AMP only plays when I strum really hard.

[joecool85]

Right.  Broken/pre-amp out to good amp input, good cord.  Good amp hums and no more. 

Ok, well then we know 100% for sure that it is a preamp issue.

[dan92y]

Right.  Broken/pre-amp out to good amp input, good cord.  Good amp hums and no more. 

Ok, well then we know 100% for sure that it is a preamp issue.

OK. Now I am out of my 'lane'.  I feel comfortable replacing parts, but I don't know what to look for that is bad.  I can't tell from the print what is pre-amp, power amp etc.  Any thoughts??

[J M Fahey]

Good, so please confirm:
1) guitar or MP3 player into Harvard power amp sounds good, although not too loud, and without hum?
2) Harvard preamp to *another* power amp hums?
3) Harvard preamp to *its own* power amp hums?
4) If (3), please touch lightly with the end of a small screwdriver, pin 2 of IC3; does it hum loud?
5) If no change please measure (relative to ground) voltages on pins 1 , 2 , 3 , 4 , 8 in IC3.
They should be close to 0 , 0 , 0 , -15 , +15 VDC.
If still no change join with a short piece of wire pin1_IC3 to node (union) R52_C29 , then touch again Pin2_IC3.
Loud hum?

[dan92y]

Good, so please confirm:
1) guitar or MP3 player into Harvard power amp sounds good, although not too loud, and without hum?
2) Harvard preamp to *another* power amp hums?
3) Harvard preamp to *its own* power amp hums?
4) If (3), please touch lightly with the end of a small screwdriver, pin 2 of IC3; does it hum loud?
5) If no change please measure (relative to ground) voltages on pins 1 , 2 , 3 , 4 , 8 in IC3.
They should be close to 0 , 0 , 0 , -15 , +15 VDC.
If still no change join with a short piece of wire pin1_IC3 to node (union) R52_C29 , then touch again Pin2_IC3.
Loud hum?

#3 HRVD preamp to own power amp = Yes quiet hum
#4 Crackles and louder hum when pin 2 of IC3 is touched
End of testing for this time.  Im guessing that chip is bad and needs replaced.  If so, where do I get the exact replacement?  I had a hard enough time choosing pots and I bet that there are even more types of IC's that will fit in the space.
Thank you very much for the tech support.

[J M Fahey]

Did you check voltages as in 5) ?
Did you put the s piece of wire I suggested after that?
As of IC3 it's easy, you can replace it by a very common TL072 or an RC4558, which is closer to the original.
Pay special attention to the small notch or dot which marks one end, the replacement should point the same way, but before changing it please answer *all* 5 questions.
Those are things you are checking for me because your amp is not on my bench, please do.
Thanks.

[dan92y]

Did you check voltages as in 5) ?
Did you put the s piece of wire I suggested after that?
As of IC3 it's easy, you can replace it by a very common TL072 or an RC4558, which is closer to the original.
Pay special attention to the small notch or dot which marks one end, the replacement should point the same way, but before changing it please answer *all* 5 questions.
Those are things you are checking for me because your amp is not on my bench, please do.
Thanks.

OK ALL voltages in step 5 are 0.00  I slipped and shorted pin 1 and 2 and the speaker popped very loudly (dont know if that matters) 
The wire between IC3 pin 1 and the node = no change when pin 2 touched there was a loud hum.

[Jack1962]

Did this amp work before you replaced the pots? other than being scratchy. if so either those are the wrong pots or perhaps while you where soldering them in you got a little bit of solder on a solder pad next to on of the pots. when you say you plugged into a poweramp did you input a signal to the amp your having trouble with to see if the preamp had a output thru the power amp?

                                                  Rock On

[dan92y]

Did you check voltages as in 5) ?
Did you put the s piece of wire I suggested after that?
As of IC3 it's easy, you can replace it by a very common TL072 or an RC4558, which is closer to the original.
Pay special attention to the small notch or dot which marks one end, the replacement should point the same way, but before changing it please answer *all* 5 questions.
Those are things you are checking for me because your amp is not on my bench, please do.
Thanks.

OK.  All the voltages were zero.  Replaced IC3.  Just had a trial run and NO CHANGE.  Officially I am about ready to give up.

[J M Fahey]

Well, if you have no +/-15v rails your preamp is OFF , just like that.
Don't know if the preamp supply is dead or something (maybe a blob of solder) is shorting the rails.
I understand it's very frustrating and share your feeling.
Because of what's happening to you and many others, I'm considering offering an alternate solution, bypassing the problem completely.
It will consist on a couple "universal" preamps, easy to mount on a half-dead amp as yours, and happily driving the still alive power amp.
For all practical means, your amp will be alive and kicking and useful again onstage.
I see there are a few power amp kit offers, which solve half the problem, but none (that I know of) to cure the preamp by transplant.
Stay tuned.

