Starting fresh DIY on JC-80

[art_lessing]

So last AUgust I took my JC-80 into the shop and asked the tech to bring it back to its original bright sound which has deteriorated to not bright, not loud. This amp used to be really loud. Anyway, he told me the transformer was putting out a low output..bla bla bla....He said he could fix it, he couldn't he charged me anyway for useless resoldering...I am a sucker.. but I have my amp back and want to do it myself with not too much experience.

Here are the symptoms:

It used to be really loud, now its not that loud. I noticed that after Vol-3 there is no more increase in volume, just overdrive and it sounds like I am using a light overdrive if I bang on the strings.


First thing: The transformer is labled with :

http://music-electronics-forum.com/a...18035805-8.jpg

120vac -------> +38v 0 -38v the actual reading is 125vac ------> +33v 0 -33v
This is constant even when I am jiggling wires and tapping on it.

Also I think that it plays loud and clear for about 30 seconds when the amp is cold.

I dont see any burnt resistors, caps, or diodes...one heat sink on the pre-amp is detached but the connection is good

http://music-electronics-forum.com/t38253/

there is a pic on this tread that shows to sinks, one has a blue chip and one has a black..on mine the black on has a loose sink.


All other connections seem to be clean, pots are clean, speaker is a new Weber 15 80W.

Also cant seem to locate any PRE-1979 JC80/60 schematics..the only ones around are the post 1979 versions where the preamp is turned 180 degrees from mine. My amp has three boards, main, pre, and effects.


Any clues?

Thanks anyone
DQ

[52 Bill]

The first thing that I would test would be the "Main In" jack. Either clean it with some DeoxIt or temporarily jumper across the normally closed contacts. See if the volume is restored.

120vac -------> +38v 0 -38v the actual reading is 125vac ------> +33v 0 -33v
This is constant even when I am jiggling wires and tapping on it.

The transformer secondary wires go directly to two small fuses. Remove the fuses from their holders and test each fuse with your ohmmeter. Leave them out for the next test.

Carefully measure the AC Voltage from each fuse holder input referenced to ground with your voltmeter set to read AC Volts. How close to 38 volts ac does your reading come to?

Next turn the amp off and reinstall two good fuses. Find the main rectifier. It will either be a single block with four leads or four separate diodes. The output side of the fuse holders should directly connect to the rectifiers. The main rectifier will have two output voltages, one positive and one negative. See if you can find the outputs of the rectifier. The output will directly connect to the two main filter capacitors.

Now set your meter to read DC Voltage and read the voltages at the two outputs referenced to ground. You should get somewhere around 40-45 volts DC.

Let us know what you find out.

[J M Fahey]

So last AUgust I took my JC-80 into the shop and asked the tech to bring it back to its original bright sound which has deteriorated to not bright, not loud. This amp used to be really loud.

When and why did you change your speaker?
Where is the original one? Is it a 12" or a 15"? Do you have a known speaker to compare (say, some known Celestion)?
That alone might explain low efficiency and dullness.

Anyway, he told me the transformer was putting out a low output..bla bla bla....He said he could fix it, he couldn't

Bla bla bla indeed, he has no clue.
Agree that Roland is inconsistent labelling its power transformers, big way.
* 38+38 VAC means hairy +/-53VDC rails ... yet schematic shows proper +/-42V DC
* 33+33VAC is about right , translates to +/-46VDC with your actual 125VAC wall voltage, would be around +/-44V rails "raw"(same as specified in Roland Cube 60) and in practice spot-on for nominal+/-42Vv rails.
In a nutshell, 33+33V AC at the transformer windings (marked 38 - 0 - 38) is fine and about what is to be expected.

You seem to be confusing/mixing AC and DC measurements, please recheck actual transformer measurements (AC) and power amp supply rails (DC) although I anticipate no problem there.

Not the first time a no clue Tech can not find the real problem, finds something "unusual" and blames it for the problem.

Slight problem is how did the amp manage to work properly before, since transformer voltages simply do NOT change?
Transformers either work/open/burn/short , pick one, they do not change voltages on their own yet stay alive, period.

he charged me anyway for useless resoldering...I am a sucker.. but I have my amp back and want to do it myself with not too much experience.

Well, you will have to troubleshoot it and know better than him

Here are the symptoms:

It used to be really loud, now its not that loud. I noticed that after Vol-3 there is no more increase in volume, just overdrive and it sounds like I am using a light overdrive if I bang on the strings.

Ok, it's clipping beyond 3.
Might be putting out less power, clipping the preamp, or you might have a very dirty Main In jack.
Try squirting a little cleaner into it and work a plug in/out a few times, ... any improvement?

