Hi,
This is my first post, but I've been reading this forum for a while.
I'm lucky to have just bought an RD110 Fifty, with the white pinwhale tolex. It's a truly great amp, and I'm looking forward to playing a gig with it.
When I bought it, the reverb wasn't working. The previous owner never used the reverb. I took out the pan, and the output transducer wire was broken. I re-soldered that, and connected it up. It worked, but it was very weak and only really noticeable on 10. I swapped the leads, swapped the ins and the outs and tried a substitute pan with no difference.
I pulled the chassis and checked the circuit. Looked clean and tidy. Checked a few solder joints and was putting it all back when I noticed a single strand of wire hanging off the RCA earth to the circuit. Ah! That must be the problem. Pulled it off (it wasn't soldered in, just kind of hanging there), fired it up and the thing whistle/squeaked/sort of feedback. Checked it again and it the whistle went, and I was left with a faint reverb with a light crackle. Switched it on today and the whistle is back. The whistle gets very loud as I turn up the reverb, independent of the volume control.
So, in summary....!
Clean channel works fine - sounds beautiful
Limiter channel works fine - sounds....well, distorted. Very slight buzz.
Turn on the reverb with the pedal and both channels have the whistle.
I'm Ok round valve amps - I've built a couple of 6G15 reverb units and repaired my Hot Rod Deluxe a few times....so, where do I start? The problem seems to be solely with the reverb channel, but it doesn't matter if the leads are plugged in or not. Why is the reverb so weak, and is the amp OK to use on the clean/limiter channel?
Everything else seems fine. It's an awesome little amp!
Another look
OK, so I've got home and looked at it again. The squeal is now a buzz - not a 50/60Hz hum - it's a harsh buzz. It only buzzes when the reverb channel is on and it doesn't matter if the reverb pan is plugged in or not.
I happened to have some 22uF 25V caps around so I replaced those - I've ordered some of the remaining electrolytics as I figure a cap job would be a smart thing to do.
I swapped two op amps over as well - made no difference, so I can assume they are OK. Next suspect is the reverb driver transistors - is there a quick way to check those?
RE: Another look
Based on described symptoms (Pan removed), the problem is not with the transistor(s). Sounds like IC 3 is picking up some ground noise and amplifying it. Check the RCA jack and make sure the center contact isn't shorted to ground. Before the symptom became a harsh buzz, you mentioned "whistle" and weak reverb signal. Two things could be causing this. Whistling sounds like oscillating and that could be from a bad solder joint on the 50pf compensation cap on IC3. Check the solder joints on C27. The low gain or weak reverb signal could be from a cold solder joint on C20 .22uF. If there is no AC path to ground (the job of C22), then the gain of IC3 will be very low (<1) and will probably attenuate any signal going through it. After all this, if you still wish to test the transistors that drive the reverb tank, read this for how to do it. http://www.answers.com/Q/How_can_you_test_a_transistor Good Luck. -mgriffin
Thanks!
Hi,
Thanks for your hints, it's given me something to look for. I'm guessing it's not something too serious, but my troubleshooting skills have never been that great. I've bought the caps to re-cap the electrolytics, I certainly wouldn't want one of those to give up and take out my amp. I'll keep this thread updated.
Some kind of progress!
Well, I thought I'd document my progress with getting a useable reverb channel.
I've checked for bad solder joins at C20 and C27, and the RCA sockets are fine.
I've replaced IC3 and the 4016 and this has improved things - the channel is not so noisy but there are two issues left:
A bad hum on the reverb channel, increasing as the reverb is turned up. This is unrelated to volume on any of the other channels. It's quite loud. Turning the reverb channel off stops the hum. Baffling, this one! I've checked earths and leads. Is it possible a bad zener is causing the problem?
The reverb is quite weak - I have to turn the reverb up full to hear any reverb at all, and this makes the humming very noticeable.
When the reverb pan is disconnected, the reverb channel oscillates (turning the reverb off at the pedal stops it). I'm not sure if this is normal or not...
Apart from that it's fine. I was playing it all evening yesterday - it sounds sooo nice! Trouble is, I play surf music so a working reverb channel would be a big help!
Have you tried reversing the plugs?
One symptom of a reverb tank plugged in backwards is weak reverb and hum. It's free to try anyways :)
Cheers,
Lars Verholt
An update...
Hi Lars - yes, I wish it was that simple! First thing I tried was reversing the plugs - second was substituting another tank. I spent some more time on it yesterday, tracking and checking the earths, and all the components around IC3. I thought an earth might be floating somewhere, but I couldn't find any. The hum is related to the reverb pot - turning the reverb up to 10 makes it worse (but turning the volume up on the clean and limiter channel does not make it louder). Turning the reverb off makes the amp very quiet indeed!
