Gallien Krueger 800rb repair - No power

[52 Bill]

For what it's worth, I had a GK400RB with a blown output transformer and GK sold me a kit very similar to yours but small
Also, now I have an 800RB with what appears to be exactly the same kit(which includes bottom panel and bolt attaching the transformer) that full compass sells. Although I haven't gotten around to the repair.

Quite a few years ago GK switched all of their transformers to torroidal types. In fact for a few years they had no replacements for the old style transformers, so a lot of amps went unrepaired unless you could source a replacement from Hammond or Plitron. These new kits include new bottom panels for bolt mounting the new transformers.

As for the low voltage readings, are you using a lamp limiter?

[beedoola]

52 Bill - I'm not using the limiter during the voltage readings.

I opened it up again just to check and I'm actually getting more like 68v on the 100w section. Still around 76v for the 300w section - measuring at the Power Supply board outputs to the PA board.

Also. I measured the AC voltage of the secondaries against and they are now measuring about the same voltage - 50V going into rectifier bridge.

[mozwell]

I stand to be corrected, but here is what i would do.
The issue is we have 66V on the 63V capacitors. This definitely needs to be corrected. We could do things to drop the DC voltage, but this would severely impact the performance of the 100W amp

Both power amps use the same transistors, the 300W amp uses 2 parallel transistors for each rail, whereas the 100W amp uses one transistor for each rail only. So we have no problem with the voltage rating of the transistor with a high rail voltage on the 100W amp.
The 15V power supply is fed from the 66V rails. TIP29C is rated for 100V so we have no problems here. As the rail is higher, we will dissipate a bit more power on the transistor, but as we are only up 6V this should be ok.
This leaves us with the voltage rating of the 470uF capacitors.
I would just change the four capacitors to 100V types, making sure you get the polarity correct when you change each one.

Set / check bias current for both power amps, check there is no DC offset on the output of each power amp & we are done.

Note, as we have higher than normal rail voltage on the 100W power amp, we will get more power out of it. This will heat up the single transistor on each rail more, so you need to be aware of this. We dont want to blow up the 100W when you crank the amp up.
I would only use an 8 ohm or 16 ohm load on the 100W amp output.

If you only have one speaker, then plug it into the 300W amp only.

DrGonz
41V in a rectangular box, according to the sch, means RMS voltage, refer note 4, bottom of the power amp sch page. It would be easier to write 41VAC or V~ or Vrms.....

[J M Fahey]

Short answer without the usual half page of babble, just woke up early (4 am here) to finish a rush job:

1) put higher rated caps there, 70/80/100V ones, whatever you can find fresh and new at Mouser or some reputed supplier

2) I´m not happy with a +/-65V rails amp and just 2 power transistors being loaded with 8 ohms.
For "100W into 8" outputs such rails should be around +/- 45V or thereabouts, +/-65V is murder.
It would be fine into 16 ohms and provide rated 100 W

I *guess* that was acceptable if used as a HF section in a biamped setup because HF would be intermittent (think a slapper Bass player) but I imagine a regular player plugging an 8 ohm full range extension cabinet there and driving it hard ..... it *will* work, no doubt, but too close to the edge of the cliff.

On the contrary, the 300W side is fine.
+/- 85V rails is still overkill but at least they use *three* pairs of transistors there.

Such high voltages are fine if PT drops under load, and was probably the case with the first EI lamination ones, but toroids are stiff buggers; idle voltage should have been lowered by 5% or 10% .

I´d *love* to see actual measured output power at clipping using this transformer: the 300W section into 4 ohms, and the 100W one into 8 and 16 ohms.

[beedoola]

bumping this as it died again.

I had tightened the screws holding the power transistors casings to the chassis/heat sink and that solved the issue.

It would for a week or so and then died.

I checked the bridge rectifier and it tests good. Having the amp plugged into my light bulb limiter I'm getting a short.

How do I go about testing the power transistors - the MJ15023 and MJ15022s?

