I'm a commercial jet engine technician.....but

[fastback4]

I know a little of the technical language you speak here so here I go. My churches bass amp Laney BC120 is old and stopped working so I said I'll try to get an opinion what might be the problem. (could be that this amp is solid state and built in 1995). I believe this amp was played without the 15" speaker attached to it as the cabinet is empty. The fuse by the on/off switch is good but the 2 side by side fuses on the front board where blown so I replaced them and within 2 seconds they glowed and blew. Here are some pictures with details that is the best I can do. I live near Florence,Ky. and Willis music has a repair person whom musicians tell me is pretty good but very busy. Thanks for reading this and may god bless you.

[Jazz P Bass]

Laney BC120

If both 3.15a fuses blew, you have a serious shorted condition.

First in line after the fuses: the bridge rectifier.
This provides the + & - 44 volt dc rail voltage.
This is what is also converted to the + & - 15 volt dc supplies.

Other than the bridge rectifier being toast, I would suspect the output transistors.

[Tom Phillips]

There are other MEF members that can better advise you about your Laney BC120. I'm sure someone will chime in soon. However, I did notice a previous discussion about the same amp at http://music-electronics-forum.com/t35706/ .
That discussion did not go into details but there is a schematic of the power amp section posted and that will be a good reference to support your repair project.

[fastback4]

By no means will this be my attempted fix. I hoped to get enough feedback to determine if this 'old' amplifier would be worth the fix. I have no idea yet what parts and labor might be.......

[Chuck H]

If you can get a shop recommendation from someone you would trust on their opinion of such things, get it fixed. It shouldn't be hard or for a qualified and experienced tech to repair and should be a lot less expensive than replacing it. Is the amp worth the cost of repair? Maybe not. Smallish, older SS bass amps don't command much resale. But cost of replacement should be the consideration since you're not likely to get a much better amp in the same class for the cost of repair. Most shops will charge you for a quote and apply that cost to the repair.

It's possible that the output transistors are bad and the missing speaker is because of it. So it may have been used as a preamp into another amp or something like that. Always a bad idea to continually plug in and use a broken amp. More hopeful, the speaker blew, they've been plugging it into another cabinet and this is the first actual amplifier failure. It might help a repair tech to know the history, it might not.

[fastback4]

Thanks to all for their response. I will ask the pastor tonight what he intends to do now that we have some opinions......

[J M Fahey]

1) it's quite repairable.

2) how much will a Tech quote?
Probably 1 hour bench time + parts (obviously with some markup).

Sorry but who cares that "used SS equipment does not command high prices"?

That's not your problem, you are not selling one but needing one.

3) Will you get a working Laney BC120 for that price or less?
Define "working" first.
I for one do not trust EBay/Craigslist seller descriptions very much.
Often it works ... sort of.

Maybe he "forgot" describing the scratchy volume controls, that it overheats after an hour and cuts off, that it picks Hip Hop Rap radio at the most inconvenient times, that jacks are rusty, that the cat peed on it and when it warms up you have to open all windows, etc.

4) Will you get it within pickup range or will you have to add packing and shipping expenses?
What about complaints to an out of town seller?

5) that's why in cases such as this, that some equipment was properly cared for and was working properly until it suddenly died, I much prefer to deal with a local Tech who worst case is reachable for a "friendly conversation" if job is not well done.

And those Laneys are good, very probably English made, simple to maintain and repair, parts schematics are available, etc.

Try that with GC or MF "brand of the day" obscure source amps, like those no name generic amps branded Acoustic, Raven or whatever.

[g1]

Due to the fact it's missing a speaker, I'll offer an alternate to Chuck's opinion and question whether it would be cheaper to fix than replace .
But of course it depends on what you will replace it with. Something new will likely be more expensive than fixing it, but on the other hand, something of similar vintage can probably be obtained quite cheaply.
The fact that the output transistors are getting warm in the few seconds before the fuses blow makes me think they may be blown.
On the other hand, it's a very simple power amp section and should not be a very expensive repair in the hands of a reputable tech.
So it comes down to the value of used gear in your area vs. the cost of repairs in your area.
I'll throw out some ballpark numbers and say $200 for repair and speaker replacement (others please comment on that number). What kind of used amps can you get for that money where you are?

