Pacer 100 hum

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zenrider
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Pacer 100 hum

Post by zenrider » Thu Feb 11, 2016 5:53 pm

I have an older circa 1984 Pacer 100. It seems to work great except at very low volume of the Master Volumn it starts a 60 Hz hum. This happens no matter if an input device is connected or not. It also happens no matter what the dial settings are for all other controls. I replaced the main power supply capacitors. I also replaced the output transistors as they were not a matched set.

I have measured all the voltages listed on the schematic and they measure good. The DC power rails show slight DC ripple, less than 2 volts. The signal looks normal on the scope. I resoldered most of the connections everywhere on the board. I cleaned all the grounds and checked for low resistance. I even ran a seperate ground jumper from a known good ground directly to the chassis during trouble shooting. I removed the reverb unit and cleaned and inspected all contact points and it is well grounded.

The amplifier sounds great as long as the Master Volume is above 2.5. Then you can hear what I would call normal noise from quiescent current and very low hum detectable by the trained ear. I would like to run it at low master volume so that I can drive it hard enough for some distortion with the gain control. At a level 2.5 on the Master Volume, that results in a neighborhood concert. The amp can be driven to distortion easy enough and sounds otherwise excellent.

Measuring with the scope, it is noticable that the DC rail power develops significantly more ripple at the low volume setting. Measuring the DC voltages during this test shows fairly stable levels at the emitters and collectors of the final four transistors. Anything in front of the master volume tap is rock solid.

This thing rocks if you run it at live performance levels. Any ideas are greatly appreciated. My wife and cat would like to be serenaded at a more reasonable indoor voice level.

zenrider
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Re: Pacer 100 hum

Post by zenrider » Fri Feb 12, 2016 4:52 pm

Still looking for help please. I read everything here that I could find about hum. None of it quite fits my symptoms.

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CharlieP
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Re: Pacer 100 hum

Post by CharlieP » Fri Feb 12, 2016 10:42 pm

Does it have a 3-Prong power cord and appropriate switch? My 50W Pacer has the double On Off On switch. Could be a source of hum if it is bad.
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zenrider
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Re: Pacer 100 hum

Post by zenrider » Sat Feb 13, 2016 8:57 pm

Yes Sir and it seems properly connected. I checked the wiring and the grounds on that too. It definitely has an effect on my home wiring and seems to like the right position, as when facing the amplifier. I checked all my home wiring and used it at other locations with the same results.

My band buddly just left and his suggestion was to just ingore it. :)

My next plan is simply going to be replacing all of the electrolytics and probably the power supply diodes. It is all cheap enough. I am just getting tired of tearing this thing down every week when I could be practicing.

I should mention that I have 40 years experience repairing many types of industrial electronic equipment. I don't know why this seems so difficult.

I thank you for the tip and welcome many more. If I magically fix it, I will post the results for everyone. Until then, all thoughts greatly appreciated.

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Enzo
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Re: Pacer 100 hum

Post by Enzo » Sat Feb 13, 2016 10:33 pm

Hum comes from many sources. First determine if it is really 60Hz hum or maybe 120Hz. Since the waveform is likely distorted, it can be hard to tell. 120Hz comes from power supply ripple. it can be lack of filtration, or it can be ground current sharing a path with sensitive signal circuits. it can also happen if the pushpull stages are not balanced. Your transformer drive is likely OK, but the final stage can still be offset.

60Hz comes from a ton of things. You can have output and power transformers linking magnetically. You can have poor grounding in certain parts.

Experience tells me that all those little 2uf/35v electrolytic caps in the signal path are dried up and should be replaced. If you order parts, get the right value, I don't have 2uf in stock, but I have a ton of 4.7uf, so I usually use those with no ill effect.

If the master hums more at zero than it does at say 2, then you have more than one source of hum, and they are cancelling by phase in the master pot.

zenrider
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Re: Pacer 100 hum

Post by zenrider » Sat Feb 13, 2016 11:28 pm

Thanks very much. Next time it comes apart, the rest of the electrolytics are getting replaced and even though the grounds are reworked, I will do them again. I suspect that 2uF cap in the feed back line from the output jack might be one culprit. The corrupt signal does not appear before the master volume. There does not appear to be a DC path to the center tap of the volume control. So, it seems strange that adjusting it more toward ground potential would cause an affect unless some of those caps are bleeding through. I will also try additional shielding around the transformers to see if I can reduce the affect.

