Fender Sidekick 35-Reverb Troubles

[Gokarna]

Hi,
I bought this amp with a blown transformer and installed the one from a Sidekick Reverb 30. As I Remember, the voltage outpot 2 x 24 volts.

The amplifier worked some time and the failed again. I figured out, the a small transistor , 2SB560F (Q9 on the pcb) was charred, i changed it with a 2SB560E (little lower max. current gain [h(fe)]), the amp worked again and failed, now I changed the transistor to a BC560, which is likely a substitute for the very hard to get 2SB560(F). I tested the amp for maximum 10 seconds it only make a very ugly hum/buzz. I found out, that the 2 big 0.5 Ohm / 5 W resistors get very hot (after just 10 sec).
Any suggestions ?

I also have a copy of the schematic here, but no scanner. I'll try to make a scan tommorow at work and post it then.

Thak you very much for your help,

Regards
Till A

[Enzo]

Yes, I think your small transistors are not the main problem. You need to check ALL the transistors, especially the output transistors, and make sure no resistors are open. ANd disconnect your speaker load until you are sure there is no DC on the output and the unit is not drawing excess current.

[Gokarna]

Hello Enzo,
thanks for your answer, just some more questions for understanding (My english isn't that good)
You mentioned the output-transistor. Do you mean only the two (big) ones, bolted to the cooling element. They've been checked by the local tech and he said , they are working. But I already have another pair, so I could change them.
Or do you mean all transistors in the signal path.
The complete circuit contains 9 transistors and, as far as I figured out, there all easy to get except the one that blew up before.

What wrote about "open" resistors. What does that mean, resistors, that are broke or not well connected.

I thought of resoldering the two "hot" resistor mentioned in my question and aso trying to measure the rectifier diode, it's called DBA-40C. Do you think, it is possible to replace this part with a self-made rectifier circuit, made out of four N4007 diodes ? Or is the whole idea about this nonsense ?

Thank you very much again,

best regards

Till A

[Enzo]

When there are only 9 transistors, they yes, I woulod check every one of them.


A resistor is a path for current. It has some value of resistance, 100 ohms 10,000 ohms, or whatever. Open means somewhere inside the resistor that current path is broken - there is an opening in the pathway, a hole if you like. An open circuit is a circuit that does not complete the path. A burnt out light bulb is open. An open resistor is the same as not having the resistor - a missing resistor.

A resistor broken in half is certainly open, but the internal resistive element can break without changing the exterior appearance of the part.

If Q9 keeps burning out, it is not because it has the wrong gain, it is not because the part is under rated. It burns up because too much currernt tries to flow through it. SO check R45 - is it close to 200 ohms? How are Q8 and R43?

If Q11 is bad - shorted or open - it could force Q9 to burn up.

Q7 is the bias transistor, it keeps the bases of the two polarity sides from turning on hard. Make sure Q7 is OK< and also the string that sets its operating point - R40, D6, D7, R41. And while you are there , might as well check the ends of that string - R39, R42 with Q5, Q6,

I tend to doubt that your rectifiers are bad - they would blow fuses.

[Gokarna]

Hi Enzo,
whow !!!!, that is a very fine instruction, thank you very, very much !

So I checked all 9 Transistors and all the resistors you mentioned, plus the two big 0.5 Ohm 5W resistors.

The result is:

-the 2SD438F is also blown

-the two 2SD613E transistors (Q10 & Q11) work, but show different results measured with a common multimeter: 318 / 110.
According to the data, this values are within; or do you disagree ?

- there was a very big surprise, when I checked the four 2SC1816GR (Q 2,3,4 and 6):

First the showed no result on the multimeter, but the I staked them reverse into the multimeter and all showed good results.

So I checked the datas on the Internet and to my surprise, I saw, that there are two different kinds of data about the 2SC1815GR. One is like shown in my own data, the other has reversed pins.
I didn't know, that this is existing !!!

Can I assume, that these 4 transistors were installed correct (there is a printed sign on the pcb), because the amplifier was working or should I also check the schematic before reinstalling them ?

- I can't check the 2SK30GR (Q1), it's a mosfet-transistor. I first have to build a little electronic circuit for that. I have a spare transistor marked 2SK30-Y, can I also install that one in case the other one is broken ?

