I like to use a variac AND a bulb limiter. That way I can get the incoming mains to go anywhere up to the full 120V at the amp’s transformer. The biulb trends to drop a few volts. On bigger amps I’ve been known to use up to 3 100w PAR lamps and sometimes turn the variac to 140 for music testing, and for tube amps which can have a sizable quiescent draw.
My worry was the inrush current popping the fuse but I suppose it will be less at low voltage. Plus as the variac is fused, I can bypass the CS800 fuse, only for testing tho.
Most circuit breakers are rated at 11x nominal current for a second or two to allow motors to start.
Huge amps I've found the 100 w bulb doesn't allow the op amp power supply to get to +-15. I used the heating element of a 1500 w room heater with a bad tip-over switch, series the power cord. About 6 ohms. The heater I used had .250 flag male terminals, which fit right in series with the male .250 flag terminals on the back of the Peavey power cord socket.
Huge amps I've found the 100 w bulb doesn't allow the op amp power supply to get to +-15. I used the heating element of a 1500 w room heater with a bad tip-over switch, series the power cord. About 6 ohms. The heater I used had .250 flag male terminals, which fit right in series with the male .250 flag terminals on the back of the Peavey power cord socket.
When you dial up a variac from zero, there is no large power up surge. You can see the caps trying to charge, so you just step it up slowly. It isn't like flipping the power switch on an amp.
The purpose of a bulb limiter is to allow you to apply power without blowing fuses, and preventing huge draw through the system. Once the amp is stable and not drawing excess current, we get rid of the bulb.
The variac can be used similarly, but it is a separate tool. Instead of watching a bulb, I watch a current meter, and I dial up the mains voltage while I monitor current. If I see it start to ramp up, I back off. I often see reports like: "I could get it up to 90 volts before it started to smoke..." That is using it wrong. The variac is not there to see how high a voltage you can get. it is there to watch the current trend as you attempt power up.
The purpose of a bulb limiter is to allow you to apply power without blowing fuses, and preventing huge draw through the system. Once the amp is stable and not drawing excess current, we get rid of the bulb.
The variac can be used similarly, but it is a separate tool. Instead of watching a bulb, I watch a current meter, and I dial up the mains voltage while I monitor current. If I see it start to ramp up, I back off. I often see reports like: "I could get it up to 90 volts before it started to smoke..." That is using it wrong. The variac is not there to see how high a voltage you can get. it is there to watch the current trend as you attempt power up.
I have fixed both the ones I've owned. I have a small box of about a dozen active parts between the two.
There's a real easy and reliable way to check a bipolar transistor or a diode with the use of a volt meter that has the diode checker function. What you are doing is checking the forward voltage of a PN junction. The meter generates a small current from probe to probe then measures the V probe to probe. This V is in the range of 0.4 to 0.75 volts, the forward voltage required to flow current. Don't turn on the power BTW. If it measures zero or infinite, the transistor is bad (unless there's some small resistance across it by design - check the schematic). I bet I have fixed 50 class A/B amps and that's the first thing I do. This measurement is polarity sensitive. For an NPN transistor put the red lead on the bass then place the black lead on the emitter. Confirm the forward drop is correct then move the black lead to the collector and do the same. My Fluke 177 beeps when it's happy. For a PNP transistor the black lead is stationary and attaches to the base. If you don't know if the transistor is NPN or PNP that's ok, just reverse the leads. Since the a good Vbe give a quick beap I can check about 20 transistors in two minutes not knowing wether they are PNP or NPN.
I had one of those big diodes in the output boards go short circuit, all triac crowbars were bad, a few driver transistors , and one small signal diode, no caps surprisingly.
Read my post "Damn crowbars".
