Harmony H306: no output

[Jonny toetags]

So I was at my friend's shop and spotted an old Harmony H306. It worked, but sounded real 'old'...basically sounded like it needed new caps and tubes. I picked it up for a great price and left.
Once I opened it up, I noticed it had been worked on (orange drops instead of the wax caps, a couple resistors changed off of the can cap.)
So basically I did the following:
-cleaned all the pots
-added ground plug
-removed the 'death cap' off of the fuse
-switched a few of the Orange Drops to Mallorys (I'm not a OD fan)
-replaced the 25uf/25V electrolytic
-replaced the can cap with a JJ 40/20/20/20 (not using the 40)
-replaced the two 220K resistors coming from the can cap (someone had put random ones in there and had them flying in the air)

Problem: powers on, voltages all seem fine, no sound from any input.
I have a B+ of about 360V w/all tubes installed. The only 'output' I get is the scratchy sound when you test voltages off of the preamp tubes.

Any insight as to what might have happened? Like I said, It was working before.
I do have a stock H305 on hand that I've been using as reference 'if needed'.

Here's a pic 'before':


Here are a few after pics. (I was short a few .02's, waiting for my order):

[MarkusBass]

The quality of the photos is not bad but still it is difficult to say whether some components are shorted, or not. For example, the resistor that is connected to the slider of VOL2 pot. Is is shorted to the ground? The same applies to the socket of the second power tube. I see there a short but maybe it is due to the quality of the photo. Also the resistor connected to the slider of the Vibrato pot - is it soldered, or not?
In general, the quality of the soldering in this amp is very poor. If you have an amp assembled in this way, you have to make sure that there are no shorts. The easiest way to troubleshoot this amp is to remove V1 and V2 tubes and provide a signal directly to 0.02 uF capacitors on the input of the power amp. Do you get output signal in this case?
Alternatively, you can find schematic (it is available on this forum) and trace the signal from the input to the output. Ths is very simple amp so this can be done in 15 minutes, or less.
I assume that you checked that all tube have correct heater voltage - you haven't mention this. You mentioned that you have +360V but this amp has at least three high voltage supplies. Are all of them present?

Mark

[Jonny toetags]

I've started going over all the soldering and checking for shorted leads.
What's the best way to provide the signal to the 0.2? Just unsolder it from the preamp and wire it direct to an input jack?

Here are a few more pics.
The only spot that makes me nervous is all those resistors coming off the preamp tube to that tagboard. Looks like a bloody mess.
Again, still odd since it worked before and I didn't touch anything in that area.
I'm not at home right now, but from what I remember the three caps on the can each had different HV.

Here are a few extra pics

[MarkusBass]

For me it is not odd. The amp worked previously and now it doesn't. The only explanation is that either you forgot some connection, or you made a short, or you wired a component differently than it was before.
On the first photo I still don't see whether the 220k resistor to the slider of the VOL2 pot is shorted to the ground, or not. The same applies to the input wire. I see that it is grounded at the first input jack. But isn't the tip of the jack shorted to the ground by this wire? I think that it makes no sense to make better photos because checking them will take several days, while tracing the signal will take few minutes only. You desolder one lead of the capacitor and provide signal from signal generator. If you have signal generator and oscilloscope this will again take no more than few minutes.

Mark

[Jonny toetags]

Alright, thanks Mark.
I have a show tonight, but will trace and check every connection/component tomorrow
I don't have a signal generator or oscilloscope
I will check back in with my findings.

[riz]

Well, you said yourself that you get "output" by touching preamp tube pins... It sounds like what you don't have is *input*. Even with no signal generator, you can trace the scratchy sounds back and see where in the circuit there is a break in the signal path. Sounds like it could even just be a miswired input jack, or an overlooked "whoops" short.

[olddawg]

You can make a signal prob with any signal source. Tape out from an old stereo works fine or headphone out from anything, a couple of alligator clips or some type of probe, and an isolation/coupling capacitor.