- All the resistors show exact results, only the two big 0.5 Ohm / 5 w resistors on position Q46 and Q47. the are measured both at 1.1 ohms.
My two multimeters (Voltcraft VC444 and GBS- MAS 830B) are know high-quality multimeters, so I think, this two resistors are also ok,
do you agree ?

- I haven't checked the diodes, will do that later the day.

Again, thanks for you fantastic answer,

best regards

Till

[Enzo]

-the two 2SD613E transistors (Q10 & Q11) work, but show different results measured with a common multimeter: 318 / 110.
According to the data, this values are within; or do you disagree ?

A little difference in gain won't matter. SOlid state amps are so heavilt fed back, they will correct any distortions from it. All we care about if that the transistors work - they are not shorted nor are they open.

2SC1816 and 2SC1816R - the R is for reverse lead order.

- I can't check the 2SK30GR (Q1), it's a mosfet-transistor. I first have to build a little electronic circuit for that. I have a spare transistor marked 2SK30-Y, can I also install that one in case the other one is broken ?

It is a JFET, not a MOSFET. Any 2SK30 you have will work. I doubt it is bad. You can remove it and the amp will work without it. Its function is a mute to keep the amp quiet for a moment when first powered up. Out of the circuit, JFETs will have a relatively low resistance between the source and drain legs. I don't know, maybe 100-200 ohms or so. The gate lead will act like a diode connected to the other two. SO it is normally ON until a voltage at its gate turns it off.

only the two big 0.5 Ohm / 5 w resistors on position Q46 and Q47. the are measured both at 1.1 ohms.

What do your meter leads measure when touched together? Subtract that from your 1.1 ohm reading. Again, all we are checking is if the resistor has burnt open or not. They are wirewound resistors and unlikely to be wrong in value.


And 2SD438. 100v 700ma rating. I have a drawer for that part, but it is empty. If I were looking around hte shop, I might sub in a 2SC2335. There are probably other subs. Or just get the right part.

[Gokarna]

Hello
today, the spare parts arrived and I installed the two new transistors (Q8 & Q9).
The amp is humming very loud, makes sound again, but not for very long, then the sound starts to get thin and crackling.
I've only let the amplifier run for about 20 to max. 40 seconds

The two big 0.5 ohm/ 5W -resistors (R46/47) get VERY HOT after just a view seconds and also the two power-transistors 2SD-613E (Q10/11) get hot.
I think, also the 10 ohm / 0.5W resistor (R48) gets hot and the cap next to it(C 25), too. But I'm not really sure about this, because they're very close to the two big resistors.

I also found a little burnt spot near the diode D13, very close to the big Cap C31 and I never soldered there - could that be the reason, why the amp is still not working as it should ?
The Diode D10 - D13 are called GZA7.5., what usual type can I use as substitude ?

Thanks for any answers and ideas,

best regards,

Till A

[Enzo]

The power transistors get hot because both are being turned on at once - you have more wrong in the amp than just those apparently.


if you don;t know what some part is, it is usually a good idea just to google the part number. That would tell you the GZA7.5 is a 7.5v half watt zener diode.

Check those diodes with a meter. Are they shorted?

[Gokarna]

Hi,
I'm back from holiday and worked again on the amp and...
now it works nearly perfectly !

I checked all the resistors from R31 to R55, all diodes from D5 to D13 and the rectifier-diode, and everything was okay, so I nearly gave up.
But then I took again a closer look on the 2SD613E transistors and their (wiring) connection – and saw, that the technican, I gave the amp to, left a real mess there.
Some of the copper was melted of the pcb, the base and of collector (Q11) were soldered together on the main pcb. I resoldered everything, cleaned it, checked Q11 again; it was bad, so I put a new one in and soldered everything together again.
And it workes well, it’s quiet loud for such a small amp. I only have to check the two inputs for the guitar again, on of them is not wired correctly.

Thank you very much again for your help, Enzo!!!

I couldn’t have done it without your support and I learned quiet a lot about reading schematics and the relationship between the electronic parts.
AND not to give away broken down devices to people, who claim to be Pro’s (the guy closed his shop last month, I’m not wondering about that fact after this experience !)

Thanks again and keep on….

Best regards
Till / Germany