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Solid State Amplifiers => Amplifier Discussion => Topic started by: Roly on December 07, 2013, 04:45:05 PM

Title: Fender Frontman 65 distortion
Post by: Roly 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
Title: Re: Fender Frontman 65 distortion
Post by: Roly on December 09, 2013, 11:28:23 AM
Bump - nobody?   :(
Title: Re: Fender Frontman 65 distortion
Post by: Enzo 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.
Title: Re: Fender Frontman 65 distortion
Post by: Roly 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).   ;)
Title: Re: Fender Frontman 65 distortion
Post by: Roly 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.
Title: Re: Fender Frontman 65 distortion
Post by: Enzo 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?
Title: Re: Fender Frontman 65 distortion
Post by: Roly 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}
Title: Re: Fender Frontman 65 distortion
Post by: Enzo 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.
Title: Re: Fender Frontman 65 distortion
Post by: Roly 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.
Title: Re: Fender Frontman 65 distortion
Post by: Roly 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.

Title: Re: Fender Frontman 65 distortion
Post by: Enzo on January 13, 2014, 11:48:17 AM
Cool.
Title: Re: Fender Frontman 65 distortion
Post by: joecool85 on January 13, 2014, 04:31:33 PM
Glad you got it fixed.  I need to get myself a scope...
Title: Re: Fender Frontman 65 distortion
Post by: Harry 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
Title: Re: Fender Frontman 65 distortion
Post by: hefty 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.
Title: Re: Fender Frontman 65 distortion
Post by: J M Fahey 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:
Title: Re: Fender Frontman 65 distortion
Post by: hefty on December 24, 2016, 01:54:57 PM
Ok that is a great point but don't have access to a scope, when I check all all the voltages they all look good as per the schematic. I chopped stick, pushed and pulled the board, looked for bad traces/connections. The only thing that would stop it was touching the same joint with the meter probe. Without a scope I thought maybe try a change out of those two transistors. I also wondered way he chose the transistors he did instead of what was used in the circuit.This was just as much for the logic behind his suggestion to use the transistor he did instead of the ones called out in the schematic I was just trying to learn something besides the issues with my own amp.
Title: Re: Fender Frontman 65 distortion
Post by: galaxiex on December 24, 2016, 02:25:32 PM
Roly was a wizard when it comes to electronics in general, and guitar amps in particular.  :)

If he used those transistors, then a few possibilities come to mind...

1. He realized that "transistors are transistors" and as long as the new ones meet or exceed the specs for the circuit, then it will likely work ok.
It's just a guitar amp...
Granted.... *some* circuits and or specialized pieces of equipment "need" a specific device and will not work correctly with any other.
That is not likely the case here.

2. He may have had those particular transistors on hand, and popped them in, knowing they meet or exceed the specs.

3. Comedy >>> He had some esoteric reason for choosing those transistors.
They are full of secret MOJO having been pissed on by Unicorns or maybe made from Unicorn tears.  ;)  :lmao:

Seriously, I wouldn't worry about it.
Replace them with the same as Roly did, or find some original part# devices,
or read some data sheets on the originals and find some suitable others to pop in there.

It's all good.
Cheers!
Title: Re: Fender Frontman 65 distortion
Post by: hefty on December 24, 2016, 04:28:11 PM
Thank you. This is along the lines I was thinking, having looked up the specs on both pairs. I think your #1 and #2 answers are right on the mark.  :dbtu: I appreciate your reply.
Title: Re: Fender Frontman 65 distortion
Post by: Harry on February 06, 2020, 04:23:59 AM
Hi ..I havent been here for few years. Just stumbled back looking for something else and saw my old post Re the Fender frontman 65 distortion problem. If Hefty or anyone else is reading this  i'd just like to make couple quick comments. Roly and i spen tweeks on the problem. It wasn't till i by chance got a  scope photo of the horrible waveform that progress was made. Roly suggested replacing the bad transistors with BD139/140 as he felt they were much better ,  better specs and he knew they were reliable. I bought the BD139/140 from Jaycar, nice and handy as well . Took bit of fiddling  and care to replace both but ......let me tell you to this day the amp  is still going strong, not a problem at all.
Roly was a great friend - mentor and i'm sure lot of people not just me , miss him. A great man and great talent.
I now take a 50 watt valve amp to gigs  as the Fender DSP 90 i have has also sh...t itself 8 times or so in 10 years . Do i like Fender amps ???? silly question !
Title: Re: Fender Frontman 65 distortion
Post by: stedutts on June 05, 2020, 12:06:52 PM
Sorry to also revive such an old thread, I also have a Fender FM65r which suddenly developed a really unbearable crackling & distortion intermittently every couple of minutes, if I messed around with on off button it stopped but then started again after a few mins and drove me up the wall!!!! I changed on/off switch but no resolve, viewed internally on pcb and found capacitor 50 had leaked, replaced with a new one but amp still did the same, after lots of investigation and searching internet after nearly giving up I stumbled upon this thread, amazingly I found a few others had had same issue and I’d after reading Roly & Enzo smart advice I decided to change transistors Q10 & Q11 to BD139 & BD140 transistors, also cleaned all pots and everything else internally and stunned the amp runs like a dream crystal clear like new and no dirty crackling pots as well. So I needed to say a huge thank you to Roly who unfortunately I hear has passed, a traffic loss to guys like us who totally appreciate his knowledge and his sharing this to help others, big thanks to Enzo and any others who participated in this thread over the years. These threads are a massive help to people like us who are stuck and don’t know what to do next. I was going to give up on the repair till I fell upon this, so a massive thanks again guys you saved me at least £100 trying to get repaired which I doubt anywhere would with it being a non repairable amp by Fender, and such little repair information available anywhere to aid us in repairing, in total the parts cost me about £3 to fix amp and that’s expensive as I only bought couple of each parts needed.
I’ll definitely try here again when I have more issues with any gear. Many thanks all. Steve Dutton, vocals, guitars & production in Manchester band The Dead Xtra’s
Title: Re: Fender Frontman 65 distortion
Post by: joecool85 on June 06, 2020, 08:23:16 PM
Sorry to also revive such an old thread, I also have a Fender FM65r which suddenly developed a really unbearable crackling & distortion intermittently every couple of minutes, if I messed around with on off button it stopped but then started again after a few mins and drove me up the wall!!!! I changed on/off switch but no resolve, viewed internally on pcb and found capacitor 50 had leaked, replaced with a new one but amp still did the same, after lots of investigation and searching internet after nearly giving up I stumbled upon this thread, amazingly I found a few others had had same issue and I’d after reading Roly & Enzo smart advice I decided to change transistors Q10 & Q11 to BD139 & BD140 transistors, also cleaned all pots and everything else internally and stunned the amp runs like a dream crystal clear like new and no dirty crackling pots as well. So I needed to say a huge thank you to Roly who unfortunately I hear has passed, a traffic loss to guys like us who totally appreciate his knowledge and his sharing this to help others, big thanks to Enzo and any others who participated in this thread over the years. These threads are a massive help to people like us who are stuck and don’t know what to do next. I was going to give up on the repair till I fell upon this, so a massive thanks again guys you saved me at least £100 trying to get repaired which I doubt anywhere would with it being a non repairable amp by Fender, and such little repair information available anywhere to aid us in repairing, in total the parts cost me about £3 to fix amp and that’s expensive as I only bought couple of each parts needed.
I’ll definitely try here again when I have more issues with any gear. Many thanks all. Steve Dutton, vocals, guitars & production in Manchester band The Dead Xtra’s

Welcome aboard, Steve!  Glad to hear the forum helped you out!  These old posts are gold sometimes, that's for sure!