Did you get a schematic taht you can post from Peavey.
If the amp is anything like the 5150, the 1 amp fuses are for the + - 25 volts.
Hello to all,
Recently I had picked up a Peavey classic vtx series combo out of a storage unit, it was sitting in the corner rotting away with the ages and had caught my eyes as most musically driven items do. What also helped was the re-cover job-(burlap and staples) - with grim results, that led me to want to save this amp. So, I dug her out- FS and all and took her on home.
Upon a thorough external exam, everything seemed to be intact- Power tubes, Reverb pan, the FS was in fair to poor condition, had all its knobs and the pots where a little stiff. External fuse was good and intact. Power cord was missing its ground prong and slightly frayed. Speaker jack was in good shape and the speakers seemed to look ok, except some age. We all know about looks- there deceiving. So, I proceed w/ plugging it in and powering it up accordingly- everything is in check- status led’s are working , no cracks or pops, funny smells, tubes are operating good from visual inspection- glowing evenly and not operating overly hot. So, I plug in the guitar and turn the standby switch. Wow, what a bummer- very weak low volume w/ a hissing/buzz. Everything is still in working properly just there is no great sound coming from this thing as I have read these things are pretty perky when working correctly. Frustration gets the better of me for a bit, and it is retired to the GARAGE!!!!
Few days later I decide to pull it out of the COLD garage, let it warm up for a few hours, throw some newer 6L6’s in her I had lying around, power up accordingly, and “damN” She was off to the races, nice and loud at reasonably low settings, full of tone variations, the FS was even working considering the shape it was in. Everything seemed fine- Then in the middle of our new found love affair, she turns and walks away, back to the comfy confines of her previous state- low volume w/ a hissing buzz!!! Thinking of course, swapping the tubes with some old ones I had laying around was the fix last time, I ran out to buy some new tubes to see if this would be the fix. Nope, same story w/ the new tubes. So, I proceed to plug directly into the back of the chassis to the “power amp in” jack. The power amp works remarkably well, although there is no volume control (normal, there is no preamp control- it’s bypassed). It sounds ok, and I spent a minute playing through it this way. “Power amp is fine”!!! Next, what I do know is that the preamp out and the power amp in jack are wired together in a relay of sorts, so to see if the connection was bad between the jacks internally, I connected them externally w/ a ¼” 90deg. Patch cord. Still the same result- Low volume w/ a hissing/buzz! That leaves me with no other option but to break her down and get it up the bench. I start this as I do any other rebuild or repair- An extensive research campaign, finding any and all info. I can about this particular amp. – Schematics, Calls to Peavey, Forums, Internet, Books, Astrological Signs and Moon Beam Deflection- Kidding!! Upon some learning a little more of this amp., I begin an internal examination of the chassis, pcb’s, wires ,internal fuses, solder contacts- everything seems to be fairly clean and in good shape no funny smells, bulging caps, fried wires . Clean it all up, flushed pots, cleaned pcb w/ board cleaner, cleaned some contacts, and replaced the a/c line cord with an exact gauge match that had a ground prong. Reassembled, same as before powering up and still the same results. Low volume w/ a hissing/ buzz.
So, I proceed to take some of a fellow owner of the same amps advice with the same symptoms and some suggestions from peavey to try and correct these problems.
First, I replaced the audio driver transistors located on top of the heat sinks on the pre-amp board.Here are some pics of the new transistors in place.
Second, I replaced the driver transistors, Here are some pics of the new transistors in place.
Fairly easy replacements, I have a bit of experience w/ PCB’s and the soldering was perfect on theserepairs,hard to locate they blend in w/ the factory contacts. Gave it a good examination to make sure everything was correct and reassembled, and began the precursory fire up and “BOOM” I’m blowing internal fuses now!
Here are some pics of the power board and the fuses that are blowing.
The amp will remain on and the power led will start from nothing and increasingly get brighter to a degree, the standby led doesn’t work at all, all other fuses are ok, and the tubes are seemingly ok-not overly bright, overly hot, no strange arcs or pops other than the two internal fuses.
I have a few ideas as to what it might be but was interested in some feedback and possible help narrowing it down. Or, quite possibly I could have overlooked something or done something wrong. I am fairly new to the Amp. Tech world itself, but have some experience and am aware of the dangers and how to avoid them. Feel free to school me on the knowledge and the no-no’s, after all any help would be appreciated. Thanks and happy holidays!!!