[DrGonz78]

You have a multimeter right? Set it to diode mode and check the legs on the transistors. Transistors have diodes in them so one way it will read OL for open line and the other way it will show a voltage drop. Usually 450 - 650mv or something like that is normal. If it is short it will read 000mv or something like 019mv. Do a search on Google to really get so more info >>
Testing transistors music-electronics-forum

[beedoola]

Can they be tested with just the pins desoldered or do I need to completely remove each transistor from the PCB?

[The Dude]

Test in circuit first. If you measure shorts, unsolder and check them out of circuit. Output transistors are generally paralleled, so if one is shorted all will measure shorted.

[Enzo]

And I like to add, there is nothing in a circuit that can make a shorted transistor appear not shorted.

[beedoola]

I’m getting a voltage drop measuring both ways - getting .375v

Removed the transistors and outside the PCB they all test good: OL and .513v or so.

If I measure the points on the PCB where the transistor pins connect I still get that .375v either way I measure it.

Thoughts?

[beedoola]

Anyone?

[nevetslab]

I’m getting a voltage drop measuring both ways - getting .375v

Removed the transistors and outside the PCB they all test good: OL and .513v or so.

If I measure the points on the PCB where the transistor pins connect I still get that .375v either way I measure it.

Thoughts?

You're getting this 0.375V using your DMM's Diode Test Function (or Ohmmeter in the 2k range, which is a 2V source and 1mA constant current source), connected between base & collector, base & emitter? First measure a known good xstr. NPN power xstr. B-E and B-C should measure about 0.48V to 0.53V one way only. C-E will be open, reversing the leads so BLK is on the base, RED is on the emitter or collector....that will measure open.

I'm not sure where you are measuring on your circuit, as you didn't specify. On the output xstrs, measuring B-E, you'll be seeing a lower value, since there is an emitter resistor from the driver stage across the B-E of the outputs.

We need to know WHERE you're finding this 0.375V in both directions with the xstr(s) out.

[beedoola]

You're getting this 0.375V using your DMM's Diode Test Function (or Ohmmeter in the 2k range, which is a 2V source and 1mA constant current source), connected between base & collector, base & emitter? First measure a known good xstr. NPN power xstr. B-E and B-C should measure about 0.48V to 0.53V one way only. C-E will be open, reversing the leads so BLK is on the base, RED is on the emitter or collector....that will measure open.

I'm not sure where you are measuring on your circuit, as you didn't specify. On the output xstrs, measuring B-E, you'll be seeing a lower value, since there is an emitter resistor from the driver stage across the B-E of the outputs.

We need to know WHERE you're finding this 0.375V in both directions with the xstr(s) out.

Yes, getting the .375v on the diode function.

Testing each transistor (also in the diode testing mode on the DMM) out of the circuit all tests good - OL in one direction and .513 or a little above on every transistor.

Using the DMM in the diode mode and touching the pads on the PCB where the transistor connect shows the .375v reading - regardless of the orientation of the probes.

[nevetslab]

Yes, getting the .375v on the diode function.

Testing each transistor (also in the diode testing mode on the DMM) out of the circuit all tests good - OL in one direction and .513 or a little above on every transistor.

Using the DMM in the diode mode and touching the pads on the PCB where the transistor connect shows the .375v reading - regardless of the orientation of the probes.

Have you checked diodes D11, D12 (flyback diodes on the LF amp, D5, D6 (flyback diodes on the HF amp)? Also Limiter connect diodes D8 & D9 on LF amp, D2 & D3 limiter diodes on the HF amp, along with D10 in the emitter circuits of the drivers for LF amp, D4 in the emitter circuits of the drivers for HF amp.

I've attached the PCB parts placement guide for the power amp below.

800RB_SM2.pdf

[The Dude]

That link is dead nevetslab. And, while we're at it, there is no valid schematic link in the thread and there are different versions of this amp. If you want accurate help, beedoola, it'd be best to post a schematic of your amp. Without it, we have no idea what you might be measuring.