Blues Deville help needed---UPDATED

[TweeDLX]

I'm working on a '94 Deville for a friend. It came in with low volume and sounding muddy/distorted on the clean channel. I plugged directly into the Power amp in, and got the same results. Running from the Pre-amp out to another amp worked OK. So, I assume the problem lies in the power amp section. No obvious burned areas or parts. I replaced a couple of resistors in the PI that were severely drifted or cracked and got volume back, but the amp is still very muddy sounding and distorted. I also reflowed all the solder joints on the tube section. I see pretty good voltages through the power supply filter caps, but one thing that bothers me is that I'm not getting a good reading for heater voltage. Measuring from pin 9 or pins 4-5 to ground on the pre tubes shows 0 volts. Same thing at the lamp, heater wire connections... Am I doing this wrong? Any other ideas as to what might be the issue here? Thanks for your suggestions.

Mike

[Raybob]

Originally Posted by TweeDLX

... Measuring from pin 9 or pins 4-5 to ground on the pre tubes shows 0 volts. Same thing at the lamp, heater wire connections... Am I doing this wrong? Any other ideas as to what might be the issue here? Thanks for your suggestions.

Mike

Set your meter to read AC instead of DC on the heaters. You should see around 6.3vac.

Does the other channel sound muddy too?

[TweeDLX]

Originally Posted by Raybob

Set your meter to read AC instead of DC on the heaters. You should see around 6.3vac.

Does the other channel sound muddy too?

Thanks for the reply Raybob. My DMM has an Auto feature that (I believe) detects whether it's reading AC/DC voltages. I'll double check in the morning. I'm hearing mud on both channels. For the record: I've swapped in two different (known good) sets of tubes all around with no change, cleaned jacks, used a different speaker. For the record, I really hate PCB's...

Mike

[Rich M]

My first thought was check the PI plate resistors as I had a HR Deluxe with a bad one. Sounds like you're one step ahead...

I'm not an expert on DMM's, but I'd be surprised if it didn't allow you to select between AC/DC. Often, you want to measure DC offsets on AC signals ir AC on DC nodes so manual selection is important. If you had no heater voltage, the amp wouldn't work at all.

Did you check couplers for leakage? You may want to check the PS caps as well.

Good luck.

[donnyjaguar]

Are you sure the speaker isn't whipped? I've had speakers partially melt down the voice coils rendering an 8Ω to about 1.5Ω.

[jay42]

The six large electrolytics are suspect, ime. Do not replace with the same brand...look for Nichicon or something other than IC.

[TweeDLX]

I have my "Test" speaker by the bench (Celestion G12H80) that works fine, but sounds like poo with this amp. Yes, I replaced the plate resistor on the PI first! I read several threads here that discussed that issue, so it was the first thing I checked. I plan to check couplers today, once I ingest enough coffee. Voltage through the filter caps is reading in the 450+VDC range. That was another "check first" area. Should I remove and test them for leakage? There are a few that have (possibly) been changed in the past, but most look like the originals.

Mike

[TweeDLX]

Originally Posted by Raybob

Set your meter to read AC instead of DC on the heaters. You should see around 6.3vac.

Does the other channel sound muddy too?

It does help if you read the instruction manual! I do have heater voltage after all... OK, so the amp still sounds crappy. I pulled one if the .1uf caps in the PI and read .123nf on the meter. Replaced that one. No change. What a pain to work on! I don't have any 100uf filter caps, but I can replace the two 22's.

[bonertone]

Check the screen grid resistors on the power amp card I have seen those fail often on that amp.
If that is what it turns out to be you would be better off replacing them with a higher wattage resistors.

[Raybob]

Originally Posted by TweeDLX

...Voltage through the filter caps is reading in the 450+VDC range. That was another "check first" area. Should I remove and test them for leakage? There are a few that have (possibly) been changed in the past, but most look like the originals.

Mike

Since some but not all the filter caps were probably changed, I would think the others need changing as well, along with the bias cap, especially since this amp is 14 years old. Definitely check them. That's most probably your problem.

[dodgeone]

Please....for god sakes.....before you tear that amp apart, read up the troubleshooting part of this web-site.
I will practically garantee your problem is amongst the ones listed therein.
Regards.

http://studentweb.eku.edu/justin_holton/

[TweeDLX]

Originally Posted by dodgeone

Please....for god sakes.....before you tear that amp apart, read up the troubleshooting part of this web-site.
I will practically garantee your problem is amongst the ones listed therein.
Regards.

http://studentweb.eku.edu/justin_holton/

That was my first research stop before posting this thread. Second was Prairiedawg's thread on the HRD here at TGP. Lots of good info that led me to the bad PI plate resistor. I'm going to change the filter caps, just because they're due. My friend's B-day was on Friday, so this is his belated "present". New IC's on the way from Doug Hoffman. I would have preferred F & T's, as they come in a smaller size. The Sprague Atoms I have won't fit.
Bonertone, thanks for the tip, but I already checked the 1.5K's, and they still be good.

Mike

[Billion81]

Originally Posted by TweeDLX

That was my first research stop before posting this thread. Second was Prairiedawg's thread on the HRD here at TGP. Lots of good info that led me to the bad PI plate resistor. I'm going to change the filter caps, just because they're due. My friend's B-day was on Friday, so this is his belated "present". New IC's on the way from Doug Hoffman. I would have preferred F & T's, as they come in a smaller size. The Sprague Atoms I have won't fit.
Bonertone, thanks for the tip, but I already checked the 1.5K's, and they still be good.

Mike

Dude- I don't have any friends like you. What a bro that would take an amp and trouble shoot some needle in a haystack problem for a birthday present.

Hope your bro appreciates it. Wait, did he save your life or something? That must be it.

Seriously-Nice to see. I have some "friends" that do little more than borrow my gear and return it broke.

[TweeDLX]

I installed new filter caps this afternoon as suggested, but it did not fix the problem. Still raspy/muddy sounding. I have a new bias cap, but haven't yet located the bias section on the board. The schem is a bit confusing, as is the layout. Another thing I've noticed is that sometimes I can get a good reading on a resistor for Ohms in-circuit, but other times not. Is it just not possible? If it isn't, I'll just continue testing for voltage drop across the resistors. Any other suggestions highly welcomed. I have a few days off coming up and would like to finish this one! Thanks for your help!

Mike

[mageerc]

Failed coupling capacitors can cause that problem...