[MarkusBass]

As I said previously, I see several suspected places that for me look like "almost" short circuit. Here is an example:



Isn't the input jack shorted to the ground?

Mark

[Jonny toetags]

So I went through the entire amp loking for shorts or missing/badly soldered components. All OK
I'm getting 6.3V on the heaters.
My 3 HV's (measured from the cap can) are 350v, 260v and 250v.
None of the bare wires around the jacks or pots are shorting.
I can get that 'scratching' sound on a few pins of the second preamp tube.
I can't get any of those noises on the 1st preamp tube.

BUT...the amp works.
HOWEVER, the only way to get it to work is to plug into all 4 inputs one at a time then go back to input 1, i have sound.
Weird or what!

[MarkusBass]

The first tube is conneted to all 4 inputs. I would say that you have problems with the power supply of the first tube, or the inputs are incorrectly wired. Since somehow you can manage to make the amp work, I would vote for incorrectly wired inputs. You know that some of the inputs in this amp are "shorting", and some are not, don't you? Just to make sure that the power supply for the first tube is OK, I would measure voltages on 220k anode resistors (on both ends) - they are between can capacitor and the tube, and on cathode resistors (they are on tube's socket). It may be something simple like missing ground for the cathode resistors.

Mark

[Jonny toetags]

The first tube is conneted to all 4 inputs. I would say that you have problems with the power supply of the first tube, or the inputs are incorrectly wired. Since somehow you can manage to make the amp work, I would vote for incorrectly wired inputs. You know that some of the inputs in this amp are "shorting", and some are not, don't you? Just to make sure that the power supply for the first tube is OK, I would measure voltages on 220k anode resistors (on both ends) - they are between can capacitor and the tube, and on cathode resistors (they are on tube's socket). It may be something simple like missing ground for the cathode resistors.

Mark

I will take some measurements tonight.
I have a clean H305 (stock) on hand I've been using as a guide. All 4 jacks seem to be wired ok. I mean, there's not much to them.
One thing I did notice, on the H306, there was a ground wire going from the middle of the 1st preamp tube to ground.
On the H305, it wasn't there. I removed it.
You can kind of make it out in this pic:

Is it ok that I removed it?

[MarkusBass]

I think that the connection was not critical but it is not nice that you modify the amp, don't tell us about it and ask what's wrong with the amp. In such a case the first rule is to bring the amp back to factory state. You can modify it only when it works correctly.
Please take a look at the attached photo. This is similar amp (Harmony 307) that I was fixing lately. Every pin of the first tube can be easily identified (and I did it for you). Soldering is slightly better. On the photo you can see every detail. On your photos it is hard to see important details. That's why I was confused whether some wires were shorted, or not.
If you measure voltages on cathodes and on anodes of the first tube, you will learn a lot.

Mark



EDIT: regarding the removed connection; please check where heater winding is connected to the ground or to some DC voltage. Without this connection the winding is "floating".

[Jonny toetags]

I only removed that ground last night, not this entire time.
On mine, the wire went directly to ground. In your pic, it looks like it goes to the heaters.

[g1]

I'll take a wild guess here that there is an input jack wiring problem, and a dirty shorting switch on one of the input jacks.
When you plug into one of the other jacks and remove the plug, it's switch does not close and the amp temporarily works.
Just a wild guess though.

[52 Bill]

Well, I searched and found a schematic that shows two inputs connected to one half of the first 12AX7 and the other two inputs connected to the other half of the tube. So, if the jacks are a problem, both sets are wired wrong.

I'm not good enough to figure out all of the wiring from the photos, because I can't see the tube sockets clearly enough. I think that you need to take a few voltage measurements at the first preamp tube in order to figure this out.

What are the dc voltages at the plates and the cathodes of the first 12AX7? What are the ac voltages at pin 9 and 4/5? I think that 4/5 are grounded but I can't see them clearly enough to know.