Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mrr3000gt on June 11, 2012, 12:36:17 pm
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Guys, long story short: like a lot of you, I do repairs for local shops. And that means its nice to nail the root cause within 1-3 hours as nobody should be charged 10 hrs of time to fix a $300 amp.
I try to steer clear of solid state anything unless its my stuff or I have no choice...So I have no choice on this VS65R.
So I have been poking around with this thing for longer than I would like to admit, and have been walking thru the circuit with my o'scope as well.
http://www.drtube.com/schematics/marshall/vs65-60-02-2.gif (http://www.drtube.com/schematics/marshall/vs65-60-02-2.gif)
The symptom is that when switched to channel 1 (clean), the amp performs normal and sounds great. At channel 2, there is bearly audible sound, and the volume control appears to do nothing (gain will increase the gain of the nearly subliminal sound).
Note that this amp appears to have a non-conventional channel switching method - but appears to be somewhat common in other Marshalls.
* All testing was with reverb tank & combo speaker.
* I eliminated the 'common' stuff: intermittant connections, bad solder, tube faulire, jacks, FX loop jumper test,..
* I firewalled problem to preamp: line out into a power amp showed that preamp out suffered from issue.
* I checked the circuit with a gtr signal with the scope: full wave response in channel 1. Channel 2 the signal appears to drop considerably in IC2 & IC5s output when switched to channel 2 (the gain/vol of ch2 just cant boost the signal, but it appears to make it to the outputs of both ICs)
* I took the time - it was quick - to check the ICs that are involved with channel switching (3 of them). I even installed 8-pin sockets to swap them - no change in amp.
* I checked the voltages of the IC3,IC4,IC5,IC6 (and the IC1 & 2). Only the exceptions to what I should have seen:
- IC4: pin 8 had no voltage
- IC2: pin 8 had no voltage
- IC5: pin 8 had no voltage
- IC6: pin 7 not +15
- IC3: pin 8 not +15
+/- 15 v deliver appears to be fine despite the ICs not detecting +15...Swapping the ICs was a quick atttempt to rule out a bad switch IC.
*D9 & D10 of preamp board appeared to fail diode test
...stumped (and also the fact I am even looking at a solid state amp...)
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I would start with cleaning all the pots , often with solid state and tube amps this is the problem. I can not open the link you provided so I have no schematic to look at.
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I would start with cleaning all the pots , often with solid state and tube amps this is the problem. I can not open the link you provided so I have no schematic to look at.
I wish it was as simple as dirty pots :icon_biggrin:
Channel 1 works just fine: its channel 2 thats bearly audible. I am trying to figure why the ICs +15 voltage is missing on the channel 2...
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> IC6: pin 7 not +15
> IC3: pin 8 not +15
What IS it? And where does it come from? Why isn't it gettiing to those pins?
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> IC6: pin 7 not +15
> IC3: pin 8 not +15
What IS it? And where does it come from? Why isn't it gettiing to those pins?
Don know yet: I have been tied up with work/tasks that I have not messed with it yesterday or today. I plan to yank the board out again and trace the +15 routes on the board to the source. From the Power board, the +15 v creation appears to be fine. Only the drive channel ICs are affected, and channel 1 works just fine. I am not convinced its the switch, but I will (again) rule it out when I trace from where the +15 is created to where its consumed.
I was hoping someone who had to fix one of these with a similar problem would chime in (wishfull thinking). I got atleast 2-3 hours I am sure the shop will charge on this thus far, and I have froze the charges (and fixing this $150 amp has become a quest).