[oc disorder]

Hi Fastback4

I'll see if I can help a bit..

EDIT: while I was writing this on the other side of the world some more posts appeared & I noticed I may have dived in the deep end
whereas you only required an overview.
As this is a "help" type forum I thought I would post my 2 cents worth anyway seeing it's all prepared.
and as there is no speaker unfortunatly it may be cheaper to buy a new complete amp.
Ah - as my teacher used to say "Read The Question" !

R27 ,R28 are two resistors that attenuate or drop the output from speaker down to headphone level
so I'm not surprised they get a little hot.
They are a higher wattage for this purpose and still should measure aprox. 220 ohms
on an ohm meter.

That muck under the capacitor is actually glue. It is used to add mechanical strength to the fixing of those
large caps to the board so the solder joins are not strained when the amp is vibrating.
It is a bass amp after all!

D10 which is cracked is a IN4007 diode across one of the output transistors and suggests an output stage fault.

I would remove it or at least prise it apart before taking measurements.

D11 should be checked too, with the diode test function on a multimeter, open circuit/infinity one way .5 aprox
the other or on low ohms with some meters open circuit one way and more or less short circuit the other.

Like JP Bass , I think its most likely the output stage or the power supply ..maybe even both !

First up with D10 prised apart measure from bass to emitter on the output transistors T11 & T 12 .

i.e. From the metal case of the TO3 transistor C (collector) to E (emitter) - the top of R23 and R24
will give you a connection without removing the board.

In circuit you shouldn't get very low readings .. if they are faulty we are looking for a very low reading
(short circuit) .. can double check by removing them and measuring again.

While you are there best check that the emitter resistors are not open circuit, as they are a very low ohmage
.22 ohms (point 22) . They will read like a short circuit i.e. less than 1 ohm.

If the transistors do measure shorted .. unsolder them and remove them.

At this stage if those 2 are shorted it's possible that is all that is wrong with the amp.

The amp could be tried briefly to see if the fuse still blows.

However often with a fault like that if the fuse doesn't blow fast enough the previous 2 transistors
Tip 31 & Tip 32 (T9 & T10) get a bit overheated driving into a short circuit and they chuck in the towel too!

And the power supply can groan under the weight of it all with a possibility of the bridge rectifier also developing a fault.

If the fuses don't blow and you can measure the two supply rails plus 44 volts and negative 44 volts looks like
only the outputs need to be replaced.

2N3773 is shown on the diagram.
These are an NPN transistor 160 volt 16A transistor.
From your photos an equivalent is fitted a MJ 15003 which is a 140 volt 20A transistor.

And thanks for the blessings , not too many of them around here - I think the majority of the guys here
are more familiar with cursing !!

[fastback4]

Everyone has a built in desire to help another. I have been blessed with many talents so I cannot resist to help another when the opportunity arises. I will print your explanation and diagnosis procedure and present it to the technician when I find one will to work on this Laney. As I said they are very very busy right now and my case isn't a big money maker to most....god bless you all

[fastback4]

I found a 'work at home' technician looking at this amp as we speak. He retired from an 'old school' stereo repair tube and ss sound amplifiers. He was pleased to receive your copied post with tracking instructions to help find the problem. He got a 20$ deposit put toward the cost's and said he might need a couple days. Thanks for your help again....Tom in KY.

[fastback4]

I did get my Laney back today and the hand written repair explanation says >>2 power transistors/ 1 tip 32C/ 1 BC237/ 1 diode / labor and parts $71.18. I played my bass for over 1 hour at half volume through a new Eminence 4ohm. 300 watt speaker and the amp remained cool and power stayed on......
Thanks for sharing your experience to help make this happen.

[Tom Phillips]

It seems that everything worked out well.
Thanks for reporting the outcome.

[Chuck H]

This is why we (and the semi retired guy that repaired your amp) do this. Less waste in the landfill and more music being made