Help Please

[Keith56]

Hello and apologies at the outset for my ignorance as I endeavour to repair my guitar amp and extend my very limited electrical knowledge.

I own an old selmer amp whose second channel has not worked for ages. Now that I have a bit of time I am trying to repair is myself and develop my knowledge of amps etc. I like tinkering as it keeps me occupied.

I have come to a bit of a dead end with the amp as I cannot see any visual failure. I have tried identifying the problem by manipulating components as my late dad did - often resluting in me getting a shock as well as him but I cannot get the second channel to come in. The first channel works fine with no issues. The reverb also. The volume control seems fine too. This leaves a component failure but i am at a loss to know where to begin on the board - If anyone can offer some guidance it would be most welcome - a few pointers would be gratefully received. I have attached a link to a photo of the unit and as you will see it seems pretty good given it is some 30 years old. I have had various amps (Audio and guitar) where a channel has gone down and it would be great to have some idea where to beging locating and rectifying the problem.

http://picasaweb.google.com/keef54/S...06749527224850

Thanks for any help Keith

[lineup]

First thing is check all fuses are all right.
Then see if power comes to each part of circuit.

Now, as you are luck to have 1 working channel
you can compare voltages in both channels.

You can come a long way by only Multimeter & some logical thinking.
If you can measure AC you may be able to track signal
and see where it gets lost.
On signal's way from Input ... to the big output transitors 2N3055

There may be some Transistor that is burnt and shortcircuited.
Look for any signs where something look burnt or as have been very hot.

[gain]

yes, good advise use the good channel to debug the bad channel. one quick down n dirty check i make before going through the entire circuit from input to output is check obvious failure points, such as the OPS transistors, driver transistors, output emitter resistors, etc. if these check OK and you are getting adequate voltage across the rails, then start at the input and work your way to the output using the good channel as a reference as linup suggested.

good luck!

[lineup]

... and I fogot another basic thing:

Check for all wires & cables works and are having a good contact.
One cable/wire may need to be re-soldered back into one point.

[Keith56]

Thank you very much for your help it is very much appreciated. I really want to use this as a learning curve as I have been frustrated and baffled as to how to run the checks you refer to in the past. I have purchased a multimeter but I am struggling to understand the information it is providing and the settings I should be using. The only one I have some confidence in is continuity! and that is shaky at times.


I will attempt to run the checks that you refer to over the weekend and will let you know how I get on

Thanks again Keith

[john_ellis]

Hi Keith

You can check transistors with an ordinary multimeter on ohms before anything else, with the transistors in situ.

Most analog meters send + out of the - wire when measuring ohms, digital meters may do this or the reverse. If you have a transistor or diode, check which way shows a reading while the reverse direction will appear open.

Having identified the "+" output, which may be the "-" wire, if you put this on the base of NPN's there should be a "resistor" to emitter and collector (check these two in turn with the other lead). The impedance you measure depends on the current the meter squirts out but will probably be in the order of a hundred ohms or so. THe point is it should not be "open" nor "short". Then check that there is not a direct short between CB or CE by placing the "+" wire on C and the other on B and E in turn. Transistors in a circuit may give a reading because there are likely to be some resistors attached somewhere, but they should not be shorted (zero ohms) .

Reverse the leads for PNP's.


This way you can tell mostly if a transistor is open or short: both junctions ought to appear like a resistor in forward, and there should not be a low resistance C to E.

cheers
John

[nigelwright7557]

Originally posted by Keith56
Hello and apologies at the outset for my ignorance as I endeavour to repair my guitar amp and extend my very limited electrical knowledge.

I own an old selmer amp whose second channel has not worked for ages. Now that I have a bit of time I am trying to repair is myself and develop my knowledge of amps etc. I like tinkering as it keeps me occupied.

I have come to a bit of a dead end with the amp as I cannot see any visual failure. I have tried identifying the problem by manipulating components as my late dad did - often resluting in me getting a shock as well as him but I cannot get the second channel to come in. The first channel works fine with no issues. The reverb also. The volume control seems fine too. This leaves a component failure but i am at a loss to know where to begin on the board - If anyone can offer some guidance it would be most welcome - a few pointers would be gratefully received. I have attached a link to a photo of the unit and as you will see it seems pretty good given it is some 30 years old. I have had various amps (Audio and guitar) where a channel has gone down and it would be great to have some idea where to beging locating and rectifying the problem.

http://picasaweb.google.com/keef54/S...06749527224850

Thanks for any help Keith

I usually break the amp into modules and check each.
1/ Transformer.
2/ Bridge rectifier.
3/ Smoothing caps.
4/ Driver board without output transistors connected by sending output back into LTP.
5/ Check output transistors.

You have to be careful not to just replace faulty components, you need to find out why they blew in the first place or you will just blow up another set !

I once built a new amp and forgot to set the bias pot and blew up 6 MOSFETS !

[burbeck]

hi Keith,
when you say that the "second channel has not worked for ages" it is normal in this kind of amp to be mono, with 2 speaker outlets that are connected together.

on many amps of this kinds there are 2 separate pre-amps, (channels) enabling 2 guitars to be used at once. each pre-amp section would normaly have there own gain controls, eq controls input sockets etc

generaly both pre-amps will be identical and will be fed into a common mixer/master volume pot, then to the power amp. because both preamps would be identical in this case, it should be relativly straight foward to fault find.

it would be very unusual for the amp to have 2 separate power amps, and from the photo i see only one

what is the model of the amp that you have? i may have a diagram for it.

also take another picture of the front panel, and any thing else you think may help here.

[lineup]

Originally posted by burbeck
it would be very unusual for the amp to have 2 separate power amps, and from the photo i see only one
what is the model of the amp that you have?
i may have a diagram for it.

Selmer had a solid state period, with transistor amplifiers.

# MERCURY 5 COMBO AMPLIFIER
The replacement for the valve powered Little Giant, with 5 watts output through an 8" speaker. Two inputs were available, each with a separate volume control, but with a common tone control.

# SELMER SS15 COMBO AMPLIFIER
A simple solid state 15 watt amp, fitted with an elliptical speaker. Single volume and tone control, plus two input sockets.

# SELMER SS30 COMBO AMPLIFIER
A 30 watt output amp, equipped with two 12" speakers. Two non-switchable channels, with simple volume, bass and treble controls.

# SELMER SATURN 60 REVERB COMBO AMPLIFIER
60 watt power output, through 2 x 12" speakers. Two channels with two inputs per channel.

I wouldn't be surprised if Keith's amplifier is here in this page:
END OF THE LINE - SELMER SOLID STATE AMPLIFIERS IN THE LATE 1960's AND 70's
http://www.vintagehofner.co.uk/selmer/black/sel8.html

The complete Selmer history, start here:

SELMER TRUVOICE AMPLIFIERS - FROM OUT OF THE MISTS OF TIME
http://www.vintagehofner.co.uk/galle...ery3/selm.html


Lineup