Hi all
I'm new here, please excuse my rather poor swiss-english
.
My PA-7E worked fine for the last few years (with EV Sentry 500) but now after playing for about 15 minutes, i had a right channel cut-off (right LED (red) was on). My PA-7E already has these aditional 1x1" PCBs. After a first try with the original crap-trimpots i replaced them with 20 turn 200Ohm and 5kOhm and could easily set the bias correctly, i have steady 40mV after 1-2 hours with cover closed. The right channel temp. (heatsink) is about 43°C (109°F), this is slightly higher than the left channel (40°C /104°F).
B U T, when playing now, after ca. 10 minutes, i still have have a right channel cut-off, but only for a very short moment (1 second, next time maybe 5 seconds), the red LED does NOT light. Before disassembling too much, my question would be: where to start best with the fault finding ?
Your help is very much appreciated, thanks in advance for every input to get my baby healthy again. Regards Walter
I'm new here, please excuse my rather poor swiss-english
.My PA-7E worked fine for the last few years (with EV Sentry 500) but now after playing for about 15 minutes, i had a right channel cut-off (right LED (red) was on). My PA-7E already has these aditional 1x1" PCBs. After a first try with the original crap-trimpots i replaced them with 20 turn 200Ohm and 5kOhm and could easily set the bias correctly, i have steady 40mV after 1-2 hours with cover closed. The right channel temp. (heatsink) is about 43°C (109°F), this is slightly higher than the left channel (40°C /104°F).
B U T, when playing now, after ca. 10 minutes, i still have have a right channel cut-off, but only for a very short moment (1 second, next time maybe 5 seconds), the red LED does NOT light. Before disassembling too much, my question would be: where to start best with the fault finding ?
Your help is very much appreciated, thanks in advance for every input to get my baby healthy again. Regards Walter
I'm not familiar with the amp or its circuitry but the two things to look at...
1/ Monitor the DC offset at the output before any speaker relays etc and see what it does when the amp trips.
2/ The symptoms seem to describe either a dry joint somewhere or a transistor failing intermittently open circuit (usually conformed by seeing a massive rise in the base/emitter voltage)
1/ Monitor the DC offset at the output before any speaker relays etc and see what it does when the amp trips.
2/ The symptoms seem to describe either a dry joint somewhere or a transistor failing intermittently open circuit (usually conformed by seeing a massive rise in the base/emitter voltage)
Hi Mooly
Thanks for your quick reply.
Measuring the DC offset will get difficult without an oscilloscope, i think....
I have a multimeter, of course and this one to check th transistors:
Peak Electronic Design Limited - Atlas DCA - Semiconductor Analyser - Model DCA55
and a second from peak to check capacitors.
Regards Walter
I don't have a PA-7E but have looked at the service manual.
When you changed the trimpots, did you follow the instructions from the service manual when setting the bias?
And it is the PA-7E (200 wpc) version you have and not the PA-7E II (225 wpc) , the latter doesn't have DC-offset adjustment (tho it's called DC Balance adjustment in the service manual.).
The red leds you mentioned must be the clipping indicators, one per channel.
For all I can see there is no indicators for the protection circuit more than that you'll hear the click from the relays when they goes off or on.
When you changed the trimpots, did you follow the instructions from the service manual when setting the bias?
And it is the PA-7E (200 wpc) version you have and not the PA-7E II (225 wpc) , the latter doesn't have DC-offset adjustment (tho it's called DC Balance adjustment in the service manual.).
The red leds you mentioned must be the clipping indicators, one per channel.
For all I can see there is no indicators for the protection circuit more than that you'll hear the click from the relays when they goes off or on.
A meter is fine for normal offset and bias measurement. The problem with the transistors is that the failure mode can be intermittent, even the very act of physically moving it or unsoldering it can cause it to 'heal' for an indeterminate time. I'm not saying that this is definitely what the problem is but its a failure mode I've encountered time and again in consumer electronics. The only way to find it is to carefully measure DC voltages when the amp is in the faulty state.
Interesting. Clipping LED's could well detect when the output approaches either rail, and that would tie in with a DC offset fault condition.
Hi Mannegizen
Yes, inputs shorted, dc at speaker output balanced to around zero mV and bias set to 40mV, i did this at least 6-7 times until i had it a steady 40mV and 0 mV at the ouput, with cover and after power on for at least 1 1/2 hours. output relay works fine, also soldered out the thermal switch and tested it in the oven; works fine. And yes, the red led is the clipping indicator. This clipping led (right channel) was red all the time and there was no signal at the out also all the time when i first had this issue. Only After setting the bias it changed as described to cut-off for seconds only and with no clipping led on. Regards Walter
Yes, inputs shorted, dc at speaker output balanced to around zero mV and bias set to 40mV, i did this at least 6-7 times until i had it a steady 40mV and 0 mV at the ouput, with cover and after power on for at least 1 1/2 hours. output relay works fine, also soldered out the thermal switch and tested it in the oven; works fine. And yes, the red led is the clipping indicator. This clipping led (right channel) was red all the time and there was no signal at the out also all the time when i first had this issue. Only After setting the bias it changed as described to cut-off for seconds only and with no clipping led on. Regards Walter
Under fault conditions, you have to measure the DC before any speaker relay. If there is DC offset, the relay opens and the speaker voltage will be zero but the voltage on the other side of the relay could be high.
Hi Mooly
That's indeed a problem; the faulty state is for a few seconds only, to be at the wrong place in the right time (ooh, that's Dr.John almost...) is almost predictable i think. If one you experienced guys has not THE idea of where to start with checking (unsoldering) components, i have no other chance than to propably start with the small additional protection board (?)
Regards Walter
Yes, the first step is to see if the problem is a DC offset issue.
