Roc Pro 1000 combo - volume and pop problems

Tube or solid-state, this section goes to eleven!
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zedsnotdead
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Post by zedsnotdead »

Hello!!!

I bought this amp used, it's manufactured on early 90's I think.
When I first powered it, I noticed that:

1- It's a loud amp. I mean LOUD! Also get a huge volume jump only touching the volume knob a bit, making it difficult to find any "in between" settings - and I live in a apartment, which makes this a real issue...
2- The controls were scratchy as hell.
3- When reverb is on, it "humms" a lot.
4- When switched OFF it makes such a loud POP, whatever the volume is at the time. I mean, the first time this happened I almos had a heart atack!! Scared th $hit out of me and my cat! Never saw him running so fast to hide under the bed. :shock:

I already took apart the amp to check values and components with the help of official schematic
http://support.fender.com/schematics/gu ... ematic.pdf.
This is what I found/did:

- All the traces and solder points seemed OK. Nonetheless I did some resoldering/reflowing on valve socket, input/output jacks, opamp pins and everything else I felt could be "better" soldered, just to make sure...
- No leaking/swollen electrolitics, no burnouts below power resitors also. I measured the cap values, namely C71-72-73-74 and they are reading ok.
- Sprayed the pots, jacks, switches (including ON/OFF switch and reverb connectors) and valve socket with contact cleaner.
- Changed the tube for a lower gain JJ 5751.
- Also noticed that the Volume Pots are log type (A), so my idea of changing them for this type so the volume could be more smooth and controlable is out the window...

Now, after all this done, I reassembled the amp and the shutoff POP is still present (my cat is also hiding everytime I get close to this amp :P )!
The reverb "hummm" is still there also.
Good news is that control pots "scratchiness" is GONE.


Now, please I need more advices.
- what else could I check??
- can I sub the A50K volume pots to say A500K and expect to get a smoother volume raise?

Thank you so much for your time!

Cheers!!

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lolbou
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Post by lolbou »

Scratchy pots is fairly common on these. You may need to change some if the scratch comes back after a short time...
zedsnotdead wrote:The reverb "hummm" is still there also.


:wink:
- Are you a mod or a rocker?
- Uh, no, I'm a mocker.

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Post by zedsnotdead »

Hi!!!

thank you for your reply. Already saw that vídeo, that's why I wrote:
- Sprayed the pots, jacks, switches (including ON/OFF switch and reverb connectors) and valve socket with contact cleaner.
Unfortunately, didn't get any better :|

I am now trying also to understand the circuit so I could find a way to "tame" the volume.
I don't know if I am right (correct me please) but U5A (TL072 on C-1 coordinate) is the first opamp after the reverb section.
R88.jpg
R88.jpg (43.9 KiB) Viewed 4934 times
So do you think reducing R88 (hence,the opamp gain) could do something to the huge volume this thing has?

Thanks!

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Post by Groovenut »

Does the volume jump on both channels?

I've had pots go intermittent and cause huge volume jumps

I would increase R89 rather than changing R88. R89 forms a divider with R90 & R91

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Post by zedsnotdead »

Groovenut wrote:Does the volume jump on both channels?

I've had pots go intermittent and cause huge volume jumps

I would increase R89 rather than changing R88. R89 forms a divider with R90 & R91
Hi! Do you have the same amp?

Yes, both volumes. It's not intermittance "per se", the volume is too sensitive I guess? Just a touch on the volume knob, like from "0" to "0.5", is a huge jump in volume, I am trying to dfigure a way to increase it more "smoothly".

Also trying to get rid of that "POP/CRANK" when switched off, and the noise when reverb is engaged... for those 2 issues I still have no clue...

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Post by Groovenut »

I dont have the amp, but I have worked on a few. Bad pots is a common issue.

The pop is common and a design flaw for the most part. You might try unplugging the guitar cable from the input of the amp and see if that helps minimize the pop. There are mute circuits to help with this in the amp but they dont usually provide the necessary reduction in pop.

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Post by DrNomis »

You could try giving all the pots a spray with contact cleaner, have a look at the wires going to and from the reverb spring-tank and see if there are any loose connections...hope that helps.... :thumbsup
Genius is not all about 99% perspiration, and 1% inspiration - sometimes the solution is staring you right in the face.-Frequencycentral.

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Post by zedsnotdead »

DrNomis wrote:You could try giving all the pots a spray with contact cleaner, have a look at the wires going to and from the reverb spring-tank and see if there are any loose connections...hope that helps.... :thumbsup
Already did. It worked ok for the pot "scratchiness". Not so good for the reverb hum...

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Post by DrNomis »

zedsnotdead wrote:
DrNomis wrote:You could try giving all the pots a spray with contact cleaner, have a look at the wires going to and from the reverb spring-tank and see if there are any loose connections...hope that helps.... :thumbsup
Already did. It worked ok for the pot "scratchiness". Not so good for the reverb hum...

Reverb circuits do tend to generate a bit of hum when they're working normally, this is because the pre-amp that gets the return-signal from the Reverb Tank needs to have a bit of gain, excessive hum would indicate an electrical grounding problem so you could try checking the signal grounds in the reverb circuit, another cause might be a faulty Reverb control pot....hope that helps.... :thumbsup
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Post by zedsnotdead »

Thank you DrNomis! :D

Yes, I will check for grounding problems and also I will try to check voltages on all testing points (TP).

