Sho-Bud Repair... Help Needed Please

[MikeBloomfield]

I'm working on a Sho-Bud; date codes on caps appear to be from 1972. Single channel w/Reverb. It came from Europe back the US - had a step-down transformer mounted in the cabinet. Anyway, it had been in storage for 20 or so years and was brought to me to check over and make it work on 120vAC. I brought it up slowly with a variac and all was well 'til I hit 115v or so: one of the 100Ω resistors burned, a .33Ω resistor opened and the outputs shorted.

The amp uses two 2N3773 outputs, which I was able to find at Mouser. I replaced the resistors and outputs and tried the amp: No output. I suspected the drivers and removed them from the board and tested them. The 40407 tested OK, the other side had a 2N4314 which tested bad. I replaced the 2N4314 with an NTE129 and tried again. The amp passes signal but it is weak and distorted. I adjusted the bias, but that doesn't clean it up and the output transistor on the side that now has the NTE129 driver (positive side?) becomes hot while the other side remains cool at idle.

I have no schematic for this amp, but have been using the schematic for the amp that uses 40636 outputs just to get some idea of a Sho-Bud circuit.

My Questions:
1. Is the 2N4314 what Sho-Bud originally used? I would have thought it would had been a 404xx.
2. What can I use to replace both drivers? NTE128 & NTE129, or are there better alternatives?
3. If the NTE129 I used is weak or "going bad", would that cause the issues (weak output, distorted, 2N3773 on that side heating up)?
4. Any idea what the correct bias current would be?

Thanks for any help offered.

[DrGonz78]

Not really trying to advocate for E-bay here... But I sometimes find old parts available for a price. This might be too much to have to pay, but it's relative to buying parts w/ shipping from digikey or mouser etc. My only intent here is to show that the original parts can be found... Best of luck.

Vintage RCA 2N4314 PNP Sil HF Amp Trans Lot of 10 | eBay

[mozwell]

Sho Bud Diagrams, Schematics and Service Manuals for FREE 2 schematics here, the second one looks to be the one you are using.

Looks to be +/- 45VDC rails for the power amp, so around 120W into 4 ohms
I do not know what the original transistors were in this amp.

Suitable drivers would be MJE340 & MJE350, also MJE15030 & MJE15031
You could also upgrade the power transistors MJL21194 plastic flat pack, MJE15003 in TO3 case

If you use these other drivers, put them on a small heatsink, and make sure the heatsink is isolated. Use a separate heatsink for each driver.

Adjust bias to measure (say) 30mA through each transistor with no signal connected & no speaker connected, you can measure across the 0.33 ohm resistor, and set bias so you measure 10mV

i would also add a small output inductor & Zobel network (R & C in series) on the amp output. I woudl use 10 ohm 2W & 100n 100VAC, These are L1, R27 & C12 on the attached sch.

As you have a bipolar pwoer supply, you could also change the power amp to to use NPN / PNP outputs. These early amp designs had to use all NPN output stages, as high power PNP were not readily available.
You could use something like the "SC480" attachment, it woudl be easy enough to modify the power amp in this way

[MikeBloomfield]

Still have not gotten it working. The Output board has "PA-1A" on the bottom. The amp does not conform to any of the Sho-Bud schematics available.
Can anyone tell me what these two devices in the white rectangle are?

[Enzo]

They look like a pair of diodes clamped to the chassis next to the power transistors. The chassis is the heat sink, and those diodes are the thermal compensation for the bias circuit.

[MikeBloomfield]

Thanks Enzo. Here are 3 pics from my scope 1. Preamp out NOT connected to Output Board. 2. Preamp out connected to Output board. 3. Speaker output into 8 Ohms

Any idea what's happening and what I need to do? Thanks!
1.
2.
3.

[J M Fahey]

Thanks Enzo. Here are 3 pics from my scope 1. Preamp out NOT connected to Output Board. 2. Preamp out connected to Output board. 3. Speaker output into 8 Ohms

Any idea what's happening and what I need to do? Thanks!
1. [ATTACH=CONFIG]21060[/ATTACH]
2. [ATTACH=CONFIG]21061[/ATTACH]
3. [ATTACH=CONFIG]21062[/ATTACH]

1) your PCB matches exactly the one shown in Sho Bud Diagrams, Schematics and Service Manuals for FREE
2) your scope waveform #3 shows lack of positive drive or current capability.
Just to check, repeat without load connected, probably it will look normal, since it will not need to supply current.
If it still looks the same, then some transistor does not stand needed voltage, I deeply distrust NTE "replacements" so I would put them out of the equation.
For a static check, short Power Amp input, disconnect load, you should measure 600/650mV across BE junctions.
Positive in NPNs, negative in PNPs.
*All* transistors must meet this condition; if some one does not, we must find why.
You may have a bad or misconnected positive driver, the top 40409 .
You can use TIP31C for 40408/40409 and TIP32C for 40410, no real need for heatsinks and of course any of the above recommended TO3 transistors, from 2N3773 to MJ15003 to MJ1502x , all beefy NPNs.
For now try to repair it as-is, rather than modding.
MJ15030/31 will also work well, in the approppriate position, but MJ340/350 would be marginal providing drive current to output transistors.
Make and use a lamp bulb limiter until the amp is pronounced good.

