CS 800 C Series driver board issue

This forum is for talking about all kinds of Peavey power amplifiers.
Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

CS 800 C Series driver board issue

Post by Lenartd » Tue Jul 19, 2022 9:41 am

Hello all. I am a posting for the first time. I have with the help of this forum and more specifically Enzo, I've successfully repaired a CS400 and a CS800 already. I am posting now because I have one more CS800 "C" series that I can only get one channel to work. Here is the background:
This amp wasn't working. It had some blown output transistors and both the SAC 187's needed replacement. Where this amp stands right now is Channel A is working and channel B will work if I move the driver board from A to B. I can't find anything different with the boards when measuring components with my Fluke DVM. I am aware that measuring things while in circuit can produce some faulty reads.
If I disconnect Channel A and power up Channel B on a dimbulb tester the light goes bright, them dims, but then starts blinking. The blinking corresponds to a voltage fluctuation on the driver board OP Amp U2 pin 8. I replaced the IC 4558, no change. U2 pin 4 is getting steady voltage, about 11 volt which I understand because its on the dim bulb. Reading across diode CR10 shows the same fluctuating voltage. It was replaced but I am still getting the same thing. I was thinking about going after Q3 and Q5 but at this point I am just guessing. It also looks like Q5 may be hard to find with its little heat sink.
So right now its a half amp. Replacement driver board are no where to be found anymore. I have a hard time giving up on this forty year old amp, but I am running out of ideas on this one.
Any advice other than boat anchor or wheel chock would be greatly appreciated. Thanks.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Tue Jul 19, 2022 6:20 pm

Remove the SAC187s and eave them until last to replace. A faulty one can cause trouble. And trouble can cause a good one to react.

Please don't throw parts at it, that is just guessing.

First, what wattage bulb is in your limiter? Too large and it doesn't protect, but too small and it won't let the amp power up. In fact right now, when you power up on the bulb and it flashes initially, then settles, what is the mains voltage the amp is seeing? I suspect your blinking is the amp trying to wake up but when it does a current surge shuts it back down. SO naturally your supply voltages can dance.

Pin 4 11v? Shouldn't that be negative 11v?

Your negative supply seems steady? Look at the +15v supply. On the print, look right along the top edge of the card, CR10, next to Q3 is the 15v zener for that supply, but look further right. 2k 5w R18 is a dropping resistor for that zener. And even further right, the main +81v rail comes into the card through 10 ohm R21, above Q9. f that resistor is damaged, then whenever it tries to conduct current, the voltage drop across it will be much higher, and COULD cause your symptom.

If you replaced CR10, I hope it was with a 15v zener, not just a plain diode.

Q5 is an MPSU60, which have not been made in many years. But most any transistor meeting the sec would do. I suspect that Q5 is OK anyway

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Wed Jul 20, 2022 8:11 am

Enzo, thanks for your insight. Here are answers to your questions:
I haven't removed the crowbar SAC187 yet because I am assuming because the channel works with the other card installed all is okay with it. But I will remove it if you recommend.
The bulb in the limiter is 150 watt and I confirmed it with my amp meter, 1.25 amps. FYI, I took the leap of faith and removed it from the bulb and it did output sound thru a cheap 8 ohm speaker. That sound however pulsed just like the bulb would flash, on and off repeatedly.
Pin 4 on IC4558 is the negative 11 volts, Pin 8 has the fluctuating positive voltage. My meter doesn't register the voltage because of the rapid fluctuation.
CR10 was replaced with a zener 1N4744A diode, no change from the original. The voltage fluctuation is present at this diode when measuring across it.
R21 measures 9.9 ohms
Now here is the interesting thing, you asked me to measure R18 on the non-working channel, it measures 2152K ohm. Working channel R18 measures 1934K ohm both within tolerance. If I measure R16 on my working channel R16 it measures 1335K ohm and very slowly after about 2 minutes rises to 1553K ohm. R16 on the bad driver board reads a steady 2022K ohm. R16 is the resistor on the negative side so I don't know what it means only that it is the only difference I have found so far.
Any thoughts?

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Thu Jul 21, 2022 10:23 am

I like to work without the crowbar because while I am trying to fix it all, I might send some DC down the road and trigger the thing, and that could cause damage.

R18 2152 OHMs, not 2152k Ness pah? And we don't sweat the small stuff, 2k is close enough

WHen you measure a resistor in circuit, and the reading slowly grows, your meter is just charging up some capacitance in the circuit. Just for science, when thi happens, quick, swap your meter leads. I'd wager the reading is now way high and sliding down instead of way low and sliding up.

