Peavey Valveking troubles

[Enzo]

Pin 4 shows voltage? On the schematic, pin 4 should be ground. I assume the 10 ohm resistor is back in? Does it have continuity to chassis?

Diodes have anodes and cathodes. The end with the line is the cathode - both on the part and on the drawing. I always remembered which end was which becuase the triangle part of the symbol was shaped like the letter A, A for Anode. I'd suggest not calling them positive and negative ends, because that depends upon the use in a circuit.

The anode of D117 can be grounded through the 10 ohm resistor in one position of switch S101.

D115 is out in the middle. completely isolated from everything else by diodes. I would not expect any resistance readings for it, it should show open to ground, in my vision. Reverse your meter leads and see if you get a "negative resistance" reading. If you do, that means ther is some small residual voltage stored in a cap somewhere confusing the meter.

[Valvehead]

Dont know where I got that from ?? ---there is just a few mv at the 4 pins of that fsw cable -- i just measured it.

yes the 10 ohm is in , to ground i get 0 ohm / 10ohm

[bnwitt]

I just had another one of these amps (VK212) in the shop. The low voltage bridge rectifier was blown due to a bad tube which blew the f203 3 amp fuse. I replaced those 4 diodes and re-tubed the amp. Then I had switching problems. Two of the relays were bad for sure so I replaced all 3 of the relays along with TR105, the U101 and all of the Qs and Ds in that circuit (17 components on the low voltage power rail not including the 4 bridge diodes). Total cost in parts was about $12 but it saved me a bunch of troubleshooting time since it only took about 45 minutes to de-solder remove and replace the components. The amp was back to normal and out the door. I know it was a shotgun repair but my time is getting too valuable to mess around figuring out which components have been damaged by a tube failure in this very poor design. Not having the tube heaters on a separate circuit from the low voltage switching and reverb power rail is a real bummer as the collateral damage can be huge. If I owned one of these amps, I’d put all of the fuses on the back chassis panel, separate the tube heaters to their own fused circuit, put in an adjustable bias circuit, eliminate the texture circuit and do the JCM800 mod. As they are, these amps will cost you if you don’t replace tubes before they blow.

[bnwitt]

Well I thought it was out the door. Now I am burning up the green Boost LED in the Footswitch when I switch the boost function on. If I relace it with the switch still engaged it will glow just fine. Switch it off and on again and the new LED burns out instantly. Seems like I might have a shorted resistor allowing too much voltage across the Boost LED. The channel select function and LED are both fine.

[Valvehead]

damn !! makes me want to throw in the towel on this one !

[bnwitt]

damn !! makes me want to throw in the towel on this one !

Now don't do that. With Enzo around you'll figure it out.

[Valvehead]

i know---he's the man !

so Enzo, can you tell me where i should have voltages to try and narrow this down ?

[Enzo]

We'll see, tonight I am kinda fuzzy.

You have switches and relays. Just start from the start. They run from +14v. Verify you have +14v more or less at that supply. Note D103, D104 parallel the relay coils. The cathode of each should be at +14. If it is ther, does any combination of switch settings make it go away?

That got power to the hot end of each coil. That end stays powered at all times. We ground the other end of a coil to energize it and switch the relay. When it is OFF a relay should have the +14 at both ends. A footswitch in the FS jack should be able to ground the bottom end of SK101,102 or SK103. SO meter the anode end of those two diodes and see if the voltagte ther goes from +14 to zero and back as you stomp the FS buttons. Leave the boost switch off or whatever you have to do so it still switches channel at this point. We are establishing a base condition, the relays do go off and on and channel switches. Yes?

How you say pushing the boost switch won;t let it change channels. SO either the relays are not energizing or they are but for whatever reason the the signal is not affected.

Now push in the switch that makes it not switch channels. DO the same measurements again to see if the relays get enegized or not. That lets us know what we are up against.

[Valvehead]

ah ha... i found something ..

measuring at anodes


Boost sw IN = d103 goes from 32v to 7v when switching ch.

boost sw IN d104 goes from 32v to 0v

next ,

Boost sw OUT = d103 goes from 32v to 21v

d104 goes from 32v to 0v

so we have an issue around D103 , yes ? or d103 itself ?

