VM 801 stereo

[William]

After finishing the VM 570 and its companion speaker the VM 102 I moved on to my next VM project which is the VM 801. This one has both amps in the main unit and the aux speaker is just a matching speaker. If you remember the VM 570 and VM 102 were a more traditional cabinet where the VM 801 and its matching speaker are MCM. Both amps for the VM 801 are pretty much the same with the exception that the main has a plug for the preamp. Both amps have a 5Y3, PP 6BQ5's, and a 12AX7. The preamp uses two 12AX7's.

So, with this stereo I have run into something I have never encountered before. I have totally gone through both amps and the preamp and now that I am ready for some final testing it won't work, it's totally dead. The power cord is good, I did check that, and I did a few other checks with things that would make sense for no power but all good. The stereo has a function switch and within that function switch is an off/on switch to bypass the changer when in AUX mode. The switch does work so that is not the problem, been there, done that, before with the Imperial. Even though when in the AUX mode with the function switch, I am wondering if the changer still hast to be plugged in to the preamp. That does not make any sense to me, why would the changer need to be plugged if it's not in use? I have studied things trying to figure out how/why that would make a difference but am drawing a complete blank, I'm just not seeing it. So, I guess I am reaching out for some advice as I really don't want to blow something up. I have attached a photo of a genuine VM schematic from Gary, and thanks for the help.

Bill

[TC Chris]

Jumper pins 1 & 3 on the 3-prong plug and see what happens?

Chris Campbell

[William]

Thanks, Chris, but the actual three prong plug has pins 2, 3, and 4. There is no actual pin 1.

Bill

[TC Chris]

Oops. 2and 3 ought to do it.

CC

[William]

Thanks, Chris, but I'm still wondering why the changer needs to be in the circuit in the first place when there is a bypass switch when in AUX mode? I can't seem to wrap my head around it.

Bill

[hermitcrab]

Bill are you saying the changer needs to be plugged in to complete the circuit? Maybe something in the motor winding? I would just hook your AC line to 7 and 8 since that is the primary side to get it going to test

[William]

I'm not sure what I'm saying anymore, all I know is that the preamp and amps won't power on using the AUX mode and in my thinking they should. The changer has three wires that go into a 3-prong plug. That 3-prong plug plugs into the preamp directly behind the function switch with its AUX off and on switch built onto the backside of the function switch. I just do not see the relationship of the changer to the AUX not allowing the system to fire up. I have not tried plugging in the changer yet as the changer that came with this unit when I bought it was the wrong one. Somewhere along the line, someone changed out the changers. I searched for correct 1200 series for over a year and actually ended up getting it from Gary. Unfortunately, it had the wrong plug so it must have originally been installed in a different model. The changer has the correct look, just the not the correct plug. The changer that came in my unit has the correct plug, though it was cobbled up to make it work and now the actual power switch is busted so I cannot try it.

With my trying to trace wires, pin 8 on the plug socket that comes from the preamp to the amp is common ground and pins 6&7 appear to be where the switching takes place, at least I think I have that right. I have been hesitant about jumping things in case I'm missing something; I do not want to blow something up.

Bill

[electra225]

If you want to power up and not blow something up, use your Variac. Power up at a lower voltage, say under 40 volts, protected by your dimbulb and Kill-A-Watt. You are wanting to test switches. Use your voltmeter for the load. Put the voltmeter across the load, so you can see when the low voltage flows like it should. If you create a dead short, the dimbulb will take the current. Does this make any sense?

[William]

Thanks, Greg, I think it makes sense. It is hooked to the Variac and Kil-o-watt, and when I slowly increase the voltage there is nothing showing on the Kil-o-watt when in the aux mode with no changer plugged into the preamp.

Bill

[electra225]

Can you jumper the changer plug? That should answer your question. If you don't get a reading on the Kill-A-Watt (unless you are like me and have it on the wrong mode ) you have an open somewhere. I don't see why the changer would affect the Aux, mode.

[TC Chris]

And if you can't figure out the schematic by staring at it, make a photocopy and use colored markers to trace the wiring out.

Chris Campbell

[hermitcrab]

Bill, have you checked the continuity of the main switch to rule it out?

[William]

Elton, there is no main switch. It's either in the phono mode or aux mode so which ever mode you select should be the main switch. The aux power switch is ok. The phono is not plugged so I'm not sure about that. I have been busy so have not gotten back to it in a few days, maybe tomorrow or Thursday, and probably Thursday as it's supposed to rain. One hiccup is/was the wrong changer so incorrect cobbled wiring. I think I mentioned earlier in the post that I have the correct changer, it's rebuilt, but I have not installed the correct wiring yet to see what that will do for making it work. I am going to try and jump a few things to see what happens. Thanks to all for your good advice.

Bill

[William]

Today it was raining so I decided to work in the basement doing my annual spring cleaning. In the spring I move everything, and in this case it's mostly console record players, away from the walls and vacuum up the dead bugs that migrate in during the year. Once vacuumed, I spray the Ortho Home Defense around the perimeter of the room and then let it dry before moving things back. I got three of the four walls done today so not bad. As I was moving things around, I happened to think of something, something that one of my other VM stereos might have in common with this VM801 that I'm working on and that being the changer and the three terminal plug that comes from the changer. Sure enough, it's the same changer with the same plug. Yippee, now I can test things out and see if my theory of the other day is correct, and it is. Now here's the theory.

VM, back in the day had a switch built into the changer that would shut off the motor but leave the amp on. There was also an input for a tuner or something. Flip the switch, hit the off/on/reject control to the on position and you had an amp without running the changer motor. Well, I got to thinking, I wonder if that switch that was located on the changer was now located in the function switch that was built into the preamp. Sure enough, that power switch at the back of the function switch does just that, but the off/on/reject control on the changer is still the main power switch. This has to definitely be an afterthought as this unit was being built during the transition phase between mono and stereo and to me it's just not a normal way to do things.

Now for the disappointing part. I had everything all hooked up on my bench including it all hooked up to my Variac that gave me all the problems with my last VM project. In between projects I took the Variac apart, cleaned everything including the copper windings, put it back together and gave it a try on my three channel Motorola table radio. Everything worked as it should with no interference from the Variac. Not the case with the VM801. At between 65 and 70 volts on the Variac, which about 20 watts, it starts to squeal and hum just like my last VM did. Now I'm wondering if it's a VM thing not liking a Variac, or there is something still wrong with the Variac. I have used this same Variac on everything that I have rebuilt over the last several years including VM with no problems. I'm confused and I think the only way I will know is to purchase a new Variac and see if I have the same problem. I guess I am on hold until a new Variac arrives, or I figure out what's going on with these last two VM stereos. One thing to remember, my last VM works fine just plugged into the wall outlet.

Bill

[electra225]

Would your dimbulb and Kill-A-Watt let you power up safely?

[TC Chris]

Is it possible that at low line voltages, the amp's tube bias is off so somehow you're getting oscillation? In other words, the problem may not be the variac, but rather the low voltage?

Chris Campbell

[William]

Not sure, Greg, but I could try that.

Chris, I never thought of that, and you might be onto something. I may just plug it in and see what happens. But then my next thought would be the WHY!

Bill

[electra225]

Low tube bias won't cause oscillation. If line voltage is low, all the element voltages will be low. Oscillation is caused by signal being fed back to where it causes trouble. I have no idea how a device used in the line power supply could cause oscillation. Would a line isolation capacitor being open or leaky allow oscillation from the Variac?

Bill, does your Variac cause issues on any other device, such as your Imperial? Hook it up to your Imperial and see what happens.

[TC Chris]

Well, he said it happened between 65 and 70 volts. I have no idea how the various voltages and polarities supplied to tubes change with extremely low voltage. I imagine at 70 V the tubes aren't conducting much, depending on the heaters and such.

Chris Campbell