250rl gallien krueger questions

[9 volt]

Hello, I'm restoring a 250rl amp head need a little guidance. Can this amp be used without a speaker load? (I'm used to valve amps e.g. have to have a load). One channel hums (volume has no effect) at this stage I suspect it is dc (blown output transistors possibly). I have read that I can test the output transistors to see if they are faulty- e.g. DMM will probably read open across all pairs of terminals. Am I on the right track? Thanks

[Enzo]

Solid state amps do not require a load. In fact if they are making DC you most definitely DO NOT want a load on there.

Channel hums? You SUSPECT DC? Don;t suspect, get out your meter and FIND OUT. DC on a speaker can only damage the speaker, plus it stressses the amp and can cause further damage.

DC on a speaker will move the cone one direction and it will stay there. It usually hums loud in the bargain. A loss of a filter cap also makes loud hum, but there will not be a large DC offset.


If your output traqnsistors are bad, and you have DC on the output, it is FAR more likeloy you will measure terminals shorted together rather than open.

ALmost all DMMs have a diode test function. We use that to check transistors for basic function. They all work about the same, so the owner manual for most meters will explain it.

[9 volt]

Thanks, I did a quick check with the dmm and have a reading of 17 dc volts on the faulty speaker output.....I will open it up tomorrow night and have check the transistors and power capacitors. Given the size of the DC voltage I figure the problem will be the output transistors rather than the filter cap. Thanks again for the fast reply!

[9 volt]

I had a look inside tonight, the main filter cap for one channel is dead, a few resistors have blown/melted and I tested the power transistors with the dmm diode test and they are faulty. The other (left channel) works so I am able to use it for comparison. There are about five main transistors in each output amp channel so I think I will will replace them all in the side that has blown.

[g-one]

Sounds like a good plan. Having the good side to compare to is always helpful.
One thing I will mention is the lack of adequate heatsinking. If memory serves, many of this model burned up from overheating. Make sure the back panel is adequately ventilated when in use. If possible, point a fan at the rear panel.

[9 volt]

Well, I finally got all my blown transistors and resistors replaced (and also a few big caps since I was in there). On the dmm all looked good (I'm using a step down transformer 240 to 115). As soon as I plugged it in I blew the fuse in my step-down.....and then another one. No power seemed to reach it. The only thing that I can see that seems irregular is that the tip33c I used was not insulated on the top e.g. the metal part of it connected the the body when screwed in. The one I took out (which tested faulty had an insulated top. As a test I removed the screw and moved the top of the transistor away from the amp frame, when I did this the lights on the front of the amp lit up for a little while then blew my last step-down fuse (which was only .5 amp ( as I ran out of 1 amp fuses). So I suspect this may be the problem but can not do any more testing until I get more fuses. Has anyone heard of two types of tip33c. Thanks

[Steve Conner]

Yes, there are two types. You've caused a major short circuit, and probably blown the power transistors again, not to mention a bunch of other parts. Get some new transistors (and insulating washer kits if you've bought the uninsulated transistors) and repeat from step 1. This time, test continuity between the transistor tabs and the heatsink before applying power.

[pontiacpete]

Wouldn't it be better to use a light bulb limiter to save on fuses?

[9 volt]

bummer......oh well got a learning curve happening. Should have ordered a few spare transistors as it'll be another few weeks (mail) before I can get some. I did a continuity test and even with the tip33c moved away from the amp case I still have continuity. Could this be because there is also a tip29c and tip30c connected to the amp case? These are the only places I can see that connect to the case.
Also, While I am waiting for the new transistors to arrive is it possible to run the amp with the tip33 and tip 34 removed? (trying to apply my valve knowledge to solid state). Prior to the repair I had one channel and prep working and now just blown fuses. Wondering if it's possible to test the other parts while waiting new transistors to see if I've caused any other damage.
Thanks again

[audiopete]

If you have continuity between power rail and ground there is a short somewhere else and the fuse blowing would confirm this. If the tip29c and tip 30c are factory installed then there shouldn't be an issue with them grounding unless they are internally shorted as well. The tip33 is definitely connected internal pin 2 to mount base - http://www.bourns.com/pdfs/tip33.pdf

[9 volt]

Got it, I'll rechecked my soldiering (though it looks fine). Ordered new ic's, sounds like I blew the new one (internal short- makes sense), hope I haven't damaged the other channel. Thanks

[9 volt]

Well the good news is my transistors finally arrived. When replacing them I noticed that both the tip33c and the tip34c needed to have an insulator put on. I've powered the amp up. No ore blown fuses!. The left channel is working fine, the right ( the one I have been working on) works well (clean signal for about a minute) then begins to get distorted. To my understanding something (diode, transistor or cap) is not handling the voltage. In this situation would it be best to do voltage checks or signal trace? Thanks again

[9 volt]

Today i did a signal trace. The signal is clean up until the base of the the tip 33c and tip34c. So this narrows down where the problem could be.

[pdf64]

From memory, I think these have some feedback derived from speaker current, so a nominal load would be needed when testing for them to operate normally.
Pete

[9 volt]

Thanks, I've just discovered that one of the a .33 5watt resistors that connects to the emitter leg of the tip33c is open, which would have to effect the bias? Slowly getting there. There is a feedback loop. Thanks, have tested with speaker, but wanted to determine where distortion was kicking in. (hence using probe)

[Jazz P Bass]

Thanks, I've just discovered that one of the a .33 5watt resistors that connects to the emitter leg of the tip33c is open, which would have to effect the bias?

The .33/ 5 watt resistor is what connects the transistor output lead to the speaker.
It is refered to as a 'ballast resistor'.
When transistors are run in parallel, a ballast resistor helps current sharing between the paralleled devices.

[9 volt]

Thanks for the advice, this is beginning to make sense! There is a small electro cap in the feedback loop which I will replace also!

[9 volt]

Well, I replaced the .33 resistor, rechecked TIP output transistors and am still blowing again. The grounds rail shows continuity with the case.....and the B rail shows a continuity like when testing a capacitor eg slow growing continuity..........starting to lose sleep!

[52 Bill]

Well, I replaced the .33 resistor, rechecked TIP output transistors and am still blowing again.

The ballast resistor could have acted as a fuse and opened up when the related power transistor shorted. Double check the transistor that the replaced resistor is connected to.

When you had the amp up and running did you set the bias on the repaired right channel?

[9 volt]

No, I haven't touched the bias trim pot, though I did have to replace the transistor that goes into the bias trim. I have replaced the both power transistors again I have built a light bulb limiter some will use this next time i test. The diodes that run parallel to the power transistors seem to test ok but I will pull one end tonight to make sure they are ok.

[9 volt]

Well tonight I found the transistor that was popping fuses. It was an mpsa56 which tested ok on diode test but was shorted from c to e. I had it up and running for about two minutes with no distortion then one output transistor baked. I haven't tried to biase the channel which may have been the problem?

[9 volt]

ok, today i finally got it running and biased the transistors, they were sitting on 15mV and are not 5mV. Ran a test (sound through it for a minute) and seems good. Are there any other tests I could do to ensure it will not blow when on for a longer period of time (rather than just seeing what happens)? Would not recommend working on one of these without abit of solid state experience, so compact and complex compared to tubes! Thanks for advice so far hope this will help someone out in the future.