[dan92y]

Well, if you have no +/-15v rails your preamp is OFF , just like that.
Don't know if the preamp supply is dead or something (maybe a blob of solder) is shorting the rails.
I understand it's very frustrating and share your feeling.
Because of what's happening to you and many others, I'm considering offering an alternate solution, bypassing the problem completely.
It will consist on a couple "universal" preamps, easy to mount on a half-dead amp as yours, and happily driving the still alive power amp.
For all practical means, your amp will be alive and kicking and useful again onstage.
I see there are a few power amp kit offers, which solve half the problem, but none (that I know of) to cure the preamp by transplant.
Stay tuned.

Its still sort of fun messing around with all this stuff.  I have a grasp of what I am doing.  Just don't know WHY.  Thanks for all the help.  I dont know what you mean by RAILS. The transformer comes in and connects to the board and a small ground bus. Again, Thanks.

[J M Fahey]

A "rail" is the informal name of a wire or conductor which carries a certain voltage or signal to many places inside an amp.
Since your preamp ICs run from +15V and -15V, it's quite common to run a couple parallel traces end to end in the board, each carrying one of those voltages.
I guess they may look like a pair of railroad tracks to some, reinforced by the fact that they "carry" something everywhere.
In the power amp, you have higher voltage "rails", between +/-*25 and +/-70V, depending on amp power.
Of course they may zig-zag everywhere if so needed, but in the schematic they are usually drawn with a couple parallel lines, so ......
What I meant is that if you had no +/-15V on *one* IC, you probably had them nowhere on the preamp; the big question is whether your power supply is not "making" them or some shorted IC (or solder blob) is killing them.
Same final result, different motives, different solutions.

[dan92y]

Well, if you have no +/-15v rails your preamp is OFF , just like that.
Don't know if the preamp supply is dead or something (maybe a blob of solder) is shorting the rails.
I understand it's very frustrating and share your feeling.
Because of what's happening to you and many others, I'm considering offering an alternate solution, bypassing the problem completely.
It will consist on a couple "universal" preamps, easy to mount on a half-dead amp as yours, and happily driving the still alive power amp.
For all practical means, your amp will be alive and kicking and useful again onstage.
I see there are a few power amp kit offers, which solve half the problem, but none (that I know of) to cure the preamp by transplant.
Stay tuned.

Its still sort of fun messing around with all this stuff.  I have a grasp of what I am doing.  Just don't know WHY.  Thanks for all the help.  I dont know what you mean by RAILS. The transformer comes in and connects to the board and a small ground bus. Again, Thanks.

I don't know if this matters or not, but...I ran a cable out from a good amp to the 'broken' one because I thought I could still use the speaker.  Out from the good on ext. speaker jack and into the crappy preamp one's input.  The dang thing came to life and nearly blew my ears out.  I had the controls set on the good amp for a descent level and when I played through the other amp it was incredibly loud. Anyway, like I said, Dont know if that makes a difference or not.  Also, is there a place where I can just get a new circuit board for this amp?  If I could swap out the whole board I would be in business, but I can find one online anywhere.  Thanks again.

[J M Fahey]

I don't know if this matters or not, but...I ran a cable out from a good amp to the 'broken' one because I thought I could still use the speaker.  Out from the good on ext. speaker jack and into the crappy preamp one's input.  The dang thing came to life and nearly blew my ears out.  I had the controls set on the good amp for a descent level and when I played through the other amp it was incredibly loud.

Well, you shouldn't do that.
Speaker out signal is thousands of times stronger than a guitar signal and can (and will) fry your preamp input.
Maybe you got lucky and didn't but don't do again.
What you *can* feed there is the good amp's line out/loop send/pre out; 3 names for basically the same thing.
What happened somewhat confirms what I suspect, your preamp is dead or at least unpowered, and that strong signal just passed through it (unamplified) into your good power amp.

Anyway, like I said, Dont know if that makes a difference or not.  Also, is there a place where I can just get a new circuit board for this amp?  If I could swap out the whole board I would be in business, but I can find one online anywhere.

Commercial amplifier boards are never available on their own, except on the *very* unusual case of the factory closing and selling remaining inventory as-is, by the pound or, for example, Behringer sells new stuffed boards as plug-in replacement for their larger power amps, on the logic that they are used by Touring bands or in big house PA systems, where they just *can't* wait for repairs.
Anyway, not available for smaller amps, even more for an older one such as this.
That's why I'll soon offer in the USA some "universal" pre and power amp boards, to "save" amps like this which are basically 90% good.
Cabinet, chassis, transformer and speaker are , literally, "the bulk" of the amp, the most expensive part and the most difficult to solve for the home constructor.
Think a month or two ahead.

Back to your Harvard, let's check why you don't have those +/-15V.
At the Power Supply you should have +/- 25V on the ends of C38 and C39 respectively. (I think you do)
Then check on both ends of R74 and R75 ; you should have + or -25V on one end and + or - 15V on the other.
Are they hot? (use carefully your finger-o-meter).
Are D19 and D21 hot? (or warm).