I also have a bunch of other pics of this amp!!!!!
Did you get a schematic taht you can post from Peavey.
If the amp is anything like the 5150, the 1 amp fuses are for the + - 25 volts.
F2,F3 are the fuses for the +/-15v supplies.
If they are blowing, look first at the four rectifiers, CR7-10, then check the filter caps C6,C7. Looking for shorted parts.
The 15v zeners could be shot, but that wouldn;t usually blow fuses.
But there is a little four pin cable over to the preamp from the power supply board. In your photos it looks like 6 pins, but I think only the center four conductors are in use. Make sure that connector is not off one pin either direction. But you could also just unplug it to see if that affects the fuse blowing.
Education is what you're left with after you have forgotten what you have learned.
Well, sorry for the slow response- holidays and all!! Anyway, funny story!! The day after my initial post, I sit down once again to get to the root of the problem on this thing- and "POOF" right in front of me is the ribbon connecting the power supply board to the pre-amp board is a little askew, It was off by 1 pin- In an elated burst of excitment, I run to the computer to report my findings and see that I had a reply on the forum- It was the answer to my problem, right on the money, althought I had already caught my "stupid mistake", I was impressed by the diagnosis and it taught me a good lesson- "pay attention"!!!! Thanks guys for your replys and all your help!!
The amp by the way sings to no end!! The variations in the tone are fun to play w/!!! Its as quiet as can be too, sitting idle you cant even tell its on.
Looks alright- but could use a new covering!! all-in-all not to bad for a storage unit find- cant beat free!! Thanks again!!
i have the same problem with my classic and ordered the drivers, the 2n3904 are still used but the mje15030 are now mje15032, they sre rated for 250vdc instead of 150vdc.
Hello all. This is my first post here. I am trying to fix this amp for my brother-in-law and I am in need of help. I have the same symptoms listed in this thread,
Sound- low and distorted
Power led comes on but not Status (I had the status light come on once, when it did, the sound was full and powerful but faded away after about 30 seconds.)
Tubes, seem to power up fine ( I don not have a tube tester)
I tried plugging the guitar straight to amp in but still had the same results, low distorted sound.
It sounds like the drivers are what needs replaced. Any advice on where to get these parts?
Smolty: It looks like you had ordered the drivers for your amp. Did you get them replaced? If so, how was it, and are you having any problems? Any help would be greatly appreciated, Brian.
Welcome to the place. This thread is so old, I doubt that the OP is still hanging around here. Why not start a new thread for your amp?
From your description, It sounds more like a mechanical problem like a broken connection, than a driver transistor. If you give the amp a good Enzo whack with your fist, can you get any change in reaction from the amp?
Start by contacting Peavey and ask for a copy of the schematic for your amp, it will help in relaying information back and forth here on the board.
Hello and thanks for the welcome. I figured I was taking a chance posting in such an old thread but it really did seem to fit my problem. I have to admit, I dont play (yet) but I am a huge audio fan (not audiophile) and I like to work with the vintage equipment. I thought as nicely laid out as this preamp is, it would be doable, otherwise this poor amp would just be sitting in my brother-in-laws storage unit.
I did try light taps with the "universal alignment tool" but no luck.
I did take it apart and do a search for loose or broken connections and once again, tried very light taps (with the handle of a screwdriver) here and there to try and reveal a loose connection but no go.
I have powered it up a couple of times now and when I flip the stand by switch, the status light turns on and I have about 30 seconds of great sound and then back to low distorted sound.
I do have a schematic for the amp now, but I am still trying to get registered with the peavey forum. Thanks for the help! I am still leaning towards the transistors as the problem, but will continue to do more search. Brian
That is different than your original post, I though that you said it only powered up the status light once.
I don't have access to the schematic right now, but a good place to start is by checking the voltages in the power stage. Look for the voltage that initially starts out fine, but slowly changes as the sound fades out. Then try and figure out what is causing the voltage to change.
You are correct sir. It has done this ( status light on) 3 times now with the same result each time. I am no expert so bear with me.
I completely agree with the voltage issue.
This is something I did not mention before, but when switching to low power on stand by (after power up) once I pluck a couple of strings the sound quickly goes from low and distorted to nada, zip, zilch. Just like once voltage is applied, something gives in, or is not operating up to spec.
52 Bill, thank you so much for the help! I do have a m.m. and understand enough to know when to say when, so I look forward to any more info/help. Thanks again, Brian
Hi Marley I'll add my tuppence worth here and add the schematic for (hopefully) others to follow.
