My Crate TD70 blows.

[Bill_Nephew]

Well, my Crate TubeDriven 70 has started to blow its mains fuse. The first replacement I popped in blew even when I pulled out the output transistors, the TIP147 and TIP142. I am thinking it could be the rectifier diodes have shorted and screwed everything up, but that is just me. Has anyone that has had this sort of problem with this amp solved it? I am really scratching my head about this one because the fuse burns before I can even measure any voltages. All that happens is a sudden green flash and the fuse is blown. It is the 3A slo-blo fuse that goes between the mains live and the live primary of the power transformer. Schematic attached.

TD-70 Schematic.pdf

[mozwell]

Use a light bulb limiter, that way nothing will be burned if there are problems in the amp.
First, some simple things to check. Is the mains voltaeg switch set to the correct voltage for your area?

measure the four diodes D18, D19, D21, D22 with a multimeter, are they ok ?
measure teh two power transistors, Q12, Q13, are they ok ?
measure the speaker, is it shorted ?

or, disconnect the transformer secondary wires from the board, taking careful note where each of them goes. inulate the end of each so it will not touch anything when they are disconnected.
Power the unit up, does teh fuse blow?

If not, the the problem is elsewhere.
Start at one end & gradually work through....
Main suspects will be
1. blown transformer (unlikely)
2. shorted rectifier diode D18 - D21
3. shorted output transistor Q12, Q13
4. shorted speaker

Good luck

[Bill_Nephew]

I am pretty sure that it is indeed the rectifier diodes. My guess is that this amp was pushed hard to the point where the diodes were flowing reverse current back through the power transformer and thus blowing the fuse. Just because it wouldn't hurt, I ordered replacement output transistors and rectifier diodes. I also will go to the hardware store tomorrow to buy parts for a lightbulb limiter. I have heard of these being used for repairing amps but never knew what exactly it does. Now that I know (thanks google!), I am definitely going to use one just as a good precaution once I load up the board with the new output transistors and rectifier diodes.

[Enzo]

Well on't guess, get out the ohm meter and measure the rectifiers. They are shorted or they are not. Rare, but a shorted filter cap can cause this issue. The power transformer can be shot, that is extremely rare, but possible.

[Bill_Nephew]

So for the rectifier diode, it should read high resistance one way and low resistance the other way, right? Something weird is going on. With D18 and D19, the resistance gradually keeps on rising on my DMM no matter which way I configure the prongs. I have the ohmmeter at the maximum setting. D20 and D21 are getting 0.01 ohms both ways. Does that confirm that the diodes are doing something funky?

[Enzo]

No.

It blows fuses, that generally means something is a short circuit. If any diode reads like zero ohms or something small like 4 ohms, then it is probably bad. When you measure resistance in a circuit, you cannot ignore all the other parts around whatever you are measuring. Your meter has no way to exclude them. The ohm meter puts a small voltage into the circuit and reads the resistance by more or less using Ohm's LAw within itself. When you are working near the power supply, that small voltage tries to charge the filter caps, so that results in your slowly climbing readings.

Does your meter have a "diode test" function? The resistance each way approach is about as crude as it gets, and can easily be fooled.

[Bill_Nephew]

Wouldn't the diode test function be just as easily flawed when I need to test the diodes in circuit? If so, I will just desolder them tomorrow so I can get more accurate readings on them.

[Enzo]

The diode test is much less sensitive to all that. The resistance scales use a voltage l;ow enough it won't turn on a semiconductor junction. The diode test is specifically designed to use a voltage capable of turning on semiconductor junction. It reads out the voltage across said junction.

Shaorted diodes will read shorted with either method. The diode test is on meters for the purpose of testing diodes better than resistance scales.

[Bill_Nephew]

I checked the diodes, and all four of them are conducting both ways with the diode test. I also unplugged the secondary wires going to the output stage, but left everything else plugged in and turned the amp on. No fuse blowing, and the tube also glowed and warmed up. That narrows the problem down to the output stage supply or the output stage itself. I am going to check that for any problems. What should I check besides the defective rectifier diodes and the output transistor pair (TIP142 and TIP147)? As for the voltages coming from the secondaries of the transformer, here's what I got:

Orange Wires (To output stage supply) : 33vAC
Red Wire (To Heater Supply) : 255vAC
Yellow Wires (To tube filaments) : 6.3vAC

A couple of minutes ago, I also desoldered one leg of each diode off of the board to effectively take it out of circuit and tested it then. Two of them actually cracked and broke when I tried desoldering and the other two were read as open both ways.

[Enzo]

In that case, replace them. Diodes are cheap. Any cracked part is bad, regardless of how it might test. And any that are open are not working either.

While they are out, measure resistance to ground from the main + and - supplies. We don;t weant to see either shorted to ground. A shorted filter, or a wiring problem would be posible reasons for that.

Bad diodes can cause problems, but then problems elswhere can destroy diodes.

If it blew fuses even with the TIP142/147 removed, then the problem almost HAS to be in the power supply - diodes in particular - unless one of the main +/-V voltages is shorted to ground somewhere.

[bluesman123]

Hope This Scchematic Helps

Hi there a few months ago I was given this schematic for my TD70 and proved invaluable I hope it helps you isolate the problem as some of the other forum members have already stated power supplies are a good place to start.If you've already sorted out the fault great but save the schematic you never know you may need it again.

[Bill_Nephew]

Thanks man. I have been looking around for ages for the board layout. I already have the schematic though. This will definitely be what I need. I already ordered replacement diodes and output transistors, so the parts from Mouser will arrive on tuesday.

[Bill_Nephew]

Update: Today, i put in some rectifier diodes with similar specs, fired it up with the lightbulb limiter I built yesterday, and voila! No fuse blowing, and I am getting +40vDC and -40vDC across the diodes that I put in as replacements. The output transistors are still out. I am going to wait for the new pair of them to come in from Mouser. So far, I have also replaced the old nasty thermal compound with some good quality Arctic Silver to improve cooling reliability. Thank you so much Enzo. I really could not thank you enough.

[anubismaximus]

Hi there a few months ago I was given this schematic for my TD70 and proved invaluable I hope it helps you isolate the problem as some of the other forum members have already stated power supplies are a good place to start.If you've already sorted out the fault great but save the schematic you never know you may need it again.

they can publish the attachment again, someone has the design of the circuit board as it comes in the service manual,

thanks

[Jazz P Bass]

Here you go: TD-70 Schematic.zip