Need Help with Hot Rod Deville--bias power supply issues(located the prob)

[Platefire]

 :BangHead:I got this in for repairs for a red plating power tube. He said he had tried other tubes but the left power tube looking from rear red plates with every tube tried. I tried my own tubes and sure enough it red plated real quick, as soon as you turn on the DC power. Checking with a multimeter
I get "0" bias voltage at the bias check point. Checking a little further up the bias supply line at C36, the schematic says 28VAC supply---my fluke reads .028VAC set on automatic. I'm using the schematic from Doug's Schematics attached. Is a blues deville and hot rod deville the same?

So next I checked the bias supply connections from PT connections on board from connector CP15 to C36 neg end and got continuity. Next I checked continuity from PT connection from CP17 to CR18. I get continuity for a second and then it cuts off---should be constant continuity I think? between these two points--believe this might be my problem??? possible bad solder joint at this point? I did try a jumper from CP17 to CR18 but still didn't bring up the bias voltage? Not sure where to go from here? Any help appreciated. Platefire

[stratele52]

...continuity between CP17 and CR18 ????

To test and fix the amp ; First ,REMOVE all output power tubes , fix bias voltage before. Tubes might be defective now  if they run without bias too long.

You should have very low resistance , "good" continuity between CP16,17 and 18 .

You must have 28 Vac between CP16 and CP17,  if not, be sure you are on the transformer wires , solder may be bad or you have a bad contact with your voltmeter test probes.
No voltage ; transformer is defective . Double test .
Good voltage ;  follow the circuit voltage to find where is the defective component or bad solder .

Good luck

[Platefire]

I wonder why only one power tube is red plating and not both since both are without bias voltage?
Also I wonder what the voltage is suppose to be from PT to connections point at board CP16 & 17?
Didn't see were the schematic said. I measured them once and as I recall was about 72VAC. Plate

[sluckey]

Is a blues deville and hot rod deville the same?

No. Get the correct schematic. There are differences in the bias circuit that will mess you up if you use the wrong schematic. Doug should have the correct schematic.

[Jack1962]

Nope they aren't the same , which hot rod deville  is it ?

[stratele52]

I wonder why only one power tube is red plating and not both since both are without bias voltage?
 

1- Check your voltmeter reading .

2- One tube is dead or slow to red plating ? I think you don't let them redplating for "long" time to watch  ?

[stratele52]

Nope they aren't the same , which hot rod deville  is it ?

+1000

First thing to do before troubleshooting

[Platefire]

My amp is dated 28 Feb 1997. The amp came with CP 17 hooked to brown from PT, CP16 to brown/yellow and CP15 to brown from PT.

According to the 1st Sche I posted 1993/1994 CP17 is suppose to recieve brown/yellow, CP16 brown and CP15 brown. I have 27VAC on both browns. The 1993 sche says CP16 should be 28VAC. I did try switch the brown/yellow to CP17 but it still red plated. So I'm wondering if I need a updated 1997 sche???

Can't find a sche that matches exactly what I'm seeing on this amp--still looking. plate

[PRR]

> the left power tube

Bad bias supply spoils *both* tubes.

*One* tube glow, look at the grid circuit. Is 220K really getting bias and moving it through 1.5K to grid? Is the 0.1u grid coupling cap leaking and pulling grid up?

i.e. with 6L6 out, and say -54V at bias adj. trimmer, there should be -53V (negative!!) at 6L6 grid. (Not quite -54V due to meter loading.)

[Platefire]

PRR there is no negative voltage coming out of the bias circuit at the bias pot R82. Schematic says it should be about -54.6. I finally went through all the schematics and the attached one looks closest to my amp. The bias circuit is failing somewhere between the PT taps board connections CP15, CP16 & CP17 and the bias adjustment pot. I'm not even sure that the PT bias taps are hooked up correctly to board connections CP15, 16 and 17. My amp has a brown/yellow center tap along with the two brown taps from PT. The schematic attached doesn't have a brown/yellow center tap?

Regarding the grid and bias resistors at power tubes, they are located on the bottom side of board
that the power tubes are mounted in and can't be seen unless you pull the board. I think if I got the bias circuit working putting out the right neg voltage at bias pot--everything downstream would fall in place. Working on the PCB boards is like pulling teeth. Platefire

[stratele52]

My amp is dated 28 Feb 1997. The amp came with CP 17 hooked to brown from PT, CP16 to brown/yellow and CP15 to brown from PT.