There's a real easy and reliable way to check a bipolar transistor or a diode with the use of a volt meter that has the diode checker function. What you are doing is checking the forward voltage of a PN junction. The meter generates a small current from probe to probe then measures the V probe to probe. This V is in the range of 0.4 to 0.75 volts, the forward voltage required to flow current. Don't turn on the power BTW. If it measures zero or infinite, the transistor is bad (unless there's some small resistance across it by design - check the schematic). I bet I have fixed 50 class A/B amps and that's the first thing I do. This measurement is polarity sensitive. For an NPN transistor put the red lead on the bass then place the black lead on the emitter. Confirm the forward drop is correct then move the black lead to the collector and do the same. My Fluke 177 beeps when it's happy. For a PNP transistor the black lead is stationary and attaches to the base. If you don't know if the transistor is NPN or PNP that's ok, just reverse the leads. Since the a good Vbe give a quick beap I can check about 20 transistors in two minutes not knowing wether they are PNP or NPN.
I had one of those big diodes in the output boards go short circuit, all triac crowbars were bad, a few driver transistors , and one small signal diode, no caps surprisingly.
Read my post "Damn crowbars".
Oh, I guarantee there were caps that were bad. Maybe not bad enough, but at least degraded. The problem with “damn crowbars” is that turn on/off thumps often trip them. Resulting in shorting out the output transistors. It probably didn’t do it when the amp was NEW, but give the caps a few years/decades and all the time constants are no longer in the same relationship to one another. Didn’t thump, now it does. Then the crowbar trips. Put in new caps, the thump goes away (or at least reduced in magnitude). Take out that dumb crowbar and replace it with a muting relay and now it’s tolerant to capacitor changes as they age. Then you don‘t have to change them till they’re really bad.
As a Peavey authorized for over 30 years, I have to say I don't recall a bad crowbar triac that didn't also have a shorted output transistor behind it.
If an output transistor blew out and shorted, the crowbar would fire. It would be seriously overloaded and short the triac out too. If a DC fault were caused by something else (ie, the VAS current source opening up, slow-charging caps, dropped mic), the triac would also fire. This would short the output stage - but not necessarily blow it. When/if the fault clears, normal operation resumes. At least that is how it is supposed to work. The output transistors were a little undersized and the triac VERY undersized in order to do this safely.
How does the CS-800 adjust its DC offset? Is there a servo running on an OpAmp?
How would one go about tweaking such a thing?
Built like a tank and zero trimmers inside.
I think I have an untouched unit. I think I am the second owner. I have not listened to it yet.
No, the DC offset is not terrible, just my curiosity.
How would one go about tweaking such a thing?
Built like a tank and zero trimmers inside.
I think I have an untouched unit. I think I am the second owner. I have not listened to it yet.
No, the DC offset is not terrible, just my curiosity.
Attachments
If you look at the schematic you posted, the second op amp upper left has speaker ground as feedback. Signal goes in the top. The first op amp, the feedback crunches down the signal against the + rail if the DDT detects too much high frequency. This turns down the gain if the highs are too hot. So centering on ground is an inmportant input.
New binding posts. The ones that fit into the boards and help mount the PCBs they are part of.
They are cheesy. I could have modified the PCB mounts and added new heavy-duty speaker binding posts but I had no justification to do that.
I wish they were better though...
They are cheesy. I could have modified the PCB mounts and added new heavy-duty speaker binding posts but I had no justification to do that.
I wish they were better though...
Attachments
I clean the heatsinks & output transistors completely with isoprophyl alcohol on cotton balls with a screwdriver and a pick. The fan I usually replace. The exit louver could use a clean. The tops and bottoms of the PCB's should be wiped with the cotton balls & alcohol. Inside & outside of the case, 409/fantastik/spicnspan rubbed with a paper towel.
Perhaps new pots and connectors. For sure new long service life electrolytic caps. I put a filter over the air inlet, sticky fiber I buy from mcmaster.com . All that, good for another 40 years.
Instead of banana/binder jacks, which are obsolete, the whole output panel could use a replacement with Speakons. The 1/4 phone jacks need to go, they are an invitation to shorts.
Perhaps new pots and connectors. For sure new long service life electrolytic caps. I put a filter over the air inlet, sticky fiber I buy from mcmaster.com . All that, good for another 40 years.
Instead of banana/binder jacks, which are obsolete, the whole output panel could use a replacement with Speakons. The 1/4 phone jacks need to go, they are an invitation to shorts.
This one is a keeper. I don't think they make them like that anymore 40+ years old. It cleaned up just fine.