I'm under the impression that carbon film resistors can fail by decreasing in resistance, but they most commonly fail by creating a break in the circuit. For this reason it is surprising to me that so many resistors could have failed by decreasing their resistance. Here are some of the values I am getting:

R31 = 63, should be 220
R50 = 877, should be 1k
R6 = 1.26k, should be 3.3k

There are quite a few more that are low like this. I'm wondering if all of them are in fact need replacing, or if I'm misunderstanding something. I've attached the schematic for reference. Thanks for any help, I really appreciate it.

Andrew

[Jazz P Bass]

Resistor measurement

The only way to be sure is to lift one end of the resistor, right?
By the way, if those output MosFets are toast, your best bet is to purchase them direct from the factory. Loud Technologies.

[ajg7]

That is certainly one way to be sure, but that route is not too much easier than just replacing them all. I'm really curious if I'm missing something, for example: is there something about the circuit around R50 (a parrellel path I missed or something I haven't considered) making my meter read low? I can follow logic only as far as my competency goes, and I'm looking to improve my understanding. I know lifting a lead is a sure way to measure resistance, I'm just wondering if there are any less intrusive solutions to this problem. Thanks for your help.

Andrew

[Enzo]

Have you removed the shorted output and driver transistors? They certainly offer unwanted parallel paths.

R31? How about shorts in Q5 and D8,9?

877 instead of 1k, and the resistor goes direct to a power supply? I wouldn;t worry about it until I have reason otherwise.

R6? It has the 1k resistor and the -40v rail in parallel. And try reversing your ohm meter leads. ANy small residual voltage sitting in the -40v filter caps will confuse your meter.


A power supply is a circuit to ground, so just because a resistor goes from some node up to a power supply point doesn;t mean it has no parallel paths.

I am not worried about resistors going low, I worry about them going open. If they do read low, it is almost always parallel paths. The only going low I ever concern myself with is when they turn to charcoal. Then your 1k might turn into 50 ohms. But that is usually obvious to the eye.

get rid of the shorted semiconductors first, and no speaker load until the thing is stable and not making DC. Go down the board and just check every transistort in the power amp. ANy that look shorted probably are - pull them. Now also check every diode too. I don;t usually worry about zener voltages, just do they rectify. If they act like diodes, chances are they will also "zene" for me. Once the circuit is working we can check for proper zener voltage.

[ajg7]

Enzo, thank you for your incredibly helpful reply, it cleared up all my questions.

[kenny_viper2]

Have you removed the shorted output and driver transistors? They certainly offer unwanted parallel paths.

R31? How about shorts in Q5 and D8,9?

877 instead of 1k, and the resistor goes direct to a power supply? I wouldn;t worry about it until I have reason otherwise.

R6? It has the 1k resistor and the -40v rail in parallel. And try reversing your ohm meter leads. ANy small residual voltage sitting in the -40v filter caps will confuse your meter.


A power supply is a circuit to ground, so just because a resistor goes from some node up to a power supply point doesn;t mean it has no parallel paths.

I am not worried about resistors going low, I worry about them going open. If they do read low, it is almost always parallel paths. The only going low I ever concern myself with is when they turn to charcoal. Then your 1k might turn into 50 ohms. But that is usually obvious to the eye.

get rid of the shorted semiconductors first, and no speaker load until the thing is stable and not making DC. Go down the board and just check every transistort in the power amp. ANy that look shorted probably are - pull them. Now also check every diode too. I don;t usually worry about zener voltages, just do they rectify. If they act like diodes, chances are they will also "zene" for me. Once the circuit is working we can check for proper zener voltage.

This is my first post, and I am currently 'trying' to fix my Ampeg BA 115, and ordered new drivers and outputs for it because that seems to be a common issue but honestly am hoping it fixes the issue of blowing the fuse. I am not too savvy on fixing PCB and amps but hopefully I have enough info on it to do it alright.

If not I might take it to you, I live in West Lansing funny enough and need the amp fixed for upcoming gigs.

