Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: frankeg on July 16, 2012, 10:15:04 am
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Hello all,
I have a 63 Fender G15 re-issue reverb unit with no reverb. The dry signal path works fine. I inserted a signal at the input and followed the fender service manual test points with a dvm. At the 6V6GT driver tube I have the correct input grid voltage (1.2 vac) and the correct cathode voltage (22.5 vdc) but no ac volts on the plate (should be 19.8 vac). I do have the correct dc on the plate and screen grid. The reverb pan is new and I swapped a known good 6V6GT with no change. With the reverb pan disconnected I measured 274 ohms on the primary of the OT (reverb tranny) - which seems very low. I plan to remove the OT and check again. Question; Do I need to use a vtvm to read ac at the plate or use a coupling cap on the dvm's hot lead? The service manaul schematic says that a dvm was used for all measurements. Any suggestions/help would be much appreciated. Thanks, fg
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OK,
So you say the clean path is good, does that mean you played through it and you get sound?
You do not need a VTVM, most any good DVM will suffice.
Measuring inductors like the reverb driver tranny and reading DCR (DC resistance) is kind of funny. so the 274R may or may not be relevant other than you have some sort of continuity between your leads. for most purposes, this is ok.
I have little personal experience with reverb units, but from my reading, a VERY common fault is bad RCA cables to and from the reverb unit.
Ray
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Thanks for the quick reply Ray!
I did play through the clean chan and it works/sounds fine. I changed the rca cables too. While I was at it, I removed the pan and inspected it. It is a new one (Accutronics) and it appears sound. I do not have a footswitch so I jumpered the jack. I was thinking that not seeing the plate volts (AC) meant that I did not have a proper load connected which led me to the transformer. I could connect an 8 ohm speaker directly to the secondary couldn't I? I'll insert a 1k signal at the input. I have a weber 10" 25w 8 ohm speaker I could use. What do you think? fg
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I do not have a footswitch so I jumpered the jack.
Remove the jumper.
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Thanks Steve!
I tried it both ways, with and without the jumper with the same results. It seemed at the time an obvious solution... I am going to hook up a speaker to the secondary tonight to see if I can get anything out of the driver. fg
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With the jumper on the footswitch you will never get any reverb because it's turned off. I was hoping you had fixed the problem but just didn't realize it yet because of the jumper.
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I am going to hook up a speaker to the secondary tonight to see if I can get anything out of the driver.
That's a valid check for the driver. Similarly, you can inject a guitar into the recovery circuit to check for proper operation. Very easy if you have a female 1/4" phone to male phono adapter.
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Steve,
I do see your point about the jumper which I now see grounds the signal. I know I didn't get any reverb with or without the jumper but I don't remember if I checked for plate volts on the driver with the jumper removed. I'll do that first. I let ya know. Thanks again for the help! fg
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Another (dopey) thing that can go bad on reverb cans is that the teeny wires from the RCA jacks to the coils can break. Physically break. If the reverb gods are really angry with you, the CONDUCTORS can break while the INSULATION remains (apparently) intact. Measure the out-of-circuit resistance across either RCA jack on the bare reverb can. Do you get some kind of non-open reading? (Hopefully others can chime in with what a typical reading would be....completely on fading memory, I seem to recall these should be roughly 2.5K ohms, but don't quote me) You may also want to give those wires just a gentle little yank at both the RCA end and the transducer ends. Really gentle! I would say that of about 5 reverb cans I have fixed, at least 3 of them just had broken wire connections.
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> seem to recall these should be roughly 2.5K ohms
The pickup end may be a few K. The driver end is typically lower, as low as 1 ohm.
In any case: infinity is bad, and dead-nuts-Zero is bad (but healthy "4-10 ohm" tanks may be so close to zero that many DMMs round-down).
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I hooked up the speaker to the reverb trans secondary and got sound which I could control with the dwell. I just used my guitar for the source. I didn't check the recovery circuit because I need to make the test cable first. In answer to my first question: I did need to use a coupling cap with my dmm to measure the driver tubes ac plate voltage. I have a "higher" end radio shack dmm not a fluke... I also can't detect continuity through the reverb pan with this dmm. It could be the pan or the internal wires as suggested or just the dmm. I'll know more once after I check the recovery circuit. I'll borrow a fluke from work to check the pan. Thanks again for the help! fg
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My money is on the tank.Look very closely at the wires to the little transducers.And really,get a decent meter.It saves so much time when you have a meter that reads what you need it to read.
