Fender HRD channel switching

[tubby]

ok, after having read all those posts on the various problems with the HRD, i'd just love to rip out the pcb and start from scratch. just by looking at the tube pcb i see at least 5 cold joints. but since i'm doing this for a buddy of mine...

it's basically a loud popping noise only when switching from dirty to clean, and only if signal is applied. the more i dig in on the guitar while switching, the louder the pop. i checked the 470k 5W droppers and the channel LED joints so far.

any input is much appreciated!

[Don Moose]

The schematic shows two relays, and this is the first place I'd look for pops.

The pop you describe - worse as you dig in - sounds like DC building up on one or more relay contacts, so I would look at the solder joints and continuity-to-gnd of R102 (connects to K1b, pin 11 --> K1b-11), R27 (K1b-9) and R42 (K2b-9).

It wouldn't hurt to verify CR18 (relay flyback diode) is soldered well.
It also wouldn't hurt to add yet another, similar diode directly to each relay's coil pins ... if it will fit well.

Hope this helps!

[tubby]

hey don,

thanks for your help!

i checked the solder joints and the Rs you mentioned, incl. the flyback diode.
nothing suspicious there. i'm afraid adding some more flyback diodes involves major scraping and drilling operations, as there's no way to get them on the pcb as it is. i hope there's a way to repair this amp without modifiying the pcb too much.

the only thing that worked so far, was attenuating the signal heavily right after C2 with a 10K to ground which is not a viable solution. i noticed that the popping increases in volume as i turn up the gain knob.

any more ideas...?

[52 Bill]

Have you checked for DC volts in the signal path at the relays? Leaky coupling caps?

[Don Moose]

Leaky coupling caps?

Ooh - good one! - Especially those leading to relay contacts.

[tubby]

thanks bill,

i checked all the coupling caps up to the PI. No DC.
man, i so wish it would have been one of those caps leaking...

just a thought: there is some slight delay on the channel switching indicator LED when switching from drive to clean, meaning it is glowing for half a sec longer after you've hit the switch. an indication that the relay contacts don't come off as quick as they should be and thus the pop?

[Don Moose]

Relays are (generally) pretty snappy - there's a coil that, when energized attracts the armature that carries the contacts. When you stop feeding the coil, a spring snaps the armature back to its relaxed position.

K1a switches between two volume controls - no real pop potential there.
K1b fiddles with some EQ settings around the tone stack, but no positions lacking a DC-to-GND path.

K2a/K2b do switch an entire gain stage (V2b) in and out. I don't see a mode there (a switched and b not, nor vice-versa) that looks like a cause for popping.

You're sure C ... is that C10 or C18 after R25? Sure that one's not leaking? C2, either?

I think we need Enzo.

[Enzo]

Only when signal is applied? It switches silently otherwise?

The main reason channel switching pops is unterminated caps. Well, more precisely an abrupt change in DC level somewhere when switched. Usually a cap that is charged or not charged gets charged or discharged. When signal is present, at any given instant the instantaneous voltage at a given point can be anything. If you are on one channel, and the signal happens to be at a peak level, then switch to another channel where a cap was not charged up to that same peak, a pop results as things equalize. At least that would be the results of my thought experiment.

Isolate the problem. Does turnig down the controls make any difference? Specifically does zero on the Master kill the pop? The signal path into the V2B gain stage is opened by teh switching relay, so there is no signal there until switched.

[tubby]

Don,

ok, i rechecked the coupling caps C2,10,18. There are 230V DC on one side, 0.0090V DC on the other. I consider this non-leaking.

Enzo,

yes, that is correct: only when signal is applied. The more signal (e.g. turning up gain, pushing the 'more drive' button or digging in on the guitar) the louder the pop when switching from dirty to clean.

Zero on (drive channel) master and clean vol. (no master in this channel) does not kill the pop. I found no interactions with the rest of the controls (bass, mid, treb, pres, reverb).

[Don Moose]

I have to agree, those caps are fine.

One last check - the with all three volumes at zero - still pops? (not quite clear from last post)

It is suggestive that, you only pop when switching from Drive to Clean (and not Clean to Drive?) with volume applied, and that it's worse with more signal.

Your earlier point - attenuating with a 10k shunt after C2 is the only real help - which I just actually caught, suggests another idea.

Does it matter which side of R13 (or 19? - the one after C2) you shunt (is one side better)? Is that resistor well soldered? Does that resistor actually measure 130k? (in non-drive mode you'll be in parallel with 1M (R42) and 50k (R23 + R25)), so you may have to do this with the amp ON and in Drive mode **VERY DANGEROUS** Consider just replacing it. **

Hope this helps **VERY DANGEROUS**!

[tubby]

hey Don,

yep, even with all vols (vol, master) on zero it still pops.
and only from drive to clean and not from clean to drive.
the amount of drive dialed in on the drive pot and playing dynamics while switching control the volume of the pop.
so, if drive is maxed (more drive engaged) and you're switching while playing, that's when you get the loudest pop.

i did a shunt from both sides of R18 (130k): no difference.
the resistors measures 128k and i touched up the joints.

i jumpered terminals 6 to 4 on relay K2A with a 1 Meg which stopped the popping, but then you can't use the drive master anymore.

this is all telling me something, but i just don't get it.

[Don Moose]

What I'm getting is that this may be an inherent 'feature' of this design.

If you have no leaky caps,
and DC paths to ground all around the relay,
and a good flyback diode,
and switching this relay - in only one direction - pops,
maybe (maybe) the relay needs replacement.

Are you sure you can't get another diode right on the coil pins?

Hope this helps!

[tim]

Hi tubby...

If it'll help, I have a HRDlx at home in my studio...When I get back tonight, I can check mine and see if does the same thing. I rarely use it because I'm a bass player but the guitarist in my band uses it sometimes and has never mentioned it as being a problem (if it's even doing it), although he typically uses it in the clean only mode and uses a pedal for distortion.

One question though so I can duplicate your situation...is this happening with the foot switch or with the mode buttons on the top of the amp?