Troubleshooting frontman: mystery on tp22 and tp26

[thz52]

Folks,
I've been in a "bad romance", a mad affair with a frontman212R from a friend.
If I fix a lot of things so far? Yes. (If I wanted to die in the process? Yes.)
What was the problem in the first place? Clean channel getting dirty.
Suspects? Someone among the path from one of the rails (Trying to avoid give too much dispensable information for now)

But I just need help to figure out why TP22 and TP26 are giving me wrong values. I can't make out.
TP22 -> 15.1 VDC and 14.8 VDC
TP26 - 2.8 VDC (always)

By the way in my country "nightmare" is how solid state amp tech are known sometimes, that's why i'm asking for a little help.
Any guess?

[DrGonz78]

Just a few things before we get started... Read this thread as best as possible. >>> http://music-electronics-forum.com/t42735/
For example, we are wanting members to list their world location since power supply voltages vary and other reasons too.

It is also good to post the schematic link so we can quickly pull up the information.
http://music-electronics-forum.com/a...-schematic.pdf

[thz52]

Just a few things before we get started... Read this thread as best as possible. >>> http://music-electronics-forum.com/t42735/
For example, we are wanting members to list their world location since power supply voltages vary and other reasons too.

It is also good to post the schematic link so we can quickly pull up the information.
http://music-electronics-forum.com/a...-schematic.pdf

Oh, thanks! So sorry for missing it. And yes, i'm always guiding me using schematic (exactly that one).

[olddawg]

There are a bunch of well known issues with that amp. It would behoove you to search the archive for previous posts by brand and model. Off the top of my head. The DC rail filter caps love to desoldering themselves. The input jacks develop bad board connections. The effects loop jacks develop issues. The diffamp pair drifts causing intermitant DC offsets and frequently requires replacement. And there is bad wave soldering that can be anywhere on the board. Sometimes just resoldering the everything is necessary. Good soldering skills and a can of freeze are your friends.

[J M Fahey]

Flamengão hein?
I guess Rio de Janeiro is 110VAC (maybe some neighbourhoods are 127V now).

What was the problem in the first place? Clean channel getting dirty.

You mean the clean channel sounds dirty or the amp stays always on the dirty channel and can´t be switched to the clean one?
Not the same thing.

But I just need help to figure out why TP22 and TP26 are giving me wrong values. I can't make out.
TP22 -> 15.1 VDC and 14.8 VDC

You mean you switch the panel button or footswitch and voltage goes from +15.1 VDC to +14.8 VDC or +15.1 VDC to -14.8 VDC ?

TP26 - 2.8 VDC (always)

You mean you push switches and TP26 stays always at +2.0V DC ? (always state polarity because hgere it is specially important)

Modern SS Fender amps switch channels and modes by rectifying roughly 30VAC into positive or negative voltages, which simplifies wiring and allows easy LED inclusion if needed.
A bad diode, switch, open wire/track or lack of 30VAC will cause switching problems.
You might also have a bad switching Op Amp.

1) check ~30VAC reach the point "Vac" on top of R119 .
measure VAC present on the bottom end of R119, at different combinations of switches being pushed; R119 dissipates some power and might have toasted . Just in case add a little solder to its PCB pads.

2) check what TP20/21/22 "truth tables" work and which show abnormal values.
Just showing what TP22 does is not enough, all of them are inter related.
And of course U7 or some of the diodes might be bad, but in general I distrust the "mechanicals" (jacks and switches) more than plain electronic parts.

Ok, test and post results.

Next time, try to search and post any related schematic or links, so we all know what are we talking about, "TP22" by itself means nothing without.

Good hunting

[thz52]

Flamengão hein?
I guess Rio de Janeiro is 110VAC (maybe some neighbourhoods are 127V now).

Yes, Flamengão!!! and right, Rio is 127V 60hz now...

You mean the clean channel sounds dirty or the amp stays always on the dirty channel and can´t be switched to the clean one?
Not the same thing.

I mean it sounds dirty! I can switch between clean and drive channel normally...

You mean you switch the panel button or footswitch and voltage goes from +15.1 VDC to +14.8 VDC or +15.1 VDC to -14.8 VDC ?

