Ok I acquired an RP65-112 with the G2 board in it from a neighbor. It sat for a very long time unused from what I was told. Now I'm going to have to rack my memory on this because I can't find my notes right this minute and money forced me to set the project a side for a while and I'm just now picking it back up. So it had very low volume at every setting imaginable so I did some poking the tubes seemed to be like white plated or had a white corrosion so I ordered new tunes and the new tubes instantly did the same thing. So I found the two very hard to match large protection diodes that sit on top of the tube sockets were bad. So I replaced everything all the caps and resistors that sat on top of the sockets. I turned it on it worked for two chords then quit. It smelled a bit warm and the mains fuse was blown.
Now I'm pretty much in the poor house and don't have any fancy test equipment other than a decent multi meter, the schematics, and my brain but does anyone have any ideas where to start. I was going to do filter caps because if I'm having component problems it couldn't hurt to replace them but what is causing my mains fuse to blow. A friend of mine looked at one of the transformers once and it seemed ok but I'm thinking they should probably be replaced as they are approaching 30 years old. Please help I have to hear this amp
Tue, 02/02/2016 - 06:09
#1
Where to start?
The 1st thing I would do is check all of the power supply voltages with the speaker disconnected and the power tubes removed. 3 things could be causing the tubes to overheat. 1. The bias voltage is set way too high (TR1). Turn the pot to the center position. 2. The control grid voltage for the tubes that comes from D9 zener diode is out of spec. 3. The driver transistors (Q1 & 2) that provide current for the tubes may have shorted out. It could be a shorted transformer blowing the fuse but more likely component failures in the power amp section are causing it to blow. If the transformer is shorted, you wouldn't hear 2 chords and the tubes would probably not glow. Hope this gets you started. Be wary of the high DC voltages in there. +700VDC hurts. Good Luck. -mgriffin
Good start.
Thanks for the info. That sounds like a real good start. Are the voltages in this that high 700dc? My Valveking holds onto about 348 or something but I think I did see a post in here of a voltage someone stated 680 something anyway I have a great respect for how powerful these suckers can be. I've watched many videos of people getting zapped by amps. I always clamp my ground before powering up when taking readings and test with one hand behind my back so I'm not tempted to get both my mitts in there at the same time.I made a discharge tool with a resistor and an led so there is a visual safe not safe and but thank you for the safety concerns as well.
Schematic Tip
I believe the RP and RD models use the same power supplies. When you're checking the power supply voltages, the RD-65 schematics GD2 and GD2A are much easier to read and have all of the voltages printed. Makes it easier to troubleshoot. http://pacair.com/mmamps3/sites/default/files/docs/2165-rd_and_2100-rd.pdf
Next ?
You already answered it griffin 155. I was going to ask you if you had any idea in what range those voltages should be. I wake up check my email and you already had the information for me. Thanks so much for help and for the schematics on RD65 I didn't have them just the RP schematics.
Ok this is what I came up with
There was a small visible sizzle at one of the protection diodes when I turned it on to test my input voltages. All my voltages were way off. Way high. So I would have to assume the transformer is most likely shorted right?
Probably Not
It would be quite rare for a shorted xfmr to have higher than expected output voltage. More likely voltages would be low to zero and smoke would be pouring out. Please provide the actual voltages you measured. As for the sizzling protection diode, I assume you're referring to one of the diodes on the tube sockets. If not, please explain. Also, check the tube sockets to see if they have any signs of carbon tracking (little black streaks) between pins. Also visually check the sockets for cracks. We're here to help you, but we need objective information. Way high and way off are not quantifiable. Feel free to reference component designations when referring to parts. For example, D9 protection diode.
short and approx. voltages
The short I previously explained I missed typed it's location it was actually at the rectifier diodes either 3&4 or 5&6. It was arcing between the traces on the board. I haven't traced them down exactly yet. As far as specific voltages the only one I remember was the 32 v line I measured like 256v?! I double checked
I hate it when it arcs
256V. Wow. That's really up there. Could it have possibly been 25.6V? That would make more sense especially if the bridge rectifier (D3,4,5,6) had a smoked diode. The arcing you mentioned might also be caused by a diode short. Test them with your DMM in diode test mode and replace any that fail the test. Also check the traces near the arcing to ensure there's no damage. Clean as best you can. Share the test results and we'll go from there. Good Luck. -mgriffin
Rectifier diodes
I'm sure it was 256v I checked it three times because I didn't want to sound like a tool lol posting something preposterous. My next step was to test the rectifier diodes but haven't had time to mess with it the last few days. I just replaced all those diodes when I had it apart the first time I guess it's not beyond the realm of possibility that I got a bad one. I will post my results when I get there.