According to the 1st Sche I posted 1993/1994 CP17 is suppose to recieve brown/yellow, CP16 brown and CP15 brown. I have 27VAC on both browns. The 1993 sche says CP16 should be 28VAC. I did try switch the brown/yellow to CP17 but it still red plated. So I'm wondering if I need a updated 1997 sche???

Can't find a sche that matches exactly what I'm seeing on this amp--still looking. plate

27 or 28 VAC same thing , just find why you don't have negative DC bias

[Glennjeff]

Very unlikely that someone or something (cue X Files theme) has altered your transformer wiring. And to my reading the schematic supplied (2 replies up) should work with your original wiring.

CP17 Brown
CP16 Brown / Yellow
CP15 Brown

Forget about the transformer hook-up.

PLEASE do this and then answer this very simple question with a  number. No stories about how hard it is to work on SS PCB's or Aunt May Bell's blueberry pie. Just a number. (I'm being a sarcastic prick for a specific and helpful reason by the way)

Remove the output valves and leave them removed until we make some progress.  Forget about them.

Disconnect the circuit at the point on the bias supply indicated by a triangle with a 5 inside of it, it is also labelled as  -54.6V -C. It goes of to the grids of the output tubes via grid leak resistors R59 and R60..

What voltage do you measure at the junction of CR15 and C43. That is at the anode of diode CR15 and the negative side of the 100 uF electrolytic capacitor C43. It should be somewhere around -75V DC.


This process should take no more than 7 steps to complete.

[Platefire]

The bias pot R82 output=0.000DCV

Can't access the junction of CR15 and C43 becasue it's a can cap with leads not exposed on the oppisite side of board--however I do have access to R76 that shows should be 74.3V on the schematic but also reads 0.000VDC. Thanks, Plate 

[Glennjeff]

Platefire,

Good, er I mean Bad.

If you have 0 V on both sides of that resistor, and the resistor does not measure as a short circuit then you most probably have a severe problem with one or more of these components.
C37
CR10
C43
CR15

It could also be solder problems or circuit track burnout associated with these components.

You will have to get access to them to fix the problem if that is the case. As they are about 5$ worth of parts you possibly should replace them all.

If you like, we can check a couple more things to make sure before you proceed.

Can you check these specific items, still with the output tubes out.

1. WITH POWER ON Check that the transformer has AC voltage on both taps to ground, ie that you have similar AC voltages between the brown wires to ground (NOT the centre tap) and then check the AC voltages between brown and centre tap (brown/yellow). They should be very similar. What is that AC voltage.

EDIT: I see that you measured about 28 V AC upthread, which is about right if a voltage doubler is being used to get 75 V DC. Won't hurt to check again.

2. If you can get to C37 and CR10 check to see what voltage, both AC and DC, is present at their junction.

3. WITH POWER OFF: Check the resistance between the junction of R76 and R83 and ground - it should be approximately 20 to 30 k (It may read a short with the test leads one way and then 20 - 30 k with the test leads the other way - that's OK)


What resistance are you measuring at that point with the power off.

If you could take high resolution photos of the parts before you start removing and replacing them, that may just save the day, especially if the circuit is somewhat different to the circuit diagram. A photograph will give us a good record of the parts used. It's a very standard power supply so as long as we have a record of the parts it should be pretty easy to reconstruct.

[Platefire]

the reading at the junction of R 83 & 76 to ground is 35.7K.

I was hoping I could get this going without pulling the board. Have very little experiance with PCB boards plus I don't have a bench or work shop to leave it lay while parts arrive--I may need to talk with customer before I proceed. My standing agreement with him is I normally don't work on PCB boards but I am willing to take a look to see if there is a simple fix I can manage. Plate   

[Jack1962]

do you have voltage at R76 ? CR15 is a diode you should be able to check voltage there if not the you should be able to at CP15 if you have voltage at CP15 but not at CR15 replace C37 , brother it's just like any other circuit don't let it get the best of ya.

[Platefire]

Wow Thanks Jack. I'm slow but I eventually get there    You gave me an idea because CR10 and CR15 join in a junction recieving their power from C37. So I temporarily jumpered a 100uf/50V cap from CP15 to the junction of CR10 and CR15---Bingo!!! Got -71VDC at R76 and 40mV at the 1ohm resistor bias check point. No red plating happening at this time. Ran my guitar through the amp and other than some scrachy volume pots, the amp sounds pretty good.