Your R21 is likely fine, but I am not yet convinced. Instead of measuring the part, measure resistance from pin 7 of the card connectors to R18. That measurs the R21 plus the copper traces and solder that are involved.

SO your positive supply seems to be motorboating. Your +15 is pulsing, but is the +80v main rail also pulsing?

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Thu Jul 21, 2022 9:11 pm

Had a setback on the repair. Probably self inflicted. Let me bring you up to speed. The B channel works and has the good driver board. I moved it over there again to confirm operation of that channel. All is fine there.

The bad driver board in now on the A channel. You asked that I work without the crowbar. The "C" series has the 98890050 board with the SAC187 on it. Rather than remove the board and unsolder the SAC187 I just unplugged the molex connector to that board to disable.

I then powered up on the dimbulb to check rail voltage. To my surprise the bulb went bright then dimmed without the pulsing. On the bulb the rails measured 60 volts and a fluctuating 35v on the other. But the bulb was staying dim. I probably need a lesson on how to properly measure the rails. The schematic calls out a +81v and -81v rail. I never see the negative value even with reversing the DVM leads. But I digress.

Here is where the setback happen. I took it off the bulb and restarted amp. The power switch lit, the DDT light came on initially then went out, and the fan started. All normal so far. I grabbed my meter, one rail measured 76v the other was still low about 35v. That's when the bad happened.

After the smoke cleared R30 the 47 ohm resistor failed catastrophic, R35, 150ohm resistor and R36, 100ohm resistors look like they got hot and now measure out of tolerance. I still need to go thru the rest of the board and do meter reads but that's all I am seeing with the naked eye.

I will replace the resistors and confirm what I can.

I am new to this forum but I have learned a lot reading post's on repairing these old Peavey amps. Perhaps others can learn from my missteps and not repeat the same mistakes. Regardless, with the help of this forum and particularly Enzo I have gotten a CS400 and another CS800 going again. This current amp may be destined to be a single channel amp.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Fri Jul 22, 2022 11:46 am

Unplugging the little board is OK way to disable the SAC187, yes.

I hope we are not putting a load on the amp until we know it is stable.

You have two power modules, one per channel. you can unbolt them from the chassis, and there is enough wire slack so you can set them to either side of the chassis, setting on end. I like to put a magazine or something between module and chassis to prevent anything touching. Now the two large main filter caps are exposed. I measure rails right on their screw terminals. They should measure about 80 volts. And ripple should be almost zero. If one were like half, I'd check to see if the cap was not well connected. HOWEVER, those rails serve both amps, so if faulty, both channels would be affected. COntrast the 15v supplies on the cards, those are for each.

Note there is a small zener board in the amp chassis, those supply ONLY the accessory sockets on the rear.

SO with one channel working, but one main rail measuring 35v, WHERE are you measuring the rails? Note the output devices, only the positive side has rail on the case (collector). The amp is quasi, so the negative side the collectors are the output bus and SHOULD have zero DC volts.

Resistors do not burn up on their own, so when any resistor burns, some semiconductor (most likely) connected to it caused that.

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Fri Jul 22, 2022 4:09 pm

Glad to hear my shortcut to disable the Channel A speaker output wasn't a factor.

I had no connected load, volume pot on zero. Working Channel B was disconnected.

After yesterday's smoke show I did the main filter cap testing you asked about back on the bulb. Both caps measured 58.3 volts on the screw terminals. Probably fine considering the amp is working with about 100vac.

As far as measuring the rail voltage, I clip my negative lead to the chassis and use the positive lead to measure the top of the driver and output transistors. Before the failure event off the bulb I measures several transistors on one row. Was getting 76vdc. When I measured the tops of the other row of transistors, "the output bus" is where my meter was picking up that fluctuating 35v until those resistors failed. I guess that's a good question, where was that rails voltage coming from? Also please explain ripple, I don't have a scope to measure waveform if that's what you mean. Just the DVM.

I have replaced the blown resistors both still need to more resistance checks on the driver board and I will test the output and driver transistors just to confirm I didn't damage something there.

Thank again for your insight.

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Mon Jul 25, 2022 9:10 am

You were correct Enzo about other semiconductors being suspect. I found Q7 and Q8 transistors also failed. MPS6530, its replacement 2N3642 and MPS6533, its replacement 2N3638 are no longer available except on ebay and I know how you feel about that. 2N4401 and 2N4403 look like possible replacements. I will contact Peavey to see what they recommend.