Firstly It would be good to narrow down the area the fault is in.
Although initially it looks like power supply-output stage fault (diminishing HT) maybe check a few things to make sure you're on the right path.
Firstly to make sure its either in the pre or power amp stages you can make use of the effects loop the preamp out/poweramp in jacks.
Connect a seperate amp input to the preamp out jack of the Classic ..guitar in to front panel jack on classic ...classic preamp out to external amplifier.
According to the schematic this should work without the standby switched to operate
low or high.
Bearing in mind the signal to the second amp will be greatly amplified so watch levels.
If thats all good it points to the power amp if not check the 15 volt supply see attached jpg.
The zener circuits do dissipate a lot of heat so it may pay to look at the solder joins
on the reverse of the power board particularly around the zeners and associated resistor.
+ 15volts is also fed to the grids of the 6L6's via R112 (100 ohms) so maybe check that
as well. i.e. measure between junction of R112,113 & 114 and ground.
If not C 50 (22uF 25v) could be faulty.
Feeding a signal (ipod/cdplayer with volume control) into the power amp in will check if the power amp is ok in isolation bearing in mind the level from the player should be low as the Classic has no volume control after the power amp in jack.
Setting your MM to the high DC volts setting as Bill52 suggested monitor the voltage at
the power board when the s/b switch is set to low and high and observe if it fades away.
Likewise with the above +15v after R112.
Thanks for all the advice. Although I cant stop thinking about this amp, it is mothers day weekend and I need to spread the love. Once I can try a few of the suggestions, I will report back. Thanks again, Brian
Just thought I would touch bases with yous guys that are helping me out with this. I really feel confident that I can fix this.
I have talked to my brother in-law to let him know where we stand, and what I need from him to help me out. You know, I love my brother in-laws' laid back attitude but sometimes its a real pain in the butt!
By that, I mean I have not been able to completely rule out the power amp. Not to worry, I know a few other musicians that might be able to help me out.
I have to say that from what little I have heard from this amp, I want it. I found out that this amp has been sitting inop. for at least the last 10 yrs! My brother in law couldnt even tell me the procedure he used to power the amp up. (power on, while stand by is in the neutral position, and then flip stand by to high or low?) Again, I am just learning to play, but I could have alot of fun with this amp! Brian
I was able to get my neighbor to help out and we discovered that the problem is in the power amp. Using my pre amp into his amp, sounded great!
Now I guess the first thing to do is have the tubes tested. Could it be that simple? I doubt it. Brian
It just ocurred to me before you get too technical..after reading your post
that if the standby switch was faulty (eg had been arcing) that this could cause the fade...
Sometimes the obvious escapes... anyway just a small point..
Also occasionaly the crimped internal wires with spade connectors can
be faulty so examine them. Sometimes I attach a meter with a continuity "beeper"
with strong alligator clips onto the spade connectors then wriggle and pull on the wires
and observe if the beeping stops.If the wire comes out can often clean the connector top and resolder covering it with a bit of heat shrink.
Have you got the user manual ?
I presume this one is the same as yours with the 2 inputs.
Peavey did make several classics I think the majority have made it to this forum over the years.
Although not your VTX here's another that was resurrected ! (4 inputs)
Need help identifying this amp
Is anyone intrested in helping me get an old peavey running?
regarding tube testing .. usually a new pair is tried or old ones put in something thats known to work. I would check the voltages first before plugging in a new set.
As the pre-amp works you should find the 15v (well a bit less) at pin 5 of the 6L6.
Another thought (geeze I've got to stop having these) in case its the last IC U8A
a 4558 causing trouble - borrow the one from the reverb (U6) and put it in U8's position. Be careful not to bend the pins and observe which way they go in.
Last edited by oc disorder; 05-11-2011 at 04:13 AM.
Good to know. I will get the gator clips/m.m and inspect the standby switch more closely.
Thank you for the manual! It is the same amp.
I have extra el34 tubes here, but from what I read, they are lower voltage tube. I am going to call a few places later today and see about tubes and testing.
I will check the voltage on leg 5. Thanks again for the help, Brian
I think the fault is more likely in the solid state componentry in the power amp.
I could be wrong....but usually tubes work or not - they don't usually fade away after 30 seconds.
Measuring the voltages will tell a lot.
