GK 250ML Blowing Fuse

[sweatyk]

Quick story. bought it 15 years ago and it worked but had a slight hum in it. Took it to a shop and they tried to work on it but managed to hand it back to me not working. Tossed it in my closet until last month. Decided that I want to see if I can get her running again. When it popped the fuse I checked out the power supply section. Found one burnt resistor. R163A which was 10 ohms. Replaced it and still blowing the fuse. Started poking around in the dual amp section looking for shorts. On one side I found a few shorted transistors a one capacitor reading 7 times more than its rating.

Q17 - TIP34C - Shorted
Q13 - TIP33C - Shorted
Q14 - MPSA56 - Shorted
C48 - 10nf - Reading 70nf

I'm a novice/hobbyist with electronics. Looking at the attached section of the schematic I have highlighted those parts. I'm wondering if anyone here has previous experience with these amps and what else I should be looking at with these faulty parts.

[Enzo]

FIRST: STOP blowing fuses. Every time you do that you risk further damage. Look up light bulb limiter, make one, and use it.


Resistors never burn up on their own, it takes current through them, most times a dead semiconductor of some sort.

If Q13,17 are shorted, it is a good bet Q12,16 were stressed, and I would replace them, regardless of how they test with a meter. And with bias transistor Q14 shorted even more likely.

Make sure the two 0.33 ohm emitter resistors on the outputs are healthy.

Right by the speaker jack is 5 ohm R124,make sure it is not open.

As to the resistor in the power supply, It makes suspect Q32,33.

[sweatyk]

FIRST: STOP blowing fuses. Every time you do that you risk further damage. Look up light bulb limiter, make one, and use it.

Thanks Enzo....I should have included more details in my post. I only blew one fuse. I do have a light bulb limiter and have been using it.

I will include the other transistors when I place an order. I did check the .33 ohm resistors and seem to be healthy.

Thanks again

[g1]

schematics & service manual:

[sweatyk]

Thanks g1......I do have that already. Not great at reading them but can usually muddle my way thru them.

[g1]

Thanks g1......I do have that already.

Just for anybody following along. Enzo mentioned a couple parts not shown in the snippets so he must either have a copy or know this one off by heart.

[Enzo]

Don't you know all the GK stuff by heart?

Yes I have the GK service disc, which covers tons. I have paper copies in my stored files somewhere.

I love those little amps


In some odd way they make me think they are the amp version of a Scholz Rockman.

I have a head version of that amp, it is about the size of The Art OF Electronics.

[sweatyk]

Enzo being you are familiar with this amp I have a question. On the schematic in the power section Q33 and Q35 do not match what is in my amps, ( I have 2 of these amps) The current amp I'm working on is amp #1.

Amp #1 Q33 - TIP32B
Q35 - TIP31C

Amp #2 Q33 - TIP32B
Q35 - TIP31B

Schematic Q33 - TIP30C
Q35 - TIP29C

Is this quite common?

The second picture is the one I'm working on. You will notice the zeners and a capacitors lifted out of the circuit. I was doing that to see if the short was coming from there before I found the output transistors on the one channel shorted. They will be reinstalled once I have the parts I'm going to replace.

[loudthud]

The difference between the TIPxxB and TIPxxC parts is the rated breakdown Voltage. If you have a way to test breakdown Voltage, you would find that many of the TIPxxB and even TIPxxA parts meet the higher TIPxxC part's spec.

[g1]

And the Tip31B & 32B are 3amp 80V spec. Tip29C & 30C are 1amp 100V spec. I don't think you are over 80V there so shouldn't be a problem. Also, what loudthud said, lower voltage rating parts often meet the higher spec anyway.

[52 Bill]

Those two transistors are the pass transistors for the +&- 16 volt supplies, so the lower voltage versions should be safe. I remember that back in the day when we were a GK dealer, I found that the pc board for one of these supplies was done wrong. I can't remember which one, but it had a cap in the wrong place or something. I reported it to them but I don't know if they ever changed it.

[sweatyk]

So my question is......do I replace parts based on the schematic or what was installed? Being it functioned and some point in its life I would vote for what was in it......unless someone has had there hands in it at one time. Which is the case for this one.

[52 Bill]

I will always opt for the parts that were specified on the schematic, unless they are no longer available. At that point I would then substitute parts of equal ratings and specifications.

There are many reasons that a tech will substitute parts in a repair job. I always assume that unless there is reason to not trust the schematic, the factory knows what parts will perform best in the circuit.

[Enzo]

I am hesitant to say match the schematic. Your schematic could be earlier than your amp version, and you wind up putting parts in it to set it back to an earlier form. A form that MIGHT be less stable. Many time designs are changed to improve reliability. For example R163a and R64A were added parts, older schematics won't have them. Someone with a newer amp and an older schematic might then decide to remove those resistors, making the amp LESS reliable.

Meanwhile, if your concern is the B versus C version of a TIP transistor, as was explained above, in your amp it shouldn't matter.

[sweatyk]

Been looking online to source the parts. Some are still available but the ones I'm having trouble finding is the TIP34C in a TO-247 case. Also both the MPSA56 and MPSA06 are obsolete. Anyone have bulk stock of these that wouldn't mind selling a few of them? I'm hesitant to buy off of eBay....been burnt there before with counterfeit parts and also had bad luck using NTE brand transistors.