I will need to source these parts to continue in making this CS800 whole again.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Mon Jul 25, 2022 9:40 pm

Q7,8 are just general purpose transistors, match or exceed the specs and most anything will sub. Oh and watch NPN and PNP of course.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Mon Jul 25, 2022 11:27 pm

Ripple refers to how smooth a DC power supply voltage might be. When AC voltage leaves a transformer, we usually rectify it. That means turn it into DC voltage. But at that point it is just half an AC waveform. SO we add filter caps to smooth it out. Totally smooth DC - like from a battery - has zero ripple, totally smooth. But in real life power supplies, there will usually be a small amount of AC-ness on a DC supply. We call that ripple, and is measured in AC volts.

You can google electrical ripple for many sources of deeper explanation.

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Fri Jul 29, 2022 10:14 am

Here is update on this CS800 driver board repair:

As I mentioned before R30, R35, and R36 resistors were replaced. Transistors Q7 and Q8 have been replaced. I also thought the IC U2 eight pin socket appeared to have a issue @ pin 5 of the socket so I replaced that socket.

I powered up on the bulb limiter, the bulb went bright then dimmed. The speaker output board still unplugged. The amp was operating at 104vac because of bulb. Measured a constant +16.07vdc at pin 8 of U2 on driver board. Pin 4 measured a constant -11.24vdc? I then measured across diode CR10, constant +15.9VDC. I measured across CR6 and received +15.4vdc which I thought was strange because that diode seems to be on the -15vdc negative side of the circuit on the schematic.

One side of rail measured 66vdc, measured on top of transistors. Other side measured 0 vdc.

I did plug in the speaker output board with no load and tried once again on the bulb limiter. This time the bulb stayed bright. I pulled the output board, the SAC187 triac ohms out okay. Not sure what's going on now but at least I no longer have the strobe effect on the limiter with fluctuating voltage in the circuit.

FYI, the DDT light was lit on power up with the speaker output board disconnect. Not sure if this means anything since the board was disconnected. Just thought I would mention it.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Fri Jul 29, 2022 5:57 pm

The DDT doesn't know if you have a speaker connected or not. The SAC187 is not the DDT.

If pin 4 of the IC has -11v, then CR6 is trying to work, the backwards polarity you measured on CR 6 must be a measurement error. One end of CR6 is ground, and the remaining end of it should be -15. That is the anode end, and it is wired directly to pin 4. If you get different voltages at pin 4 and the hot end of CR6, there is something wrong with the measurements.

Terms: rail. The rail means the power supply. On the positive side it is connected to the transistor collectors, which is their cases. On the negative side (since the amp is quasi-complementary) the negative rail connects to the EMITTERS. The cases on the negative side are not at a rail voltage. The negative side cases are however wired to the output bus - the line to the speaker. The negative side emitters are accessible though, they are connected to those big 10 watt resistors next to them.

WE must clear up the power supply problem before anything else will work right.

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Sat Jul 30, 2022 10:22 am

You were correct Enzo, measurement error on my part. Both driver board CR6 and U2 pin 4 measure -11.47vdc and -11.52vdc respectively. U2 pin 8 is still good at +16.37vdc. I tested the same points on my working channel B and I'm getting the approximate required positive and negative 15 volts while on the 150watt bulb limiter.

Is it possible the CR6 zener diode could be creating this issue? When I compare diode and resistance values with the working board they are nearly identical. I have already replaced CR6 and CR10 once with the recommended 1N4744.

I have tried to reason a solution by studying the schematic but there is a lot going on with this circuit for a novice like myself.

Lenartd
Member
Posts: 15
Joined: Thu Jul 14, 2022 10:38 am

Re: CS 800 C Series driver board issue

Post by Lenartd » Sat Jul 30, 2022 10:24 am

False alarm on the DDT light on Channel A. It does come on at startup but does go out after a short time.

User avatar
Enzo
Member
Posts: 9535
Joined: Fri Sep 12, 2008 10:24 pm
Location: Lansing, Michigan

Re: CS 800 C Series driver board issue

Post by Enzo » Sat Jul 30, 2022 11:11 pm

Could CR6 cause a low voltage? Certainly. It is a 15v zener, but if it doesn't "zene" at 15v, your supply goes wrong.

A zener is a diode first. SO it needs to check one way and not the other like any diode. But it works by a controlled voltage breakdown in the reverse direction. If you exceed 15 v across it is will shunt anything over 15v. Your meter uses maybe a volt to check resistance. So there is no way for your meter to test whether the 15v zener function is happening. And if your zener is breaking down at 11v instead of 15v, then that is what you will get. Your meter will never know that.

If you already replaced it, it OUGHT to be good, but no guarantee. otherwise it is possible something is loading down the -15v supply. Take U1 and U2 out of their sockets, does the -15v recover or is it still low? Also, and maybe do first, the input circuit, the ICs on the volume control board, could be loading us down. Disconnect that four pin connector that carries the 15v rails over to the small board, does -15v recover?

Post Reply