If you notice in the small print on the right of he schematic
Vi (Idle) = 40mv max 20mv min ..10mv MAX difference between sides
This is measured across firstly R126 and then R 130 those 2 big wire wound resistors
on the left of the transistor heatsink. (often referred to as cement resistors).
Comparing one reading to the other there should not be a difference of more than 10 mv .
These are the emitter resistors and this gives us a clue as to how hard the transistors are working
ie how much current is flowing through them as by ohms law current flowing through the
resistor produces a voltage across it.
Current = voltage divided by resistance if you want to do the math's for fun !
Negative probe to the outside edge which is ground.
The other important small print voltages (Vk) are easily measured between ground and tube
pins 1 and 8 (the respective junctions of the collectors of Q6,7 and the cathodes of the 6L6's ).
These two separate readings should not have a total difference of more than 10 volts.
By the way this is where the signal enters into the cathode ...unusual design ..same as
Music Man amps.
Vk (idle) = 95v max 75v min 10v MAX difference between sides
Another measurement should be taken from the bases of Q6 and 7.
Facing the amp with the transistor tab away from you and the legs towards you
its the leg to the left.
You should get about half a volt there .560 volts (notice they left a decimal point off the diagram).
The other measurements they provide are 1.90v at junction of R119 and diode CR39
and 1.24v at junction of R121 and diode CR32 (which is also junction of R122 & R127).
1.10v should be found at junction of R116 and CR28 and also junction of R136 and CR37.
See attached diagram.
When you said "I did take it apart" I presume you removed all the knobs and pot nuts
and flipped the main board over and inspected all the solder joins ?
Again Q6 and Q7 and the two emitter resistors R126 and R127 are the main suspects as
they are prone to get hot.
Good luck on your misson !!!
Hello again. I have not had time to open up the preamp yet. In the mean time, I have left it plugged in and have periodically powered it on to see how often the status light would illumate when the standby switch was flipped, it was at least 30% of the time. I did this to see how long it would stay on if I did not strum the guitar. It still remains on for 15-20 seconds. Does this help to point out the standby switch as the problem?
oc disorder: I plan on following your advice and I will let you know how it turns out. Thanks again, Brian
I doubt the s/b switch is the problem..but need to eliminate these silly obvious things now rather than later.
Still need to do what Bill52 suggested in post 9.Measure the voltages particuarly the high voltage.
The status light is illuminated by Q1 after its base senses the HT via 2 x 220k 1 watt resistors.
Its possible that the components here could be faulty (i.e. faulty indication) which is why we really need to know
what is at the HT terminal.
What happens when you remove the tubes and switch it on. Does the status light stay on ?
It could be just something on the supply board. Faulty capacitors - diode breaking down under load..Ht fuse not making
proper connection loose connection.
Without any meaningful readings is impossible to figure out whats going on from where I'm looking.. can only make educated or otherwise... guesses. CR38 and CR 31 could also be suspects ..maybe breakdown under load but then the 500mA fuse should blow.
Ok, I have the preamp out and I am getting all my ducks in a row. I am printing out the instructions to read over tonight and then start measuring tomorrow. I am sure I will have some pretty basic questions about what to do so please bear with me. I really do appreciate the time and help I have received from members of this board. Oc disorder especially! Brian
I have just removed the tubes (preamp is out) and powered the amp on then switched the standby to high power. Status light is on and is staying on. When I flip standby to neutral, light fades out. When switched to low power, the light will not come on.
Where should I begin to measure? The tube sockets? Brian
I was hopeing someone else would chime in with some objective comments but they seem to be letting us go...??
It seems that there is definitely a problem with the HT supply.
I hope there is not a problem with the output transformer .. as it is the initial recipient
of the HT before it (the HT) goes to the anode of the 6L6's.
Its possible we wont get meaningful readings until we have a known 500 or so volts on the tubes.
Have a close look at the ends of C3 and C4 the two main capacitors on the power board. They usually have a point at one end that bubbles/bulges when the cap is under stress sometimes leaking electrolyte through that hole.
If that is the case I would order 2 either sprague or F&T 100 microfarad 350 volt axial
(leads out either end) electrolytic capacitors and fit them immediately.
However refit the tubes and measure at the tube socket.
You should get 525v at pin 3 ..if you are at all apprehensive about touching probes onto
such lethal voltages connect one end to a clip you can connect to the chassis and only use one hand to touch with the other probe with the other hand in your pocket !
That way you can't absentmindedly rest one hand on the chassis while the other is
poking around 500v.