[Enzo]

The diodes are not likely the problem, they are just a lot easier to get to than the relays themselves. SInce they are in parallel, whatever is on the diode will also be on the relay coil.

I am a little concerned the schematic says 14v and you get 32v. Where are you grounding your meter? Then again the 14v is not regulated.

With all power off, measure resistance from the anode of D103 to chassis. No footswitch. Press the channel button. How many ohms you get? It should switch between open in one position and 10 ohms the other way.

[Valvehead]

grounding meter right to chassis.

so--measured d103 power off--

580 ohms - other end 3 ohm

boost switch in = same

do not get an open reading

[Enzo]

Press the channel button. How many ohms you get? It should switch between open in one position and 10 ohms the other way.

What happens when you push the channel button?

As long as you are in there, meter on diode test, do you have good drops across each of the four diodes just above the relays and boost switch there?

[Valvehead]

ok-sorry--i had a few beers last night ..

here is what i have -

Diode test-

d103 .2v / .2v

d104 .2v/ .2v

d116 1.2v/ .5v

d117 .9v/ .3v

Ohms testing :

ch. sw. IN , boost off
d103 anode 3 ohm cath 143 ohm
d104 anode 1 ohm cath 143 ohm

ch sw OUT , boost off
d103 anode .5 ohm cath 250 ohm
d104 anode 0 ohm 250 ohm


if i do the same test with the Boost sw on , i get he same results

i hope that makes sense

[Enzo]

No those readings don't make sense.

Look at the circuit. The power supply (it says 14v, but I don;t care right now if it is 32v, 32v will energize a relay too)is connected to the top of the relays. Relay SK101 SK102 are the channelk switchers, and they are the ones with D103 across their coils. To get a relay to switch, with power on the top end (the cathode end of the diodes) we then ground the bottom end to pull current through the coil energizing the relay. Turning the channel switch on and off OUGHT to interrupt this ground path.

Your readings show me 3 ohms versus half an ohm. Well 3 ohms won;t even slow it down, so with low resistance like that, essentially that coil must be energized all the time.

Here is your circuit path. +14v, through the coil over to S101 pin 5 (and 4). When the switch is open, that is as far as it goes. With the switch closed, the path continues through pin 1 of the four wire cable to the ring contact of J3 (footswitch jack). From there, through the cutout contact to the 10 ohm R170, back through pin 4 of the four wire cable, and then to chassis ground. For some reason your switch doesn't seem to interrupt this path. Maybe the switch is bad.

I am not sure what readings you took on D116 D117. I was looking for test of those plus D115 D110. To test a diode with diode test, you put the probes on the ends of the diode. If these are readings to chassis, that doesn;t work. I just want to know if each of those four diode acts like a diode. Roughly half a volt forward, and very high or open reading in reverse.

Back to your path. I wonder why we do not get the 10 ohms. Is the FS jack one with a metal bushing that touches the chassis? If so, that explains it. And that bushing becomes the end of the path at chassis ground. But I wonder why we cannot get an open on the one switch setting.


You know, before I program a series of tests on the path, just check the push switches. Each switch has two DPDT section. The upper unit uses the front six legs, and the lower the rear six. They are laid out just like a slide switch, so for example on the top is a row of three on the left running fore to aft, and the other three in a row on the right. In each row of three, the center pin switches between the front or rear contact in the row of three. You can visualize the action as the switch has in and out positions. Top and bottom sections are independent. You want to get on each of the four three-pin sections and verify it switches between front and rear in each grouping.

Look at the factory schematic layout, the circuit board traces are shown. Note on switch S101 the rear left three and the front right three are wired so all the pins of those groups are wired together. SO no setting will show open, all will read short. But the other two groups should test right. And over on S102, the left rear group are wired together the same way. But the remaining three groups should act as a switch. Your readings seem to indicate the S101 right rear not opening.

[Valvehead]

ok , diode mode on my meter i get -

D110 .4v / 1.3v

d115 .4v / 1.25v


i measured the channel switch and it doesnt seem right --im trying to understand your explanation --and I think the switch might be bad.

That being said , guess what I happen to have here in my parts bin (brand new) ? (i will install it and see what happens)