Hot Rod Deville what isn't working proper

[norris2002]

Hello.

I was talking to an old friend, and he told me that his 1996 hot rod deville has stopped working and he was going to take it to a tech, but i asked him to let me have a look first. I only had a few hours to mess around with it last night, and there are some improvements, but still fully working.

There was no sound at all. I checked the preamp out into my champ, and that worked fine, so started looking at the PI and power amp sections. The 82k plate resistor on the PI was reading open circuit, I replaced that and the sound came back, but at no where near the 60W volume that you would expect. also, the sound has quite a flubbery distortion.

Checked voltages and B+ was running about 80V high (514V rather than 432V), the heaters at 7v and bias at -54. Checked the transformer wiring, and it was set up for 230V, and our wall was reading 242V. swapped the transformer wiring for 240V, and this has brought the B+ to 488V. still no improvement. Used a variac to bring the B+ to what should be expected, and still no improvement.

I then checked the voltages around the PI plate resistors, and they were reading something like 270 or 300 on the plate. I will not be able to get back to the amp for about a week, but does anyone have any idea?

other checks that I have done: filter caps, all ok. Run a scope with a small aerial around the pcb / speakers, no crazy parasitics going on. All tubes have been swapped with known good replacements to test out.

any suggestions would be greatly appreciated.

Neil.


http://schematicheaven.com/fenderamps/h ... eville.pdf

[norris2002]

After looking around on the interwebs, it looks like these PI plate resistors are a known problem in this series of Fenders. I will tell my friend it would be a good idea to source new 82k and 100k resistors at 1W, preferable carbon comp or other high voltage type, and we will swap them all over when we get the chance.

The amp has always sounded a bit ratty since he has had it, and seemed to give up the ghost completely before he gave it to me.

[billyz]

The Pi plates are unreliable in some of these Fenders. I always check them. None of the other resistors are CC though. I would use any good quality type there. I usually use 1/2w with no problems.

Another common fault is the two ceramic resistors for the channel switching circuit. 15v + and -. They unsolder them selves from the board. Since you are at it, I would resolder them.
I suspect it could be the ROHS solder used, it is brittle. I always use the leaded variety and never have a return. Some take to other measures like elevating them off the board, different values etc. even remounting them elsewhere. and resolder all the connections on the board for the pots and especially the input jacks. I even replace them with switchcraft jacks for the ultimate in reliability

These are always biased very cold, run them up to 60-70%. one other thing I do after approval from the owner is to increase the NFB resistor to approximately 20:1 ratio. They always like it.

Flubby sounds could be the Tubes or speaker. Also, use a really good PI tube.

[norris2002]

I was planning to re bias it and change the v1 cathode cap at least after i got it running better. I will check out the solder joints you mentioned and ask him to consider a change to switchcraft style jacks. It has that style on the speaker out, but not the input. I guess that is sub-assemblies for you.

Thanks for the input Billyz, just need to find the time to get the amp back on the operating table.

Neil.

[norris2002]

Well, I finally got back underneath it, and had a good root around. Some of the solder pads looked less than desirable, so I touched up all the ones that looked bad. I then replaced the other PI plate resistor, as these are known to go also, re soldered the 5W resistors in place, resoldered and checked continuity on the Power tube sockets and checked the speaker output jack for shorted contacts (there was a stray bit of solder that looked bad, and the contacts could do with being spread out a bit).

replaced all the tubes and fired it up, and it worked again!!

Next step is to re-bias and replace some other components. Other than the following modifications, is there anything else people would recommend for this line of amps?

-change C1 (1st stage cathode cap) from 47uf to 25uf
-change drive channel 1st coupling cap from 15nf to 0.022uf


I will have a look at the neg feedback (increase R69 to something like 500k or 1M??)

[norris2002]

Apologies for resurrecting this thread again, but I need some more help.

I resoldered quite a few pads and replaced a couple of parts (PI resistors), and on testing the amp (with PCB hanging out) it worked as normal. After fixing the PCB's back into the chassis, and tightening up the front panel, the amp sounds fuzzy and low in volume. This makes me think that there is a bad solder joint somewhere in there, and fixing it back into normal position has opened it up.

What component is likely to cause this problem. There is continuity on the tube sockets, and I am 95% sure the problem is in the Power amp section. Any help would be greatly appreciated.

