Strum and Drum

fronobulax

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So it amuses me that the dining room currently holds 2 bass amps, 2 guitar amps, a 5 string bass, an acoustic electric bass, two Guild electric basses, a D-25, an non-Guild acoustic and a serious 88 key - " can play this on stage" keyboard. It saddens me that one of the amps does not work.

The amp was given to me by a friend who was moving out of his house. It had belonged to his son who has no interest in it. I plug it in, turn it on and nothing happens. The fuse is not blown. While I am not intimidated by "no user replaceable parts" I am hampered by my complete lack of knowledge about what to do next.

The label says
Model NBR: GA920P
Input Volts: 117 AC
Input Watts: 41
Cycles: 50-60
Serial Nbr. LK 2083130
Fuse Size: 0.35 A
plus comments about being fully solid state and "made in Japan for Strum & Drum, Inc." The face plate says "National Tremolo Reverb Amplifier Solid State".

This ebay listing seems to be what I have. I presume the cabinet is 2x8 but I have not measured the speakers nor run the risk of blowing them by testing with one of the other amps.

Google search results seem unhelpful.

Suggestions, advice? A schematic would obviously help. Failing that a warning of the hazards I am likely to encounter if I pull the cover would mazimize my chances of still being around to post to LTG. And I don't think it would be considered a win if Mrs. Fro bought her F-30 with the proceeds from a life insurance policy.

Thanks.
 

capnjuan

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fronobulax said:
... Failing that a warning of the hazards I am likely to encounter if I pull the cover would mazimize my chances of still being around to post to LTG.
Hi Frono; the more common risks are putting lots of time into a project whose tone doesn't justify the time (corollary: sinking the time but not getting any sound at all).

Like all electronics, solid state equipment has filter caps that hold a DC charge; this risk is mitigated by the fact that solid state gear is high current/low voltage so those power supply caps are commonly limited to about 50V DC which means that if you get bitten, you only get bitten by 50V ... not 450V.

If you have a VOM you can start with the power switch. Set the meter to ohms resistance, amp unplugged, touch or connect the leads to either side of the power switch; in the 'off' position, it should read infinite resistance, in the 'on' position, it should read 0 resistance. If it reads infinite resistance irrespective of position, then the power switch is bad.

Of course, something else could be burfed up too ... if you want to mess with it, pull the chassis out and take/post a few pics of the component side ... from different angles with good light. If the switch is good, the next stop is the power transformer; seeing if it puts out secondary AC to feed the rectifier. If yes, then you'd check the rectifier diodes ... and so on.

But fixing these can be like trying to solve equations with three unknowns; 'no workee' can be a function of several co-existing problems. The items above are intended to proof the power although even if the power is fine, there could still be a break / short / electronic goober in the audio chain ...


Happy to help if you want to fool with it. J
 

fronobulax

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I especially appreciate that no reference was made to the fact that making sure the power switch actually closed a circuit was an obvious thing to do.

So I tested the power switch and got nothing. I opened things up. It seemed pretty clear that someone had been there before me because the main screws turned easily and were not completely seated.
IMG_2798.JPG

When I opened it up I immediately noticed two things. First one of the leads from the power cable was not connected to anything. In the picture above you can see the free end in the UL. To its right is the power switch on the front panel. There was also a capacitor with a free end. One end was connected to something on the circuit board. The other was free. You can see that up and to the right of the transformer or down and to the right from the power switch.
IMG_2799.JPG

Close up. Rotated because I can't correct that at the moment for some stupid reason. The unconnected power cable lead is out of focus above the on/off switch. The unconnected capacitor is above and to the right of the power switch.

I'm losing access to a camera so I figured I'd better post something soon. I will continue to poke around and see if there are obvious disconnects. The sleeve from the power switch has screw holes on two sides. One is connected to the power cable and the other is empty. One hypothesis is that the free lead and the capacitor were connected to the now missing screw.
 

capnjuan

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Well ... you can terminate the black line supply wire on the other side of the switch, ignore that cap for the time being, hook up a speaker, and flip the switch with a screwdriver with an insulated handle, chopstick ... or what-have-you and see what happens.

If it doesn't self-destruct and you want to fool with it, it needs a three-wire power cord ... the new green ground wire going under that transformer hold-down nut ... with a ring or spade connector crimped on it.

If it doesn't blow itself up and it still doesn't work, without a schematic, determining whether the rectifier is putting out DC or whether there is bad connection at a jack or control is going to be about as you can go troubleshooting.

Those are the output transistors fastened to the vertical shield that also serves as a heat sink. The 4 diodes (black cylinders w/ silver bands on one end) at the edge of the circuit board nearest the transformer form the rectifier and the largest aluminum can adjacent to the diodes is the multi-section can cap. You can see the dis-assembly/re-assembly headaches trying to update it ... flippity dippity with the circuit board.
 

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Not that I know jack squat, when I rescued the Dirt Road Special amp, it had a couple of 4700 mf electrolytic caps feeding the main output transistor. Both of those had failed, one in spectacular fashion. After I went to radioshack and picked up the (in stock :shock: ) replacement parts, it fired right up. Look for a bulge or discoloured marks on those big blue caps.

Other than that, I got nuthin'.
 

fronobulax

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I'm not where the amp is at the moment, but when I temporarily connected the power lead to the switch, toggled the switch and measured the resistance at the power plug I saw a change from zero to infinity so I know what the next step will be. I think the floating capacitor's anchored end just went to a ground on the circuit board but that can be safely ignored. More later once I patch things up and apply power.
 

capnjuan

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The little ceramic cap was there to filter out electronic hash that might have ridden in on the incoming power. I wonder why the line was disconnected from the switch in the first place; preparation for a 3-wire cord ... troubleshooting shorts ....? Not knowing the answer ... the business with the screwdriver or chopstick ... a little (.........) is in order.

Orientation:

frono05orientation.jpg
 

taabru45

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Captn.....you never fail to impress me......so many times I could have used you by my side, or rather I'd like to have been by you're side while YOU did it right... :lol: Steffan
 

capnjuan

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Hi Steffan: well ... I've had my share of screwups ... an old Dynaco ST35 with its can cap going off like a roman candle ... and more recently, an amp sitting here ... a victim of my well-intended meddling. Anyway, sometimes fixing this stuff can be just a matter of getting lucky :wink:
 

fronobulax

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Just to follow up, I connected the one lead and got the power light to light but no sound. At this point I don't want to mess with it until I have a permanent solution to the loose connection and that, in turn, depends upon getting a three pronged power cable and connecting it. Thanks for the help so far and you'll obviously hear from me when the project gets back to the top of my list.
 

capnjuan

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Hi Frono; I dig your new avatar 8)
 

cjd-player

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fronobulax said:
So it amuses me that the dining room currently holds 2 bass amps, 2 guitar amps, a 5 string bass, an acoustic electric bass, two Guild electric basses, a D-25, an non-Guild acoustic and a serious 88 key - " can play this on stage" keyboard.
Mmmm. Snack while you play.

Works for me. :D :D :D :D
 
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