I've never seen one of these amps, and haven't even seen the circuit of one and so I don't want to lead you up the wrong path but that said these kind of faults are likely to be either a dry somewhere or a transistor that runs hot which could typically be the VAS (voltage amplifier stage) of the circuit.
If the fault is intermittent and you can't get it to do it then you sometimes have to second guess the cause and replace the suspect parts 'on spec'.
So look for semiconductors that run hot, look for areas on the board that may be discoloured through heat... these are the parts and areas that are going to be most suspect.
I've never seen one of these amps, and haven't even seen the circuit of one and so I don't want to lead you up the wrong path but that said these kind of faults are likely to be either a dry somewhere or a transistor that runs hot which could typically be the VAS (voltage amplifier stage) of the circuit.
If the fault is intermittent and you can't get it to do it then you sometimes have to second guess the cause and replace the suspect parts 'on spec'.
So look for semiconductors that run hot, look for areas on the board that may be discoloured through heat... these are the parts and areas that are going to be most suspect.
Hi Walter,
I was authorized warranty and have worked on these amplifiers. Can you tell me why you decided to begin replacing things before any troubleshooting was done? The bias current should not have been touched until you had the initial problem sorted. Also, when you replace the controls, the bias settings should be set near the minimum current so that you don't cause overheating before you can get things adjusted to a safe value.
You have intermittent DC offset as Mooley mentioned. Before you begin doing things, you have to understand what you are working on. Those small add-on PCBs are there to increase the allowed current before current overload protection cuts in. They are not for protection. The use of these add-on PCBs increases the output current available by "backing off" the normal protection current limiting.
I am afraid to ask how you tested the thermal cutouts "in an oven". Please tell me that you didn't put the entire amplifier in an oven!
So, test #1, per Mooly
-Chris
I was authorized warranty and have worked on these amplifiers. Can you tell me why you decided to begin replacing things before any troubleshooting was done? The bias current should not have been touched until you had the initial problem sorted. Also, when you replace the controls, the bias settings should be set near the minimum current so that you don't cause overheating before you can get things adjusted to a safe value.
They are not crap trim pots. Multi-turn controls are not meant to pass any current through the wiper contact. Please replace those controls with the same thing that was in there before. Use parts from a good manufacturer, not ebay vendors.
You have intermittent DC offset as Mooley mentioned. Before you begin doing things, you have to understand what you are working on. Those small add-on PCBs are there to increase the allowed current before current overload protection cuts in. They are not for protection. The use of these add-on PCBs increases the output current available by "backing off" the normal protection current limiting.
I am afraid to ask how you tested the thermal cutouts "in an oven". Please tell me that you didn't put the entire amplifier in an oven!
So, test #1, per Mooly
Do that and report. Do not get creative, do not desolder anything unless you are replacing the bias controls with the original type. I have to ask, were the original controls defective, or did you just decide to replace them?
-Chris
Hi anatech
The trimpots were crap, indeed, or defective if you prefer; casing broken and absolutely impossible to adjust bias with them, too "nervous". this is the type i put in:
Trimmer Cermet 200 Ohm linear 500 mW, BI Technologies, 67PR200LFTB kaufen | Distrelec Schweiz
Thermal switch 75°C: No, i didn't put the amp into the oven; i pulled the oven over the amp (LOL). Seriously: i desoldered the switch, put some silicone cable and and tested it in the oven, works fine, 75°C off, ca. 60°C on. I didn't quite get you right: Multi-turn controls not letting pass any current through the wiper contact ?? how that ? (technical data sheet says "slider current 100mA") i suppose it wouldn't work at all if so; i have no problem with it when you know it better from your experience, don't misunderstand me , please. Regards Walter
The trimpots were crap, indeed, or defective if you prefer; casing broken and absolutely impossible to adjust bias with them, too "nervous". this is the type i put in:
Trimmer Cermet 200 Ohm linear 500 mW, BI Technologies, 67PR200LFTB kaufen | Distrelec Schweiz
Thermal switch 75°C: No, i didn't put the amp into the oven; i pulled the oven over the amp (LOL). Seriously: i desoldered the switch, put some silicone cable and and tested it in the oven, works fine, 75°C off, ca. 60°C on. I didn't quite get you right: Multi-turn controls not letting pass any current through the wiper contact ?? how that ? (technical data sheet says "slider current 100mA") i suppose it wouldn't work at all if so; i have no problem with it when you know it better from your experience, don't misunderstand me , please. Regards Walter
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Do you get distorted sound before it cuts out?
Did you play very loud when it did?
When turning the amp on, does both protection relays turn on at the same time? (should be five seconds delay, a shorter delay can be caused by bad caps in the protection circuit.)
Have you tested with another preamp?
(I don't think a bad preamp can cause this, but it doesn't hurt to rule it out.)
As Mooly wrote, bad solders can cause intermittent problems like this.
A flashlight and a magnifying glass helps a lot.
Q208-211 is the ones I would check first.
Second the solders on the protection relay.
(Tho I've never seen bad solders on a protection relay but a couple of times in power supplies, so it doesn't hurt to rule it out.)
Did you play very loud when it did?
When turning the amp on, does both protection relays turn on at the same time? (should be five seconds delay, a shorter delay can be caused by bad caps in the protection circuit.)
Have you tested with another preamp?
(I don't think a bad preamp can cause this, but it doesn't hurt to rule it out.)
As Mooly wrote, bad solders can cause intermittent problems like this.
A flashlight and a magnifying glass helps a lot.
Q208-211 is the ones I would check first.
Second the solders on the protection relay.
(Tho I've never seen bad solders on a protection relay but a couple of times in power supplies, so it doesn't hurt to rule it out.)
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