Gallien Krueger 800rb repair - No power

[beedoola]

I wanted to know how to go about troubleshooting this amp. It won't power on. How can I test the power transformer to see if that is the issue?

[DrGonz78]

A simple blown fuse will render an amp unable to power on, is it blown? Many suspects to investigate before jumping right into testing for bad power transformer. If fuse is blown then transistors or a bridge rectifiers top the list of suspects right off the bat. You got a schematic for this exact amp? Sure we can test the power transformer but first just have to ask if the fuse is intact. A bad transformer will surely take out fuses but many other things to consider first. If no fuse blown then it might a problem that deals with simple power connections or switches.

[Enzo]

It is the last thing on the list of possible failures, they are VERY reliable, transformers are.

However, transformers can;t function if they get no power, so is mains voltage appearing right at the transformer primary wires? Is there continuity through the mains wiring? In other words, unplug the amp from the wall, turn its power switch ON, and measure resistance between the two main prongs of the power plug. You SHOULD see the resistance of the transformer primary, not open. Also measure for continuity through the primary winding right at the transformer wires. If there is any issue, then go individually through teh fuse holder, the power switch, etc.

If all seems OK on the primary side, then see that proper voltages come out the secondaries, and no fuses open.

[nickb]

Schematics

Volume_1 - see page 14 for PSU. If the fuse is OK check for specified voltages on points A-G. If the fuse is blown, disconnect the PSU, replace fuse and use a lamp limiter before re-powering, then check those voltages again.

Volume_2

[beedoola]

So I picked up the amp today. Got it for $30.

The guy included an additional transformer with it. The one in the bad seems to be the original. There seems to be a new one in the amp.

The fuses included are 15A 250V. Are those right.

Upon opening the amp, the red wire from the transformer was disconnected.

The fuse holder only has only wire going to it - coming from the power switch. The Brown wire from the transformer went to the power jack (AC cord jack). The other end of the power switch went to the other lug on the power jack. And the 3rd lug on the power jack is connected to ground.

The rest of the wires from the transformer are wired to the main board - see last picture.

[beedoola]

I tested the resistance of the Red and Brown wires on the transformer and got 7ohm.

[DrGonz78]

I tested the resistance of the Red and Brown wires on the transformer and got 7ohm.

Well that's a positive sign there indeed. It is not open (OL) so the thermal fuse in the primary is intact too. The red primary wire not connected would appear to connect to the fuse but that is an assumption. However, 15 amp fuse is overkill and looking at the schematic it says 8 amp fuse. Have you built a light bulb limiter or have variac with ammeter? You are not going to simply plug straight into the wall to power up for the first time. Also, do some ohms tests on the "supposedly" damaged original power transformer, what is the resistance on it's primary winding?

Edit: BTW is that fuse holder just unscrewed or is it busted loose?

[beedoola]

I'll measure the other transformer.

I haven't built a limiter but I think I have everything at home to build one so I will.

I loosened the fuse holder to pull the transformer up to get a better look at how the AC connector was wired. The fuse holder is intact. I went and got some 7amp 250v fuses at Radioshack.

EDIT** I tested the other transformer - the one NOT in the chassis and it read 1.1 ohms.

[mozwell]

If this is a replacement transformer, there is a chance the output voltages may be wrong. Who knows what was used to replace the original one.
When you power it up, FIRST disconnect the secondaries from the amp & leave them floating. Maybe connect each one them into a separate pole of one of those 12 way screw terminal strips so you can be 100% sure they don't touch anything or short together.
Power the amp up through a light bulb limiter & measure the AC volts of each secondary and the primary. You can then check these voltages are correct before reconnecting them to the amp.
It would be a pity to damage something else in the amp because the transformer was replaced with the wrong type, when 10 minutes & a quick check can verify all is ok first.
To quote Enzo, never think of a reason NOT to check something.

[DrGonz78]

Also to mention that while you are checking the voltages of the replacement transformer that perhaps it would be wise to check original transformer too. You might get a surprise when it works just fine without a fault. Either that or just a very bright light bulb in your limiter. I bet that it is possible that someone thought just replacing the power transformer will get the amp to stop blowing fuses. If it ends up that way then what a score for $30!! Now if the old tranny is dead and the new one is not the correct one then that would be a shame. Either way you'll know soon enough.