Maybe get your neighbour to come over while you do it and he can pull the plug if you light up !
You will only get the 95 volts on pin 1&8 if these is 525v on the tube at pin 3 .
Thats why the "big one" is the place to start.
The other smaller voltages are independant of this, so give or take a few millivolts should read ok.
If you have a diode test function let the amp discharge for a few minutes (if the transformer unplugs from the power board unplug it or remove the HT fuse) switch the s/b to centre position and measure each diode Cr1,2,3&4.
The diode test function should show a reading one way (.5?? to .8?? approx)and no reading the other.
Have fun !!!
Just to be sure, I need to check and see if the tube is getting its 525 volts? I can do this.
I bought a chinese tube amp (el34 tubes) and within 1 month, one of the tubes went t.u. and took out the resistor. I guess I got lucky it wasnt worse. When I was trying to find the problem, it was suggested to check the high voltage at the tube socket (450 volts). I had planned on waiting for a friend but went ahead and checked it myself, no problems.
Yes, it does get the pucker factor going but thats ok. I will use a gator for the ground and only work with one hand. I may not do this today ( health problems=insomnia) and I want my head straight for this.
I inspected the 2 caps and they look good, no bulging or sign of fluid. I really appreciate the time you are putting into this, Brian
Hello again. I took some measurements at the tube socket and this is what I got:
Pin 5 had 15 volts
Pin 3 (on either tube socket) was showing 2 volts! Does this mean it is the power transformer?
I also measured the resistance of r126 and r130 and they measured at 5.6 and 5.7 ohms.
Tips for the next step? Brian
I guess it's time to send out a search party to find the missing 523 volts !
Don't think it could be the power transformer as it's unlikely to be intermittent.
"I also measured the resistance of r126 and r130 and they measured at 5.6 and 5.7 ohms."
Well thats good we now know that those emitter resistors are ok.
Well it definitely has a problem with the HT supply.
I've attached a simplified circuit of the area you need to concentrate on.
Some times a simplified circuit makes it eaiser to see whats going on
I have also attached a diagram of the connections from the transformer
to make sure there is not something silly there.
Check that fuse is 0.5A or half an amp slow blow (T 0.5A). (T=timed or time lag or slow blow)
I'm not sure if one of the wires is grey or green that leads to the fuse.
Make sure these correspond to what you have.
Try that 2 volt measurement again check it on pin 3 again than check it where the wire from the tubes goes onto the power board*. I marked it on the diagram.
It's where CR3,CR4 and C3 meet so any of those exposed wire "ends" will do
to touch the probe on.
I presume you are measuring while the switch is on the high power setting.
Do the same again on the low power setting- measuring at pin 3 and at * (above).
While it is measuring 2 volts DC , set your meter to measure high AC volts and you can check the AC voltages coming from the transformer where they join to the power board G , BK & R. Between G & R will be the max ac voltage (400 and something)and between BK and G and also BK and R should be approx half of the reading of G & R.
My guess is when you have 2 volts DC on Pin 3, the transformer will still be putting out its required high AC voltage.
Now remove the tubes and do all 4 DC measurements again.
Hi pwr 1.Pin3 tube
2.terminal on pwr board (to tube BD)
LoPwr 1.Pin3 tube
2.terminal on pwr board (to tube BD)
Check the AC again if you want to.
Normally here I would suggest new tubes but.....!!!!! if there was a serious short shorting out 523 volts , I would expect the fuse to go.
Seems like an intermittent connection or one of the bridge rectifier diodes intermittent CR1,2,3&4.
Looking at the simplified diagram apart from the transformers and tubes there are only about $20 worth of bits that could go wrong.
Must confess starting to run out of ideas.
I have not finished with all testing yet but I did re-check the high voltage on pin 3. This time it reached 34 volts before it stopped and dropped back to 2 volts. This was with the standby switch in the high power position. I didn't know if this would help point to something specific but I did want to check and make sure I had measured the right pin correctly. Thanks, Brian
Have we ruled out a faulty standby switch?
Not to sound like a complete idiot, but how do I jumper it. I have blue, red and black wires (in that order from top to bottom). What 2 do I use to jumper? Thanks for your input, Brian
In electronic test equipment, a jumper cable is used to make a temporary contact between two points for the purpose of testing a circuit.
Ok, I jumped the standby switch both ways, high power and lower power and I am still having the same problems.