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Re: Pacer 100 hum

Post by BowerR64 » Tue Feb 16, 2016 5:01 am

I bought an older Marshall Mosfet lead 100 from a pawn shop about a year ago and when i tried it at the shop it had a bad hum but i got it anyway because they are kinda rare and i really wanted it.

Once i got it home it was still noisy and humming loud. I slowly went in stages cleaning everything. FIrst i sprayed all the input and out put jacks push a cable in and out several times. I could hear a little improvement, next i took it apart and sprayed all the pots and then tried it and again i could hear more improvement.

The last thing that finally got rid of all the hum was when i pulled the leads to the reverb tank and the RCA jacks that plug into the tank itself.

It was weird the whole amp was dirty and had a really loud hum but once i cleaned everything it was like in stages the amp got completely clean and all the noise was gone.

Enzo is right it still can be other things like psu filter caps tube sockets, a bad ground but a can of deox-it cleaned several used amps i picked up and its fairly easy to do so i always like to start with something easy then to put it into the shop or start replacing parts.

zenrider
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Re: Pacer 100 hum

Post by zenrider » Tue Feb 16, 2016 5:14 pm

Thank guys. Today I purchased fist fulls of components. Besides reworking all the connections again, I plan on replacing all caps. I also got all the resistors in the final stage and matched them all to within 0.1%. I noticed someone has replaced the 0.68 ohm emitter resistors with 0.47 ohm. This could offset the performance curves of the output transistors (which were already replaced). I also got a new master volume pot and new power supply diodes. I also purchased a grab bag of NPN and PNP transistors. About the only active component I could not get today was the JFET. I am going to shotgun all the caps and check each Xsistor. Also, make sure the grounds are super clean and get rid of the rivets that hold the circuit board brackets and clean that up and reattach with lock washers and screws. Also planning on rerouting any grounds that are not connected to a common point to eliminate ground loops. I am also going to verify my mains home ground is in good order. I have some circuits in the shop that are known good with GFI, so I am certain those circuits should provide a good earth to the system.

I will keep all posted of the progress and always appreciate any further in sight or experiences. I have been playing the amp all week and it is nice and clean as long as we keep the master above 3. And I can still drive it to a nice crunch with the guitar controls cranked all the way up and the gain set anywhere above 70%. I am good with that setting, but 3 neighbors down, they aren't really Zeplin types.

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Re: Pacer 100 hum

Post by zenrider » Mon Feb 22, 2016 6:44 am

All the caps now replaced. All of the output section resistors were precision matched and replaced. Transistors between volume control and output are tested good. Output transistors previously replaced. Master volume control replaced. All grounds are now connected to a solid, single point at the connection of the line ground. All jacks removed, cleaned, grounds cleaned. When I plugged it in, same problem.

This thing is silent at a setting of 3 or more on the volume. Everything works great and sounds excellent. All controls work great. Just still won't run at indoor levels. All my neighbors enjoyed Aerosmith and Johnny Winter SRV renditions yesterday.

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CharlieP
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Re: Pacer 100 hum

Post by CharlieP » Wed Feb 24, 2016 10:26 am

zenrider wrote:All the caps now replaced. All of the output section resistors were precision matched and replaced. Transistors between volume control and output are tested good. Output transistors previously replaced. Master volume control replaced. All grounds are now connected to a solid, single point at the connection of the line ground. All jacks removed, cleaned, grounds cleaned. When I plugged it in, same problem.