Edit: BTW the readings of 1.1 ohms across the primary of the old tranny made me wonder if it is still good. At least the primary thermal fuse is not blown. Also, the new toroidal looks like it has two other black primary wires is that true? Or I am seeing other wires that lead up to front of the chassis power on switch.

[mozwell]

Ive never worked on one of these, but from the pictures The green wires seem to attach to the board through a hole in the metal bracket. It is VERY important that these green wires do NOT touch the bracket, and that the 4 leads from the bridge rectifier next to the green wires do NOT touch the bracket. If one of these is shorting to the bracket, this could be the only problem with the amp

OK
Use a light bulb limiter for all these tests.

Secondaries
green - black - green, these power the 100W amp with +/-60V rails, & provide the +/-15V power. I would expect 42VAC or so between either green & black, and 84VAC between the two green wires.
blue- black - blue, these power the 300W amp with +/-85V rails. I would expect 61VAC or so between either blue & black, and 170VAC between the two blue wires

Also, there are some diagrams for the power supply that show different primaries for 115V and for 230V mains
Page 20 shows wire colours for this, Brown, Red, Orange, Yellow
For 115V connect Brown & Orange together, these go to the mains switch. Connect Red & Yellow together, these go to the IEC inlet Neutral
For 230V connect Brown to the power switch, connect Yellow to the IEC inlet Neutral, and connect Red & Orange together (but don't connect them to anything). Insulate this connection so it cannot touch anything

It may be that the transformer primary has not been connected correctly, so "it wont power on", and someone thought to replace it to fix it.
Once we verify the transformer is correct, we can reconnect the secondaries to the amplifier & go from there.

Mains fuse is 8A for 115V supply & 4A for 230V supply

Before you reconnect the transformer secondaries, with a multimeter set to diode range, check BOTH the bridge rectifiers to see all the diodes are ok
If you can, measure the power transistors for both amps. We are looking for shorts between C and E on the power transistors. An easy way to do this is to measure from +85V to the 300W speaker positive terminal, and from -85V to the 300W speaker positive terminal. Also measure from +60V to the 100W speaker output terminal & from -60V to the speaker output terminal. You can use diode range, you should measure open one way & 0.6V the other way.

[beedoola]

The transformer is a Keen Ocean model TTO-17005-00.

Do I need to remove all the secondary wires to test them?

I saw the differences on the specs for the 115 and 220v power supplies.

This new transformer has a brown and red input wires.

When I wire the thing up to test it, the Red should go to one of the IEC pins and the Brown should be on the other side of the power switch - like in the diagram for the 115v wiring diagram.

[DrGonz78]

Do I need to remove all the secondary wires to test them?

Yes you need to remove all the secondary wires to test AC voltages. You do not want any voltages entering the circuits to the amp yet. Testing the other power tranny will be done in the same fashion and use a light bulb limiter at first just in case, at least on the questionable tranny.

Ive never worked on one of these, but from the pictures The green wires seem to attach to the board through a hole in the metal bracket. It is VERY important that these green wires do NOT touch the bracket, and that the 4 leads from the bridge rectifier next to the green wires do NOT touch the bracket. If one of these is shorting to the bracket, this could be the only problem with the amp

Yeah good eyes!! Yeah those green wires need to be installed much better than they are and definitely not touching the chassis. I did find a picture online of GK 800rb that has some sort of insulators in those holes too.

[J M Fahey]

I chimed in to answer bedoola but see he has not indicated where he lives, so I canīt.
Sorry.

[beedoola]

Dr. Gonz

That black wire you see in that pic isn't part of the primary. Just the red and brown. That black wire is from the power switch.

The transformer that is in her appears to be the original or the original type. The one that came with it looks older but has a different mounting/bolt pattern which isn't reflected in the chassis. Perhaps someone got one from another older 800rb thinking that it could be put in the current transformer's place.