Just a thought
I didn't get a chance to pull those diodes out and check them yet. I was going over the schematic trying to nail down labels on the actual board locations and I noticed C43 a .05 microfarad cap that runs right to ground off of that leg of the transformer in that rectifier circuit. Anyone think that could be a culprit of my voltage spikes and strange behavior?Trying to get my ass in gear and start getting some actual work in on this thing. I'll post after I test the rectifier diodes and that cap.
Here's what I came up with so far
So I checked rectifier diodes D3-D6 and and the .05 microfarad C43 nothing is shorted there. When I pulled the diodes I noticed that the trace pads weren't in the best shape so I did a little closer inspection of the traces. I found what appears to be a broken trace between D16 & R67 on that 32V line which runs to pin 5 which I believe is grid one of the tube. The broken trace was creating a intermittent connection I do believe. Anyways I think I'm onto something finally. Any thoughts ,concerns,or suggestions would be more than appreciated. Thanks in advance guys you've been a great help. Oh I tried to attach a picture of the broken trace but I couldn't get the file size down small enough to attach it.
Broken trace
Incidentally the broken trace wasn't at the location of the arcing I witnessed. It was on the same line but the trace wasn't burnt it was weird it seemed almost like a flaw in assembly.
One of the protection diodes
One of the protection diodes was shorted in my RD115 One Hundred when I got it. The fuse was blown, I replaced it, and ran the amp through a bulb limiter, to check why. I was able to find the bad diode, but lifted both to test, working fine then. They can be tested with your MM after removing the tube.
My buddy told me about the
My buddy told me about the light bulb limiter trick and I always forget it.
Did that first
I had a bad protection diode when I first started on it. I replaced them and then I found I had a problem No. 2 lol.
Fancy equipment
FWIW, an amp issue like this would be a relatively easy thing to diagnose, and ultimately to fix, if one has some equipment at hand. It doesn't have to be very expensive. My oscilloscope is a $20 flea market find (a good one, yes). My variac (variable transformer) was another relatively inexpensive, but super useful piece.
Cheers,
Lars Verholt
Equipment
I've had my eye out for both of those items but haven't been able to come across anything that cheap or anything in my price range as I am on a super tight budget. Most of the amps I repair are more common and have common problems and I haven't really needed anything other than a multimeter or a tube tester occasionally. This one not so much.
Dang Lars, I thought I was
Dang Lars, I thought I was the only one with throw away scope! Mine's an old Bell and Howell school model from the 60's. Works just fine for audio Mike.
Scopes and such
I saw an instructable that had plans for a little box that you plugged your phone into and it turns your phone into an almost fully functional scope and signal generator. It should serve my purpose just fine so I'm going build it and an ESR meter and I should be good to go. Talk about on the cheap lol.
Input voltages
Ok so I made a current limiter after making the trace repairs and checking all my rectifier diodes to run the amp through with the tubes out and speaker disconnected. Now my household outlet voltages are and always have been very low. It fluctuates between 56-89 volts depending on what I have running in the house. Right now I'm running a bunch of electric heaters so we are down between 50 and 60 volts at the wall so by the time it went through the current limiter I was at 48.2 vac at the main in the amp. So the limiter functioned properly and didn't signal any shorts. Most of my voltages tamed down although low were acceptable except 2 and I'm not sure if it might be because the tubes aren't in to complete the circuit but I'm not sure. First questionable voltage was at the orange/yellow or yellow/red wire that runs off the transformer to the power indicator led then off to pin 4 which is grid 2 of the tube. It had a fluctuation in it. It would start at 176 volts drop to 3 then start over at 176 again. I'm going to try to test it at another location now that I studied the schematic a bit. The second was one of the heater voltages was absent. I only had .003 volts at pin 2 of the tube according to the schematic I should have 6.3 with 120v coming in. I was going to double check my measurements today and possibly put the tubes and speaker back in and run it through the limiter to see what happens to those voltages. Any thoughts would be greatly appreciated. Everyone just kinda disappeared.