[dan92y]

I don't know if this matters or not, but...I ran a cable out from a good amp to the 'broken' one because I thought I could still use the speaker.  Out from the good on ext. speaker jack and into the crappy preamp one's input.  The dang thing came to life and nearly blew my ears out.  I had the controls set on the good amp for a descent level and when I played through the other amp it was incredibly loud.

Well, you shouldn't do that.
Speaker out signal is thousands of times stronger than a guitar signal and can (and will) fry your preamp input.
Maybe you got lucky and didn't but don't do again.
What you *can* feed there is the good amp's line out/loop send/pre out; 3 names for basically the same thing.
What happened somewhat confirms what I suspect, your preamp is dead or at least unpowered, and that strong signal just passed through it (unamplified) into your good power amp.

Anyway, like I said, Dont know if that makes a difference or not.  Also, is there a place where I can just get a new circuit board for this amp?  If I could swap out the whole board I would be in business, but I can find one online anywhere.

Commercial amplifier boards are never available on their own, except on the *very* unusual case of the factory closing and selling remaining inventory as-is, by the pound or, for example, Behringer sells new stuffed boards as plug-in replacement for their larger power amps, on the logic that they are used by Touring bands or in big house PA systems, where they just *can't* wait for repairs.
Anyway, not available for smaller amps, even more for an older one such as this.
That's why I'll soon offer in the USA some "universal" pre and power amp boards, to "save" amps like this which are basically 90% good.
Cabinet, chassis, transformer and speaker are , literally, "the bulk" of the amp, the most expensive part and the most difficult to solve for the home constructor.
Think a month or two ahead.

Back to your Harvard, let's check why you don't have those +/-15V.
At the Power Supply you should have +/- 25V on the ends of C38 and C39 respectively. (I think you do)
Then check on both ends of R74 and R75 ; you should have + or -25V on one end and + or - 15V on the other.
Are they hot? (use carefully your finger-o-meter).
Are D19 and D21 hot? (or warm).

OK I said before I am not an electrician/tech/etc.  I assumed you meant check the ends of the 38 and 39 caps. to ground.  If that is NOT the case, that would explain the sparks.  Am I doing this correctly???

[dan92y]

I don't know if this matters or not, but...I ran a cable out from a good amp to the 'broken' one because I thought I could still use the speaker.  Out from the good on ext. speaker jack and into the crappy preamp one's input.  The dang thing came to life and nearly blew my ears out.  I had the controls set on the good amp for a descent level and when I played through the other amp it was incredibly loud.

Well, you shouldn't do that.
Speaker out signal is thousands of times stronger than a guitar signal and can (and will) fry your preamp input.
Maybe you got lucky and didn't but don't do again.
What you *can* feed there is the good amp's line out/loop send/pre out; 3 names for basically the same thing.
What happened somewhat confirms what I suspect, your preamp is dead or at least unpowered, and that strong signal just passed through it (unamplified) into your good power amp.

Anyway, like I said, Dont know if that makes a difference or not.  Also, is there a place where I can just get a new circuit board for this amp?  If I could swap out the whole board I would be in business, but I can find one online anywhere.

Commercial amplifier boards are never available on their own, except on the *very* unusual case of the factory closing and selling remaining inventory as-is, by the pound or, for example, Behringer sells new stuffed boards as plug-in replacement for their larger power amps, on the logic that they are used by Touring bands or in big house PA systems, where they just *can't* wait for repairs.
Anyway, not available for smaller amps, even more for an older one such as this.
That's why I'll soon offer in the USA some "universal" pre and power amp boards, to "save" amps like this which are basically 90% good.
Cabinet, chassis, transformer and speaker are , literally, "the bulk" of the amp, the most expensive part and the most difficult to solve for the home constructor.
Think a month or two ahead.

Back to your Harvard, let's check why you don't have those +/-15V.
At the Power Supply you should have +/- 25V on the ends of C38 and C39 respectively. (I think you do)
Then check on both ends of R74 and R75 ; you should have + or -25V on one end and + or - 15V on the other.
Are they hot? (use carefully your finger-o-meter).
Are D19 and D21 hot? (or warm).

OK I said before I am not an electrician/tech/etc.  I assumed you meant check the ends of the 38 and 39 caps. to ground.  If that is NOT the case, that would explain the sparks.  Am I doing this correctly???

I got more courage and tried some more.  R-74 and 75 I would say are probably warmer than intended.  Not burning but pretty warm. NO voltage on either end but on one end they both sparked pretty well.  D 19 and 21 are NOT HOT. NO voltage and on D 21 when I touched the probe on one end the speaker popped VERY LOUDLY.  Scared the ***** out of me.No blown speaker or fuses though.