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Author Topic: Fender Frontman 65 distortion  (Read 6884 times)

Roly

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Fender Frontman 65 distortion
« on: December 07, 2013, 04:45:05 PM »
Okay, now I need some help.

I have someone writing to me who has been struggling for weeks with an odd fault on a Fender Frontman 65.  After very extensive checking of all the usual suspects it comes down to this; the amp randomly starts distorting, and when a meter probe is lightly touched on to TP20 (join of D23 and D24) it suddenly comes good again.

I now suspect high frequency instability of some sort, possibly due to an aging component such as a bypass cap, but I'm wondering if any of the brains trust has encountered this before and has any clues.

cct attached
If you say theory and practice don't agree you haven't applied enough theory.

Roly

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Re: Fender Frontman 65 distortion
« Reply #1 on: December 09, 2013, 11:28:23 AM »
Bump - nobody?   :(
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender Frontman 65 distortion
« Reply #2 on: December 09, 2013, 06:02:39 PM »
Hey, I don;t work weekends, relax.

If you think instability, scope the output.  If RF is at TP20, it would be on the output too.


Does touching it anywhere else trigger its return?


usually when I touch a meter to something and it wakes up, I think the node is somehow unterminated and the probe allows it to discharge back to a normal state, at least for the moment.

What does the distortion on the output look like?  Clipping?  one side or symmetrical?  If so clipping at what level?

How about can you leave a meter attached to that point and will it eventually go back to the symptom?   And then with that meter still there, touch ANOTHER meter to the node - does that still wake it up?  If so, does the original meter show anything unusual?

ANything is possible, but I tend to doubt the aging cap idea, the amp is only 10 years old or less, and the modern caps and low voltages of this circuit don;t usually stress caps much.

Roly

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Re: Fender Frontman 65 distortion
« Reply #3 on: December 10, 2013, 04:29:11 AM »
I'm relaxed, but he's up to 70-something e-mails.  No CRO, but I've got him building an RF probe.

I'm baffled and grasping at straws; it's a dogsbreath circuit, looks like it was designed by the cleaner while the engineer was out to lunch.  I've seen a few posts complaining about a strange distortion with this model, which is where he started, but it's proving very elusive.  Was hoping one of you guys was going to say "Oh yeah, THAT..." (and I still live in hope).   ;)
If you say theory and practice don't agree you haven't applied enough theory.

Roly

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Re: Fender Frontman 65 distortion
« Reply #4 on: December 14, 2013, 10:53:27 AM »
He has finally got a CRO in it during the fault condition.

Bottom half clipped off, signs of a parasitic on the top half, and it apparently doesn't look too good even when the fault is not apparent.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender Frontman 65 distortion
« Reply #5 on: December 14, 2013, 11:11:36 AM »
Some thoughts, in no particular order:

Does it matter if a load is present, in other words can it make a good output waveform unloaded, but it collapses under load?

When I see half a waveform, my first thought is almost always the output stage, like broken trace/solder to the V- side output transistor or emitter ballast.

The symptom is like gold, when it stays.  A lot of guys get the symptom (when it is not always there) and cannot resist the urge to do whatever thing stops the symptom.  Like probing some point.  We want to avoid doing that thing and explore as far as possible with the symptom present.   We have that ugly form, at the output?  So is it at the base of BOTH side outputs?  At the driver bases?  Farther back?

Roly

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Re: Fender Frontman 65 distortion
« Reply #6 on: December 14, 2013, 11:21:20 AM »
I think this is loaded.

I've asked him to check back along the signal path.

I didn't expect the clipping, and I expected the parasitic to be much higher in frequency, so I'm still at a bit of a loss, and welcome your fresh input.


{Harry! - git in here}
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender Frontman 65 distortion
« Reply #7 on: December 14, 2013, 12:49:55 PM »
I am on the other side of the world, but my guts tell me that the parasitic stuff will probably go away when the gross clipping symptom is cured.   The feedback loop nature of the solid state amp means it is constantly trying to correct itself, while it clearly lacks the ability to put a correct waveform on the output.

Roly

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Re: Fender Frontman 65 distortion
« Reply #8 on: December 15, 2013, 02:22:17 AM »
Yep, I'm inclined to agree.  Pondering on the circuit my feeling now is some sort of intermitant in one of the two protection circuits, and my guess is it will be the driven protection which I assume is some form of SOAR protection.

Hopefully the chap who is being driven mad by this will be coming on board here shortly.
If you say theory and practice don't agree you haven't applied enough theory.

Roly

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Re: Fender Frontman 65 distortion
« Reply #9 on: January 13, 2014, 10:24:26 AM »
{Closing the circle}

And sure enough...  Q11 which is part of the signal drive to the SOAR protection has been found to be intermittent E-B.

It appears that this was causing the protection on one half cycle to kick in, but not all the time.