Also, there is a 1ohm resistor connected between the tied cathodes of the power tubes, and a diode in parallel with this. do i need to disconnect the diode to read the bias on this 1ohm resistor? it would show the combined mA across both tubes wouldnt it?

Hope someone can give me some good pointers.
Neil.

[billyz]

Also, there is a 1ohm resistor connected between the tied cathodes of the power tubes, and a diode in parallel with this. do i need to disconnect the diode to read the bias on this 1ohm resistor? it would show the combined mA across both tubes wouldnt it?

Hope someone can give me some good pointers.
Neil.

Look at the power tube board far left outside corner. You should see a bias test point. This is mv reading for both tubes. It equates to the current draw in ma's. You do not need to lift the diode.

I have had to, several times, carefully look at the backside of the main circuit board when I have a problem that was not cured the first go around. I almost always find another bad solder joint. Usually very hard to see. Use a little dab of flux on each joint to get it to flow right. Try to wiggle the component see if it is a little loose. If it worked with the board out then I would look at the standoffs too.

BTW, I never change those cathode or coupling caps you mention. The feedback resistor R69 I would change to 100K if , R68 is 4.7K. If you go to 500K then you might as well disconnect it ( you might like that ).

A lot of techs don't like to work on these. I have done so many now that it is not that big of a deal . You learn a few methods to quickly pull a board out and flip it back. I think I do 2-3 a week and it is almost always the same issues, bad solder joint, open PI resistors or bad tubes, and of course the input jacks, now and then a bad speaker.

[ampdoc1]

I'd replace the FETs Q1 and Q2. These perform the mute function when changing channels. I've seen these go bad, and usually the sound is completely dead. But, I suppose they could fail without shorting completely.

ampdoc1

[norris2002]

My understanding was that these fets were used to bring in different bypass caps on the 3rd (drive) stage and 4th stage (by bypassing the large value resistors)

I should get a chance to look at this again in the morning, and have a good look for bad joints. I think i remember seeing the a pad on one of the 5W 15v supply resistors having lifted. could this cause the bad distortion and low volume on the normal channel. My understanding is that if the amp is in the normal channel, then there is no input from the ss devices (except reverb send and receives) and the rectifier diodes. nothing much to do with the 15V supply at least. Please, correct me if I am wrong.

I want to get this amp working with the limited access I have, as my friend cannot afford right now to send it to a tech. It should be a simple problem, but with PCB, it becomes much more difficult to trace faults.

Thanks again.
Neil

[billyz]

I should get a chance to look at this again in the morning, and have a good look for bad joints. I think i remember seeing the a pad on one of the 5W 15v supply resistors having lifted. could this cause the bad distortion and low volume on the normal channel. My understanding is that if the amp is in the normal channel, then there is no input from the ss devices (except reverb send and receives) and the rectifier diodes. nothing much to do with the 15V supply at least. Please, correct me if I am wrong.

When ever I see a lifted pad, I scrape back the trace to find some good copper and bridge it over with solder, might need some flux on it too. If the pad is lifted, the resistor is going to flop around and eventually break off. Since those 5w resistors are big and heavy this could be one for an off board mount.

[Structo]

A person could install a small eyelet but crimping the bottom of it would be problematic.

[ampdoc1]

Norris, I'm sure you are correct. I was thinking of the Blues Deluxe and it's heir. Had a lot of them go dead silent when the FET controller died! That and the PI resistor.

ampdoc1

[norris2002]

I was referencing some of the voltages, and the top and bottom of the OT read at about 480V, not the 34V that the schematic states!!! This cant be right! Do i have a blown output transformer, because that sounds expensive.

I will pull the contacts and read the resistances across the OT. I guess that as long its not reading 0 ohm its ok? This is really confusing me.

[Structo]

The B+ is 480v, which is connected to the center tap of the primary on the OT.
It calls for 94v at CP19 and CP20.
I am not sure what they mean by that but since the primary is getting 480v I would expect to see close to that on the plates of the 6L6.

[norris2002]

I must have mis-read the voltage as 34V not 94V (I have an A4 printout, so it is a bit or a struggle to read some of it). I did think it odd that it stated such a low voltage on the plates of the output tubes.

Low volume and flabby distortion does sound a lot like PI problems to me. I will check out all of the circuit around the PI again, and see if I can make it work again. It did work briefly, And I am sure it is something simple like a bad connection somewhere. I may just suck up all the lead free solder on the PI components and replace it with better leaded solder.

Neil.