[Enzo]

Well, I am here M-F from 6:30PM until dawn - musician hours.

I am not sure what days they allow West Side guys into the South Side.

I'm on ML King blvd a block south of Jolly. I am in the same building as His Productions recording studio.

Shiawassee Technical Services
5223 So. MLK
Lansing, MI 48911
517-882-2544

[kenny_viper2]

I was wondering...I tried to power up the amp and it makes a long "hummmmmmmmmm" noise and then I saw a tiny little ploom
of smoke start to come from the amp. I powered it down and saw where
the speaker chord connects to the circut board, the connector itself on
the speaker chord was a little bit 'brown'. Not charred, but light brown.
Any ideas? I am pretty decent at working with electronics and wish I could
just bring you the amp, but lack of money is forcing me to try myself.

Thanks Enzo

Well, I am here M-F from 6:30PM until dawn - musician hours.

I am not sure what days they allow West Side guys into the South Side.

I'm on ML King blvd a block south of Jolly. I am in the same building as His Productions recording studio.

Shiawassee Technical Services
5223 So. MLK
Lansing, MI 48911
517-882-2544

[52 Bill]

I'll suggest two things:
One, disconnect the speaker from the amp before you damage it.
Two, start a new thread so that everyone can follow along without the earlier posts.

It sounds like you have dc on the output of the power amp.
What sorts of test equipment do you have access to? A multimeter?

[DrGonz78]

By the way, if those output MosFets are toast, your best bet is to purchase them direct from the factory. Loud Technologies.

Hey Jazz (or others) just working on purchasing some of these MosFets and was wanting to know why you recommend Loud Technologies as the source? I would guess that might be obvious but would like to order online now without having to call Loud. I have found some of these from digikey that would work, but just wondering if they were just as good.

These links are my source and if you ordered on digikey which one (brand) would you choose or should they really only be ordered through Loud? Thanks to anyone who can point me in the right direction here as I am placing order over the weekend. Thanks.

Source links below...
Discrete Semiconductor Products | FETs - Single | DigiKey

Discrete Semiconductor Products | FETs - Single | DigiKey

[J M Fahey]

Transistor counterfeiting is rampant.
I guess the idea is to avoid EBay sellers or smaller desperate_for_profit suppliers; Digikey *should* be fine, but of course nothing beats buying straight from Manufacturer, if available.
The basic idea is that for any parts reseller, most important is difference between purchase/sale $$$, , while a Manufacturer earns by selling working amps, he *will* pay a few cents extra for confirmed good parts.
I always buy from the local branch of Arrow or from a guy who usually has good stuff from Avnet, but I have been burnt *many* times by others.
Just now, I'm getting a lot of stuff back because of bad TL062, which last only a few days under +/-15V rails, yet never had problems with same batch when powered with 9V batteries .
I had bought them for pedals and onboard electronics, and because I run out of TL072 , used a few TL062 in line powered stuff.
Anyway, by datasheet specs they *must* stand +/-15V .... but clearly they do not, so I think my supplier bought a marginal batch, real cheap.
Oh well.

[DrGonz78]

Yeah I looked on Ebait and was just thinking that these parts must be bottom of the barrel in regards to what is the best. I think Digikey might be good and might still wait till Monday to call Loud. The real ticker for me is price but that should be about the same across the board really. Thanks Juan for your always consistent advice here.

[Jazz P Bass]

Mosfet gain matching may or may not be critical in this amp.
If the manufacturer has them available I will go with their parts.
As I understand it (you can verify the fact with Loud) the parts are 'matched' when purchased through Loud Tech.

[Enzo]

It matters more in some amps than in others, but I agree^^^. You will probably get real parts from Mouser, but they will not be matched. The LOUD folks will sell you a matched set.

[DrGonz78]

Yeah that is food for thought right there! Now I have a better understanding of how replacement transistor parts can be considered as a matched pair from the factory. Thanks guys!