Thermal Switch Kicking On/Off (Peavey XR-600B series 400HB)

[kevtronic]

Hello;

I have a Peavey XR-600B series 400HB that was blowing fuses after a couple hours running. I replaced all the P600D diodes in the power section as a couple were reading low, so I figured might as well change them all out while I was at it. I also replaced the two 1000uf electrolytic caps in the power section as the values were fairly far off and the esr was testing about twice as high as my meter states they should be. Now, after running about 3hrs it doesn't blow the fuse, but the thermal switch starts kicking on and off. The actual load on the speaker jacks themselves was 3 ohms per jack for the testing period.

I'm hoping someone may be able to guide me through bugging this issue out whether I'm on the correct track or not...

I've attached the schematics for viewing...

Thanks...

[Enzo]

Running hot either means the amp is overloaded or underbiased, or something is leaky.

Isolate the problem. I doubt the preamp is involved. Unplug the umbilical to the front panel so the PA runs alone.

Is there any DC offset on the output? I don't care about a few millivolts. DC on the speaker will run the amp hot. If ther is no DC there, fine, but leave th amp unloaded.

Measure the mains current draw - that is easier than trying to measure draw off the power rails. At idle I'd expect well under 50 watts. if it is substantially higher, that points to bias. yes, I know it is not adjustable. But we can fix it.

The peavey schematic has plenty of voltages on it, so go check them. In the center of the drawing is CR13 in the dashed line, that is the dual diode on the power transistor board. To me, it divides the schematic - I refer to the left half and right half. Unless there is DC on the output, then any excess current problems are most likely to be on the right side.

[kevtronic]

Thanks for the quick response and the info.

Is there any DC offset on the output?

When the amp first turned on I was getting a fluctuation between about 35 to 45 millivolts.
It dropped and stabilized to 21.5 millivolts after a minute or so...

leave the amp unloaded

Hopefully this isn't too stupid a question, I just want to make sure I'm doing things correctly, but by "unloaded" do you mean no speaker load or no input load? I tested the DC offset with the preamp disconnected and with no speaker load...

Measure the mains current draw

Do you mean the current draw through the power cord before the transformer? If so, is a clamp type ammeter OK for this test or should I break the connection and do the inline/series current test? Testing the "mains current" on an amplifier is a first for me...

Thanks again for the help, it's much appreciated...

[g1]

Yes, unloaded means no speaker connected.
And yes, current draw before the power transformer.
If your current clamp is accurate at low current levels (less than 1 amp), then that will work. Otherwise, you can remove the fuse and connect your current meter in line there (assuming it is fused so you still have some protection).

[kevtronic]

Yes, unloaded means no speaker connected.
And yes, current draw before the power transformer.
If your current clamp is accurate at low current levels (less than 1 amp), then that will work. Otherwise, you can remove the fuse and connect your current meter in line there (assuming it is fused so you still have some protection).

I removed the fuse and connected the amp meter there.
I got 120.7 vac and .325 amps, so my power is about 39 watts...

[Jazz P Bass]

So the amp draws 39 watts at idle.

Sounds about right to me.

On your 3 hour/ 3 ohm test, what power level did you run the amp at (output power at the load).
The amp output is rated at 320Watts RMS down to 2 ohms.
(That's a lot of heat to dump without a fan)

The thermal cycling may be an indication that the circuit is working.
(The breaker will remove all power to the amp. No lights, no nothing)
So what temperature is it kicking in & out at?
The breaker is rated at 84 degrees Celsius (184F).

XR600B.pdf

[kevtronic]

So the amp draws 39 watts at idle.

Sounds about right to me.

On your 3 hour/ 3 ohm test, what power level did you run the amp at (output power at the load).
The amp output is rated at 320Watts RMS down to 2 ohms.
(That's a lot of heat to dump without a fan)

The thermal cycling may be an indication that the circuit is working.
(The breaker will remove all power to the amp. No lights, no nothing)
So what temperature is it kicking in & out at?
The breaker is rated at 84 degrees Celsius (184F).

[ATTACH]35365[/ATTACH]

Thanks for the info and input...
I set it up with 3 ohm 200 watt resistor loads on each speaker out. I wasn't measuring the output power and I haven't kept and eye on the temp of the breaker. But, I have to say I think I should re-test doing this. When re-doing the test as stated above should I have the pre-amp section connect or disconnected? If the preamp section is connected are there any settings you would suggest or possibly running a signal?

[Jazz P Bass]

Two 3ohm loads is 1.5 ohms.

Yes, of coarse hook in the preamp.

Run a sine wave, 100mv's/ 1K, into any input.
Use one of the 3 ohm loads.
Crank the output until it clips.
At that point the DDT circuit should engage.
Measure the Vac voltage on the load.
V x V /Load = Power

I think the amp is fine.

[kevtronic]

Your right. Duh moment here on my part, as the output jacks are wired in parallel and one or both jacks can be used. Well, I will crank it up some time this weekend and retest it. I hope you are also correct in that the amp is fine. I will post the results after I test it. Thanks again for the help and info...

