Valve Amplifier fault - nasty buzz.

Hi,

My HRD has developed a fault and I'm pretty good with electronics, safety (discharging caps etc) and soldering, but I don't have the circuit knowledge to work out which components are faulty. Could someone here offer any help please ?

The symptoms are that it's making a horrible buzz on the 'drive' & 'more drive' channels, as well as letting some sound through on the clean channel when the volume is at zero.

Here's the full schematic....

And here's just the preamp section:

Here's a Wav file of the noise it's making...

Be careful when listening to it, it starts off very quietly then gradually gets loud - so don't turn up your speakers because you can't hear it very well when it starts !

The increase in oscillation frequency is caused by me turning the DRIVE knob from 12 down to 0. Turning the BASS knob from 12 to 0 also has a similar effect. Adjusting any of the other controls does not cause a change in frequency - only in volume / tone of the buzz.

This is happening *only* on the Drive channel, and gets nastier with the MORE DRIVE button pushed.

The recording was done with nothing plugged into the amp.

With the amp on the clean channel, you can hear a little output from the speaker with the VOLUME set to zero - which it never used to do. Apart from this anomaly, the amp behaves fine on the clean channel and I used it for practice this week.

I've tried swapping positions of the preamp valves and also a different set of power amp valves - no difference. I've also tried resoldering R78 & R79, which apparently have a tendency to develop dry joints, and all the big electrolytic caps.

I'm more than happy to give test point readings to anyone who can help... the schematic gives TP voltages so hopefully that might be of use.

Anyone ? Many thanks in advance - I really can't afford to take this anywhere to be repaired.

Hi,

My Fender guitar amp has developed a fault and I'm pretty good working with electronics, safety (discharging caps etc) and soldering, but I don't have the circuit knowledge to work out which components are faulty. Could someone here offer any help please ?

The symptoms are that it's making a horrible buzz on the 'drive' & 'more drive' channels, as well as letting some sound through on the clean channel when the volume is at zero.

Here's the full schematic....

And here's just the preamp section:

Here's a Wav file of the noise it's making...

Be careful when listening to it, it starts off very quietly then gradually gets loud - so don't turn up your speakers because you can't hear it very well when it starts !

The increase in oscillation frequency is caused by me turning the DRIVE knob from 12 down to 0. Turning the BASS knob from 12 to 0 also has a similar effect. Adjusting any of the other controls does not cause a change in frequency - only in volume / tone of the buzz.

This is happening *only* on the Drive channel, and gets nastier with the MORE DRIVE button pushed.

The recording was done with nothing plugged into the amp.

With the amp on the clean channel, you can hear a little output from the speaker with the VOLUME set to zero - which it never used to do. Apart from this anomaly, the amp behaves fine on the clean channel and I used it for practice this week.

I've tried swapping positions of the preamp valves and also a different set of power amp valves - no difference. I've also tried resoldering R78 & R79, which apparently have a tendency to develop dry joints, and all the big electrolytic caps.

I'm more than happy to give test point readings to anyone who can help... the schematic gives TP voltages so hopefully that might be of use.

Anyone ? Many thanks in advance - I really can't afford to take this anywhere to be repaired.

df

buzz at mains frequency F , 2*F or 3*F ? or random

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

There is much more in the drive circuit than valves. There are a couple FETs an op-amp, zener diodes, caps etc... All of which could and probably are responsible for that motorboating.

I could track this down in 10 minutes or less with that schematic and my oscilloscope but you my friend and fellow guitarist haven't a snowball's chance in hell with this repair.

Save some money up and take it to a tech. Use the clean channel + a drive stomp box for now.

Here's a web page with some of my gear;

Well, thanks for that vote of confidence ! ...but I *will* fix it. ..of that I am certain. I've successfully repaired many things 'remotely' with the help of kind folk like Sam Goldwasser. This is only an amplifier after all.

Err...Ok..... Thanks.

Ok well I'll be waiting for your post to describe what you and Sam found wrong. I sincerely hope you can do the repair, sorry if my reply sounded otherwise.

Keep in mind I repair amps and other pro-audio gear for my living so I'm a bit jaded when one (you) basically admits he has no idea what he is doing besides being able to avoid getting knocked on his ass by HV.

You're welcome indeed!

Well, thanks for that vote of confidence ! ...but I *will* fix it. ..of that I am certain. I've successfully repaired many things 'remotely' with the help of kind folk like Sam Goldwasser. This is only an amplifier after all.

Err...Ok..... Thanks.

Sadly, I'm with Meat on this one. I fix these things for a living, and I had one of the same model doing exactly the same thing a few months back. I spent literally DAYS on it, and was no nearer when I had finished. Eventually, as a result of some complicated negotiation between the owner of the shop it came via, the Fender rep, and Fender themselves, their service department agreed to take a look at it (normally, they will only take stuff back that is in warranty, and this was out).

Some weeks later, it came back, and was apparently fixed, although I didn't see it again myself to confirm this. We tried to find out what exactly the problem was, but all we could get out of them was a vague-ism along the lines of "it was a couple of resistors".