You guys are amazing! Plate

[Glennjeff]

Great stuff.

Still have to wonder why only one tube was red plating. Might be an idea to check all wiring and clean and  retension sockets around the output tubes and output transformer.

All the best.

[Platefire]

I'm tempted to just scab a 100uf/100V axle from CP15 around the bottom C37 to the leads of CR10 & 15 junction with just a lite dab of solder. They are only about 1" apart. It could always be removed easily if someone else had to pull the board for something else, then could go ahead
and replace with original. Would this be terribly bad practice or reasoning? Platefire 

[sluckey]

Be professional.

[Glennjeff]

You already know the answer, It's bad practice, it could work OK.

You need to make sure the faulty component is no longer in circuit at least, one day it might decide to change from open to short circuit or whatever.

I'm not sure what your reluctance at dealing with PCB's is. Would you care to explain why you don't want to pull it apart and do it correctly / professionally. One day, if you keep doing this type of work, you're going to have to "get over it", why not today ?????

All the best.

[Platefire]

I did some pretty extensive searching and could not find a replacement for C37 radial Electrolytic. It's "IC" which I suppose is a Illinois. It's about 1/2" diameter and about 1" long. I've looked on the net at Doug's, AES and general google searches and haven't turned up nothing. Where do you find these caps?

My reluctance with PCB's is just lack of experiance and lake of facilities to properly work on them.
Plus I will have to do it super slow to do it right because of my lack of experiance and there is no way I could charge my customer for the labor I would have in it. Knowing this paticular customer as I do, if I gave him a price for scabbing it in and doing it the proper way, he would tell me to scab it--honestly. I really like doing things the proper way and practice because I like to feel good about my work and take in consideration future owners of the amp that they not end up with a hacked amp. Platefire

[Glennjeff]

Know your problem , "client wanting the cheap way out and not paying for time", quite well.

C37 100uF 100V - Doug has one on his page, brand doesn't matter, axial leads are better if your going to wing it.  If your going to do a "bush mechanic" job, do it well, and always remember - heat shrink, silicone rubber and superglue are your friends.

OR

Do it with the PCB for experience sake and squeeze whatever you can out of the client (in a nice way).

Glad you found the problem.

All the best.

[alerich]

I'm with you, Plate. Hate PCB amps with a passion. I don't do this for a living. I'd starve. I mostly work on and build my own gear and do the occasional repair or re-bias for friends but I am really picky about what I'll work on. I installed a simple relay activated solo boost in my Mig60 and posted some pics on my Facebook page. One of my buds asked if I could essentially re-engineer the awful tone sucking effects loop on his Marshall TSL head. I have nightmares just looking as pictures of those amps online.

Glad you found the trouble and thanks for sharing.

[Jack1962]

you can replace that cap with any good cap of the same value , I would use the radial type because that's what belongs in the amp (axial caps are not better than radial caps) I would remove the board a do the replacement . Quality First Brother 

[sluckey]

Mouser has a huge selection of 100µF/100V radial caps...

http://www.mouser.com/Passive-Components/Capacitors/Aluminum-Electrolytic-Capacitors/Aluminum-Electrolytic-Capacitors-Leaded/_/N-75hqw?P=1z0wrkmZ1z0x1d4Z1z0z7l5

[6G6]

Yeah.
If Mouser dosen't have, it probably doesn't exist. 

[alerich]

Be professional.

+1. If you are going to do the job do it correctly. A local shop here did a cap job on a blackface Super Reverb and just snipped the leads of the old caps and soldered the leads of the new caps to those of the old caps. It looks sloppy and haphazard. I don't need anyone to do amp work on my gear but they also do luthier work but they'll never see one of my guitars in their shop. Your reputation is important. Do it right.