[kevtronic]

I need to get my self both a signal generator and an oscilloscope. I've been putting it off, but I think it's time to take the plunge. I am on a budget, but I also want to make sure I purchase equipment I can use for the long term repairing amplifiers, etc. I would prefer equipment not hooked up to a computer or phone. At the moment for signals I use cd players and software tone generators.

[Enzo]

Clearly then you did overload the amp, and it probably is OK and was thermalling as designed.

A note: sine waves are great for looking at gross distortion of for clipping, nut it is an unrealistic signal for the amp. A sine wave is hard on an amp. Look at music, it is a lot of lower level stuff with peaks on the beat like kick drums. Sine wave means ever half cycle is at peak output continuously. Music is just peaks with rest periods, so to speak. If you want to run extended testing, either use music maybe 2/3 power, or sine wave at reduced power. Oh short term isnt an issue, but a couple hours at ful out sine wave ought to thermal a lot of amps.

[kevtronic]

Clearly then you did overload the amp, and it probably is OK and was thermalling as designed.

A note: sine waves are great for looking at gross distortion of for clipping, nut it is an unrealistic signal for the amp. A sine wave is hard on an amp. Look at music, it is a lot of lower level stuff with peaks on the beat like kick drums. Sine wave means ever half cycle is at peak output continuously. Music is just peaks with rest periods, so to speak. If you want to run extended testing, either use music maybe 2/3 power, or sine wave at reduced power. Oh short term isnt an issue, but a couple hours at ful out sine wave ought to thermal a lot of amps.

Thanks for the info Enzo. I didn't use a sine wave as I don't own one that's not software based. What I did do is use a BB King CD and set the input, by chance, at about 2/3 for about 5 hours. Now that the amp has a proper load it hasn't hiccuped as far as I can tell. Here is what I was getting in the last half hour off the output voltage wise: DC millivolt from about 4 up to 31, bouncing all over. The DC usually ran between about 4 and 21 mv. AC volts from about 4 to 12.5 (bouncing all over). Occasionally, not too often, both the DC and AC would go OL...

Thanks for all the help. Now that the fuse blowing and thermal issues are resolved I'll be doing some sound quality testing...

I really want to get and O-scope and signal generator pair. A decent pair for mainly working on amps/pa's. Although I do mess with other electronics. However, it just seems confusing on which on to spend my budgeted money on. I want to get it right the first time. Something I can put on the bench, learn to use properly, and continue to use repairing amps/pa's etc. I know this subject has been beat to death. I'm thinking either the Rigol O-scopes between 50/100 mHz. I'd wouldn't mind getting one of the old CRO O-scopes, but I'm just not well versed enough to be sure I'm getting the right one the first time...

Thanks again for the help/info - you guys are great...

[Enzo]

Those few millivolts are not a concern at all. perfectly normal.

[km6xz]

Yes, you need a signal generator and scope, and a Variac. A lot of different repair processes depend only on these. What is your budget? A good older portable scope from Tektronix or Hewlett Packard such as the very common Tek 465b or m models or HP 180. Go to a Ham radio swap meet or you can find lots of them on eBay ( Tektronix 465B: Oscilloscopes | eBay ) in the $200 range. They last a long time, were top portable scopes in their day so have the highest quality parts inside. The low cost digital scopes like Rigol have nice features like math functions but are more fragile particularly working at higher voltages(tube amps) and like all digital gear, is essentially throwaway if it fails.
The most useful signal generator I have is also one of the lowest cost. I have a test gear junkie like many here are amp or guitar collectors so I have had or have, every good generator made from the 1940s on. I suggest the Heath model IG-5218 or the older version IG-18. They have every feature needed for a bench tech in one unit: Accurate frequency calibration, accurate meters level meter full range from -50dbm to +20dbm, with and without 600 ohm termination, and DBv calibration, low distortion and enough output to test headphones and speakers directly, square wave output and wide coverage from 0.3hz to 111khz with leveled output. There are lots of generators which have any one of those features but none I know of which have all of them in one compact box. The big plus in getting some of the old Heath gear is they are intended for hobbyist construction and have assembly/repair manuals which would make them the easiest to repair of any gear out there. I have generators with lower distortion but the IG-5218, with minor mods is consistently 0.0016% ThD at 1khz with the strongest harmonic 80db down from the fundamental.
A cheap Chinese metered Variac, at least 4 amps like this one: Brand New TDGC ? 0 5km 500VA Variable Output AC Convertor Speedy Fast Shipping | eBay
If you can spare the money get one that is metered for both current and voltage. You can build a digital current meter that is quite accurate with a 3 or 4 digit AC ammeter module like this one can be used instead of the more expensive current metered variac.
Enduring AC Digital Ammeter Voltmeter Panel Amp Volt Meter Red LED Display Tbus | eBay You want one with battery or other full time power since most of those very low cost meter assemblies take power from the AC mains it is measuring, so it goes dead below 80volts. Current pulled at 1 volt is just as important as measuring current on working normally low current drain unity.

Those 3 items, for less than $400 is the basis of a very competent workbench.

[kevtronic]

Thanks for the help, tips, and info...
It seems to be working and sounding great now...