I think I might do a vídeo about this, showing you guys the inners of the beast and also what the problems are... It might help. right?

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Post by phatt »

Been a long time But have worked on the Performer 1000 (Same circuit)

Things to try;
Bridge the FX loop with a short lead and try switching the level send return switch,,, it might help.
while FX is bridged,, does the FX blend knob make any diff to level, If I recall it can work as master Volume when not needed for FX.
Grovenut already mentioned removing the lead from input *Before switch off* Did that stop or help the thump?
Phil.

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Post by zedsnotdead »

Guys, sorry for my english, really...




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Post by DrNomis »

The fact that your first mod to your amp can control the loudness suggests to me that the cause of the large jump in volume is located somewhere in the schematic before the location of the mod, it's likely that the volume control pot needs replacing.... :hmmm:
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Post by phatt »

Hey Zed your English is fine, :thumbsup
Heck I know a lot of Aussies who can't pronounce Anything,, often pronounced as AnythinK, which is not a word. :slap:

First up,
These Amplifiers ARE LOUD and are not much use for small rooms.
I doubt there is much wrong with it but as you have shown in the Vid you just insert a pot and use it as a master volume control.
If that silly External speaker switch was removed R90 could be replaced with a master Volume pot of say 50k. 50k or 100k would work fine there.

The Bad hum is a clue you most likely have a missing ground somewhere. The bad pop might be related to the missing ground connection.
With guitar cable removed the preamp is in mute mode and should be close to dead silent, maybe a small pop when you turn it on.
Start by checking you have continuity from Earth pin of power cord all the way back to the Common on the circuit board.
Looks like all the pots are loose for repair and that can cause bad hum. Check R163 is in fact in place and connecting. You are doing well keep learning :thumbsup

One thing I do first is check the Earth pin is in fact connected,, you would be shocked if you knew how much equipment is not grounded. :shock:

Found a unit just last week,, a plastic powered pa speaker no earth wire was Never installed on the metal back plate which is also circuit common, Yikes! :evil: :evil:
Phil.

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Post by zedsnotdead »

Thank you so much phatt!!! :D
I know my limitations... but I do my best. If anything is not clear, please tell me.

Do you happen to see the written "notes" on part 2?
I added several of them while messing around with the amp after, just to update the vídeo. But when I play it here they don't show up! wth....

Anyways, upadate is in order:
After I shot this couple of vídeos, I messed around both mods and concluded that R89 mod gives a dull tone compared to R88// mod.
My main concern about this is that I I wanted to have the option to get the original value (470k) using a knob. I could accomplish this with a switch to break the parallel connection, but instead I modified the 500K pot. Opened it and used nail varnish on one end of the wiper so it would break the connection when it's fully CW.
Works like a charm!

The POP and "humm" is still there, of course.
The "hummm", like phatt suggested, I also suspect it's caused by bad grounding issues... thing is my mains does not have "earth" connection. I will validate this suspicion as soon as I can.
The POPPP, will see if it gets better also.

I will post a follow-up video on this soon.

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Post by mwelch55 »

I have a Fender Roc Pro 700 and it has the same problem. After looking at the schematic, I noticed that volume and gain pots are linear instead of Log. I think this was a design flaw. If the volume pot was changed to a log taper pot, it would prevent this sudden jump in volume. I haven't done that, but I think it would resolve the problem.

Mike

(First time post, long-time lurker)

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Post by zedsnotdead »

mwelch55 wrote:I have a Fender Roc Pro 700 and it has the same problem. After looking at the schematic, I noticed that volume and gain pots are linear instead of Log. I think this was a design flaw. If the volume pot was changed to a log taper pot, it would prevent this sudden jump in volume. I haven't done that, but I think it would resolve the problem.

Mike

(First time post, long-time lurker)
Hi!!
Well, both volumes pots in my 1000 are log... so, can't do much about it :hmmm:

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Post by gambitpawn1 »

I have a Fender Roc Pro 1000 as well, yes it is TOO loud, so I plan to do the master gain/volume mod on R88 that you did (awesome job on coming up with that one.) I found on my amp, it only makes the hum when I have the reverb on. When I open mine up, I will try a different reverb tank and see what happens.

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Post by zedsnotdead »

Keep us updated ;)

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Post by phatt »

Yes The Pro 1000 I worked on had weak / feeble reverb and does add noise as you turn up the reverb dial.
No the tank is the last thing to cause hum,,, the circuit is the offending issue.
Replacing the tank will achieve nothing and may cause even more problems.
Of course check the RCA connections and cables as they can be a cause of excess hum.
Sadly a lot of reverb circuit designs are not well researched and THAT is the most common cause of weak reverb,, Not the tank!
Assuming all else is in order then the excessive hum/buzz as you turn up the reverb depth control is a sign that the circuit is a crap design.
Unless the designer did a lot of maths and ran some real world breadboard testing then it's a good bet the Driver circuit is not well matched to the tank.
remember these amps are made to a price not a standard so you get what you pay for.
Without major rework of the reverb setup there is little that can be done to resolve that problem.
Phil.

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