Sorry for that! You all were really kind, I appreciate it! Well... When I switch it goes from +15.1VDC (Normal Channel) to -14.8VDC (Drive Channel)

You mean you push switches and TP26 stays always at +2.0V DC ? (always state polarity because hgere it is specially important)

Yes! Whatever I do measurement always give me -2.8V VDC (clean or drive) and at more drive I got -9.0VDC

Modern SS Fender amps switch channels and modes by rectifying roughly 30VAC into positive or negative voltages, which simplifies wiring and allows easy LED inclusion if needed.
A bad diode, switch, open wire/track or lack of 30VAC will cause switching problems.
You might also have a bad switching Op Amp.

1) check ~30VAC reach the point "Vac" on top of R119 .
measure VAC present on the bottom end of R119, at different combinations of switches being pushed; R119 dissipates some power and might have toasted . Just in case add a little solder to its PCB pads.

2) check what TP20/21/22 "truth tables" work and which show abnormal values.
Just showing what TP22 does is not enough, all of them are inter related.
And of course U7 or some of the diodes might be bad, but in general I distrust the "mechanicals" (jacks and switches) more than plain electronic parts.

Ok, test and post results.

Next time, try to search and post any related schematic or links, so we all know what are we talking about, "TP22" by itself means nothing without.

Good hunting

I got 33VAC on R119 in any condition.

TP20 - OK!
TP21 - OK!
TP23 - OK!

What I've done so far: Replaced both TL072, Replaced C55, C56, C57, C59 and C60 capacitors, Replaced D34 and D36 diodes... I've changed R144 and R145 to 480 ohms now (because 330R that i replaced too was still giving too much heat dissipation and damaged pads (I had to redo all traces from that path).
Well, a important detail: It comes and goes. Before everything, when I was touching D24 and some others neighbours with probes the problem disappear but come back later…
When I turned off it sounds good few seconds before mute (during discharing, right?)

Thanks in advance! All your answers helps a lot (all people from forum)

Doug.

[thz52]

Hello, is anyone there?

[nickb]

Put it into 'clean' mode. Now,

What is the voltage at the gate of Q4?
Do the the dirty controls have any effect?

[thz52]

Put it into 'clean' mode. Now,

What is the voltage at the gate of Q4?
Do the the dirty controls have any effect?

When clean I got around -10.4VDC
When dirty (drive) I got -0VDC

Yes, they do... but pots goes noisy like a mix of crunch and pop sound.
It doesn't seem a malfunction from these pots but a reflection of circuitry issue (or both)

What about those voltages from Q4?
Oh... When I turn it on with guitar connected in Pwr In or in some of inputs the amp sounds fuzzy and then turns clean then not so clean...

[Randall]

I had two of these amps come in recently at one time from the same customer. Both had bad or broken connections at ceramic resistor R144. Hows that for coincidence? Repairing both of those connections fixed both amps with different issues. Glean from that what you may.

[thz52]

I had two of these amps come in recently at one time from the same customer. Both had bad or broken connections at ceramic resistor R144. Hows that for coincidence? Repairing both of those connections fixed both amps with different issues. Glean from that what you may.

What were symptoms? I'll take a look into it... thanks a lot for share it!

[nickb]

When clean I got around -10.4VDC
When dirty (drive) I got -0VDC

Yes, they do...<snip>

What about those voltages from Q4?

The voltages are correct BUT you switched it into dirty mode ( I said clean) so now I don't know if you tested the dirty controls in clean mode or dirty? If the dirty channel controls are doing something when you are in clean then Q4 is bad.

Oh... When I turn it on with guitar connected in Pwr In or in some of inputs the amp sounds fuzzy and then turns clean then not so clean...

Check the +16V and -16V supplies on pin 4 and 8 of the opamps. This is new information. I suggest the get the power amp in clean first and work backwards from there.

... but pots goes noisy like a mix of crunch and pop sound.
It doesn't seem a malfunction from these pots but a reflection of circuitry issue (or both)

Sounds like an infestation of breakfast cereal to me

Let's get the circuit issues straightened out and then we can come back to this.