Q11 replaced with a BD140 NPN BD139 it now seems to be working perfectly.  (actually both Q11 and Q10 (PNP BD140) were replaced for safety)
{edit to correct blooper}

Hope this helps anybody with a similar strange problem.

« Last Edit: November 01, 2014, 10:18:09 AM by Roly »
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender Frontman 65 distortion
« Reply #10 on: January 13, 2014, 11:48:17 AM »
Cool.

joecool85

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Re: Fender Frontman 65 distortion
« Reply #11 on: January 13, 2014, 04:31:33 PM »
Glad you got it fixed.  I need to get myself a scope...
Life is what you make it.
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Harry

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Re: Fender Frontman 65 distortion
« Reply #12 on: January 15, 2014, 07:18:41 PM »
Hi Everyone !
                    Yep finally got here . First of all i need to thank the one and only Roly for his help and guidance over the last 12 months or so not only with this  crazy intermittent problem but also with the two valve amps i've built . . And let me say i think its great people like him  and Enzo and others help those with problems.
This is roughly the chain of events with this amp..
The Fender Frontman 65 r was  just out of warranty when i first   saw it . This amp was bought new in 2011 . It seems these amps are old now in USA . The printed circuit has 2005 stamped on it  . I offered to look at it simply to save the friend some money. I have a DSP 90 which has over the last 8 years stopped 6 times so figured it would be a  fairly simple problem. Boy did i get sucked in ! The amp was distorting badly  at  odd times . I replaced two  very high resistors and ran the amp over few days . All good. Then  it started to distort again. Further searching i found the main PS caps were bad ...... they are 2200uf caps..one was reading 3 uf ...yes THREE the other one 1900uf  and had one leg loose inside , good for a 13 months old amp !! . so figured great  found the problem. Replaced the caps , all went well for 6 weeks .Then it started distorting again.  At this time the owner wanted to take it to a Fender specialist which was fine by me ! After 4 weeks owner was told ' can't fix it '  .. So  then  went directly to Fender  .just out of warranty too bad cant  do anything  and if tech mr XX cant fix it neither can we  !! So the amp landed back in my lap ! I was in contact with another Tech that knows these amps very well , but he slowly vanished ... I think he gave up . A few other techs (friends ) i spoke to didn't help either , just would say "oh thats a bad problem' ..You don't say !! After lots of e mails between Roly and myself, and lots of testing and taking out parts to check them etc etc  . Finally it's now  fixed . This problem was extremely hard to  fix mainly because of the very intermittent nature . I could turn the amp on in a distorted state pick up  the meter leads and by then it would be fine again. Or some days turn it on and be good all day. I hope i never come across another one like it ! As for Fender ....good way to get a bad reputation me thinks . And .. a set of new  Fender PU's for my guitar ...two failed inside 6 weeks !  Thank you Roly  and Enzo .
                                                                                                                 Harry

hefty

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Re: Fender Frontman 65 distortion
« Reply #13 on: December 23, 2016, 08:02:56 PM »
Sorry to revive and old thread but all the info in this tread is what my question is about. I picked up a Fender Frontman 65r for 40.00 dollars that has the exact problem that was fixed in this thread. I bought it knowing it had this problem but wanted to use the cab for a speaker enclosure for a spare  scumback speaker I had. I could save the  time of building my own and I wanted to see if I liked the sound of my freash built 18 watt tube amp with the speaker.  the Fender cab was like brand new also  so I figured I would maybe build a small tube Fender type amp after I was done with auditioning the speaker in the cab. Well long story but before pulling the speaker I tried the amp and I had a lot of fun with it. The amp was a big surprise for me a hard core tube lovin amp person. Well until finally one time turned it on and low and behold this exact problem started. I finally knew what the person who sold it to me was talking about. I found this thread in my search for answers. I could of written word for word the symptoms and touching the same nodes in the circuit and have the amp reset and work again until the issue started again. Now for my question Why did Roly  use the BD139/BD140 transistors instead of original 2SA1013/2SC2383 transistors that where used in the circuit. You can still get these two NPN/PNP transistors. I was going to ask Roly but found out about his passing. I found his web page and read a lot of his info. This was a sad day to the electronics world for he seamed like a very helpful and was a great resource to us younger dabblers. so that is what I was wondering. I have a few thoughts but still would like any one else's thoughts. Thanks for any help and again thanks to Roly and people like him.

J M Fahey

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Re: Fender Frontman 65 distortion
« Reply #14 on: December 24, 2016, 12:13:05 AM »
Sorry to revive and old thread but all the info in this tread is what my question is about. I picked up a Fender Frontman 65r for 40.00 dollars that has the exact problem that was fixed in this thread.
Sorry but that´s very unlikely.
You can never know by *symptoms* alone but only after some testing.

Same as going to a Doctor and asking him "give me those orange pills you gave my cousing Syd because I also wake up with a terrible headache and pain in the gut"
About a Million different illnesses cause head and stomach pain yet they are all different and require different treatment.  :tu: