Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Leevi on February 18, 2021, 12:16:59 am
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I didn't find schematic for this Twin which is a reissue model. Any idea what it could be?
No tremolo. Just two pretty much identical channels and reverb.
Also half power switch that drops B+ from 450V to 220V and changes bias.
There is a huge 100Hz ground hum which I should start to troubleshoot.
/Leevi
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Hi Leevi
Could be a version of "The Twin" (1989?)
Try C110 in PS, Node B, 22uF 500V Surge. I had the same symptoms some years ago, if they are the same PCBs.
Also Check R129, 30k 10W, they go open.
Kind regards
Mirek
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Looks like the '94 Twin.
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Looks like the '94 Twin.
Yes that looks similar
https://lh3.googleusercontent.com/proxy/GrS7zMfU6eZ8TRMHAKsKYPeTUSSP6NHIODLfLV4Y4Qa_0DnA77bqMUctPOwTFtu-gPseCbrVLyROfKmiR1hdC98n9R5krwctaNF_vmc1Rk8avVr7RQCpHLKXxe5Mw1__not4h6ahAQO-rA
Try C110 in PS, Node B, 22uF 500V Surge.
I already changed the cap, no affect
/Leevi
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Schematic:
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-94-Twin-Schematic.pdf
/Leevi
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Ok Leevi
Please disregard my earlier suggestions as they were for the earlier model (The Twin)
The only problems on the 94 Twin that I have seen were Fractures/Dry joints on rear PCB Jack Sockets and faulty relay K3.
Also distorted output if R140 goes open/high resistance.
Found these amps to be fairly quiet and very punchy if you go directly into "Power Amp In" Socket.
Kind regards
Mirek
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Thanks Mirek,
Plugging into the power amp in did not change the situation.
The problem is somewhere in the grounding/filtering/bias ripple/heaters, contacts but probably not in the signal path?
After pulling out PI the amp is dead quiet.
I also noticed that the schematic I attached does not fully match with the circuit.
Among others the filter cap C52 is missing and the minus pole of the C51 is connected to the plus pole of the C39.
This is factory setting I believe.
Working with the PCB is a nightmare :laugh:
/Leevi
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Working with the PCB is a nightmare
Totally agree. I suppose gutting it and putting in a turret board circuit is not an option? I did that for a friend's DR reissue after he experienced several cracked traces on the main board. Of course if I had charged him more than just the materials it would have been an expensive proposition.
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Wow Leevi!! :w2:
I have just looked at your pdf of the schematics and PCB layouts and understand your comments on C52 and C51.
Do you know if you have rev A or rev B,C,D? (and rev E?)
I would be confident the problem is after Power Amp In and from what you have indicated possibly in PS HT and/or Bias.
I would try to restore PS to the original and check bias DC and Filtering (add temp 100u100V across main bias C58 for proof)
Please look at rev E Layout Notes: Note 4) Instances not used C52, R102, R110, R111
"Mongrel of a job with PCBs" but check all the large Electrolytics solder joints and large resistors on the power supply PCBs.
I admit not long after Fender put "The Twin" and "94 Twin" onto the market, I started to dislike FMIC and where they took Leos efforts!
I have also found sloppy trimming of wires on tube sockets touching chassis and sometimes no solder at all, in this era.
Check the soldering on the multi-pin connectors on PCBs (Power Supply PCBs) and check that every wire of Ribbon cable, is correctly fitted to the IDC blades on top of connectors (especially connectors under cover)
Good luck :dontknow:
Kind regards
Mirek
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C42, C43 & R104 - are either or both of those 22uF leaking?
--pete
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C42, C43 & R104 - are either or both of those 22uF leaking?
YES! It was C42!
Thanks DummyLoad and thank you Mirek for your contribution
/Leevi
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C42, C43 & R104 - are either or both of those 22uF leaking?
YES! It was C42!
Thanks DummyLoad and thank you Mirek for your contribution
/Leevi
hopefully you replaced them both. those 22uF 500V IC are prone to fail.
--pete
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OK, Thanks
/Leevi
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Congrat !
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Thanks for the update DummyLoad and Leevi.
...and well done Leevi.
Gives one a little more confidence in dealing with the latter Twins :laugh:
Kind regards
Mirek
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One thing I was wondering in the circuit are the Bias checking contacts that I have never seen before.
There are contacts with resistors where parallel diodes (CR3 and CR2) are used across the resistors.
It is probably meant for a smart way to set bias i.e. the forward voltage (Vf) of the diode is higher than the cathode voltage at idle?
When playing hard the diodes start to conduct and no current will go through the resistors.
/Leevi
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The most likely reason for the diodes is that in the case of a power valve shorting, they protect the 1ohm cathode resistors. Thereby avoiding a warranty repair.
I don’t envisage that the diodes forward voltage won’t be reached under normal loading conditions.