Heater Voltage
Try measuring the heater (filament) voltage between pin 2 and 7. Pin 2 by itself won't show much since it connects to ground. For the tube accelerator grids (pin 4), if your main house voltage is as low as you say (50-60vac), 176V at pin 4 is in the ballpark. When your house voltage gets back to 120vac, pin 4 should be about 350vdc assumeing the Hi/Lo front panel switch is set to Hi. If the switch is set to Lo, then tube voltages for pin 4 accelerator grid and pin 3 plate will about 15-20% lower. The Hi/Lo switch does not affect the Low Voltage supply voltages. Anyway, you're doing good by persevering. Nothing will work right until the Hi Voltage and Low Voltage supplies are in spec. After that, you'll verify that all 3 zener diodes are regulating properly. Two should be +/- 16Vdc (for the ICs) and one should be +22Vdc (for the tube control grids pin5). Hold off putting the tubes back in until all of your Power Supply numbers are in spec. It would be a shame to white plate the tubes again. Don't worry. We're paying attention like the KGB. -mgriffin
Progress
Thanks so much mgriffin I really appreciate your experience and help. Ok the 176 I pulled was an AC voltage I pulled right off the transformer and after looking at the schematic a second time it's well with in spec. The fluctuation just threw me for a curve. Everything check out right on the money. I had +15.52 VDC at D10, -15.56 VDC at D11 and 21.64 VDC at D9. I had 378.0 VDC at pin 4. Is 378 to high? Thanks again.
Dustin
Next Steps
Hi Dustin. Sounds close enough for government work. With speaker disconnected, go ahead and put the tubes in and switch the power on. If the tube plates start glowing bright red to white, shut it off fast and let us know. If that happens, you'll need to test the power transistors that feed the cathodes of the tubes. I'll try to talk you through that if needed. If the tubes behave normally, turn the amp off and connect the speaker. Plug your guitar in, power it on and see if it works. If all seems good, the next step will be to check and adjust the bias. It's easy to do. Good job BTW. -mgriffin
Are you sure you want to
Are you sure you want to power it up without a speaker load?
It won't hurt
It's OK to power it up for a short time with the speaker disconnected. This won't damage the output transformer or the tubes. Just want to make sure the tubes are idling properly and the plates aren't glowing from too much current.
The output jack is shorted
The output jack(s) is(are) shorted if no speaker is hooked up to it in order to protect the OT and tubes.
Sincerely,
Lars Verholt
Thanks, Lars, I didn't know
Thanks, Lars, I didn't know if there was a shorting jack, (like Leo used in his earlier amps).
Re read
I just now re read your comment Lars and I misunderstood it the first time. Is that what the D14&D15 diodes do short the output jacks if there is no speaker connected?
Transient absorbsion/suppresion
Hi Dustin, D14 and 15 lead very boring lives. For the most part, they do nothing and are just along for the ride. Their job is to absorb a large negative voltage spike (transient) that could happen if the hi voltage power supply were to suddenly and catastrophically fail. They also absorb minor negative voltage spikes that may occur during cycling of the power and/or standby switches of the amp. If you're interested in the science, here's a link that will help explain. https://www.google.com/patents/US4047120 If you look at the diagrams, they're not that far off from MM power amps. -mgriffin
How long are we talking
Just a couple of seconds I'm assuming I always thought that was a big no no but then again there are protection diodes for that purpose. I put some pretty beefy diodes in too cause I couldn't find an exact replacement so I wanted to ensure I was protected. Thanks Mgriffin
Power transistors
I'm a bit apprehensive about turning it on without the speaker hooked up. Do you think I should just check the power transistors. I'm not sure but I think I might have made a little pass fail transistor tester if I can find it. I can't remember if I made it for opamps or transistors. I think my problems all started with a bad connection to the speaker which took out one of the D15 or D14 protection diodes that sit on top of the tube sockets. That was where I started way back when I first got the amp and were the parts I replaced that I referred to in my initial post along with the caps and resistors and stuff. Anyways off to the bench I go. I'll keep you posted. Thanks guys.
I didn't mean to cause a rumble.
Hi Dustin, I didn't mean to cause a rumble. But as you can see, plenty of attention is being paid. Powering up the amp with speaker unplugged from the jack and no input signal will not hurt anything. This is because no amplified AC signal will be present at the output transformer primary side which implies no signal on the secondary side where the speaker connects. But for heavens sake, don't run the amp full bore with a guitar and the speaker unplugged. That would be like holding the gas pedal to the floor in your car with the transmission in neutral. With that said, go ahead and hook up the speaker when you install the tubes. Watch the tubes closely when you power it up and ensure the tube plates don't start glowing like the sun. If the tubes behave nicely. plug in a guitar and take it for a ride. If you want to test the power transistors, here's a quick and easy test you can do. We want to make sure that they're not internally shorted from emitter to collector and you can use your DMM to do this without removing them. With the amp powered off, measure the resistance from the transistor metal mounting tab (collector) to the 3.9 ohm resistor where it connects to the emitter. The resistance should be very high (Mohms) if the transistor is not shorted. If it is very low (near 0 ohms), either that transistor is kaput or the insulator patch under the transistor is compromised and shorting the collector to chassis ground. If one is bad, it is good practice to replace both. I apologize for throwing you a curve ball. It will be interesting to hear your results. -mgriffin
No Rumble
No rumble here my man I just didn't understand how the protection circuit worked till Lars explained it. One of those protection diodes was where my problem first started I think though.
