Generic Amp?

[BigClick]

Hi,

I have an old (Circa 1974) solid state guitar amp that doesnt work, I cant find any service sheets for it but I was hoping it would be a fairly generic peice of kit.

There is a power suply section consisting of a transformer, bridge rectifier and a giant capacitor, a two-channel pre amp board, a power amp board and what I think is a reverb board.

Is it possible to diagnose the problem just using principles rather than having the service sheet?

Further is there a such a thing as a generic two channel guitar amp module about that I could substitue for the old boards?

thanks

[David]

Realistically, there is only so far that you can go without the circuit diagram. You don't say what tools you have available, but assuming a reasonable set, you could try:

1) look for obviously faulty components (burnt resistors, leaky or bulging capacitors, loose or broken wires)
2) check fuses, capacitors put straight across the mains. If fuses have blown, be cautious about replacing these without looking first for the cause
3) check for presence of DC from the power supply section. If present, follow it through the rest of the system
4) Check voltages throughout the systems - is a voltage present at the transistor collector, base, and emitter? There should be about 0.7V between B and E, and higher on the C.
5) use a signal generator - put output to input of power amp. If sound, go backwards towards the guitar inputs.

I believe Maplins has various guitar-related modules you could try, otherwise Amazon or Ebay. Hope this helps

[BigClick]

Realistically, there is only so far that you can go without the circuit diagram. You don't say what tools you have available, but assuming a reasonable set, you could try:

1) look for obviously faulty components (burnt resistors, leaky or bulging capacitors, loose or broken wires)
2) check fuses, capacitors put straight across the mains. If fuses have blown, be cautious about replacing these without looking first for the cause
3) check for presence of DC from the power supply section. If present, follow it through the rest of the system
4) Check voltages throughout the systems - is a voltage present at the transistor collector, base, and emitter? There should be about 0.7V between B and E, and higher on the C.
5) use a signal generator - put output to input of power amp. If sound, go backwards towards the guitar inputs.

I believe Maplins has various guitar-related modules you could try, otherwise Amazon or Ebay. Hope this helps

Thats great, Il give it it a go and see where it leads me.

If the original pre-amp section is ok i see that Maplins do a decent power amp module.

... Questions;
1. If the original PSU section isn't good enough for a new power amp can I use two power supplies in the same chassis (pre-amp and power amp)?
2. Should I be able hear anything from the pre-amp section with headphones?; assuming the pre-amp section is working of course
3. What is the best way to restrict the output from the pre-amp seeing as the original amp was 30Watts and the Maplins is 200Watts?

Again, many thanks for your response.

[David]

1) Yes. Common the chassis connection, but keep the two Vcc outputs from the PSUs separate, so that each feeds its own module.
2) Unlikely if the headphones are modern low - res ones, possibly if they are high - res ones. Better to apply a signal to the inputs (check this site. Under "links" you should find easy audio oscillators. Kits are probably available.
3) easiest is to put an attenuator between the pre-amp and the power amp. Use a potentiometer with the preamp output connected across the whole of the track, and the input to the power amp taken from the slider. The value of the pot will really depend on the configurations of the preamp output and the power amp input, but start with something in the 50 - 100 K range and experiment.

[BigClick]

1) Yes. Common the chassis connection, but keep the two Vcc outputs from the PSUs separate, so that each feeds its own module.
2) Unlikely if the headphones are modern low - res ones, possibly if they are high - res ones. Better to apply a signal to the inputs (check this site. Under "links" you should find easy audio oscillators. Kits are probably available.
3) easiest is to put an attenuator between the pre-amp and the power amp. Use a potentiometer with the preamp output connected across the whole of the track, and the input to the power amp taken from the slider. The value of the pot will really depend on the configurations of the preamp output and the power amp input, but start with something in the 50 - 100 K range and experiment.

Thanks again I will see how I get on

[BigClick]

Hi all,

The 1A mains fuse had blown so I replaced it but on power up the transformer started humming quite loud and was also getting very hot.

So I disconnected the transformer output form the chassis and it was giving out 49.1 volts without humming or getting hot

Next I tested the bridge rectifier and found a leaky diode (Multi-metre was reading 596 in diode mode accross the AC terminals in one direction)

I popped into Maplins and bought a 6a bridge rectifier (£1.38) and replaced the old one.

This time the transformer didnt hum and i got 67 volts on the filtered side

I powered down and put the dc fuse in line and whilst I was doing this I heard some crackles from the speaker (good sign I think)

Once the fuse was in I powered up and the amp works like new No need for replacement IC modules now.

Many thanks for all your help.

[David]

Well done! I'm delighted to have been able to help.