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Hughes and Kettner Warp 7 100W Head, Loud hum and no guitar sound until maxed

Started by batbob, March 02, 2018, 11:22:59 AM

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batbob

Hey,
That voltage was indeed a bit weird. I've not got confused with the BCE pinout but the voltages are different than I measured the other day.
T3 Base is 44v, Collector at 44v and Emitter at 2.5v
Not sure why that happened unless I hadn't grounded the meter lead properly or the probe was not touching the solder.
Doesn't help though really does it I guess?
This is how it stands now.

g1


batbob

No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Don't know what the next move is though.
Thanks all, for your advice and guidance.

phatt

I'm not the expert but from my experience;
The base of T3 would not be at Zero as there is always some slight DC imbalance there, maybe 50mV or so. Just a thought?
It may have already been mentioned but have you checked the Speaker output for DC volts?
Phil.

g1

Quote from: batbob on March 07, 2018, 02:34:53 AM
No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Excellent, we can use those numbers later.

For now, most of those DC voltages look good except T3.  Can you measure from E to B (one probe on each leg).  We are looking for about 0.6V difference between the two.

g1

Please also recheck T5 and T6 voltages, I think B & E are mixed up, and T6 should be -1.1V, not +.

batbob

Phil,
I rechecked the voltages around T3 and there must have been an issue with me not connecting correctly. The Collector was 2.5v with Base and Emitter both at 44 v

g1, I measure the voltages on T3 Emitter-Base whilst the amp was on and it was about right, at 0.530v.
I did the diode check on the DMM on T3 too to see if that identified anything, there was an oddity in the Collector to Emitter of 1.75v (whilst amp was switched OFF)
The rest were (- means black lead and + means red lead on my Fluke 79III)
B- > C+ =0.53v
B- > E+ = 0.53v
B+ > C- = OL
B+ > E- =OL
C+ > E- = 0.53v
C- > E+ = 1.75v

I also rechecked T5 and T6, traced the circuit to make sure I was right on the E,C and B.
The measurements with the amp on are consistent with the above post apart from the  emitter on T6 should have been -1.1v (you're spot on), with the amp off E>B drop of each is 1.1v

There's 30mv DC on the speaker output.

This has me foxed....

g1

Quote from: batbob on March 07, 2018, 06:33:19 PM
g1, I measure the voltages on T3 Emitter-Base whilst the amp was on and it was about right, at 0.530v.
I did the diode check on the DMM on T3 too to see if that identified anything, there was an oddity in the Collector to Emitter of 1.75v (whilst amp was switched OFF)
The rest were (- means black lead and + means red lead on my Fluke 79III)
B- > C+ =0.53v
B- > E+ = 0.53v
B+ > C- = OL
B+ > E- =OL
C+ > E- = 0.53v
C- > E+ = 1.75v
I'm sorry but I'm getting confused as to what readings are amp on, which are amp off, and whether the diode checks are with transistor in or out of circuit.
The leads of T3 are E,C,B (left to right) looking at the printed face.
If there is .53V from B to E running, then one should be around 44V and the other around 43.5V
Please check again the exact DC voltage for the three terminals of T3 with amp running.

phatt

Quote from: phatt on March 07, 2018, 08:01:08 AM
I'm not the expert but from my experience;
The base of T3 would not be at Zero as there is always some slight DC imbalance there, maybe 50mV or so. Just a thought?
It may have already been mentioned but have you checked the Speaker output for DC volts?
Phil.

Opp's sorry meant to say base of T2  :duh:-[

@ G1, Q, with only 30mV's on the output,,,,is it possible the amp might be working but with a cracked trace on the output? Or even a frayed speaker wire/cold solder joint.
Reason I say,, *Batbob* mentioned it breaks through on big signal.
I've had that happen and it sends you round the twist. :grr
Phil.

batbob

Cool. I'll get on those further tests when I get home from work.
With regards to whether readings were taken in circuit and amps on/off
The amp on was t3 e-b .53
The other voltages in that post are t3's readings in circuit and amp off.
I'll also inspect the tracks from output backwards to see if I can spot a crack or dry joint
Thanks all for your help.
Whereabouts are you guys based?

phatt

Quote from: g1 on March 07, 2018, 01:33:24 PM
Quote from: batbob on March 07, 2018, 02:34:53 AM
No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Excellent, we can use those numbers later.

For now, most of those DC voltages look good except T3.  Can you measure from E to B (one probe on each leg).  We are looking for about 0.6V difference between the two.

I agree, adding the amp would have to be ON otherwise readings won't be valid.
Only saying that because *batbob* mentions power off readings as well as on readings which makes no sense to me. Or have I missed something? xP
Maybe probes are not fully contacting giving false readings?

@ *batbob* if that test fails them T3 has an issue.
If T3 checks out ok then Amp might be working but you likely have an intermittent connection.
One common issue is the speaker braids, the 2 flexible wires that go from speaker terminals across to the speaker cone. They often fail by fraying right at the solder point.
It is often fixable.
Phil.


batbob

Thanks Phil I'll take a look later.
In terms of output the sound is the same through the headphones or the speaker cab which is a seperate 4x12.
I'll take a look when I finish work and get home. Thanks for the help.

phatt

Well if the Headphones cut the speakers off then that Hphone socket is a prime suspect for intermittent operation. :trouble
If it's one of those plastic encapsulated types that are pcb mounted then look for hairline solder cracks around the pins.
Phil.

g1

Batbob, I'm in Canada. (rural southern Manitoba)
Sorry if we missed trying the headphone output, good catch by Phil.

Phil:  Agree that it is probably some bad connection bursting through when sufficient signal is applied.  But, first I wanted to be very sure it was not that T3 is turned off and is only turning on when big signal applied to it.  Seeing 44V listed for both base and emitter, I'd like to know the exact voltages there.  The .53V 'across' was listed for both power on and off conditions, so I was confused about which it was and would still like to see the exact numbers with power on.

batbob

Cool stuff, Canada eh? Thanks for taking the time you guys to help me out.
I've rechecked voltages on T3 with amp on and no input, double and triple checked pinouts.
T3 Emitter > Base was 511mv (.511v)
E>C was 41.3v
C>B was 40.9v
This sounds like the T3 was behaving normally.

T2 Base = 0.006v, Collector 44v, Emitter -600mv

With the headphones in it breaks through at higher signals (higher gain & volume)
With the 4x12 Cabinet plugged in it also does the same thing.

With the talk of dry joints and other broken joints I'm just going to heat up every single connection on the board to ensure there are none left.

The pinouts I've been working from have been scavenged from the web and are as follows:


Thanks again.