When I measure out of the power board into the tube socket, I get the same rise and fall of voltage(anywhere from 2 up to 34 volts) as I was measuring at pin 3.( I have not checked the ac power in yet)
I did measure pin 5 again and noticed only 1 tube was receiving 15 volts, the other was 5.
So its looking like faulty diodes ..the fuse is 500mA ? just quickly swap the two tubes and measure
at pin 5 again..see if the 5 volts stays with the tube or the socket.
Having done that lets check the first port of call and measure the AC voltages before it turns into 525DC.
Yes sir, the fuse is a 500mA. I swapped the tubes and the 5 volt measurement stayed with the socket. I assume that is normal.
Now, onto the ac voltage.
The wires that are along the edge of the power board (as in your diagram) are 3-orange, 3-red, and the last 2 are yellow. I am unsure how to measure each one of these. Do I keep the ground from my m.m. on the chassis ground and test G, BK, and R with the red probe. Or is BK the ground for G and R? Brian
To measure these ac voltages simply connect to the ac wires.
For some practice set mm to AC volts put one probe on yellow wire and the other on the other yellow wire ..you should read 6.3 volts approx.
As it is AC there is no + or - for measuring purposes.
When you have 3 wires coming from a transformer one of them should be marked with a stripe to show it's the centre tap. (Otherwise one of them should read the same with respect to the other two).
Now on the orange wires or low voltage supply for the pre-amp between the left one and the centre one (should have white stripe) should be approx 20volts AC.
Likewise From the centre wire with white stripe to the yellow one on the right
should read 20v AC.
Therefore from the two outer yellow wires you should get approx 40v.
We know that this supply works as the pre-amp works ... the voltages are my guess's and are only approximate.
So now we measure the HT or red wires the same way. Outside to centretap x 2approx 200v AC then outside to outside approx 400v.
The two pin 5 voltages should be the same but lets leave this for the moment 'till
we get the HT going.
Ok, I got the ac measured and all looks good.
The 2 yellows are at 6volts
The low voltage(orange) measured 20 volts each and 40 together.
The high(red) voltage measured 185 volts each leg and 370 volts together.
Does this point to the diodes even more, now? Thanks again for walking me through this. Brian
Good. As the heater measures 6 volts I presume this was measured with the tubes in ?
If not try it (by the way with tubes in should really have speakers connected ).
So now you can measure 370 volts AC going into a bridge rectifier and 2 volts DC coming out..maybe 34volts on a good day!
It can't be a heavy load or short circuit on the DC out as the 500mA fuse would normally blow.
Therefore all the solder joins ,pc tracks, components,and connections including the fusehave to be checked as it appears there is a very high resistance somewhere.
Actually something is staring me in the face.
It's the chassis ground side of the HT supply which is also the ground for one side of the heater supply and CR31 and CR38 at the tube sockets.
It seems from the layout of the power supply there are three wires that go in a connector to the Tube Board which we don't have a layout picture of .
They are the HT 535v wire ,the combined ground and heater wire and the other heater wire.
The "combined ground and heater wire" should end up connecting to the chassis , one pin of the tube socket and the grounds of CR31 & CR38 at the tube board.
Check thats the case ..if you have a continuity "beeper" in your mm one probe on the second heater winding from the right on the power board and the other to the chassis , should show connected or on ohms scale a very low reading.
When checking that earth connection may be less confusing to remove the tubes and heater fuse.
Check the fuse holder F1 and fuse for any corrosion or oxidation - rub with emery /fine sandpaper if necessary check the soldering underneath and the solder plated copper tracks for any hair line cracks.Especially check around the diodes CR1,2,3&4 .
If you look closly at the diodes you may see a small crack or rupture (hopefully !) .
Were you able to do this ?
Well maybe for the cost $1.30? you could buy 4 x 1N4007 diodes and put them in.If you have a diode test function let the amp discharge for a few minutes (if the transformer unplugs from the power board unplug it or remove the HT fuse) switch the s/b to centre position and measure each diode Cr1,2,3&4.
The diode test function should show a reading one way (.5?? to .8?? approx)and no reading the other.
Note the stripe on one end.
With the amp off and discharged - check either with continuity "beeper" or a low ohms setting all over that power board to see that all the tracks and connections are ok.
Bear in mind the transformer windings will give a very low reading as will the heaters if the tubes are plugged in.
I would pull out the fuses and say.. check between the cr2 lead and the cr4 lead and see if they are joined together as they should be via that track.
Check from there to the fuse holder end they are supposed to be connected to etc.
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