It seemed to me that the dirty channel is designed with altogether too much gain in it, in the first place, made even worse when the 'More Drive' button is engaged. Personally, I think that the HRD is one of the worst designs that Fender have ever come up with, but others may think differently.

I wish you luck with it, and if you happen to find out what it is, or luckily drop on the problem, please come back and tell us.

Arfa

df

I would try touching a grounded 1nF, HV cap to various points until a change of oscillation frequency and then change whatever gives the gain in that part of the circuit.

I only read down to wav file ref before, sound card in my other pc

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

Trust me - I tried all of that to no avail. The problem is in the preamp section, around the opamps somewhere, but no amount of additional decoupling or alteration in the gains of the relevant stages, made any appreciable difference to the problem. I checked every component in the area, both visually, and electrically. I did at one point find a resistor which was not the value that it stated on the schematics, but changing it to the correct (?) value, again made no difference to the problem, which I kind of expected would be the case anyway, as the amp had apparently worked correctly before developing this problem.

Arfa

df

That buzz is the classic case of motorboating which is usually caused by a decoupling capacitor gone bad. The concept is that your amp begins to oscillate due to feedback, and the feedback path is in the power supply. The power supply connects all of the different stages of the amp together. It should be DC, but if a decoupling cap goes bad, the power supply will have AC. Any high gain stage can amplify that AC which puts more AC on the power supply which in turn gets amplified, and you get the idea, it oscillates. I would suspect any of the filter caps in the power supply, especially the

16 volt supply, in section A4 of the schematic. Check that the 16 volts is 16 volts(plus and minus 16 volts). It would be common for one of those zeners to go bad or one of the 470 ohm 5 watt resistors to go bad which would destroy the voltage regulation. The 16 volt supply powers the opamps. Test or replace all four of the caps in the 16 volt supply. You also have four caps in the high voltage section right above the 16 volt power supply. The later sections such as the 345 volt B+ would be used to power the preamp tubes. If those caps open up, you probably would get motorboating too. bg

That is a pretty fair description of the problems that would normally be associated with an unstable amp, but assuming that this example is the same as the one of the same model that I had exhibiting *identical* symptoms, then a power supply filtering / rail decoupling issue such as a bad cap, is not what is causing it. It certainly wasn't on the one that came across my bench. The rails remained very nearly as clean when the problem was occuring, as when it wasn't. Bear in mind also, that the problem only affects the dirty channel. You can drive the clean channel as hard as you like, with any ancilliary control settings, without the amp exhibiting a problem, which 99% knocks out the power supply, as a cause. As I said in my earlier post, it eventually went back to Fender's own service department, but the only comment that they made was that it was "a couple of resistors", which probably means that they couldn't find the problem in any reasonable period of time either, so just jammed a replacement board in it, and came up with the 'couple of resistors' story ...

Arfa

Well, I managed it - despite all the naysayers. It was one of the

22uF caps - C33, C35 or C36. I replaced all three and the problem is gone !

Many thanks to all who tried to help, and ner-ner-ner-ner-ner to all who doubted my abilities as a mere mortal. ;-)

Good for you. I like a happy ending.

Oh yeah? Been to a chinese restaurant lately ?

:)

"

Likewise. It's written down now, so if I get another, I'll try those caps. It's got that 'feel' about it, that it might become a stock fault.

Arfa

I ordered carryout the other day. Hot and Sour soup and Hunan Beef with a pork eggroll.

Additional note.

I have had a look at the schematic now to check what those caps were, and I was surprised to find that they were the HT filter caps. Although I didn't actually change the ones in the example that I had with the same problem, I did give them exhaustive checks for ESR and value, and they checked out just fine. The belief that they were indeed OK, was borne out by the fact that the rails at 'x', 'y', and 'z' were all fundamentally clean, and became only very little worse, when the amp was unstable. It's also hard to see how a problem with filtering on these rails, which feed all the tube HT supplies, would affect only the dirty channel. Given the fact that these caps are relatively small in value in the first place, I guess that it's possible that only a small reduction in performance of one that filters the rails to the preamp tubes, might be enough to cause a stage to become unstable, when it's working harder under 'drive' (dirty) conditions as opposed to when it's switched to 'clean'. If this is indeed the case, then it's a pretty lousy bit of design work on Fender's part, that a stage could go unstable with so little provocation.

Just as a matter of interest, did you manage to actually fault-find the problem to these caps, and if so how, or was it a case of gut instinct and shotgun replacement ? Always interesting to know how others arrive a solutions to odd problems.

Arfa

Didn't ask for the happy ending ?

"

I repaired a Trace Elliot Vellocette today that had a poor solder joint on the board connector for the 6.3v filaments. Pulled the harness plug off the standoff pins soldered into the board and one of the pins came with it. The pin was dark so it had been hot from the poor connection at the land.

I missed it on visual, but wasn't really looking for an intermittent as it had no filiment voltage whatsoever.