[Platefire]

Well the wife and I are taking a short vacation for a few days and I'll ponder this over. Thanks Sluckey for the cap link, no shortage there  


So---on the board it would have to be removed and laid over on it's back side to de-solder. First would I need to remove the chassis from cab or just disconnect the board with the backplate off? Just off the top of my head, I would remove all the pots and jacks nuts/washers/knobs through the top of chassis. Then it would be unplugging/removing all the connections to board and labeling them where they go or maybe not if you could just lay the board over on something without unplugging them? Then the screws holding the board to chassis. Finally laying the board over as much as ribbon cables would allow then remove component and install new--check continuity of bias circuit to new cap leads--then do everything you just didi in reverse. Anybody got a general proceedure with tips and pointers wrote down for this type of operation? I always take a lot of notes on something like this so I can get it back together right. So can you test this live with the board out before it goes back in or not? Plate  

[super&plexi]

hi, and congrats, I'm in tail end of one of those cheap p.c. headaches. As usual, the advise is spot on, and if i may add, if it's fender type w/separate small board for pots on face, and LG. main board, for components,

 sometimes(quite often) the jumpers going from board(small) to board(large), are piteous, laughable, and some other words I wouldn't use around my Biker Friends. they will break, and if not while you are working on them,

Soon after, if Mr. Murphy has any pull at all, which is usually the case. I have found that these kind of amps can actually bring you closer to a client, sometime's, being honest and upfront with them like you are, and possibly working out a pre arranged barter, as in; they can be, and quite often are a lot more work than seems, but if easy, low$ if a pain higher$, or "what do you have to trade?''.

  I'm simplifying of course, and you may be way ahead of me, feel free to set me straight, but I used to do this kind of stuff when we had a store, and customers seemed to be very loyal, and you can come up on real nice things on the back end, coming out far ahead of just straight cash up front.

 we haven't had dealings, but always nice to see pride, as it is obvious you do care about your work and image, in that you bolster that feeling of Yeah Hon, I could fix it myself, but this Guy's gonna do probably a better job of it, and we take it easy, go fool around while it get's taken care of,  I trust him. not enough of that anymore.           

[phsyconoodler]

Plate,you can do this.Once you carefully remove the board(not all the way out).Once you get it folded back you can easily get at everything. Equipment? All you need is a magifying lamp,desoldering braid and patience.

[Platefire]

Hay Phsyco

Thanks for the vote of cofidence! Maybe I can shop for a magnifing lamp on my little trip. First stop in Baton Rouge at SonLife Ministries to hear Bob Henderson play and maybe meet him. I never seen a guy with so many strats. He's always an inspiration. He's also a great sax player and singer.

I really need to take on this PBC thing because seems like more and more people are finding out I repair amps and I don't know of anyone else in the area that does it. If word gets out that I work on anything, it could turn into a flood   Thanks, Platefire

[sluckey]

I really need to take on this PBC thing

There you go! Just be careful, slow, and methodical. It ain't gonna be your money cow to begin with. But you'll get more proficient. And people will appreciate a job that's well done.

BTW, there's no way I'd ever do this stuff for a living! 

[LooseChange]

PC Boards can vary a lot. Some will disintegrate as soon as you apply heat. There is a huge learning curve when repairing this kind of stuff.
You will get pc boards with missing traces or the pads are lifted. It takes a lot of patience and experience to fix these things without making it worse.
There is a well known tube amp guy up here who believes he is cursed when it comes to pc boards and won't touch them ever.

[phsyconoodler]

there is a well known tube amp guy up here who believes he is cursed when it comes to pc boards and won't touch them ever.

 Hahahaha !!!

 I used to think that way but after modding and repairing umpteen dozen Hot Rod Fender amps I have no issues at all.I find them rather easy now.Now surface mount amps I don't even touch.I think they are an abomination to the tube amp world.

[Platefire]

Loose Change----That's not too encouraging  

Got some questions on the radial cap to be removed/installed:

1-So the bad cap is flush up against the board so I can't see the leads. Do I need to pull the board and remove the old cap so I can see the distance between the old leads and order a cap
where the distance between the leads will match the holes in the board? The leads on the new cap matching the existing holes is a concern to me---should it be? or just bend to fit as normal?

2-What about de-soldering/removing old and soldering in new cap??? PCB soldering practices?

3-how careful do I need to be when folding the board down to work on it---in flexing those 3 old ribbon cables? About 16 years old--1997

Phsyco share some of your PCB practices!! Platefire  

[LooseChange]

Sorry! You've got lot's of repair experience.
1. If you get a cap that's a radial and is the same size, it will be really close and most likely fit. You need to pull the board to remove the cap. Don't worry about the new cap fitting. You can always bent the leads and use some silicone to keep it in place if needed.
2. I heat the solder joints and pull the part out. Some will break the part off. I don't do that. (if removing parts where you can see the leads, cut them away from the part and just unsolder the leads and pull out) Remember if the part discolored the pcb due to heat... Be extra careful about heat exposure to the traces.
3. Those ribbon cables are pretty tough. Just be gentle.