I think we have success!!!!!
So this morning I got up bright and squirrelly got my game plan on and long story short we have sound. I put a set of tubes in and ran it through the bulb limiter and at first it indicated a problem and I thought uh oh. Pulled some voltages they were all off so I pulled the tubes and put a different set in and good to go. I haven't run it full power yet without the bulb limiter but all my voltages checked out. I had 336.4 VDC on pin 4 of one tube and 336.1 VDC on the other. Cathode voltages was 51.8 VDC on one and 51.1 VDC on the other. So I plugged a guitar in and we got some sound. Now I'm going to check the bias and start making her look pretty. Unfortunately I think I might have to sell this one when I'm done. Haven't decided if I'm going to sell this amp or my Gibson hybrid but one of them will have to go . Thanks again for all the guidance you guys are great Lars, Mgriffin
Good job
Good job Dustin. I think we all learned something. -mgriffin
Thanks that means a lot
Thanks that means a lot seriously I really enjoy doing this and to get a good job from someone as knowledgeable as you is more help than you can imagine.
Tube Bias
A thought about tube bias. I read the MM service bulletin on the bias adjustment procedure and I set mine to 37.6mVDc on one side and almost 25 on the nose on the other tube 25.1mVDC. Now in my Valveking bias isn't adjustable. You have to use a quad matched set of tubes but when I got it whoever tubed it adjusted the bias by changing a plate resistor I think. It was supposed to be new! Anyways it put my rectifier diodes in jeopardy and took it out when the tubes went but it was biased. Anyway to get the bias to match perfect couldn't one adjust the value of one or both of the emitter resistor? Would that put anything else in the stream at risk and would it improve sound at all? I haven't even had a chance to really play with the amp yet so I really don't know how she sounds or if something else might be weak. I know it has some scratchy pot issues that hopefully can be cleared up with a squirt or two of some good contact juice. I don't know just a thought. Thank again guys for all the info.
Good question
It would be hard to get different resistor for the 3.9 ohm since the next available value would be too far off. What you're seeing with the mis-match bias adjustment between tubes is not because the tubes are mismatched. It's caused by the resistor network (the 470 Ohms) that make up the transistor bias circuit not being perfectly matched. Also, the 3.9 ohm emitter resistors aren't perfectly matched. All of these little mismatches add up to skew one side of the push-pull circuit being slightly hotter than the other and your bias measurements bear that out. I believe when your amp was manufactured, 5% carbon comp resistors were used. Use and age take their toll these so it's doubtful that any of those resistors are spot on. Will this aging make things sound better or worse? Probably not. When I restored my RD65-112, I saw the similar bias mismatch as you have. But if you haven't figured it out yet, I'm a retired electronic engineer and I was having none of that. I knew that if I replaced all of those OEM carbon 5% resistors with matched 1% metal oxide resistors, the two bias measurements would track much closer. Also. replacing the power transistors with a matched set would get them even closer. So I did it and now the bias measurements track each other very closely, always within 1mV of each other. Unlike the earliest MM amps that have a "classic" tube power amp design (and require matched tubes), when MM started using the circuit with a transistors controlling the current passing through the tubes, this eliminated the need for matched tube sets. It's a very clever design. You get that classic tube sound without all inherent tube mismatch problems. I hope you enjoy your now working RP65. -mgriffin
When I set the bias on mine,
When I set the bias on mine, it seems the longer I let it idle, the closer the values got. I put a set of JJ's in it, nothing spectacular.
Junk tubes
I shouldn't say that these look like old sylvanias who knows they could be some rare old stock I don't know they came in it when I got it at the yard sale. I got a nice set of electro harmonix for it but when I fixed the protection diodes with the short that I had it cooked them. I'm going to re-tube it anyway so as long as bias isn't crazy outta whack for now is fine. I didn't let specifically set and idle for a real long time but the tubes should have been good and warm.
think I decided to keep it
I think I'm going to hang onto this and sell my Gibson. I generally play tube amps because of their distortion and my Gibson is hybrid but is a tube preamp. So I'm going to keep the MM and build a tube based distortion unit based on the design of the Gibsons preamp. Then I have the best of both worlds.
Good info
Thanks for the info on Transient suppression circuits Mgriffin. That is certainly good info to have I saved that to my Pdf library for sure. Now I was wondering if it wasn't to much trouble if you or anyone else on here could explain how the phas0r circuit in these amps works. I'm sure if I study hard enough I could figure it out but it would be great to get a professionals take on it for sure.