In this amp, do NOT connect a cable from Line Out (speaker derived) to Main In (between Pre and Power amp) or you'll probably damage it.

First thing: The transformer is labled with :

http://music-electronics-forum.com/a...18035805-8.jpg

120vac -------> +38v 0 -38v the actual reading is 125vac ------> +33v 0 -33v
This is constant even when I am jiggling wires and tapping on it.

I sort of explained it above, it's an unexplainable but real Roland quirk.

Also I think that it plays loud and clear for about 30 seconds when the amp is cold.

Well, that might show a problem ... but you should inkect some aurio tone, scope it , check how much it puts out before clipping and then see whether it meets specs or not.

I dont see any burnt resistors, caps, or diodes...one heat sink on the pre-amp is detached but the connection is good

http://music-electronics-forum.com/t38253/

Ok, attach it, even if it seems to work without.

there is a pic on this tread that shows to sinks, one has a blue chip and one has a black..on mine the black on has a loose sink.

Colour is not important, but fix that loose heatsink.

All other connections seem to be clean, pots are clean, speaker is a new Weber 15 80W.

Also cant seem to locate any PRE-1979 JC80/60 schematics..the only ones around are the post 1979 versions where the preamp is turned 180 degrees from mine. My amp has three boards, main, pre, and effects.

You'll have to work with what's available.
In any case, I guess an early model was later integrated, to simplify production, into 1 or 2 large boards, that does not necessarily mean big electric design changes.

Any clues?

* try it with another good speaker or cabinet, a marshall 4 x 12" if available
* drive the power amp (plug into Main In) from another known good Pre Out (say, from a Peavey or something) , do you get satisfactory volume and definition that way?
If so, you have a Preamp problem.

*** just a hunch: check that the loose heat sink does not mean that a driver transistor has a broken leg.
Use good light and a loupe if necessary. ***
Do you mean one of these silver thingies on the left?

[art_lessing]

http://music-electronics-forum.com/a...-wp_001197.jpg

I think they are the same as the two on the left in this pic..not my amp ..simliar pic

[art_lessing]

here's an update...and thanks for all the help

I checked the inputs and they are good
I reattached the heat sink
still has early break up, more so when my guitar volume is turned up all the way....I compared it to playing through a 60 watt Concert amp in perfect condition. The concert amp has none to very little distortion from a really hot passive pickup, but the JC sounds like I am playing through a mild fuzz face if I really thumb pluck my e-string or bang on all the strings with a hard pick.

here are pictures


this is the 1976 power board with detached sink (fixed) , notice no fuses and backwards from 1979...I cant find the schematic for this either...it turns out to be the same board that the '75 jc-120 uses but with half the components

would this section be easy to test? Where would I attach a signal?...I have a loop station that I use for my typical signal volume when testing stuff. I have it set to a volume that is equal to when I am playing without it. The output is equal to me standing there playing my guitar. I don't have a signal generator, or scope.

[52 Bill]

would this be easy to test? or would it be easier to replace all resistors, caps, diodes or are these parts hard to find now?
is there any way to tell where the pre-amp outs are from the picture?

The board on the top right of you photo is the power supply board. The two heatsinks are for the power transistors that are used in the low voltage regulators.

The board below it with the chorus switch is the FX board.

The preamp/power amp board is to the left and is not fully shown in your photo. Better photos of the entire chassis will be of help to id the chassis if nothing else.

You shouldn't try fixing this by randomly replacing things. Test things and try to isolate the problem to a specific area of the amp. Then fix the problem.

[art_lessing]

here is the whole thing..

[g1]

This may or may not be your issue, but you will not hear a tube amp overdrive the input like a solid state amp when using high guitar volume/hot pickup. The tube will overdrive and compress a fair bit before it becomes audible. The older SS stuff will have very objectionable distortion the instant the input is overdriven.

[52 Bill]

That photo really doesn't help much. According to the web, the JC-80 is a single channel 60 watt amp with a single 15' speaker.

So based on the looks of the boards, it's a JC-120 with one less pre and power amp.

Unfortunately there are no FX loop jacks and the line out is derived from the power amp output. The only way to listen to only the preamp is to take the signal from the wiper of the volume control.

Do you have some way to listen to this output? A second amp of some sort?

[art_lessing]

yep it is the jc-120 minus 60....they used the same boards and everything..the power board even has a bunch of unused holes for the components of the other 60 watts...the volume pot tap in is great idea...I assume that would be to test whether the pre amp sounds like fudge without the power amp. So where would I send that signal....to another amp's input or to something like an AUX in on a stereo or set of speakers?