[52 Bill]

Juan is right, it look exactly like the schematic referred to above.

The board looks like it has had some questionable repairs done to it. For example, the schematic shows three 100 ohm resistors in the driver section. I see two 100 ohm resistors and a 10 ohm 1 watt. The over sized ballast resistor is also poorly installed.

I'd suggest checking all of the parts and compare them to the schematic to see if the resistors are right or wrong and whether or not the pc card contacts were cleaned when the board burned.

[Enzo]

And do not focus laser-like on finding bad parts. An open connection between parts is just as damaging as a bad part. And just as likely.

[MikeBloomfield]

I installed the TIPs in place of the 404's on the schematic (except for the matched pair) and have 2N3773s for the outputs. The AC current draw is fine, but the 100 Ohm resistor between the 40408 and 40410 on the schematic is burning up. Also, I had to substitute a different pot for the bias trim pot, since it was oddly fluctuating between 0 and 6 meg ohms. I'm using a 4.7k pot set to about 48 ohms. But I can't read any voltages 'cause that resistor is burning up.

Ideas?

Hey guys, thanks for all your help!

[52 Bill]

Double check your work, pinouts are different on those transistors. Are you certain the replacements are in correctly? NPN where NPN should be? PNP where PNP should be?

Did you test the two bias string diodes?

For the bad bias trimmer, I'd install a 100 ohm resistor and add the temporary trimmer in parallel to it. You really don't want to have the bias string go open.

[J M Fahey]

Agree and add: that

100 Ohm resistor between the 40408 and 40410 on the schematic is burning up.

because it's dropping way more than the 1V max it expects.
Why? : it is the BE resistor for the top TO3 transistor.
The top TIP31 feeds its base and it amplifies current 20x and drives the speaker.
Now, if top 0.33 ohm resistor or top TO3 or the connector/wiring are open, the poor TIP tries to drive the speaker on its own, through the poor 100 ohm resistor ... failing miserably and chopping the upper half of the wave .
To be able to measure something use a lamp limiter, it's faster than your hand adjusting the Variac, and check these connections, also parallel 100 ohms with the trimmer pot.
As Enzo says, I also suspect some bad connection, including that 40 y.o. edge connector.
Use a rubber eraser on the card edge "fingers" until copper is clean, then wet it with a little WD40 or something and wipe it once with tissue paper, so it stays *very lightly* oiled ("naked" copper oxidizes in a few hours) and move it in/out the connector a couple times, to scratch it clean.
Forget all this if the "fingers" are factory gold or nickel plated, of course; in that case just wipe them with a clean rag with a drop of WD40 and then with another clean rag, no oil.

Once your amp looks DC stabilized, recheck you have 500/600mV at the TO3 base on the transistor itself.
Also check that its socket makes good contact, it may be worn or weak.
Also check across the 100 ohm resistor.

PS: I liked this amplifier, maybe I'll build one just for kicks.
There's also "clean" in the Guitar World.
Already have the power section, mine is basically the same, and the preamp may be kludged in one afternoon or two.
And the reverb brings fond memories, it's the same as published in Popular Electronics in 1968, and it was one of my first commercial products.
PCBs made with my Mom's nail enamel and Letraset front panels on hand brushed aluminum

[52 Bill]

And the reverb brings fond memories, it's the same as published in Popular Electronics in 1968, and it was one of my first commercial products.
PCBs made with my Mom's nail enamel and Letraset front panels on hand brushed aluminum

I remember that project! I wanted to build one to install on my 1949 Supro amp. SWTP had a lot of great DIY projects back then.

[MikeBloomfield]

Hi Juan. Got back to this amp tonight. I replaced the outputs with MJ15024s. New 100Ω BE Resistor for the 40408, .33Ω bias resistors, 100Ω bias trim resistor.
The amp is stable, but still cutting off the positive wave.
See attached pic for B-E voltages. The matched pair of 2N4249s both read 631mv. I now suspect the 40408 is bad. Can I safely replace it with a TIP31C?
Thanks again for your help!

[soundguruman]

What you gotta watch out for, is those edge card connectors go bad.
Are you sure you have clean connections and continuity on all those plug-in points?
make sure you are getting a connection, all the way from the wire on the card connector, to the components on the circuit board.
Check ALL the plug in points, on ALL the boards, with yer ohm meter.
a lot of times on old amps, you gotta bypass those plug in connections, and solder the wires straight to the board.

If you wanna try clean the card, take it out of the connector,
rub the edge connections with a white gum eraser. Get all the old crud off of the contact points.

You are losing the + voltage in yer wave form, so something is loading it down or dropping it....a bad leaky electrolytic cap? A burned edge card connection? A reverse leaky transistor or diode?
pay close attention to burn marks on the fiberglass also. If there is any. Those burn marks are carbon, which can form an "extra" resistor on the circuit board.