The HR series have pretty good boards and should not create any probs for you.

[Platefire]

The attached scan of layout drawing shows the left end of main board just as you see it on the left side when you take off the back plate. All those power supply connections can be un-plugged and moved out of the way with the exception of P5 & 6 Reds from PT. Those connections are soldered and with short leads kind of restrict your ability to fold down the board to get access to leads of C37 like phsyco talked about. The bad C37 is shown right at the bottom of scan. I haven't tried to remove the board yet, just been looking to see what things I see that might be an issue when it comes to getting access to the back of board. This is the main hurdle I see.  

Just a question? When desoldering a radial electrolytic like LooseChange said he did, you can only desolded one lead at a time, so when you warm up one connection the other will still be soldered holding removal back. Only way I can see doing it that way is rocking it side to side to get one side out first and then just warm and pull out the other one. Or use a solder sucker to pull out the solder on one side.

Like I said, I just got my light out last night and took a close look to begin to plan my proceedure and this is a couple of things I saw. I know what your going to say, just do it! Plate  

[sluckey]

I'd use a solder sucker or wick (or nice PACE desoldering station) to remove the solder from each lead.

However, it's only two leads and they are only 1/4" apart. Grab the cap with one hand. Now put your iron tip on one lead. As soon as the solder melts start rocking/pulling the cap and quickly move the iron tip to the other lead. Keep on rocking/pulling. You may have to go back and forth once or twice. Just always move the tip as soon as the solder melts. You don't want to leave the tip on the board or you risk damage. I don't recommend this procedure on cheap boards. My Twin Reverb Reissue has a cheap board and I will not be removing components this way.  But then I have the proper desoldering equipment.

[Platefire]

Best I can tell, Mouser doesn't take pay pal and that's how I purchase my repair parts.

I found some Marcon radials on e-bay. Does anybody know about these. Think they are from Japan, sold in the usa.

Also how about Lelon caps? at this link

http://www.parts-express.com/pe/showdetl.cfm?partnumber=020-1312

[Willabe]

Best I can tell, Mouser doesn't take pay pal and that's how I purchase my repair parts.

I keep a separate checking account at a small local bank that doesn't require a minimum balance so it's free. They give me a debit/charge card for it. I only keep enough money in it to buy parts over the phone or on line. That way if anyone gets into the account it's only a few bucks and my house hold bill money is safe.


                  Brad     

[Platefire]

Yow, the two caps brands I listed in previous post are I think both from china. From my research Electrolytics from china are not good but I read those from Japan are genrally good. Found a panasonic from West Florida components.

Up to now I've been able to get all my parts using paypal but I guess the radial electrilytics are little more rare. The size is 1/2" Dia x 3/4" long. Plate

[Willabe]

Try Digikey. Just as big or bigger than Mouser.


             Brad     

[LooseChange]

Plate, FWIW... Get a PayPal Extras credit card. That has solved all my problems. I use the credit card when PP is not accepted and I pay the CC bill with PP online. Always pay it off and never paid a fee.

[Platefire]

Well I prayed hard all the way through but got the PCB board out, installed my cap and re-installed without any major problems and the amp seems to be working fine. The hardest part
was getting the top of the pot shafts to clear the top edge of chassis opening. It was really tight. I was cued off on this by this site http://www.bustedgear.com/repair_Fender_deville_jack_2.html
that was really helpful becuse I was prepared for that issue. Once I got the board cleared and flipped over it was pretty easy changing the cap. So I appreciate all the encouragement and pointers. This is my first PCB board of this size to deal with. Thanks a bunch!!!   Plate

 

[LooseChange]

Nice job! Just wipe the sweat off your hands before you flip that standby switch. :)

[DummyLoad]

 

good news old man! see, ya can teach old dogs new tricks!

grats...

--pete

[Platefire]

I hope this don't mean I'll be expected to work on PCB boards from now on?     

[phsyconoodler]

Plate,you are now a member of the Fender HRD PC board club. Welcome the new member people!

[Platefire]

The initiation was "Trial by Fire", when I thought the waves of PCB board was was to carry me assunder and my last breath was vanishing away at the last second I reached for the top of the chassis and the board fell in place and I was again at the surface gasping the solder smoke filled oxygen knowing it was over and I survived. I hereby accept entrance into the HRD PC board club.  Plate