This thing is silent at a setting of 3 or more on the volume. Everything works great and sounds excellent. All controls work great. Just still won't run at indoor levels. All my neighbors enjoyed Aerosmith and Johnny Winter SRV renditions yesterday.
Renumber 3 to 1, problem solved! :lol: :lol: :roll:
Charlie
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zenrider
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Re: Pacer 100 hum

Post by zenrider » Thu Feb 25, 2016 5:09 pm

That is not a bad solution, really. I am not opening it up again until something smokes. Which I am pretty sure is not going to happen since I am using it for practice in a small room. It is taking up way too much practice time to mess with thing. If I were to get paid by the hour, I could have picked up a really nice amp with lots of modern built in features. Matter of fact, I am headed over to Peavey product pages.

polaris26
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Re: Pacer 100 hum

Post by polaris26 » Sun Feb 09, 2020 6:42 pm

I hope nobody minds my resurrecting this thread. I recently purchased a second Pacer to use as part of a stereo setup, and I just got around to powering it up today with an external speaker (am in the process of acquiring a replacement speaker for it). On power-up, it was doing something similar to what the OP was describing (I think). The key is that it was not a hum (60/120 cycle) but a lower frequency oscillation of about 45 Hz as measured on my Fluke 87 meter. This would happen with the Master volume pot down all the way and would clear up once the Master got turned up a little.

Unlike my first Pacer, this amp has been worked on. It looks like someone went through it and replaced at least all of the electrolytic caps and some of the small signal transistors. Fortunately, the workmanship and parts are good quality and I found nothing obvious to complain about.

I went crazy for a few hours trying to figure out why this thing is doing what it's doing. The one thing I did notice is that the bias/quiescent current was very low on the output transistors. I tried powering up the amp with a non-inductive 8 ohm test load and the oscillation did not happen. I tried putting a large non-inductive 8 ohm resistor in parallel with the speaker, and that stops the phenomenon also. I tried disconnecting the 2.2uF feedback cap and this also stopped the amp from oscillating.

Long story short, I ran some LTSPICE simulations on the output stage and the sim oscillates like a banshee also, at a similarly low (almost subsonic) frequency!

I speculated that the lack of loading on the coupling transformer due to the low bias/idle current might be allowing the output stage to oscillate, so I bumped the bias resistors from 15 to 18 ohms (my other Pacer's resistors have already drifted over time from the nominal 15 ohms to about 17.5 ohms anyway). The oscillation is now gone, although on power up there is a little bit of a brief vibration like it's wanting to oscillate. I wonder if it's an issue of marginal design? I suspect a smallish resistor of about 100 ohms across each coupling transformer secondary might also damp the oscillations, but I'm going to leave it as-is for the time being.

Anyone else have experience with this sort of phenomenon?

Regards,
Dave

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Pappy B
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Re: Pacer 100 hum

Post by Pappy B » Tue Feb 11, 2020 11:25 am

I have a Pacer that has a very low hum when at idol. I recapped, replaced power resistors, replaced diodes and relocated the reverb pan to the bottom of the cab. I made a bracket that kept the correct orientation of the pan and made new cables. It still had a hum that increased as the reverb was turned up. I tried routing the cables every way I could and eventually remounted the pan back onto the chassis. Now the hum is very low and constant and can't be heard with any signal applied. It doesn't bother me and I don't bother it. This is one of my most played amps.
The more I learn the more I realize how much I don't know.

zenrider
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Re: Pacer 100 hum

Post by zenrider » Sun Apr 17, 2022 12:11 pm

I am breaking this out of hibernation. I am going to run it on loud then disassemble again to see if I can finalize this solution. At this point, I suspect a shunted winding in the output transformer. I got a new test meter and scope. I will update. Thank you again everyone.

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Re: Pacer 100 hum

Post by The_Chris » Fri Sep 16, 2022 6:39 am

I just had my PACER serviced. It's been down for 20 years. It made that low oscillation sound - like I was using an octave pedal all the time. It was unusable. Luckily, I had a bunch of other amps, so it sat on a shelf.

I just retired to Greece, and wanted everything working, so I gave it to an electrical engineer who was a guitar player. I can't remember how he got rid of the oscillation. Maybe it was just caps. He might have replaced an opamp.

However, I have always had THIS problem. At very low volumes, the signal starts to break up. They way I'd describe it is - you hit a note, or a chord, and as it's dying down, it gets to the point of where it just starts breaking up. Like the amount of power isn't jumping across a connection. Of course, at loud volumes it's great, but as it dies down, this happens.

Any ideas?

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