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My money is on the tank.Look very closely at the wires to the little transducers.And really,get a decent meter.It saves so much time when you have a meter that reads what you need it to read.
Will do. Totally agree about the meter. Thanks, fg
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OK,
Just to cover all bases, when you test a tank with an ohm meter, you'll check from the center contact of the RCA socket to the out side of the socket. Not all tanks have a common shield to tank chassis. some are isolated.
The "in" should read about 8R. The "out" at about 1K.
There is no continuity between the in and out.
Just throwin that out there.
Ray
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OK,
Just to cover all bases, when you test a tank with an ohm meter, you'll check from the center contact of the RCA socket to the out side of the socket. Not all tanks have a common shield to tank chassis. some are isolated.
The "in" should read about 8R. The "out" at about 1K.
There is no continuity between the in and out.
Just throwin that out there.
Ray
Thanks Ray, I did not know that!!! fg
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OK, I checked the tank as Ray suggested and got continuity. I ran a 1k tone from the tank output cable into the recovery circuit and got a attenuated tone out. I checked the dwell control while I was at it and it works. I jumpered the frequency divider circuit made up of C9, C10 and the tone control by going to the "ac" side of coupling cap C8 to the mid wiper of the mixer and got glorious reverb and lots of it. I removed that jumper and jumpered C10 (250 pf) but got the attenuated signal again. I can't check C9 without removing the board because it's soldered tight against the board. I'm thinking that C9 is leaking causing a loss of signal to ground.
Has anyone here removed the boards before? Are there any pitfalls I need to be aware of? As always, I appreciate your help.
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My money is on C10. Or a bad solder connection or circuit board trace. Just take your time and be careful. And don't use any more heat than required.
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I can't think of any particular pitfalls. Congrats on narrowing it down to the tone control caps!
Just make careful note of how things went together before you pull it apart.
Unsoldering the cap is probably easier with 3 hands. It would be nice if the board was held vertical, you applied heat to both of the cap's legs with the iron at one time, and you had a 3rd hand to pull the cap free from the other side of the board. You can probably easily rig something to hold the board vertical for you though.
It would also be nice if you had to desoldering braid or a solder sucker to remove the existing solder from those pads and the holes before installing your new cap. New flux/solder helps make sure you don't wind up with cold solder joints.
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New twist folks. There are 2 boards, the main board and a smaller board that contains the pots, jacks and the capacitor's in question. I loosened all the pot and jack nuts and for some reason decided to check the sound before going further. Well? You guessed it! The reverb was back... So I played a little with various degrees of tightening and it definitely makes a difference. The service manual states that the pot brackets are physically connected to chassis/earth ground through R22 which places 15 ohms of resistance between audio ground and the pot mounting brackets for isolation. With the board removed, R22 provides a ground reference (15 ohms) for the pot brackets. Does it make sense if C9 is leaking and providing a ground then I have a ground loop which is "fixed" by lifting the bracket ground? Or do I have another issue here? BTW, the reverb is slightly distorted and not as clear as it was when I jumpered C8 to the mixer pot so I still believe C10 and C9 should be replaced.
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I'm thinking bad solder joint or broken pcb trace that is being flexed.
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Yeah, I was going to ask if there was a chance the Tone or Mix pots caused the board to flex, straining an intermittent connection?
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Board flexing issue... That makes sense. Ok, I'll go over the board with a magnifying glass tonight. I guess an intermittent connection would also cause the distortion I get when the defective connection is made up but not solid? Thanks again guys! I'll let you know...fg
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Fixed!!!
The board with the pots and jacks on it is only 1" wide and it sits vertically in the chassis. The stress of tightening the pots/jacks would affect the pots/jacks themselves but works against the strength of the board. Think "joist" here. With the board removed from the chassis (dangling from two ribbon cables) I was getting 8 ohms of resistance from the pot brackets to ground. The manual says it should be 15 ohms. After replacing the two caps (C9 & C10) mentioned earlier, the reading went to 15.4 ohms. Seems that I had a parallel path? At any rate, I now have clean sounding reverb with plenty of volume and range. Thanks to all who helped out!!!
BTW, the caps came from Doug....fg