week volume

[Tubelube]

My Fender Deluxe Hot Rod developed a blown speaker, a friend not knowing what happened blew a fuse becaues of the blown speaker. He replaced it with a 7 amp fuse which did not blow this time, the tube heaters all lit up when I got the amp back. I replaced the speaker as it had an open circuit, I then replaced the fuse with a slow blow 3 amp. The amp has sound but the volume is low on both channels, all the controls seem to work. I checked the tubes on a tube tester and none of them are shorted and they all test out ok. I suspect a bad output transformer, am I correct? Thanks!

[jestaudio]

My Fender Deluxe Hot Rod developed a blown speaker, a friend not knowing what happened blew a fuse becaues of the blown speaker. He replaced it with a 7 amp fuse which did not blow this time, the tube heaters all lit up when I got the amp back. I replaced the speaker as it had an open circuit, I then replaced the fuse with a slow blow 3 amp. The amp has sound but the volume is low on both channels, all the controls seem to work. I checked the tubes on a tube tester and none of them are shorted and they all test out ok. I suspect a bad output transformer, am I correct? Thanks!

May be worth checking the joints around the output tubes and flying lead, I had one a few months ago with a similar issue which was just bas joints due to the excessive heat

[Phil_S]

To test the OT, start by disconnecting the speaker and then by pulling the power tubes. (OK, before you do anything, unplug the amp and then make sure you know about safety concerns.) I like to disconnect the OT center tap from the OT, too. Then with your meter set to Ohms, measure the windings. The primary is connected to the power tube sockets at pin 3 for 6L6's. Use clips on your meter for better contact. The primary winding should read about, but not exactly, the same on each side of the center tap. If it is infinite or open, the primary is toast. The secondary will read very low Ohms and some meters won't give a reliable reading. Even so, the secondary (speaker leads, or if a jack, between the tip and sleeve) should not be open or infinite.

Given that your output is weak, there may be a partial internal short that does not exhibit itself on the Ohms test. To go further, this is the test to do: http://www.premierguitar.com/articles/14403

When you are done diagnosing the OT, good or bad, you will still need to determine why the speaker blew. Failure to address that will only lead to more trouble.

And tell your friend his $1 fix with the 7A fuse was the most expensive $1 he ever spent on his amp!

[Teleguy61]

+1 both above responses.
HRDxs are notorious for solder joint failure around the power tubes, which are mounted on a pc board.
I had a joint fail during a show, one output tube failed, amp started passing square waves and blew a JBL D120 on me.

[ampdoc1]

I agree with everything suggested. Absolutely resolder the tube sockets first!!

When I had a shop years ago, I saw a number of these amps, and they almost all had identical additional problems.

Before you pull the tranny, try testing and replacing as needed:

R57, (82k) at pin 1 of the Phase Inverter Tube (V3) - usually OPEN;

& Transistors Q1 and Q2, (J111)s - SHORTED


a'doc

[Structo]

If it has a "week volume" try it next Thursday and see if it improves.

Sorry I couldn't resist.

Check screen resistors.

[Tubelube]

Thanks guys! I will let you all know what I found out by next tuesday (1-21-2014). Thanks from all of us Phil_S for that awsome transformer test, much appriciated!

[tribi9]

If it has a "week volume" try it next Thursday and see if it improves.

Sorry I couldn't resist.

Check screen resistors.

7 days with low volume makes one week.

[Leo_Gnardo]

I've worked on about a hundred of these & similar Fenders, yet to have a bad OT. +1 more on checking those tube socket PC board connexions, also failed 82K R is sorta half common. ALSO signal likes to get lost at the power amp in jack. ALSO also the resistors knocking down voltage for the zener regulators like to heat up to the point they de-solder themselves from the board. Look for +15 and - 15 volt supplies - if one is missing that would stop the signal too. And how about an input jack broken, or its legs broken off the PC board.

Please in the future develop the good habit of NEVER putting an oversize fuse in, or letting your "friend" do it. Has a way of making repairs get expensive, fast. Never a good thing to do. Example: a 25 cent silicon rectifier went bad in one of my customers' Mesa Triple Rectifier. He put in an oversize fuse. Power transformer wrecked. Repair bill over $300. That's an expensive fuse. Let's not go there OK? Learn from other people's mistakes.

[Structo]

Yep, pcb mounted jacks are a recipe for failure.

Cracked solder joints or broken traces, etc.

[Tubelube]

Test results for the (weak volume) problem, I re-soldered the power tube sockets and re-powered the amp, I could hear the guitar strings being amplified for about 3 seconds then the PC board started to fry around pin 3 of the V4 (6L6) tube. Pin 3 is directly connected to the brown wire of the out put tranny. I was able to turn the amp off and blow out the fire, about a 1/4 inch diameter area around pin 3 is charcoal. R81 and R80 are toast also. The 3 amp slow blow fuse survived this traumatic occurrence! I next tested the output tranny's primary, after unplugging the three wires of the output tranny's primary I tested from the red center tap to brown wire of and got a reading of 81 ohms. Then I tested the red center tap to blue and got a reading of 109 ohms. They are supposed to be within 15 to 20 percent accuracy. I am not sure how to do the math but that's a spread of 28 ohms, not sure if that's good or bad? R58 reads 98 ohms in the circuit. R57 reads 81.4 ohms in the circuit. Q1 and Q2's DSG pin out reads as follows: Q1-D to S is shorted on VOM, D to G measures 12 mega ohms then goes down in value from there as it was tested in circuit. S to G measures 14 mega ohms and goes down in value from there. Q2 has about the same measurements as Q1. If the red and black leads are reversed on the VOM both Jfets measure infinite resistance from D to G and S to G but shorts from D to S of each transistor. All the tubes still check out ok as far as shorts go and they read good on the other tests. I am starting to suspect one of the filter caps. What should I try next? Thanks!

[Teleguy61]

Do you have a convenient window, at least four stories above a paved street or parking lot?

[Tubelube]

If I can't figure it out soon I just might find a tall tree to drop on it

[Tubelube]

I forgot to add that R58 and R57 could have been bad before the last power up, I could not see them with the circuit board screwed in place. It seems when the bad solder joint was fixed the interaction of the tube caused the fire works. Could the tube, output tranny and a filter cap all be bad at once?

[Structo]

Do you have a convenient window, at least four stories above a paved street or parking lot?

Dude, I just about blew coffee all over my desk!