Replacing the opamp and the 4016 eliminated the crackling and weird noises, so that's good, all there is left is a hum and a weak reverb. I see a lot of posts about weak reverb on this model - is it normal, maybe?
Plying the amp at a gig would be fine as the hum never gets any louder if you turn up the guitar, but at home it's kind of annoying. I've run out of inspiration now, so it's back together in one piece.
Re: An Update
Hi Ron,
Have you tried bypassing the reverb tank with a "known good" RCA cable? You obviously won't have any reverb, but you can verify if the hum is or is not still there. You can also use the reverb control to change volume. If the hum is still there with the bypass cable, then take a look at the reverb driver transistors using the procedure I mentioned earlier. One other thing before you tear into the driver circuit: Have you ohm'd out the reverb cable pair for good continuity? They can get damaged when a ham handed helper yanks on the coax wire instead of the plugs. Check all four signal paths: ground to ground and center to center on both cables. Hang in there and you'll get it. -mgriffin
Any word on the patients
Any word on the patients condition?
An update...
Well! We finally had a band practice where I can turn it up and see how it performs and sounds with the rest of the band. There were two things apparent:
1) It's very loud. Volume on about 3 and it was giving a solid sound that had plenty of presence
2) It sounds absolutely amazing. The bottom end has plenty of growl and bass, and the tops are bright without being piercing.
I can't believe how tight and responsive the bottom end is compared to my HRD. Also how nicely balanced the mids are - It just sounds like a great, great amp. And it's so small! I love it.
As for the dodgy reverb - there's no improvement. Thanks for asking!
I tried jumping the input and output and it certainly sounded interesting, but a hum was still present. I've tested most of the components, and all seems good so far. There is one oddity - the voltage on IC3 is +8V and -8V with no IC, but this drops to around +7V and -7V with a chip in the socket. Is this normal, or possibly part of the problem? The IC doesn't seem to be getting very hot. I have replaced the IC (this cut out a lot of the noise, but the hum remains). My next step is going to be to replace the 18V zeners in the power supply - could a faulty zener cause the voltage drop and the hum?
I'm looking forward to having a usable reverb channel as I love playing this amp.
Zeners
Good observation. 8V going to 7V with IC-3 seems excessive. Did you check -8V and both + and -16V to see if they get pulled down. The 2 reverb driver transistors rely on +/-16V. Did you replace the 6 electrolytic caps in the low voltage power supply? Make sure all caps are installed with proper polarity and resistors and zeners in that power supply are in spec and in good condition. Also, put your DMM in AC mode and see if any AC is on the power supply lines. If there is, you may have a bad diode (D12-15} in the power supply. BTW: Zeners (D10-11) live a horrible life. They literally eat the excess voltage when regulating and that energy gets converted to heat. Over time, they will degrade. Does anyone remember old British motorcycles? They used a huge zener diode to regulate battery charging voltage. What were they thinking? -mgriffin
Harumph
Thanks for your suggestions - I replaced three zeners, two 1N5353s and the other one, checked the bridge rectifier diodes and glared at it for a while. Still no difference. Very puzzled now, I really thought the zeners would be the problem!
Haven't checked the voltages though, I was a bit short on time and it needed to be back in one piece. Really can't work out why it's only on the reverb channel. Is it possible the reverb pan (which looks like a new one) has an earthed case and there's some kind of earth loop going on?
On the plus side, I played a gig with it and it sounded awesome. A lot of people commented on how good it sounded. Mind you , with what we play, there's not much else to comment on but the guitar tone!
Hum
Hi Ron, Are both cable shields down in the foot switch well grounded to the case? Don't know first hand, but I've heard stories that unshielded foot switch cables can cause reverb hum in MM amps. There's a current thread talking about this. http://pacair.com/mmamps3/node/2055 Rock on. -mgriffin
An update
I thought I'd close this thread as I think I've found the problem: Filter caps. I thought I'd replaced them, but I'd only replaced some of them, I think I remember I couldn't find any 2uF 350V. In the end I lived with the hum and used an outboard reverb. It still sounded good.
Recently, though it started sounding awful. Distorted, harsh and not very loud. I replaced the three main filter caps that I hadn't replaced previously and that cleaned the amp up - no hum or noise, and the reverb sounded really nice. Only problem is the distortion on the clean channel remains, so something else must have been damaged. I've started a new thread about that problem.
Answer is though, if there's hum on your reverb, replace the filter and power supply capacitors. If you're in the UK, I can tell you where to get them from.